Chapter Iv. the Plant Formation

METHODS OF INVESTIGATION AND RECORD

=198. The need of exact methods.= The use of instruments in the study of the habitat has made it evident that the loose methods of descriptive ecology were altogether inadequate to the accurate investigation of the formation. This feeling has been heightened by the recognition of the fact that vegetation exhibits both development and structure, and is, in consequence, open to exact methods of inquiry. In the search for feasible methods, it was quickly seen that the quadrat, first used for determining the abundance of species, furnished the key to the problem. Accordingly, the principle underlying it, viz., that of intimate detailed study and record, was developed and extended in such a way as to give rise to a number of methods of precision. These have been applied in the field for several years with signal success, and they are here described in the conviction that they constitute a satisfactory system, if not, indeed, the only one for the exact study of formations.

There has been a growing appreciation of the fact that the superficial methods of descriptive ecology made it impossible to build upon such a foundation, and they, indeed, were making actual progress in the field of ecology more and more difficult. Ecologists have now begun to see clearly that precise methods are as indispensable in the habitat as they are to the study of the structure and modification of the plant. For some reason, however, they have been slow to perceive that accuracy in the investigation of the cause, the habitat, is a fruitless task unless it be followed by corresponding exactness in the study of the effect, the formation. After having urged the fundamental necessity of instrumental methods, for six or seven years, both in season and out of season, the writer does not feel called upon to further plead the cause of the quadrat. The final acceptance of the instrument was inevitable if progress were to be made in the habitat, and it is just as obvious that the quadrat must be accepted if the study of the habitat is to bear fruit in the interpretation of the formation. The use of the quadrat does not mean that the general methods of descriptive ecology are all to be discarded, whether they have value or not. The statement that quadrat methods are indispensable signifies merely that they must be used for research work in the development and structure of vegetation. They are not necessary in reconnaissance, nor do they displace general methods of real value. The use of the latter in even a supplementary way will gradually be discontinued, however, as fields become smaller by reason of increase in the number of workers, and as the need for precise methods becomes more universally felt.

The quadrat constitutes the initial concept from which all the methods have grown. In itself, it has given rise to a variety of quadrats applicable to the most fundamental problems of vegetation. From it have come, on the one hand, the migration circle, and on the other, the transect. The latter in turn has yielded the ecotone chart, and the layer chart. All of these are based upon direct and detailed contact with vegetation itself, and permit accurate recording of all the results obtained.

QUADRATS

=199. Uses.= In its simplest form, the quadrat, as the name implies, is merely a square area of varying size marked off in a formation for the purpose of obtaining accurate information as to the number and grouping of the plants present. As indicated above, it was first used for determining the abundance of the various species of a formation. This made it possible to ascertain the relative rank of the species of layers and formations, and enabled one for the first time to gain some idea of the minute structure of a bit of vegetation. The results were at once applied to the task of establishing a numerical basis for abundance, and of working out a new system of abundance to correspond. The quadrat method was also used to determine the character of seasonal aspects, and to yield a knowledge of the exact differences in diverse areas of the same formation. Incidentally, the determinations of abundance were made the basis of an actual census of certain alpine formations. This, while it was extremely interesting to find that a square mile of alpine meadow contained approximately 1,500,000,000 plants, was confessedly destitute of ecological value. The most important applications of the quadrat idea were made by Clements in the chart, the permanent and the denuded quadrats. The development of these was due to the fact that zones or formations permit of comparison upon floristic as well as physical grounds, and that a detailed record of their structure is necessary for this purpose. Similar comparisons are necessary for the consocies, zones, and patches of the same formation, and the quadrat becomes an indispensable means for studying alternation and zonation. For the investigation of invasion year by year, and especially for succession, the method of permanent quadrats is imperative, and the denuded quadrat an invaluable aid. Changes, which would otherwise be incompletely observed and imperfectly recorded, are followed in the minutest detail and recorded with perfect accuracy.

=200. Possible objections.= The use of the quadrat has led to the criticism that it is needlessly detailed and thorough, and that, after all, the space covered is but a minute part of the entire formation. The first objection is one that has also been urged against the use of instruments of precision in the habitat. It is always brought forward by those who have not used instruments, and as witnesses they are of necessity incompetent. No one who is familiar with the instrument or the quadrat by actual practice has felt that the methods based upon them were too thorough. In no case has the writer ever listed or mapped a quadrat without discovering some new fact or relation, or clearing up an old question. It can not be denied that quadrat methods require both time and patience, but this is true of any kind of research work that is at all worth while. Every ecologist, moreover, that has the interests of his field at heart and deprecates the present slipshod work, will appreciate the necessity of methods which seem like drudgery to the mere dabbler.

The second objection, that the quadrat is at best but a small bit of the area under investigation, seems at first to be a valid one. It can not be gainsaid that the actual space studied is insignificant as compared with the whole formation; still, it must be obvious that even a single quadrat can add at least some facts of value, which can never be obtained by the best of general methods. Furthermore, if the formation be an actual and not an imaginary one, a single quadrat will be in some measure representative. In the more homogeneous ones, it will have much the same value that a type specimen bears to the species established upon it. In formations which are less uniform, its value is correspondingly reduced, so that in formations which show marked zones, consocies, or patches, it becomes necessary to locate a quadrat in each. In the matter of representation alone, the graphic method of the quadrat map with its close-focus detail photograph, is far superior to anything that can be obtained by the ordinary description and photograph. Finally, the scientific study and recording of succession, and particularly of competition, is an impossibility without the aid of the permanent and denuded quadrat. The stoutest champion of the practice of walking through a formation, and jotting down impressions, can not avoid their use if he would attack these problems, and, once familiar with the quadrat, his objections to the drudgery of thoroughness will soon vanish.

Kinds of Quadrats and Their Use

=201. Size and kinds.= The unit size of quadrat is the meter, and when the term is used without qualification, it refers to the meter quadrat. To make them strictly comparable, and exactly divisible, unit quadrats are always grouped in squares; thus a major quadrat is a square of four units, and a perquadrat one of sixteen units, or four meters square. Quadrats of greater size are necessary in woodland and forest, where the rule, however, is that the woody plants alone are recorded for the whole quadrat, the herbaceous growth being listed or mapped for but one or two representative units. For special purposes, quadrats of 3, 5, 6, etc., meters may be used, but they are much less convenient. Quadrats are further distinguished with respect to their use. A list quadrat is one in which the plants are merely listed and the number of individuals of each species indicated. Chart quadrats are those in which the area concerned is accurately mapped on plotting paper. Both list and chart quadrats are rendered permanent by careful labeling, so that their changes can be followed from year to year. The greater value of the chart causes practically all permanent quadrats to be of this type, and for the same reason only permanent chart quadrats are converted into denuded ones.

=202. Tapes and stakes.= The lines for marking out quadrats are made of strong white tape, ⅝ inches wide. This is doubled and sewed firmly at both edges. Under moderate stretching, the tape is carefully marked off into decimeters, and eyelets 5 mm. in diameter are set in at each end and at the marks. This can readily be done by any shoemaker at slight expense. The usual lengths are one and two meters, as these are most frequently used, and they can also be easily combined to make larger quadrats. The tapes are slightly longer than one meter in order that the distance between the end eyelets may be exact. The tapes of the larger forest quadrats should be divided into lengths of one meter, as these permit ready plotting and also make it possible to interpolate a meter quadrat for the study of the undergrowth at any point. The intervals of the tape are numbered from left to right, as conspicuously and clearly as possible. For this a waterproof ink or paint is very desirable. For holding the tapes in position, hatpins, nails, and meat-skewers have been used with more or less satisfaction. The ideal stake, however, is one which holds the tape close to the ground, and can be readily moved. It is merely a stout wire, 3 mm. in diameter and 8 inches long, looped at the top, sharpened at the tip, and with a small ring of solder 3 inches from the tip.

=203. Locating quadrats.= In staking a quadrat, the end tapes are invariably placed so that the numbers read from left to right, and the side tapes so that they read down. In mapping, a fifth tape is stretched parallel to the top, and as each decimeter strip is marked, the outer tape is shifted to delimit the new strip. Indeed, the side tapes can be placed alone, and the plotting tapes moved down one at a time as the mapping proceeds, but it is usually more satisfactory to locate the quadrat exactly and to square it first, a task most easily done by enclosing the whole quadrat, and then using a fifth tape. In the case of list quadrats in open vegetation, the measuring strip is unnecessary, but as a rule it facilitates counting, as well as mapping.

The List Quadrat

=204. Description.= This, as the simplest form of quadrat, is employed primarily to ascertain the abundance of species in a formation or during a particular aspect of it. Since this can be obtained readily from the chart, the list quadrat has fallen more and more into disuse, except where it is desired to determine abundance alone, or to aid in deciding whether a chart is really representative. The size depends almost wholly upon the nature of the vegetation. When the number of trees is to be determined, a quadrat of 10 or 50 meters is necessary. In ordinary herbaceous formations, the usual size is 2 meters, while the meter quadrat is used when the plants are especially small and crowded, as in alpine meadows. The location of the quadrat is based upon the general rule, but since its especial task is the determination of the greatest variable in vegetation, viz., number, it is necessary to use more quadrats, and to place them in areas which show the greatest differences in the mixture of species. For example, it was found that a half dozen list quadrats, when carefully located in the prairie formation, gave results almost identical with those obtainable from a larger number. With a little experience, the various degrees of mixture can be picked out superficially, and the corresponding number of quadrats established. If a single list quadrat is to be made for a formation or station, such a time should be selected as will make it possible to cover the greatest number of plants. Fortunately, this usually falls near the middle of the summer, when the remains of spring plants are still in evidence, and the autumn ones are sufficiently developed to be recognizable. In taking the census of different aspects, the quadrat should be made as near the middle of the period as is possible.

=205. Manner of use.= In listing a quadrat, i. e., counting the individuals of each species, the plan followed is to list the smaller, less conspicuous plants first, since they are apt to be tramped down. As a rule, the outside tapes and the taller species afford sufficient landmarks. When this is not the case, the measure tape is used, and the individuals of all species are checked as they are found, while in the first method one species, rarely two, is taken at a time. In cases of peculiar difficulty, it may be permissible to pull or break plants as they are counted, but ordinarily this can and should be avoided. Clusters, and bunches of stems from the same root are counted as single plants, and the number of stems indicated by an exponent. In the case of bunch grasses, each bunch counts as one plant.

=206. Table of abundance.= The species are arranged in the final list in the order of their numerical importance, and are divided into groups which correspond to the different degrees of abundance. The latter are arranged in two series, based upon the fact that association is by groups or by individuals. The table of abundance, based upon a 2–meter quadrat rather than upon the 5–meter one, by means of which the earlier results were obtained, is as follows:

Social exclusive, no other species of vascular plants present

social inclusive, above 100

gr^1 gregarious^1 100–50 copious^1 cop^1

gr^2 gregarious^2 50–25 copious^2 cop^2

gr^3 gregarious^3 25–10 copious^3 cop^3

sg subgregarious 10–5 subcopious sc

vg vixgregarious 5–1 sparse sp

It is obvious that the above outline is faulty inasmuch as it takes no account of the height and width of the individuals. This is a serious defect, and it constitutes one of the many reasons why the list quadrat should be replaced by the chart quadrat. The prairie formation affords an unusually striking illustration of this. A single quadrat may be filled by ten plants of Psoralea floribunda, and at the same time contain 22,000 plants of Festuca octoflora. Yet the former is conspicuous and controlling, the latter plays an altogether insignificant role. This difference is readily shown by comparing a plant of each. The one is 3 × 3 feet, the other 3 × ¼ inch. Such figures furnish a valuable check upon mere number, but make the brief, graphic designation of abundance difficult. An attempt has been made to solve this problem by roughly determining the space occupied by the plant, by means of the formula, height (πR^2) × abundance. This would give Psoralea a value of 210, and Festuca one of 1.6, which much more nearly represents their real importance in the formation. Abundance or numerical value is a floristic concept entirely, and has little place in ecology unless checked in the way indicated. The whole problem, ecologically, depends upon an intimate knowledge of competition, and its solution in consequence is at present impossible.

The Chart Quadrat

=207. Description and use.= The detailed labor required in mapping makes it advisable to use the meter quadrat. An additional reason of much importance is furnished by the desirability of securing a detail photograph of the quadrat. This is impossible with field cameras, which should not exceed 6½ x 8½ inches, and are indeed most serviceable in the 4 × 5 size, if the area be larger. In open formations, the major quadrat of 2 meters can be used if necessary, but this is very rarely the case. Forest quadrats of ten meters square are easily charted, but detail photographs can not be made of them. Larger quadrats are impracticable; they can be counted but not mapped to advantage. The location of the chart quadrat must be decided by the structure to be studied. Its greatest service is in connection with zones and societies of the same formation, which can be easily compared in the chart form. In fact, the chart quadrat may well be regarded as the fundamental method for inquiry into zonation and alternation. It is an important aid in delimiting areas from the contiguous formations, and in determining the relationships of mixed formations. It is also used to record the character of the different aspects, but this is done more satisfactorily by the permanent quadrat.

=208. The chart= used is a decimeter square, and the scale is consequently 10 : 1. It is outlined on centimeter plotting paper, and the centimeter squares are numbered at the edges to correspond to the intervals of the quadrat, i. e., the top and bottom lines are numbered from left to right, and the side lines from top to bottom. These outlines are ruled in quantity and used as needed, or the forms can be furnished by the printer. In practice, a special quadrat book the size of the chart has been used. The need of a second book may be avoided by outlining two charts on the plotting sheet, and filing the latter in the field record book. In the few cases where 2–meter quadrats are desirable, four charts are used, care being taken to label them so that they can be combined whenever necessary. Ten-meter quadrats are recorded on the decimeter chart also, each meter interval corresponding to a centimeter, i. e., the scale is 100 : 1.

=209. Mapping= is invariably begun at the upper left-hand corner of the chart, and is carried across the strip marked off by the plotting tape, decimeter by decimeter. As soon as this strip is completed, a second one is formed by moving the top tape to a position one decimeter below the plotting tape, which then becomes the upper one. This is repeated until the last strip is reached. Little difficulty is experienced in locating each plant exactly, as the decimeter interval is small, and the centimeter square which corresponds is divided into twenty-five tiny squares. Each plant is put in whenever possible, but mats, turfs, and mosses are merely outlined in mass if the individuals are not distinguishable. This holds true of all large rosettes and mats, even when they are single plants. Symbols were formerly used for indicating the various species. They have the advantage of requiring little space on the chart, and the disadvantage of necessitating constant reference to the legend. They are at present replaced by initials. By this plan, the decapitalized first letter of the generic name is used if no other genus found in the quadrat begins with the same letter. If, however, two or more genera begin with a, for example Agropyrum, Anemone, and Allium, the most abundant one is indicated by a, and the others by the first two letters, as an, al. In case two species of the same genus are present, the species initial is used in connection with that for the genus, as ac and ar for Agropyrum caninum and Agropyrum richardsonii respectively. It is rarely necessary to exceed two letters for any species. Plants which regularly have several stems from the same root are indicated by the initial and an exponent as a^3. Seedlings are represented by a line drawn through the letter. Usually the chart sheet affords sufficient space below the chart for the legend. When the list of species is long, the back of the sheet is used.

=210. Factors and photographs.= Each chart is numbered, and the formation, station, and date indicated. The constant factors, altitude, slope, and exposure are ascertained and recorded on the sheet. The variable factors are read in each quadrat whenever possible, and in addition to being preserved in the record book, are noted on the chart sheet along with the base reading in the formation for the same time. This facilitates the interpretation of the differences found when two or more charts are compared. Chart quadrats are regularly photographed. For this purpose a long focus 4 × 5 camera with a telephoto lens is used. At the proper distance this will make a view of the same size as the chart, thus making possible an exact comparison of the two. The chart and photograph serve as mutual checks, as well as complements, since the former shows number, position, and arrangement, and the latter, height, form, position, and arrangement. The view is usually made by placing the camera directly in front of the middle of the lower tape, at such a distance that the side tapes fall just within the limits of the ground glass. The swing is always used in order that the focus may be uniformly sharp. Surface views of the quadrat can be taken by means of a device which permits the camera to hang downward from the tripod, or by means of a tripod with a swinging platform. Such views are especially valuable for the study of competition, since they give a clear idea of the spread and density of the various plants. They are difficult to make unless the vegetation is low and nearly uniform in height. The usual photograph is much more serviceable in regular quadrat work.

The Permanent Quadrat

=211. Description and uses.= As stated heretofore, either list or chart quadrats may be rendered permanent in order that they may be followed from season to season or from year to year. As a matter of fact, however, an area which is to be studied repeatedly really demands charting, and in practice chart quadrats alone are made permanent. This is done simply by driving a labeled stake at one corner of the quadrat, and locating the latter definitely in relation to a conspicuous landmark. When one is in residence for several years, practically all chart quadrats are converted into permanent ones, since the work already done in the chart quadrat is so much accomplished towards the permanent one. This is not necessary when one wishes merely to compare different areas of stable formations. As a rule, however, some change is constantly being wrought by invasion or competition, and the amount and direction of this can only be revealed by the permanent quadrat. The latter has a fundamental value for all kinds of invasion, but it is absolutely indispensable in studying complete invasion or succession, and in discovering and recording the gradual effects of competition. It is in the detailed investigation of these dynamic phenomena that the paramount importance of the quadrat is most evident. If the experience of several years be taken as conclusive, no other method is capable of revealing the minute changes as they are occurring.

The permanent quadrat is regularly 1 meter square, a size determined both by the exigencies of charting and photographing. When ecograph batteries are used, the quadrat is located as close to the latter as is possible. Otherwise, the quadrat itself should constitute a station for making factor observations. This connection is absolutely essential, since the quadrat is used expressly to determine the structural changes, which are produced by physical factors, and the reaction of vegetation upon them. Permanent quadrats are established in different formations or stages of a succession to trace the invasion of new species and the dropping out of old ones in response to competition. They serve to distinguish the proper formation, which represents a particular stage of development, from the mixed formations which precede and follow, and also to determine the exact course as well as the rapidity of the change that follows each reaction. When applied to different examples of the same stage, and to all the different stages of a succession, the whole development of the latter may be minutely traced and definitely recorded. The importance of following the changes from aspect to aspect is much less, since these are periodical rather than dynamic. They are an essential feature of structure, however, and it has been the practice to make at least one series of aspect charts from each permanent quadrat.

For tracing the invasion and competition of lichens and mosses, which play a primary role in initial formations, a subquadrat is used. The size varies, but it is usually smaller than the quadrat, although the latter is entirely available in the case of the large foliose lichens. For the crustose and smaller foliose forms, a subquadrat 2 decimeters square is used, and for the larger forms and tufted mosses, one of 5 decimeters. In the case of ground forms, tapes are employed, and the quadrat is permanently staked. On rocks and cliffs, where moss and lichen stages are most common, tapes are impracticable, and the quadrat is permanently outlined with paint. Charts of lichen quadrats are made to the usual scale of 10 : 1.

=212. Manner of use.= Permanent quadrats are mapped and photographed in exactly the same way as chart quadrats. As soon as this has been done, a labeled stake is driven at the upper left-hand corner, so that its edge indicates the exact position of the quadrat stake, and a smaller one is placed at the opposite corner to facilitate the task of setting the tapes accurately in later readings. The label stake bears merely the number of the quadrat and the date when it was first established. It is firmly fixed and allowed to project just enough to enable it to be located readily. Its position requires careful landmarking when the quadrat is to be visited year by year. In forest formations, this is readily done by blazing, but in grassland it is necessary to have recourse to compass and pacing, or to erect an artificial landmark. After several charts have been made, a permanent quadrat attains a high value, and every precaution must be taken to prevent losing its exact location. At the second reading of a quadrat, whether in the succeeding aspect or year, the tapes are placed with reference to the stakes, and a chart and photograph are made in the usual manner. These are labeled and dated like the original ones, but they are numbered to indicate both the quadrat and the series, e. g., 15^2 indicates the second chart, and photograph made of quadrat 15. The date indicates whether the readings are by the aspect or the year, though this may be shown also in the name of the series itself. It is clearly an advantage to have the two successive charts of a quadrat upon the same sheet, and to file all the charts and photographs of the same permanent quadrat together, and in the proper order.

Since much of the value of a permanent quadrat depends upon its use as a station for observing physical factors, it is unprofitable to establish a large number. The results of invasion and competition can be ascertained by the quadrat alone, but these should be merely preliminary to seeking for their causes. Clearly, a quadrat should be established for each battery of instruments, while additional ones should be located only in so far as they can be visited often enough to give an insight into the factors that control them. In view of the fact that the most important factors, water-content and light, are less variable than humidity, temperature, and wind, it will suffice if visits are made once a week. This is especially true when it is possible to refer the more variable factors to the continuous records of a base station. While all the results determined for permanent quadrats are preserved in the field record, a record of them is also kept on the reverse of the chart sheet for convenience in interpreting the different charts.

The Denuded Quadrat

=213. Description.= This is primarily a permanent quadrat from which the plant covering has been removed, after it has been charted and photographed. What is practically the same thing is obtained by establishing a permanent quadrat in a new soil, or in one recently laid bare and not yet reclothed with plants. These, however, are merely permanent quadrats, in which the first chart and photograph furnish a record of the habitat alone. They are of great importance in succession, and will be more fully discussed under experimental vegetation. The denuded quadrat is of the usual size, 1 meter, though the smaller lichen quadrats are also denuded. The location is subject to the conditions already indicated, especially with reference to physical factors. The denuded quadrat, however, is particularly adapted to the study of invasion and the resulting competition. Consequently, when migration is markedly from one direction, a series of denuded quadrats throws a flood of light upon the actual steps in invasion. Denuding is a valuable aid in succession, but it must be clearly recognized that, while permanent quadrats register the exact course of the succession, denuded ones can merely furnish facts as to the probable courses of stages not now in evidence.

=214. Methods of denuding and recording.= Permanent quadrats may be denuded at any time during the time they are under observation. The best results, however, are to be obtained by establishing the two side by side, or at least close together. In this way, they are mutually supplementary, and furnish the most evidence possible with regard to the procedure of invasion and competition. Another advantage is found in that the same observations of climatic factors will do for both, though water-content and soil temperatures are necessarily different. A quadrat which is to be denuded is first mapped, photographed, and labeled exactly like a permanent quadrat. The vegetation is then destroyed. This is usually done by removal, though it may also be burnt, destroyed by flooding, or in some other manner. The method will depend upon the use which the quadrat is to serve. If it is to throw light upon the vegetation of an area in which denudation has affected the surface alone, the aerial parts only are removed by paring the surface with a spade. When the disturbance is to be more profound, the upper seed-bearing layer is removed, and the underground parts dug up. In the interpretation of a secondary succession, the denuding cause is made use of in a fashion as nearly natural as possible. Ordinarily, the plants are removed just below the top of the ground by a spade, leaving the underground parts undisturbed. This method has yielded very interesting results.

Quadrats have been denuded in the fall after the majority of the plants have completed their growth. This is largely owing to the fact that other field work is less pressing at this time. Denudation can be done as well in the spring, though the invasion will be slower in this case, since the seeds which have accumulated will be partly or entirely removed. During the first season the denuded quadrat should be mapped every month, and, if the invasion be rapid, photographed also. In open formations, especially those of a xerophytic nature, a single chart and photograph made at the end of the season are sufficient. In a few cases of this sort, indeed, no invaders have appeared until the second year. Beginning with the second season, a single record taken near the close of the growing period will suffice. Denuded quadrats are labeled, dated, and filed exactly as other permanent quadrats, but it should be noted that the first member of the chart and photograph series is that which records the original vegetation of the area denuded.

=215. Physical factors.= When denuded quadrats are single, their physical factors must be observed in the usual way. If they are associated with permanent ones, the ordinary readings are made for the latter, and those factors which are affected by exposing the soil are alone taken for the denuded area. These are the water-content, soil and surface temperatures, and in some stations at least the humidity near the surface. As everywhere, water-content is the most important, but the temperature at or near the surface has a marked effect upon germination. Because of its bearing upon the latter, the surface water-content is usually determined also. This has been done by taking a surface sample 2 inches square and 1 inch deep. Denuded quadrats naturally show considerable differences from year to year as the action of the invaders becomes more pronounced. To this fact is due much of their value as aids in interpreting succession.

Aquatic Quadrats

=216. Scope.= The preceding discussion of quadrat methods is based wholly upon their use in terrestrial formations. Wet meadow and dry bog are the wettest places in which quadrats have been used. It is clear, however, that with certain necessary modifications, quadrats can be used as successfully, though not as conveniently, in many water formations as in land ones. The tapes need to be raised above the surface of the water by longer stakes, and photographs often taken from a boat, but otherwise the usual methods apply, at any rate for bogs and shallow bodies of water. In lakes or streams the tapes might be attached to buoys or floats. The determination of factors is made as usual. Permanent quadrats are feasible in many cases at least, and denuded quadrats are not altogether impossible.

TRANSECTS

=217. The transect= is essentially a cross section through the vegetation of a station, a formation, or a series of formations. It is designed primarily to show the order of arrangement of species in zones and societies, but it also serves as a record of the heterogeneity of any area. In the form of the layer transect, it furnishes a graphic method of representing the spatial relations of the species in layered formations, e. g., forests, ponds, and lakes. It is merely a logical extension of the idea underlying the quadrat, and the transect is, indeed, little more than an elongated quadrat. An important difference, however, lies in the fact that the former normally traverses areas more or less unlike, while the latter is always located in a homogeneous one. Furthermore, the transect is plotted with especial reference to the topography. With respect to dimension, transects are classified as line, layer, and belt transects, and the latter may also be permanent or denuded.

The Line Transect

=218. Description and method.= A simple transect is sometimes made by establishing the points between which it is to be run, and then recording the plants pace by pace along this line. This is satisfactory where the striking changes in structure are desired. A more accurate method is ordinarily used, since it gives detailed results, and at the same time brings out the more general features. For this, use is made of a tape of proper length which is divided into decimeters. Tapes of 10, 50, and 100 meters are used, and if they are furnished with eyelets, transects of intermediate lengths may be run with them. When longer transects are desired, as in the case of forest formations, tapes of 500 or 1,000 meters should be used with eyelets a meter apart. The transect is located in the area to be studied by running the tape from one landmark to another, fastening it here and there by means of quadrat stakes. Previous to this, the shortest distance between landmarks is ascertained when the transect runs through a depression or upon a level surface. In the case of an elevation, the height is ascertained by a barometer, the length and angle of the two slopes obtained, and the length of the base line determined from these data. The field record of the arrangement of the plants is made entirely without reference to the surface line. The vertical lines on the centimeter sheet are taken to correspond with the tape, and the individual which touches the latter on either side is recorded to the right or left respectively and within the proper square. The species are indicated as for quadrats. A single row on either side may be taken alone, but the double series serves as a desirable check. After the record is made, the topography of the transect is drawn carefully to scale. This drawing is made upon the scale of 100 : 1 for transects of 10 meters or less, and of 1000 : 1 for those that are longer. The combination of this drawing with the line series of plants can not be made advantageously in the field. For the shorter transects, meter sizes of centimeter plotting paper can often be used to advantage. In this event, the topographic line is drawn to the scale of 10 : 1 and the series of plants transferred directly to it. In the case of transects between 10 and 100 meters, the scale of the drawing is increased from 1000 : 1 to 100 : 1, so that each decimeter of the original series is compressed into a centimeter. For the longest transects, corresponding reductions must be made, but in these it will be remembered that the series is plotted by meter instead of decimeter.

=219. The location and size= of line transects are determined by the purpose for which they are designed. Short transects are valuable for detail, but they can be used to advantage only where changes in arrangement are taking place rapidly. They are especially adapted to the study of minute alternations and to the zonation of small ponds, streams, ditches, roads, blowouts, etc. Longer transects can not furnish the same detail, on account of the amount of time necessary, but they are invaluable for the zonation and alternation of larger areas, such as the consocies, formation, and formation series. They are of particular importance for the record of zonation, since they afford a clue to the topographic symmetry of the area. The location of a transect depends upon the area to be studied, though it should always run through a portion as typical as possible. The general direction is ascertained by means of the compass, and when there is a measurable difference in elevation it is taken by the barometer or otherwise.

The points at which ecotones cross the transect are carefully indicated upon the chart. They serve as stations for simultaneous readings of physical factors, though in the majority of cases water-content readings alone will determine the reason for the ecotone. Photographs of line transects should be made while the tape is in position, in order that the superficies of the series may be as evident as possible.

The Belt Transect

=220. Details.= This differs from the line transect in that it is wider, and consequently affords a more accurate record of the arrangement of plants. While both give the actual facts of distribution, the line transect necessarily ignores the minor lateral deviations in position. These are brought out in a strip of some width, and the belt transect thus gives a more correct view of the variations which result from competition in an area physically homogeneous. The width of such transects depends upon the length, and the character of the vegetation. The standard width is one decimeter in herbaceous formations, and one meter in the long transects which are used in woodlands. In open vegetation, especially in the initial stages of successions, the width may often be increased to advantage, but ordinarily the amount of work necessary to run a belt transect of some length limits the width to one decimeter.

The location of a belt transect, the choice of landmarks, the determination of direction and elevation are made exactly as for the line transect. The topographic map is made in precisely the same way also, the scale used depending upon the length. Two tapes, however, are employed, and these are placed so that they mark off a strip just one decimeter wide. Every few meters, or oftener if need be, they are checked by a decimeter rule, and fixed firmly in place by quadrat stakes. The arrangement of the plants is recorded as for the line transect, except that the record covers a decimeter strip just as in quadrat work. Accordingly, an interval of a centimeter is left on the sheet between the successive portions of the strip, in order that the latter may be put together without confusion when the topographic map and the plant series are combined. The record should invariably start in the upper left-hand corner and read down. The map and the centimeter strip recording the plants of the transect are combined on a common scale as already indicated for the line transect.

The ecotones of zones are shown on belt transects by single cross lines, and those of consocies by parallel cross lines. In taking photographs of the transect, it is desirable to use guidons to mark these points clearly. The same device may also be used to indicate the course of the transect, when the tapes are completely hidden by the plants. Physical factor readings should always be taken, and, as before, they are best made at the intersections of the ecotones.

The Permanent Transect

=221. Advantages.= Both line and belt transects, after they have been recorded, should be rendered permanent, in order that they may serve to indicate the changes of a heterogeneous area from year to year in the same detailed fashion that the permanent quadrat does for homogeneous ones. For historical as well as for physical reasons, the ecotones of zones and of consocies are subject to change from year to year, and the amount and direction of this change can only be ascertained from annual records made in exactly the same spot. By means of the permanent transect alone the very origin of such areas can be followed from one stage to another of the succession. Moreover, the transect is equally valuable with the quadrat in making it possible to follow every step of the minute changes wrought by competition.

=222. Details.= The transect is made permanent by blazing the landmarks at either end, if these already exist, or by erecting them when it is necessary. A label stake is driven at each end, on which is painted the number and date of the transect and its length. Each stake should also indicate the exact direction in which the other lies. The position of the ecotone is indicated by smaller stakes bearing the number of the transect and the date when the ecotone was found at that point. These are left in place, and in a few years show very graphically the change in position of the zones. For the first season, permanent transects afford results of great value when recorded for each aspect, but after this an annual visit will suffice. The details of mapping, plotting, etc., are identical with those indicated above, with the addition that all charts and photographs must bear the number of the reading as well as that of the transect. Physical factor observations are taken as often as the charts are made, and the results noted on the back of the chart sheet for purposes of ready comparison.

The Denuded Transect

=223.= The denuded transect bears exactly the same relation to a permanent one as that which exists between the denuded and the permanent quadrat. While the permanent transect records the actual mutations due to changing physical factors or to competition, the denuded transect throws needed light upon the mobility and ecesis of the various species, and upon the nature of the competition between them. Denuded transects may be established wherever it seems desirable, after the strip has been properly charted and photographed. The most valuable results, however, are secured by locating each one alongside of a permanent one. The best plan is to locate and chart two permanent transects a meter apart. A single view is then made of the two. One of them is denuded together with a strip 2 decimeters on either side, resulting in a denuded transect 5 decimeters wide. In charting this during succeeding years, the entire width may well be plotted as long as the vegetation is open, but after it has again become well established, it is necessary to save time by confining one’s attention to the central decimeter strip. Photographs can be made either of the permanent and denuded transects singly, or of the two together. The latter method has certain obvious advantages. Climatic factor readings can be made for both transects in common, but all those factors which are affected by the exposure of the soil surface must be observed in each.

The Layer Transect

=224.= This is a modification of the line transect, by means of which the vertical relations of plants are also shown, especially the tendency to form layers which is so regular a feature of forest formations. Owing to the difficulty of charting in three planes, belt transects do not lend themselves to this purpose. Because of the greater complexity, layer transects can rarely exceed ten meters in length except in those formations where layering is little or not at all developed. The simplest method is to establish a line transect in the ordinary way, and then to record the height of each plant as its position is noted. This is done by means of a measuring stick ruled in decimeters, which can be moved from interval to interval along the tape, or better, by two such sticks connected by tapes a meter long at every five decimeters of the sticks. These should be two meters high for woodland, and one meter for grassland. Layer transects often run on even surfaces, but if this is not the case, the usual data for a topographic map should be taken. The final chart is constructed on the scale of 10:1, the height of each plant being indicated by a vertical line equal to .1 of the observed height. A photograph of a representative meter of the transect is taken when the measuring sticks and rods indicated above are in position. Physical factor readings, principally of light, but often also of humidity, temperature, and wind are made at the height of the various layers when these are present.

ECOTONE CHARTS

=225.= The contour lines of zones and consocies are of the utmost importance in recording the structure of vegetation. They do not permit such accuracy as do quadrats and transects, but this is hardly to be considered a disadvantage in view of the fact that ecotones are rarely sharply defined. In establishing the ecotones of zonation, the width and the length of the base, i. e., the area of excess or deficiency, or as much of it as is to be considered, are determined. This base may be road, ditch, pool, lake, or stream, or the peak or crest of a hill, ridge, or mountain. When the zonation is bilateral, meter tapes are run at right angles to the base, at proper intervals, and the points and the distances where the ecotones cross are noted. In the case of radial zones, the tapes are run in the four cardinal directions, and if the base be large, in the four intermediate ones also, the intersections being likewise noted. From the data thus obtained, the zones may be outlined with a fair degree of accuracy. If the series be an extensive one, it is charted to the scale of 100:1; in cases of small areas, however, the scale of 10:1 will give better results. Whenever the zones show clearly enough to warrant, a photograph is also taken. Water-content readings are of paramount importance in the interpretation of zones. Samples should be taken at all intersections, and the resulting values indicated at the corresponding points upon the chart. When the zones are broken up into alternating patches in consequence of asymmetry in the topography, the ecotones of the latter are traced in a similar fashion from the center of each as a base, the absolute position of which is ultimately determined with reference to the ecotone lines already established.

THE MIGRATION CIRCLE

=226. Purpose.= The migration circle is designed to record the invasion of species, since it operates outward from an individual or a group of plants as a center. As migration takes place to a certain degree in all directions, a circle is better adapted to the purpose than the quadrat. From the very nature of invasion, migration circles should always he permanent in order that the yearly advance may be accurately noted. Circles of this character are important aids in the study of any vegetation, except, perhaps, one that has practically become stabilized. Their great value, however, is found in succession, where it is necessary to trace the movement of new individuals away from the original invaders as centers of colonization.

=227. Location and method.= The size of the migration circle is largely controlled by the density of the vegetation, and in some degree by the height of the species also, since this determines the trajectory of the disseminule. In close formations, a circle of 1–, rarely of 5–meter radius can best be used, but in the more open initial stages of succession a radius of 5, 10, or, in exceptional cases such as open woodland, even 25 meters, affords the best results. The location should always be made with a plant or group of plants of the species to be studied as a center. This migration circle differs from the quadrat in that it is used to show the movement of one, rarely two or three species, and not the position of all the plants within it. The center is permanently fixed by driving a labeled stake with the number of the circle and the data. Two tapes the length of the radius are used for recording. These are provided with the usual eyelets, 5 decimeters apart, and are fastened on a peg in the top of the central stake so that they move readily. At the outer ends they are staked 5 decimeters apart by a tape of this length when the radius is 1 meter, and 1 meter when the radius is 5 or 10 meters. The record forms must be especially prepared on blank sheets about 9 inches square. The scale is 10:1 for circles of 1 meter, and 100:1 for those of 5– and 10–meter radius. In the former, concentric circles are drawn about the center at intervals of 5 decimeters, and radii are drawn to the circumference at the same interval. In the larger circles, the intervals are 1 meter. Each segment of the circle is read by means of the two tapes, and the position indicated with reference to the concentric lines and radii. When but one species is read, a tiny circle is used to denote the position of each plant. If more than one is used, the symbols are those already indicated for the quadrat. One tape is left in place and the other with the segment tape is shifted to a new position, and the resulting segment is read as before. The exact position of the base radius is fixed by a label stake, in order that the segments of successive years may exactly correspond. The record sheet is labeled, dated, and filed. By folding at one edge, it may be filed in the regular field book.

=228. The denuded circle= is established in the same way as a permanent one. The original position of the individuals of the species under consideration may be recorded or not, depending upon the use to be made of the results. The safest plan is first to read the circle in the usual way, and then to denude it. The latter should be done in such a way as to remove all the disseminules from the surface in so far as possible. It is essential also that this be done before the seeds are mature and begin to be scattered. The central plant or cluster is of course not removed. In special cases, all the plants of the species are allowed to remain to serve as centers of colonization. The successive yearly readings of the denuded area are made exactly as for a permanent circle. Permanent and denuded circles, like quadrats, should always be established near each other so that they permit of ready comparison under similar conditions.

=229. Photographs= of migration circles furnish the most detail when the camera is placed just behind the central group in such a way as to show its relation to the other individuals or clusters of the circle. In the denuded circle, or when the plants stand out conspicuously from the bulk of the vegetation, it is not necessary to use guidons, but in other cases the latter greatly increase the value of the picture. Factor readings are less important for migration circles than for quadrats and transects. The factors of principal importance are those that deal with migration and ecesis, i. e., wind, water-content, and soil temperatures. The former may be determined for both circles in common, but the conditions that affect ecesis must be observed separately for each.

CARTOGRAPHY

=230. Value of cartographic methods.= Chart, map, and photograph are records indispensable to the systematic study of vegetation. They serve not merely to preserve the facts ascertained, and to permit their ready comparison, but they also put a premium upon accurate methods, and consequently bring to light many points otherwise overlooked. For ecology, they have the value which drawings possess in taxonomy, in that they make clear at a glance what pages of description fail to indicate. They are the fundamental material of comparative phytogeography, and in all careful vegetational study their use is no longer optional but obligatory. Hence it is obvious that cartographic methods should be clear and simple, and that they should be uniform, so that charts and maps of widely separated formations may be directly compared without difficulty. It is not to be expected that uniform methods will come into general use immediately, but a proper appreciation of the obligation that rests upon every ecologist to make his results both easily comprehensible and usable will serve to produce this very necessary result. In the treatment that follows, as elsewhere, no attempt is made to describe the general cartographic methods used by other ecologists, notably Flahault. The methods employed by the author form a complete system, which has proved valuable, and for various reasons it alone is discussed here.

=231. Standard scale.= The question of the scale to which charts and maps are to be made is of primary importance. The general principle is that the ratio between area and drawing should be as small as possible. Moreover, charts and maps of the same character should always be drawn to the same scale, unless a good reason to the contrary exists. The ideal scale is 1:1, which is manifestly an impossibility. This is approached most nearly in the quadrat chart where the scale is 10:1. Charts of definite areas are made on a scale as large as possible, while maps of formations, regions, etc., are necessarily drawn upon a very small scale. General maps designed to show the distribution of species and formations, or the vegetation of continents, are usually not drawn with reference to a scale at all. While it is manifestly impossible to use the same scale for charts and maps, it is feasible and desirable that they be constructed upon scales readily convertible into each other. This is most satisfactorily accomplished by means of the decimal system, and the various type scales are 10:1, 100:1, 1000:1, etc. The first two or three scales are used for charts of quadrats, transects, and circles; the remaining ones are employed in making maps of large areas. No attempt has been made to draw an absolute line between charts and maps, but an endeavor is made to restrict the term chart to the record of the number and position of plants, while maps deal with the arrangement and location of formational areas. It is hardly necessary to point out the reasons why all charts and maps should be based upon the decimal system of scales. Experience will furnish the very best of arguments.

=232. Color scheme.= The first requisite for the graphic representation of formations, regions, etc., is that each class of formations be invariably indicated by the same color. It is also necessary that the colors and shades be easily distinguishable, and it is at least desirable that they be referred to the different classes in some consistent sequence. Uniformity in all these points is greatly to be desired at the hands of all ecologists. Here, as in the case of the standard scale, uniformity will be found the more desirable the more impossible it is made by ignoring it. In the use of color to represent regions and provinces, on maps too small to indicate formations, the color of each division is represented by the color of its dominant formation; thus the prairie province is colored ochroleucus on account of the color used to represent prairie formations, the boreal-subalpine zone atrovirens on account of the typical coniferous forests, etc. No endeavor has been made to take account of the various types of formations, e. g., the different coniferous forests, as this is a problem to be worked out for more local maps in various shades of dark green, etc. The following color scheme which has been based upon the points made above is proposed as a satisfactory solution of the problem. The color standard used is that of Saccardo’s Chromotaxia.

I. Hydrophytic Formations: blue

1. Marine: cyaneus

2. Brackish: ardesiacus

3. Freshwater: caeruleus

4. Swamps and marshes: caesius

II. Mesophytic Formations

A. Forest formations: green

1. Coniferous forests: atrovirens

2. Broadleaved evergreen forests: viridis

3. Deciduous forests: flavovirens

B. Grassland formations: yellow

1. Meadows: melleus

2. Prairies: ochroleucus

C. Culture and waste formations: red

1. Fields: ruber

2. Groves and orchards: atropurpureus

3. Wastes: purpureus

III. Xerophytic Formations: brown

1. Deserts: isabellinus

2. Plains and steppes: avellaneus

3. Saline formations: umbrinus

4. Arctic-alpine formations: testaceus

=233. Formation and vegetation maps= are detailed maps of a single formation or a series of them, showing the formational limits, and when the scale is not too small, the ecotones of zones and consocies. In the cases where the topography is level, as sometimes happens in mapping single formations, the chain and pedometer must be used to ascertain the size of the different areas. Indeed in all mapping of vegetation, the methods of surveying are directly applicable. Over large areas, however, it is not necessary that limits be drawn with mathematical accuracy, and for the purposes of the ecologist, the plane table and camera are satisfactory substitutes for the surveyor’s transit, at least in the present aspect of the subject. When the formation or group of formations is commanded by an elevation of some height, the latter is used as a base. A plane table is established upon it and the topographical and vegetational features are recorded in the usual way. This map is usually supplemented by a series of views from the same base. Indeed it has come to be recognized that a complete series of photographs of this kind give a more valuable record than the plane table, and that the construction of an accurate map from them is an easy matter. Since the camera saves much time and energy also, it is used almost exclusively to furnish the data for map making. In hilly, and especially in mountainous regions, the photographic method is indispensable. Its application is extremely simple. A central hill or mountain is selected, and from it a series of views is taken so that the edge of one exactly meets the edge of the other. This is an extremely important matter, and demands much nicety of judgment. The camera is kept in the same spot, and after each exposure it is turned as the operator looks through it until a landmark at one edge just passes from view at the other. As soon as the new position is determined, the tripod screw is turned to hold the box firmly in position. In case of a slight jar, the exact position should again be obtained. If the series is accurately made, the resulting prints will give a complete panoramic view of the region, without overlap or omission. For this purpose, a 6½ × 8½ camera is desirable, since the topographic and vegetational features are larger and stand out more distinctly. A large camera requires fewer changes of position, and hence saves time and reduces the chance of error. A 4 × 5 camera serves the purpose sufficiently well, though it requires a little more care in operation on account of the greater number of exposures necessary. This may be avoided in some degree by the use of a wide-angle lens if the depth of the area is not too great. Whatever camera may be used, a telephoto lens is a very desirable adjunct, since it enables one to choose between three different sizes of the view without changing the position of the camera. To avoid possible confusion, the exposures are always made from right to left, and the plates are used in the numerical order of their holders. For the same reason the landmarks are described and numbered in their proper order. The prints obtained are mounted on a card in sequence. The view map may be preserved in this form, or it may be reduced or enlarged by making a copy to the size desired. Outline maps of topography may be traced from the resulting negative, and the formations filled in by means of the proper colors. The most satisfactory method, however, is to have the original views or the copy printed “light” and to color the formations just as they appear there, with all the wealth of topographic and vegetational detail. If a detailed topographic map alone is desired, this is traced directly from the large copy.

=234. Continental maps.= A method of determining the general outlines of regions, provinces, and vegetational zones as a preliminary to their detailed study has been used successfully for several years. This is based upon provincial and continental maps on which are traced the geographical areas of the species of genera typical of the various formations. Detail topographic maps of the prairie province and the North American continent have been used for this purpose. A number of the facies of extensive and representative formations of the different portions of the continent are selected and grouped according to genera. One map is devoted to each genus, unless the number of species is large. In this case a number of maps are used, since the limits are apt to become confused. The range of each species is determined from all the reliable sources, and a corresponding line is drawn upon the map to delimit its geographical area. The limits of the area of each species are drawn in a different color, and the name of the species printed in the same color in the legend. Although this work has as yet been done only for the trees of North America, and for the grasses and principal species of the prairie province, it promises to constitute a final method for the limitation of vegetational divisions. It is clear that if the original data concerning ranges are accurate, the increasing study of formations will do little more than rectify the detailed course of the limiting line, since in most cases facies and formations coincide in distribution. The limiting line or ecotone of a zone or province is a composite obtained from the limits of certain representative facies and principal species, and checked by the limits of species typical of the contiguous vegetations. Thus, the boreal-subalpine zone is clearly outlined by combining the limits of Populus tremuloides, Larix americana, Pinus banksiana, Abies balsamea, Picea mariana, Picea canadensis, and Betula papyracea, and checking the results by the areal limits of the hardwoods and grasses to the southward.

PHOTOGRAPHY

=235.= The camera is an indispensable instrument for the ecologist. Although it has too often been employed to give an air of thoroughness to work of no ecological value, it is as important for recording the structure of vegetation as the automatic instrument is for the study of the habitat. No ecologist is equipped for systematic field investigation until he is provided with a good camera and has become skilful in its use. For this reason, it is felt that a few hints concerning photographic methods and their application in ecology may not be out of place. No written advice can take the place of experience, but certain elementary suggestions and cautions will greatly shorten the apprenticeship of one who does not have the good fortune to be taught by a professional photographer. To the student of ecology, the camera is not a toy. It must be understood and operated with as much thoroughness as any other instrument, and when this is done, the results will be equally certain and desirable.

=236. The camera and its accessories.= Although two cameras are desirable whenever it is possible to obtain them, a single one will meet all the requirements of field work. This should be 4 × 5 inches in size, since it is much more convenient and will do all the work that a larger camera can. In the comparatively few cases in which larger views are needed, the 4 × 5 negatives can be readily enlarged. The smaller instrument is less expensive in operation because of the cheapness of the plates, and it gives a negative of the proper size for lantern slides and for reproduction. A 6½ × 8½ camera is valuable in special cases, such as making a series of photographs for maps. In the writer’s own experience, the 6½ × 8½ camera, although used exclusively at first, has been almost completely supplanted by the 4 × 5. The best field camera is of the folding type with a good stout box. It must be what is known technically as a long focus instrument, which enables small objects to be taken natural size and permits the use of a telephoto lens. It should be provided with a swing and also a reversible back by which the position of the plates can be changed instantly. The lens must be of the telephoto pattern, which makes it possible to use the front or back lens either alone or in combination. The chief advantage of this is that the image, when distant, may be made of three different sizes without changing the position of the camera. Generally speaking, the high-priced rapid lenses are the best, since it is exceptional to get the desired length of exposure in vegetation, on account of the ease with which the plants move in the wind. Before buying such a lens it is desirable to test its rapidity and depth of focus, since it is not necessarily better than some of the lenses furnished with good cameras. The lens should be provided with an iris diaphragm capable of being stopped down to 128 or 256. The shutters furnished with the ordinary lenses are satisfactory, since “snap-shots,” i. e., instantaneous exposures, are practically never possible for plants. The automatic shutter of the “Premo” camera is an especially convenient form. All shutters should be carefully tested before using to determine the exact time value of the exposures indicated. It is not uncommon for the exposure at 1 second, or at other points, to have a value quite different from the one indicated. When this is the case, it is evident that it can not be known too soon. The camera should have at least a half-dozen double plate-holders. These are numbered consecutively so that the figure uppermost when the holder is in the camera will indicate the number of the plate exposed. A carrying case is desirable on a long trip when all the plate-holders must be taken, but ordinarily it is a disadvantage, since the camera box will carry two or three holders. The camera cloth should be as small and light as possible, and at the same time opaque. The most satisfactory one for the field is the rubber cloth. The tripod should be a happy combination of lightness and stability, a condition more nearly reached by the aluminum tripod than by any other. It should have not less than three joints in order to facilitate the use of the long focus upon objects near the ground.

=237. Choice of a camera.= There is not a great deal of choice between the moderate-priced cameras of the various makers. A field camera is restricted to certain special uses, and hence is more serviceable when attachments useful only in portraiture or instantaneous work are absent. Even the ray filter, which has some value in the indoor photography of flowers, is useless in the field on account of the long exposure required. From considerable experience, “Premo” and “Korona” cameras have been found to be very satisfactory instruments. Doubtless the same statement would be found true of all the standard makes, but they have not been used by the writer. “Premo” cameras are made by the Rochester Optical Co., Rochester, N. Y., and “Korona” cameras by the Gundlach-Manhattan Optical Co., Rochester, N. Y. When two or more cameras are used, the best results can be obtained if they are of the same make, since the details of operation are then the same. The reduced liability of making a blunder is often offset by the fact that a different pattern will permit of a wider range of use. Any standard brand of plates will produce good negatives when skilfully used; at least, this has been proved in the case of the Cramer, Hammer, Seed, and Stanley brands. Every professional photographer has his favorite brand of plate, but the ecologist will do well to give the various kinds a thorough trial, and then to invariably use the one which gives him the best results. Thus, while it seems to be less popular with the profession than the others mentioned, the writer has obtained at least as satisfactory results with the Stanley plate as with the others, and consequently now uses it exclusively, since it is cheaper. The one important point is to make a final choice only after personal experience, and then to always use plates of the same brand, and preferably of the same rate of speed.

=238. The use of the camera.= To the ecologist, objects to be photographed fall into two categories, viz., those that move, and those that do not move. For practical purposes, areas sufficiently distant to render the movement imperceptible belong to the latter, as well as those, such as rock lichens, many fungi, etc., which can not be stirred by ordinary winds. The treatment accorded the two is essentially different. A fundamental rule of ecological photography is that detail must receive the first emphasis. The ecological view should be a picture as well as a map, however, but when one must be sacrificed, artistic effect must yield to clearness, and accuracy, i. e., technically speaking, contrast must give way to detail. Leaving apart the necessity of securing a sharp focus, which holds for all work, detail or definition depends directly upon the aperture of the diaphragm. Detail is increased by decreasing the size of the aperture. This in turn increases the length of time necessary for a proper exposure, and consequently the danger that the plant will be moved in the midst of the exposure. When the movement is negligible, the invariable rule should be to reduce the aperture to its smallest size, and to expose for a corresponding time. In all cases where the plants are close enough to show even a slight blurring on account of the action of the wind, the time of exposure must be reduced, in the hope that a short period of quiet will suffice for it. This reduction in time must be compensated by increasing the aperture of the diaphragm, and hence the amount of light which strikes the plate. The proper balance between the two is a matter of considerable nicety. It depends much upon the vagaries of the wind, and can readily be determined only after considerable experience. Although regions naturally differ somewhat in the nature of their winds, much experience in prairie and mountain regions warrants the primary rule that views of vegetation and plants subject to movement are not to be attempted on windy or cloudy days when it can possibly be avoided. Even on reconnaissance, a poor picture is no better than none at all, while in resident work a time will come sooner or later which will permit the making of a view satisfactory in all respects. There may be occasional instances when one is rewarded for keeping the camera trained on a particular spot for hours, and for wasting several plates in the hope that still moments will prove to be of the requisite duration. As a regular procedure, however, this has nothing to commend it.

Various methods have been tried to reduce or eliminate the trouble caused by the wind. Canvas screens have been used for this purpose with some benefit. When the picture is worth the trouble, a tent may be erected to afford a very efficient protection. This is too prodigal of time and energy, however, to be practicable under the usual conditions. Flashlight exposures on still nights are sometimes feasible, but the disadvantages connected with them are too great to bring them into general use. The best procedure is to bide one’s time, and to take quadrats, transects, and other detail areas, as well as many plant groups, at a time that promises to be most favorable. Single plants can often be moved in the field so that they are protected from the wind, or so that they are more strongly lighted. Slender, or feathery plants are usually very difficult to handle out of doors. The best plan is to photograph them in a room that is well and evenly lighted, or, best of all, in a stable, roomy tent.

=239. The sequence of details.= No photographer ever escapes blunders entirely. At the outset of his work, the ecologist must fully realize this, and accordingly plan a method of operating the camera which will reduce the chance of mistake to a minimum. The usual blunders which every one makes sooner or later, such as making two exposures on one plate, drawing the slide before closing the shutter, allowing the light to strike the plate through the slit in the holder, etc., can be all but absolutely avoided by a fixed order of doing things. This order will naturally not be the same for different persons; it is necessary merely that each have his own invariable sequence. The following one will serve as an illustration. As a preliminary, the plate-holders are filled, after having been carefully dusted, and the slides are uniformly replaced with the black edge inward. It is a wise precaution to again see that all the slides are in this position before leaving the dark room. This will ensure that a black edge outward always means that the plate has been exposed. The tripod is first set up and placed in what seems about the proper position. The camera is next attached to it, and the front and back opened. The bellows is pulled out, a short distance for views, and a longer one for detail pictures, and fastened. It is necessary to move the diaphragm index to the largest aperture and to open the shutter at “time.” The next steps are to orient the view or object, and to bring it into sharp focus upon the ground glass. The first is accomplished by moving the entire instrument, changing the position of the tripod legs, swinging the camera upon the tripod, or by raising or lowering the lens front. It is often desirable also to change the position of the object on the plate by use of the reversible back. In views with much distance, the foreground is brought into sharp focus. In close views, especially of quadrats, the swing is used to increase the distance for the foreground, and the focus is made upon the center. After focusing, the shutter is closed, the indicator set at the time desired, and the diaphragm “stopped down” as far as possible. Plate-holder 1 is slipped into place, care being taken not to move the camera by a sudden jar. The camera cloth is dropped above the holder and allowed to hang down over the slide end. The slide is drawn and put on top of the instrument, the black edge always up. The exposure is made and the slide replaced with the black edge outward. This point should receive the most critical attention, as a blunder here will often cause the loss of two negatives. The plate-holder is returned to the receptacle, or merely placed in the back of the camera, which is then closed. The number of the plate, the name of the view or object, the condition of the light, the length of exposure, and the aperture of the diaphragm, as well as the date, are recorded in a notebook for this purpose. The shutter is then opened at “time,” the diaphragm thrown wide open, and the front of the camera closed. When distances are short, the camera is often carried upon the tripod. As a rule, however, it is usually removed, and the tripod folded. In making subsequent pictures, the plates should always be used in their numerical order.

=240. The time of exposure= is obviously the most critical task in the manipulation of a camera. The time necessary for a proper exposure varies with the season, the hour, the condition of the sky, the light intensity of the formation, the color and size of the area to be photographed, and, finally, of course, with the aperture of the diaphragm. Fortunately for the ecologist, the variation in light intensity during the season, and even during the greater part of the day, is not great, and can ordinarily be ignored. The beginner will make the most progress by determining the exposure demanded by his instrument for taking a general view in full sunlight and with the smallest stop of the diaphragm. In standard cameras with lenses of ordinary rapidity, this is usually about one second. This will serve as a basis from which all other exposures may be reckoned until one has worked through a wide range of conditions and can recall just what time each view requires. On completely cloudy days the time required is five to ten times that necessary on a clear day; filmy clouds and haze necessitate an exposure of two or three seconds. The more open forest formations demand an exposure of about five to ten seconds on a sunny day, while the deeper ones require two or three times as long. A close view requires more time than a distant one, since the light-reflecting surface is much smaller. Quadrats require two or three seconds, and individual groups frequently take a longer time. The color of the vegetation plays an important part also: a dark green spruce forest requires twice as long an exposure as the aspen forest, and a grassland quadrat takes more time than one located in a gravel slide. In this connection, it is hardly necessary to point out that the lighted side of objects should always be taken, never the shaded one. The exposures indicated above are based upon the smallest stop. The reasons for using this whenever possible have already been given. When a larger stop is necessary, the exposure is decreased to correspond; for example, a quadrat that takes three to four seconds at 256 can be taken at 64 in one second. As a rule, the sun should not be in front of the camera, but, when necessary, views can be made in this position if the sun is prevented from shining directly into the lens.

=241. Developing= is as important as exposing. Indeed, it may well be considered more important, since a properly exposed plate may be spoiled in developing, while an under-exposure or over-exposure may be saved. Owing to the ease with which plants move in the wind, the ecologist is obliged to reconcile himself to many under-exposures, which can be converted into good negatives only by skilful developing. Every base station should have a good dark room, equipped with running water when possible, a good ruby lantern, and the proper trays and chemicals. Prepared developing solutions are alluring because of their convenience, but after an extended trial of several kinds, the writer has reached the conviction that pyrogallic acid, or “pyro,” is by far the most satisfactory in working with vegetation. Of almost innumerable formulae, the following gives excellent satisfaction and is convenient to use.

I. II.

500 cc. water 500 cc. water 30 grams sodium sulphite 5 grams pyrogallic acid 30 grams sodium carbonate

For developing, equal parts of I and II are mixed, and a few drops of a 10 per cent solution of potassium bromide added, unless there is reason to suspect that the plate has been seriously underexposed. The fixing bath is a concentrated solution of sodium hyposulphite, “hypo,” to which a few drops of acetic acid are added. It should be replaced every week or two, depending upon how much it is used. A tray of water is kept at hand for bringing out the detail in underexposed negatives, and a second tray is used for washing. The “pyro” and the bromide solution should always be within reach, the former for accelerating, and the latter for retarding the development of unsatisfactory plates.

The image will begin to show on a properly exposed plate within one to three minutes after it has been put in the developer. If the image appears almost instantly, and then recedes quickly, the plate is badly overexposed, and should be thrown away. In case it “comes up” less quickly, indicating that it is not greatly overexposed, it can be saved by the addition of more bromide. When the image does not show till the end of five to ten minutes, the plate has been underexposed. It is then necessary to add more “pyro,” taking care not to pour it on the plate, and, after the image appears with its striking contrast, to leave the plate in water until as much detail as possible is brought out in the shadows. In the case of a normal exposure, when greater detail is desired, the negative is left for some time in water, and when contrast is sought more “pyro” is used. Negatives with unusual detail lack “snap”; they are “flat,” and fail to make artistic pictures. Contrast, on the other hand, often obscures detail, and the best results can only be obtained by a happy combination of the two. The most important maxim in developing is that the process shall be continued until the image has become indistinct. The universal tendency of the beginner is to remove the negative the moment the outlines grow dimmer, and the result is a thin, lifeless negative. It is almost impossible to develop too far, if the image is not allowed to disappear. Negatives of this sort are “thick,” and though they print more slowly, produce brilliant pictures. A large quantity of the developing solution is used with single plates in small trays, and is allowed to act without rocking the tray. Much time is saved, however, by developing several plates together, and to avoid using a large quantity of the solution, the tray is gently rocked from time to time. This movement is particularly necessary at the beginning, in order that the plates may be covered evenly, and at once. Fifty cubic centimeters of the solution will develop three or four 6½ × 8½ plates, and twice as many 4 × 5’s. After the developer has once been used, it is kept for several days to restrain overexposed plates. As soon as the plate is developed, it is rinsed in water, and placed in the fixing fluid, until the white opaqueness is entirely removed. The “hypo” is then washed out by immersing the negatives for one to two hours in running water. If the latter can not be secured, the water in which they are placed should be changed frequently. The negatives are then air-dried within doors, in a place free from dust. Finally, they are filed away in negative envelopes, each bearing the name and number of the negative, and preferably also, the time and other exposure data.

=242. Finishing.= On account of the time demanded by other field tasks, it has not been found desirable to make and finish prints in the field. This, with the making of lantern slides, enlargements, etc., may well be turned over to a professional photographer. It is the custom to make a proof of each negative to meet the casual needs that arise in the field. For this purpose, solio “seconds” are used, since they are both cheap and satisfactory. When an urgent demand for a finished print does arise, it is met by using “velox” paper, which can be exposed in the dark room, and then developed and fixed exactly like a plate. Two standard papers for views are “solio” and “platina.” The former gives brown tones, and is used for contrast and brilliancy, hence it is especially good for printing from negatives that have too much detail and too little contrast. “Platina,” on the contrary, yields soft gray tones, and softens contrasts.

FORMATION AND SUCCESSION HERBARIA

=243. Concept and purpose.= A formation herbarium is a collection of exsiccati, in which the species are arranged with respect to their position in the formation, instead of being grouped in genera and families. Its primary purpose is to furnish a record of the constitution and the structure of a formation or a series of formations. At the same time, it affords the basal material for developing the subject of comparative phytogeography. It is impossible for one ecologist to visit many remote regions, to say nothing of spending a period sufficient for obtaining even a fair knowledge of the vegetation. He can at the best acquire an acquaintance with but few regions at first hand. In consequence, a method that brings a vegetation to him, with its structure carefully wrought out by years of study, is of the highest value. Time, as well as distance, sets a narrow limit to the number of formations which one man can investigate critically in a lifetime. It is no longer possible for a botanist to explore vast regions, and to bring back results which have anything more than a very general value. This fact, far from restricting the comparative study of vegetation, will serve to make it more accurate and systematic. The exact results of numerous resident investigators, expressed in formation herbaria, with the proper series of quadrat maps and photographs, will be worked over by men who are themselves specially acquainted with a particular vegetation. Comparisons will be founded upon a definite basis, and the relationship of various vegetations can then be expressed in precise rather than general terms. It is hardly too sweeping to assert that accurate work in the field of comparative phytogeography can be done only in this fashion. The value of formation herbaria in class work is evident. On account of the limitations of time and distance, classes can touch but few formations, and these at every time except the growing period. For these reasons, an accurate and complete formational record that can be consulted or studied at any time is almost indispensable to class study in the development and structure of formations.

=244. Details of collecting.= Formational collections, unlike the ordinary sets of exsiccati, can not be made upon the first visit to a region, or by a single journey through it. The determination of formation limits, and of developmental stages, of aspects, layers, abundance, etc., must necessarily precede, a work which alone takes several years. Moreover, collecting itself requires more than one year in a region containing numerous formations. This is exemplified by the Herbaria Formationum Coloradensium. The preliminary study for this was made from 1896–1899, the collecting was done chiefly in 1900 and 1901, while additional numbers were added in 1902–3. For the purposes of the formation herbarium, specimens should be collected and pressed in such fashion as to show all the ecological features possible. Plants must be collected both in flower and in fruit, with the underground parts as perfect as may be. Seedlings and rosettes should be included whenever present. In pressing, one or two leaves should be arranged with the lower side uppermost to admit of the ready comparison of both surfaces. Opened flowers are valuable for flower biology, while seeds and fruits are desirable for showing migration contrivances. The ferns, mosses, and lichens of the formation should be fully represented, together with the more important fungi and algae. The number of photographs taken for each herbarium should be limited only by considerations of time and expense. The ideal series consists of a general view of each formation, showing its physiographic setting, nearer views of each of its aspects, detail views of its consocies, societies, and layers, and flower portraits of all the constituent species. Such a series can only be obtained by residence through a long term of years, and in most cases general and aspect views, with portraits of the facies and a few of the striking principal species, must suffice. Quadrat and transect charts, together with formational maps, are extremely desirable, and, indeed, all but indispensable.

=245. Arrangement.= The arrangement of species within each formation herbarium is based upon the structure of the vegetation. The primary groupings are made with reference to time of appearance and abundance; when definite zones, associations, or layers are present, they must likewise be taken into account. In the Colorado collection, the first division is into three aspects based upon the period of flowering (aspectus vernalis, aestivalis, autumnalis). Within each aspect, the species are arranged with respect to abundance in the groups, facies, principal species, and secondary species. Each group is placed in an ordinary manila cover, which bears a printed label indicating the aspect and the group. The species labels give, in addition to the name, date, and place of collection, the phyad or vegetation form, the geographical area, the rank of the species, the aspect, and the formation. To these may well be added data concerning migration contrivances, seed production, pollination, period of flowering, etc. The photographs are mounted on the usual herbarium sheets, and placed in the proper order in the various groups, and a similar disposition is made of quadrat and transect charts, and such physical factor summaries as seem desirable.

=246. Succession herbaria.= The arrangement of formation herbaria may follow the classification of formations with respect to character, region, or development. The first is the most convenient for purposes of instruction, and has distinct advantages in permitting a close comparison of the vegetation of different habitats. The second basis, which is the one used in the Herbaria Formationum Coloradensium, is peculiarly adapted to mountain vegetation in which the zones are usually very distinct. The arrangement of herbaria in a developmental series, however, is the most logical and the most illuminating, since the structure of the ultimate formations is not only made plain, but the stages in their development are also laid bare. Such succession herbaria are the natural outgrowth of formational ones. Indeed, the latter should be made merely the starting point for these in all regions where the causes which bring about successions are active. Where weathering is still an important factor, as in mountains, the initial and intermediate formations which lead to the final grassland or forest are often in evidence. After a formation herbarium of each stage has been made in the way indicated, a succession herbarium is obtained merely by arranging the various herbaria in the sequence of the developmental stages. Thus, in the Colorado collection, the subalpine formations are arranged according to altitude in the following series: (1) the pine formation, (2) the gravel slide formation, (3) the half gravel slide formation, (4) the aspen formation, (5) the balsam-spruce formation, (6) the spruce-pine formation, (7) the meadow thicket formation, (8) the brook bank formation. Of these, five belong to the same succession, and it is possible to indicate the development of the spruce-pine forest by arranging these five formations in their proper order in a succession herbarium, as follows: (1) the gravel slide formation, (2) the half gravel slide formation, (3) the pine formation, (4) the balsam-spruce formation, (5) the spruce-pine formation.

DEVELOPMENT AND STRUCTURE

=247. Vegetation an organism.= The plant formation is an organic unit. It exhibits activities or changes which result in development, structure, and reproduction. These changes are progressive, or periodic, and, in some degree, rhythmic, and there can be no objection to regarding them as functions of vegetation. According to this point of view, the formation is a complex organism, which possesses functions and structure, and passes through a cycle of development similar to that of the plant. This concept may seem strange at first, owing to the fact that the common understanding of function and structure is based upon the individual plant alone. Since the formation, like the plant, is subject to changes caused by the habitat, and since these changes are recorded in its structure, it is evident that the terms, function and structure, are as applicable to the one as to the other. It is merely necessary to bear in mind that the functions of plants and of formations are absolutely different activities, which have no more in common than do the two structures, leaf and zone.

=248. Vegetation essentially dynamic.= As an organism, the formation is undergoing constant change. Constructive or destructive forces are necessarily at work; the former, as in the plant, predominate until maturity, when the latter prevail. Consequently, it no longer seems fruitful to classify the phenomena of vegetation as dynamic or static. The emphasis which has been placed upon dynamic aspects of vegetation has served a useful purpose by calling attention to the development of the latter. Although it is a quarter of a century since Hult, and more than a half century since Steenstrup, by far the greater number of ecological studies still ignore the problem of development. This condition, however, can be remedied more easily by insisting upon an exact understanding of the nature of the formation than in any other way. It is entirely superfluous to speak of dynamic and static effects in the plant, and the use of these terms with reference to the formation becomes equally unnecessary as soon as the latter is looked upon as an organism. The proper investigation of a formation can no more overlook development than structure, so closely are the two interwoven. Future research must rest squarely upon this fact.

=249. Functions and structures.= The functions of a formation are association, invasion, and succession: the second may be resolved into migration and ecesis, and the third, perhaps, into reaction and competition. Formational structures comprise zones, layers, consocies, societies, etc., all of which may be referred to zonation, or to alternation. The term association has been used in both an active and a passive sense. In the former, it applies to the inevitable grouping together of plants, by means of reproduction and immobility. Passively, it refers to the actual groupings which result in this way, and in this sense it is practically synonymous with vegetation. Invasion is the function of movement, and of occupying or taking possession; with association, it constitutes the two fundamental activities of vegetation. It is the essential part of succession, but the latter is so distinctive, because of the intimate relation of competition and reaction, that clearness is gained by treating it as a separate function which is especially concerned with development. Association, zonation, and alternation are structural phenomena, which are in large part the immediate product of habitat and function, and in a considerable degree, also, the result of ancestral or historical facts. It is a difficult matter to determine in what measure the last factor enters, but it is one that must always be taken into account, particularly when the physical factors of the habitat are inadequate to explain the structures observed. Structurally, association regularly includes both zonation and alternation. As there are certain typical instances in which it exhibits neither, the treatment will be clearer if each is considered separately.

ASSOCIATION

=250. Concept.= The principle of association is the fundamental law of vegetation. Indeed, association is vegetation, for the individual passes into vegetation, strictly speaking, at the moment when other individuals of the same kind or of different kinds become grouped with it. It is then (and the same statement necessarily holds for vegetation) the coming together and the staying together of individuals and, ultimately, of species. A concrete instance will illustrate this fact. In the development of the blowout formation of the Nebraska sand-hills (Redfieldia-Muhlenbergia-anemium), association begins only when the first plant of Redfieldia flexuosa is joined by other plants that have sprung from it, or have wandered in over the margin of the blowout. Henceforth, whatever changes the blowout formation may undergo, association is a settled characteristic of it until some new and overwhelming physical catastrophe shall destroy the associated individuals. It will readily be seen that association does not depend upon particular individuals, for these pass and others take their place, but that it does depend essentially upon number of individuals.

Association involves the idea of the relation of plants to the soil, as well as that of plants to each other. It is synonymous with vegetation only when the two relations are represented, since there may be association such as that of a parasite with its host, which does not constitute vegetation. But it will be seen that the relation of the parasite to the host is practically identical with the relation of the plant to the soil or stratum, and the two concepts mentioned above become merged in such a case. From this it follows that association results in vegetation only when the two ideas are distinct. The concept of association contains a fact that is everywhere significant of vegetation, namely, the likeness or unlikeness of the individuals which are associated. In the case of parasite and host, this unlikeness is marked; in vegetation, all degrees of similarity obtain. As will be evident when the causes which lead to association are considered, alternate similarity and dissimilarity of the constituent individuals or species is subordinate as a feature of vegetation only to the primary fact of association.

Since association contains two distinct, though related, ideas, it is of necessity ambiguous. It is very desirable that this be avoided, in order that each concept may be clearly delimited. For this reason, the act or process of grouping individuals is termed aggregation, while the word association is restricted to the condition or state of being grouped together. In a word, aggregation is functional, association is structural; the one is the result of the other. This distinction makes clear the difference between association in the active and passive sense, and falls in with the need of keeping function and structure in the foreground.

=251. Causes.= In considering the causes which produce association, it is necessary to call in evidence the primary facts of the process in concrete examples of this principle. These facts are so bound up in the nature of vegetal organisms that they are the veriest axioms. Reproduction gives rise immediately to potential, and ultimately, in the great majority of cases, to actual association. The degree and permanence of the association are then determined by the immobility of the individuals as expressed in terms of attachment to each other or to the stratum, such as sheath, thallus, haustoria, holdfasts, rhizoids, roots, etc. The range of immobility is very great. In terrestrial plants, mobility is confined almost entirely to the period when the individual lies dormant in the seed, spore, or propagative part, which is alone mobile. In aquatic spermatophytes, the same is true of all attached forms, while free floating plants such as Lemna are mobile in a high degree, especially during the vegetative period. Among the algae and hydrophilous fungi, attached forms are mobile only in the spore or propagative condition, while the motile forms of the plancton typify the extreme development of mobility. The immediate result of reproduction in an immobile species is to produce association of like individuals, while in the case of a mobile species reproduction may or may not lead immediately to association. We may lay down the general principle that immobility tends to maintain the association of the individuals of the same generation, i. e., the association of like forms, while mobility tends to separate the similar individuals of one generation and to bring unlike forms together. With the mobile algae, separation of the members of each generation is the rule, unless the individuals come to be associated in a thallus, or are grouped in contact with the substratum. Flowering plants that are relatively immobile, especially in the seed state, drop their seeds beneath and about the parent plants, and in consequence dense association of the new plants is the rule. In very many cases, however, this primitive tendency is largely or completely negatived by the presence of special dissemination contrivances, which are nearly, if not quite, as effective for many terrestrial plants as the free floating habit is for algae. From this point, the whole question of mobility belongs to migration, just as the adjustment between the parent plants and their offspring, or between plants established and the mobile plants to be established, belongs to competition.

If association were determined by reproduction and immobility alone, it would exhibit areas dissimilar in the mass of individuals, as well as areas dissimilar in the kinds of individuals. Some areas would be occupied by plants of a single species, others by plants of several or many species. This tendency of association to show differences is, however, greatly emphasized by the fact that vegetation is fundamentally attached to and dependent upon a surface that exhibits the most extreme physical differences. For this reason, new differences in association appear, due not only to the morphological differentiation of vegetation forms, but also to the changes in the degree and manner of association produced directly by the different habitats. Association might then be defined as a grouping together of plant individuals, of parents and progeny, which is initiated by reproduction and immobility, and determined by environment. It is a resultant of differences and similarities. In consequence, association in its largest expression, vegetation, is essentially heterogeneous, while in those areas which possess physical or biological definiteness, habitats and vegetation centers, it is relatively homogeneous. This fundamental peculiarity has given us the concept of the formation, an area of vegetation, or a particular association, which is homogeneous within itself, and at the same time essentially different from contiguous areas, though falling into a phylogenetic series with some and a biological series with others. From its nature, the plant formation is to be considered the logical unit of vegetation, though it is not, of course, the simplest example of association.

=252. Aggregation.= As indicated under the causes of association, the process by which groups of individuals are formed depends entirely upon reproduction and migration. In short, aggregation is merely a corollary of movement. The simplest example of this process occurs in forms like Gloeocapsa, Tetraspora, and others, where the plants resulting from fission are held together by means of a sheath. Though called a colony, such a group of individuals is a family in the ordinary sense. Practically the same grouping results in the case of terrestrial plants, especially spermatophytes, when the seeds of a plant mature and fall to the ground about it. The relation in both instances is essentially that of parent and offspring, although the parent soon disappears in the case of annuals, while among the algae its existence is regularly terminated by fission. The size and the density of the family group are determined by the number of seeds produced, and by their mobility. These are further affected by the height and branching of the plant, and by the position of the seeds upon it. The disseminules of immobile species fall directly beneath the parent, and the resulting group is both uniform and definite. A similar arrangement is caused likewise by offshoots. An increase in mobility brings about a decrease of aggregation, since the disseminules are carried away from the parent plant. Perfectly mobile forms rarely produce family groups for this reason. It is evident, however, that mobile perennials sometimes arrange themselves in similar fashion in consequence of propagation by underground parts. Consequently, it is possible to state the law of single aggregation, viz., that immobility promotes the grouping of parent and offspring, and mobility hinders it.

If all species were immobile, the family group would be characteristic of vegetation. Since the great majority are more or less mobile, aggregates of this sort are the exception rather than the rule. Mobility not only decreases the number of offspring in the family group, but it also spreads disseminules broadcast to enter dissimilar groups. It leads directly to mixed aggregation, by which individuals of one or more species invade the family group. Once established, the newcomers tend also to produce simple groups, thus causing an arrangement corresponding essentially to a community. Such collections of family groups are extremely variable in size and definition. This arises in part from the nature of simple aggregation, and in part from the varying mobility of different species. Mobility alone often produces similar communities by bringing together the disseminules of different plants, each of which then becomes the center of a mixed group. In the case of permobile species, several disseminules of each may be brought together. The resulting area, though larger, is practically the same. At present, it is difficult to formulate the law for this method of grouping. It may be stated provisionally as follows: mixed aggregation is the direct result of mobility, and the greater the mobility the more heterogeneous the mixture.

The constitution of all the major areas of a formation is to be explained upon the basis of aggregation by the two methods described. The relative importance of family groups and communities differs for every formation, and the exact procedure in each can be obtained only by the detailed study of quadrats. The problem is further complicated by competition and reaction, particularly in closed vegetation. For this reason, aggregation can be studied most satisfactorily in a new or denuded area, where these processes are not yet in evidence.

Kinds of Association

=253. Categories.= In the analysis of association, it must be kept clearly in mind that the concrete examples from which all generalizations must be drawn are often in very different stages of development, and are of correspondingly different ages. For this reason it has seemed best to consider the primary relations of association in general in this place, leaving the treatment of the effects of invasion, succession, alternation, and zonation to be taken up under these topics.

Various categories of association may be distinguished, according to the dominant physical factor concerned or the point of view taken. These will fall into two series, as we consider the relation of plant to plant with reference to some object or characteristic, or the grouping of plants together in response to some dominant factor. In the first series may be placed association with reference to substratum, to the ground (occupation), and to invasion; in the second belong light and water-content association. It should be noted that these are all actual associations in nature, and not concepts such as the vegetation form, within which plants from widely different associations may be classified. Naturally, it does not follow that it is not logical or valuable to group together those plants, such as hydrophytes, sciophytes, hysterophytes, etc., which have a common relation to some factor, but belong to different formations.

=254. Stratum association.= Plants manifest independent or dependent association with reference to the stratum to which they are attached and from which they derive food or support. Independent association is exhibited by those holophytic species of a formation which are entirely independent of each other with respect to mechanical support or nutrition. It is characteristic of the greater number of the constituent species of formations. Dependent association is manifested in the relation between host and parasite, stratum and epiphyte, support and liane. Warming has distinguished six kinds of associations: parasitism, helotism, mutualism, epiphytism, lianism, and commensalism. Commensalism corresponds to the primary principle of association which has given rise to vegetation. Homogeneous commensalism is the term applied to social exclusive plants, in which the patch is composed of a single species. Such association is extremely rare in nature, and if the most minute forms be considered, probably never occurs. On the other hand, heterogeneous commensalism, in which individuals of more than one species are present, is everywhere typical of vegetation. Warming regards saprophytism merely as a specialized kind of parasitism, an opinion that may well be defended. Helotism, however, is also a mere modification of parasitism, if it is not indeed parasitism pure and simple. Mutualism is an altogether vague concept, including parasites, epiphytes, and endophytes of doubtful physiological relation. Pound and Clements treated lianes, parasites, and saprophytes as vegetation forms, relating herbaceous creepers and twiners to the lianes, and dividing the fungi and lichens into nine groups. Whatever the value of these divisions may be from the standpoint of vegetation forms, they represent the same relation between plant and nutritive stratum, and with respect to association should be merged in one group. Schimper was the first to perceive the essential similarity of all such groups from the standpoint of association. He terms these plant societies (Genossenschaften), retaining the four groups already established, lianae, epiphyta, saprophyta, and parasiticae. It is evident that dependent association comprises extremely divergent forms, from the slightly clinging herb, such as Galium, to the most intense parasite. The distinction, however, is a clear one, if restricted to that relation between plants in which one acts as a mechanical support or stratum or as a nutritive host for the other.

=255. Ground association.= The first division of formations into open and closed was made by Engler and Drude. Open formations were defined as those having incomplete stability and heterogeneous composition, while closed formations have a more definite uniform stamp. What is true of formations is equally true of vegetation, so that association may be regarded as open or closed with reference to the density and thoroughness with which the plants occupy the ground. In open association, the ground is slightly or partially occupied, readily permitting the entrance of new plants without the displacement of those already present. Such an arrangement is characteristic of the early stages of a formation, or of a succession of formations. It produces unstable open formations, which arise, usually after denudation, in sand-hills, blowouts, gravel slides, dunes, flood plains, burned areas, etc. In closed association, occupation of the ground is complete, and the invasion of new species can occur only through displacement. Closed association results in stable, closed formations, such as forest, thicket, meadow, and prairie. As open association characterizes the early stages of a succession of formations, so closed association is peculiar to the later or last stages of all such successions. In short, open formations represent certain phases of the development of vegetation, while closed formations correspond to the relatively final structural conditions. It is a fundamental principle of association that every succession from denudation, or from newly formed soils, begins with open formations and ends with a closed formation. The causes leading up to open and closed association are intimately connected with development, and hence are considered under invasion and succession.

=256. Species guild association.= Drude has distinguished a kind of association peculiar to invasion, in which there is a successive or concomitant movement of certain species of a formation into another formation or region, resulting in species guilds (Artengenossenschaften). The association in this case is largely one of community of origin or area, and of concomitant migration. It is especially characteristic of areas adjacent to formational and regional limits. Fundamentally, it is merely the grouping of plants which are invading at the same time, and consequently it differs only in degree from what occurs in every invasion where more than a single individual is concerned. Accordingly, this type of association has little more than historical interest. This must not be construed to mean that it does not occur, but that it differs in no essential from the ordinary grouping of invaders.

=257. Light association.= The constituent species of formations show two fundamentally different groupings with respect to light. In the one case, the individuals are on the same level, or nearly so, in such a way that each has direct access to sunlight. Such an arrangement is characteristic of most grassland and herbaceous formations. In the case of desert formations, there is often considerable difference in the height of the plants, but the distance between them is so great as to admit of direct illumination of all. This arrangement may be termed coordinate association. In forests, thickets, and many herbaceous wastes, the height and density of certain species enable them to dominate the formation. In a dense forest, the trees receive practically all the light incident upon the formation, and the shrubs, herbs, fungi, and algae of lower habit and inferior position must adapt themselves to the diffuse light which passes through or between the leaves. The same is equally true of dense thickets and wastes, except that the vertical distance is less, and the diffuseness of the light is correspondingly modified. In these formations, the dominant trees, shrubs, or herbs, the facies, constitute a primary or superior layer. The degree of subordinate association, as a result of which inferior layers will arise, is entirely determined by the density of the facies. In open woodlands, which are really mixed formations of woodland and grassland, the intervals, and usually the spaces beneath the trees also, are covered with poophytes, showing an absence of subordination due to light. This is the prevailing condition in the pine formation (Pinus ponderosa-xerohylium) of the ridges and foot-hills of western Nebraska. When, however, the trees stand sufficiently close that their shadows meet or overlap throughout the day, the increasing diffuseness begins to cause modification and rearrangement of the individuals. By photometric methods, the light in a forest is found to be least diffuse just below the facies, while the diffuseness increases markedly in passing to the ground. The taller, stronger individuals are consequently in a position to assimilate more vigorously, and to become still taller and stronger as a result. Just as these have taken up a position inferior to that of the facies, so the shorter or weaker species must come to occupy a still more subordinate position. This results, not only because the light is primarily weaker nearer the ground, but also because the taller plants interpose as a second screen. The complete working out of this arrangement with reference to light produces typical subordinate association, which finds its characteristic expression in the layering of forests and thickets. Layers tend to appear as soon as open woodland or thicket begins to pass into denser conditions, and up to a certain point, at which they disappear, they become the more numerous and the more marked, the denser the forest.

In the Otowanie woods near Lincoln (Quercus-Hicoria-hylium), layering usually begins at a light value of .1 (1 = normal sunshine in the open). Thornber has found the same value to obtain in the thickets of the Missouri bluffs. In these, again, layers disappear at a value of .005, the extreme diffuseness making assimilation impossible except for occasional mosses and algae. A number of herbaceous plants are present in the spring, but these are all prevernal or vernal bloomers, which are safely past flowering before shade conditions become extreme. In the Fraxinus-Catalpa-alsium, all inferior holophytic vegetation disappears between the light value of .004 and that of .003. The spruce-pine formation (Picea-Pinus-hylium) of the Rocky mountains, with a light value of .01, usually contains but a few scattered herbs, mostly evergreen; in some cases there are no subordinate plants other than mosses and hysterophytes. The lodge-pole pine formation (Pinus murrayana-hylium), with light values often less than .005, is nearly or quite destitute of all but hysterophytic undergrowth. Such extremely dense formations are examples of coordinate association merely, since the formation is reduced to a single superior layer, in which the individuals of the facies bear the same spatial relation to incident light. In layered formations, in addition to the subordinate relation of other species to the facies, there is, of course, a kind of coordinate association manifested in each layer.

=258. Water-content association.= Schouw was the first to give definite expression to the value of the water-content of the soil for the grouping of plants. He established four groups: (1) water plants, (2) swamp plants, (3) plants of moist meadows, (4) plants of dry soils. The first he termed hydrophytes, introducing the term halophytes to include all saline plants. Thurmann recognized the fundamental influence of water-content upon association, and further perceived that the amount of water present was determined primarily by the physical nature of the soil. He distinguished plants which grow in soils that retain water as hygrophilous, and those found upon soils that lose water readily as xerophilous. Those which seemed to grow indifferently upon either were termed ubiquitous. The latter correspond in some measure to mesophytes, but they are really plants possessing a considerable range of adaptability, and do not properly constitute a natural group. Warming proposed the term mesophytes to include all the plants intermediate between hydrophytes and xerophytes. He recognized the paramount value of water-content association as the basis of ecology, and upon this made a logical and systematic treatise out of the scattered results of many workers. Schimper placed the study of vegetation upon a new basis by drawing a distinction between physical and physiological water-content, and by pointing out that the last alone is to be taken into account in the study of plant life, and hence of plant geography. Accepting the easily demonstrable fact that an excess of salts in the soil water, as well as cold, tends greatly to diminish the available water of the soil, i. e., the chresard, it is at once seen why saline and arctic plants are as truly xerophytic as those that grow on rocks or in desert sands. An anomalous case which, however, physical factor records have explained fully, is presented by many plants growing in alpine gravel slides, strands, blowouts, sandbars, etc., in which the water-content is considerable, but the water loss excessive, on account of extreme heat or reduced air pressure. The effect of these conditions is to produce a plant xerophytic as to its aerial parts, and mesophytic or even hydrophytic as to subterranean parts. Such plants may, from their twofold nature, be termed dissophytes; they are especially characteristic of dysgeogenous soils in alpine regions where transpiration reaches a maximum, but are doubtless to be found in all gravel and sand habitats with high water-content. With these corrections, the concept of water-content association, which owes much to both Warming and Schimper, but is largely to be credited to Thurmann, becomes completely and fundamentally applicable to all vegetation.

Up to the present time, the general character of the habitat, together with the gross appearance of the plant itself, has been thought sufficient to determine the proper position of a plant or a formation in the water-content classification. Such a method is adequate, however, only for plants and formations which bear a distinct impress. For an accurate classification into the three categories, hydrophytes, mesophytes, and xerophytes, it is necessary to make exact determinations of the normal holard and chresard of the habitat, and to supplement this, in some degree at least, by histological studies. Except in the case of saline, acid, and frozen soils, the holard alone will be a fairly accurate index, especially in habitats of similar soil composition. For an exact and comprehensive classification, however, and particularly in comparative work, the chresard must constitute the sole criterion. As the latter has been ascertained for very few formations, and in Nebraska and Colorado alone, the present characterization of many plants and formations as hydrophytic, mesophytic, or xerophytic must be regarded as largely tentative, and the final classification will be possible only after the thorough quantitative investigation of their habitats.

The water-content groups, hydrophytia, mesophytia, and xerophytia, include all formations found upon the globe. The exactness with which this classification applies to vegetation is made somewhat more evident by dividing mesophytia into forest and grassland. This is based primarily upon light association, but it also reflects water-content differences in a large degree. The groups thus constituted represent the fundamental zonation of the vegetative covering with respect to water-content. Ocean, forest, grassland, and desert correspond exactly to hydrophytia, hylophytia, poophytia, and xerophytia. The difference is merely one of terminology: the first series takes into account the physiognomy of the vegetation itself, while the other emphasizes the causative factors.

THE DEVELOPMENT OF THE FORMATION

=259.= A strict account of development should trace the results of the various activities of vegetation in their proper sequence. This is aggregation, migration, ecesis, reaction, and competition. These functions are so intimately and often so inextricably associated that it is hardly feasible to discuss development by treating each one separately. In consequence, the two fundamental phenomena, invasion and succession, which they produce, are taken as the basis of the discussion. These, moreover, are different only in degree; succession is merely complete, periodic invasion. Nevertheless, the subject gains in clearness by a separate treatment of each.

INVASION

=260.= By invasion is understood the movement of plants from an area of a certain character into one of a different character, and their colonization in the latter. This movement may concern an individual, a species, or a group of species. From the nature of invasion, which contains the double idea of going into and taking possession of, it usually operates between contiguous formations, but it also takes place between formational zones and patches. More rarely and less noticeably, there may be invasion into a remote vegetation, as a result of long carriage by wind, water, birds, railroads, or vessels. Movement or migration, however, represents but one of the two ideas involved in invasion. Migration merely carries the spore, seed, or propagule into the area to be invaded. In ecesis, the spores or seeds germinate and grow, after more or less adjustment, and in case the latter becomes sufficiently complete, the new plants reproduce and finally become established. With all terrestrial plants, invasion is possible only when migration is followed by ecesis, because of the inherent differences of formations or of areas of the same formation. In the case of surface floating forms, such as Lemnaceae, and of the plancton, ecesis is of much less importance, on account of the uniformity of the medium and the lack of attachment, and migration is often practically synonymous with invasion.

MIGRATION

=261.= Migration has been sometimes used loosely as a synonym for invasion, but it is here employed in its proper sense of removal or departure, i. e., movement, and is contrasted with ecesis, the making of a home, the two ideas being combined in invasion, which is a moving into and a taking possession of. An analysis of migration reveals the presence of four factors, mobility, agency, proximity, and topography. Not all of these are present in every instance of migration, as for example in the simple elongation of a rootstalk, but in the great majority of cases each plays its proper part. Mobility represents the inherent capacity of a plant for migration, and in its highest expression, motility, is in itself productive of movement. As a general rule, however, modifications for securing mobility are ineffective in the absence of proper agents, and the effective operation of the two will be profoundly influenced by distance and topography.

=262. Mobility= denotes potentiality of migration as represented by modifications for this purpose. It corresponds, in a sense, to dissemination, though seed production also enters into it. Its most perfect expression is found in those plants which are themselves motile, Bacteriaceae, Oscillatoria, Volvocaceae, and Bacillariaceae, or possess motile propagules, such as most Phycophyta. On the other hand, it is entirely undeveloped in many plants with heavy unspecialized seeds and fruits. Between these two extremes lie by far the greater number of plants, exhibiting the most various degrees of mobility, from the motile though almost immobile offshoots of many Liliaceae to the immotile but very mobile spores of fungi. It is thus seen that motility plays a relatively small part in migration, being practically absent in terrestrial forms, and that it bears a very uncertain relation to mobility. In analyzing the latter, contrivances for dissemination are seen to determine primarily the degree of mobility, while the number of seeds produced will have an important effect in increasing or decreasing it. A third factor of considerable importance is also involved, namely, position with reference to the distributive agent, but any exact knowledge of its importance must await systematic experiment somewhat after the methods of Dingler, but with air-currents, etc., of known velocity and direction. The time is not distant when by such methods it will be possible to establish a coefficient of mobility, derived from terms of position, weight, resistant surface, and trajectory for definite wind velocities or for particular propulsive mechanisms.

=263. Organs for dissemination.= Plants exhibit considerable diversity with reference to the part or organ modified, or at least utilized, for dissemination. This modification, though usually affecting the particular product of reproduction, may, in fact, operate on any part of the plant, and in certain cases upon the entire plant itself. In the majority of plants characterized by alternation of generations, the same individual may be disseminated in one generation by a reproductive body, and in the other by a propagative one, as is the case in the oogones and conidia of Peronospora, the spores and gemmae of Marchantia, the fruits and runners of Fragaria, etc. Special modifications have, as a rule, been developed in direct connection with spores and seeds, and mobility reaches its highest expression in these. It is, on the other hand, greatly restricted in offshoots and plant bodies, at least in terrestrial forms, though it will now and then attain a marked development in these, as shown by the rosettes of Sempervivum and the tumbling plants of Cycloloma. For the sake of convenience, in analyzing migration, all plants may be arranged in the following groups with reference to the organ or part distributed.

1. Spore-distributed, sporostrotes. This includes all plants possessing structures which go by the name of spore, such as the acinetes of Nostoc and Protococcus, the zoogonidia of Ulothrix, Ectocarpus, etc., the conidia, ascospores, and basidiospores of fungi, the tetraspores of red seaweeds, and the gemmae and spores proper of liverworts, mosses, and ferns. These are almost always without especial contrivances for dissemination, but their extreme minuteness results in great mobility.

2. Seed-distributed, spermatostrotes. This group comprises all flowering plants in which the seed is the part modified or at least disseminated. The mobility of seeds is relatively small, except in the case of minute, winged or comate seeds.

3. Fruit-distributed, carpostrotes. The modifications of the fruit for distribution exceed in number and variety all other modifications of this sort. All achenes, perigynia, utricles, etc., properly belong here.

4. Offshoot-distributed, thallostrotes. To this class are referred those plants, almost exclusively cormophytes, which produce lateral, branch-like propagules, such as root-sprouts, rhizomes, runners, stolons, rosettes, etc. Migration with such plants is extremely slow, but correspondingly effective, since it is almost invariably followed by ecesis.

5. Plant-distributed, phytostrotes. This group includes all plancton and surface forms, whether motile or non-motile, and those terrestrial plants in which the whole plant, or at least the aerial part, is distributed, as in tumbleweeds and in many grasses.

=264. Contrivances for dissemination.= Any investigation of migration to be exact must confine itself to fixed forms. For these the degree of perfection shown by dissemination contrivances corresponds almost exactly to the degree of mobility. Because of the difficulty of ascertaining the effect of ecesis, it is impossible to determine the actual effectiveness in nature of different modifications, and the best that can be done at present is to regard mobility, together with the occurrence and forcefulness of distributive agents, as an approximate measure of migration. The general accuracy of such a measure will be more or less evident from the following. Of 118 species common to the foot-hill and sand-hill regions of Nebraska, regions which are sufficiently diverse to indicate that these common species must have entered either one by migration from the other, 83 exhibit modifications for dissemination, while 8 others, though without special contrivances, are readily distributed by water, and 4 more are mobile because of minuteness of spore or seed. Some degree of mobility is present in 73 per cent of the species common to these regions, while of the total number of species in which the mode of migration is evident, viz., 95, 66 per cent are wind-distributed, 20 per cent animal-distributed, and 14 per cent are water-distributed. It need hardly be noted that this accords fully with the prevalence and forcefulness of winds in these regions. Of the species peculiar to the foot-hill region, many are doubtless indigenous, though a majority have come from the montane regions to the westward. The number of mobile species is 121, or 60 per cent of the entire number, while the number of wind-distributed ones is 85, or 70 per cent of those that are mobile. Among the 25 species found in the widely separated wooded bluff and foot-hill regions, 2 only, Amorpha nana and Roripa nasturtium, are relatively immobile, but the minute seeds of the latter, however, are readily distributed, and the former is altogether infrequent.

The following groups of plants may be distinguished according to the character of the contrivance by which dissemination is secured:

1. Saccate, saccospores. Here are to be placed a variety of fruits, all of which agree, however, in having a membranous envelope or an impervious, air-containing pericarp. In Ostrya, Physalis, Staphylea, the modification is for wind-distribution, while in Carex, Nymphaea, etc., it is for water-transport.

2. Winged, pterospores. This group includes all winged, margined, and flattened fruits and seeds, such as are found in Acer, Betula, Rumex, many Umbelliferae, Graminaceae, etc.

3. Comate, comospores. To this group belong those fruits and seeds with long silky hairs, Gossypium, Anemone, Asclepias, etc., and those with straight capillary hairs or bristles not confined to one end, Typha, Salix, etc.

4. Parachute, petasospores. The highly developed members of this group, Taraxacum, Lactuca, and other Liguliflorae are connected through Senecio and Eriophorum with the preceding. These represent the highest development of mobility attained by special modification.

5. Chaffy-pappose, carphospores. In this group are placed those achenes with a more or less scaly or chaffy pappus with slight mobility, as in Rudbeckia, Brauneria, Helianthus, etc.

6. Plumed, lophospores. In the fruits of this class, the style is the part usually modified into a long plumose organ, possessing a high degree of mobility, as in Pulsatilla, Sieversia, and Clematis.

7. Awned, ascospores. These are almost exclusively grasses, in which the awns serve for distribution by wind, water, or animals, and even, according to Kerner, by hygroscopic creeping movements. The mobility in many cases is great.

8. Spiny, centrospores. This group contains a few representatives which possess a moderate degree of mobility by attachment, as in Tribulus and Cenchrus.

9. Hooked, oncospores. The members of this group are extremely numerous, and the degree of mobility as a rule is very high. All exhibit in common the development of hooks or barbs, by which they are disseminated in consequence of attachment, though the number, size, and disposition of the hooks vary exceedingly.

10. Viscid, gloeospores. In these, the inflorescence is more or less covered with a viscid substance, as in species of Silene, or the fruit is beset with glandular hairs, as in Cerastium, Salvia, etc.

11. Fleshy, sarcospores. These are intended for dissemination by deglutition, largely by birds; the effectiveness of the modification depends in a large degree upon the resistance of the seed envelope to digestion. The mobility varies greatly, but the area over which migration may be effected is large.

12. Nut-fruited, creatospores. This group includes those plants with nut fruits which are carried away and secreted by animals for food.

13. Flagellate, mastigospores. These are plants with ciliate or flagellate propagative cells, i. e., zoogonidia, as in Protococcus, Ulothrix, Oedogonium, Ectocarpus, etc., or with plant bodies similarly motile, Bacteriaceae and Volvocaceae.

=265. Position of disseminule.= The position on the plant of the organ to be disseminated, i. e., its exposure to the distributing agent, plays a considerable part in determining the degree of mobility. In the majority of plants, the position of the inflorescence itself results in maximum exposure, but in a large number of forms special modifications have been developed for placing the spores or seeds in a more favorable position. In both cases, there are often present also devices for bringing about the abscission of the seed or fruit. It is, moreover, self-evident that the height of the inflorescence above ground or above the surrounding vegetation is likewise of considerable importance in increasing the trajectory. It is yet too early to make a complete classification of contrivances for placing disseminules in the most favorable exposure, but the following will serve as a basis for future arrangements.

1. In all operculate Discomycetes, and especially in the Ascobolaceae, where the asci project above the hymenium, the spores are raised above the surface by tensions within the apothecium. This might be regarded as dissemination by expulsion, if it were not for the fact that the spores fall back into the cup, unless carried away by the wind.

2. In Gasteromycetes and in certain Hepaticae, the spores are not only elevated slightly above the sporophore by the expanding capillitium or by the mass of elaters, but they are also held apart in such a way that the wind blows them out much more readily.

3. In Bryophyta, the sporophore regularly dehisces by a slit, or is provided with a peristome. Both structures are for the purpose of sifting the spores out into the wind; by reason of their hygroscopicity, they also insure that the spores will not be shaken out in wet weather.

4. In a few grasses, such as Stipa and Aristida, the twisting and intertwining of the awns lift the floret out of the glumes, and at the same time constitute a contrivance readily blown away by the wind or carried by attachment.

5. In certain Compositae, the involucral scales are reflexed at maturity, and at the same time the disk becomes more or less convex, serving to loosen the achenes. This result is also secured in certain species by the drying and spreading of the pappus hairs.

6. The scapose Liguliflorae, Taraxacum, Agoseris, etc., are characterized by the elongation of the scape after anthesis, with the result that the head is raised to a considerable height by the time the achenes are mature.

7. Carpotropic movements, though primarily for another purpose, often serve to bring seeds and fruits into a better position for dissemination.

=266. Seed production.= The relation of spore or seed-production to mobility is obvious in the case of mobile species; in the case of immobile ones, it is just as evident that it has no effect, though it may still have considerable influence in increasing migration. In the case of two species with equally effective dissemination contrivances, the one with the largest seed-production will be the more mobile. On the basis of the relation of seeds to flower, two groups of plants may be distinguished, one, Polyanthae, in which the flowers are many and the seeds few or single, as in Compositae, and the other, Polyspermatae, Portulaca, Yucca, etc., in which the number of seeds to each flower is large. So far as the actual number of seeds produced is concerned, polyanthous plants may not differ from polyspermatous ones, but, as a rule, they are much more highly specialized for dissemination and are more mobile. The number of fertile seeds is also much greater, a fact which is of great importance in ecesis, and which, taken in connection with mobility, partially explains the supremacy of the composites. Among the fungi and algae, the amount of spore-production in a large degree determines the mobility, since these forms are intrinsically permobile.

=267. Agents of migration.= In the last analysis, however, the possibility of migration depends upon the action of distributive agents; in the absence of these, even the most perfect contrivance is valueless, while their presence brings about the distribution of the most immobile form. In short, migration depends much more upon such agents than upon mobility, however perfect the latter may be. It is, moreover, evident that the amount and extent of migration will be determined primarily by the permanence and forcefulness of the agent, as indicated by its ability to bring about transportation. Finally, as will be shown later, the direction and rapidity of migration depend directly upon the direction and intensity of the agent.

Migration results when spores, seeds, fruits, offshoots, or plants are moved out of their home by water, wind, animals, man, gravity, glaciers, growth, or mechanical propulsion. Corresponding to these agents, there may be recognized the following groups:

1. Water, hydrochores. These comprise all plants distributed exclusively by water, whether the latter acts as ocean currents, tides, streams, or surface run-off. In the case of streams and run-off, especially, mobility plays little part, provided the disseminules are impervious or little subject to injury by water. Motile plants, or those with motile cells, which belong entirely to this group, may be distinguished as autochores, which correspond closely to mastigospores.

2. Wind, anemochores. This group includes the majority of all permobile terrestrial plants, i. e., those in which modifications for increasing surface have been carried to the extreme, or those which are already permobile by reason of the minuteness of the spore or seed. Saccate, winged, comate, parachute, pappose, plumed, and, to a certain extent, awned seeds and fruits represent the various types of modifications for wind-distribution.

3. Animals, zoochores. Among terrestrial plants, dissemination by attachment represents essentially the same degree of specialization as is found in wind-distributed plants. The three types of contrivances for this purpose are found in spinose, hooked, and glandular fruits. Dissemination by deglutition and by carriage, either intentional or unintentional, though of less value, play a striking part on account of the great distance to which the seeds may be carried. Dissemination by deglutition is characteristic of sarcospores, and distribution by carriage of creatospores.

4. Man, brotochores. Dissemination by man has practically no connection with mobility. It operates through great distances and over immense areas as well as near at hand. It may be intentional, as in the case of cultivated species, or unintentional, as in thousands of native or exotic species. No other disseminating agent is comparable with man in respect to universal and obvious migration.

5. Gravity, clitochores. The members of this group are exclusively colline, montane, and alpine plants, growing on rocks, cliffs, and gravel slides (talus), etc., in which the seeds reach lower positions merely by falling, or more frequently by the breaking away and rolling down of rock or soil masses and particles. Dissemination by this method is relatively insignificant, though it plays an important part in the rock fields and gravel slides of mountain regions, particularly in the case of immobile species.

6. Glaciers, crystallochores. At the present time, transport by glaciers is of slight importance, because of the restriction of the latter to alpine and polar regions, where the flora is poorly developed. In the consideration of migrations during the glacial epoch, however, it plays an important point.

7. Growth, blastochores. The mobility of species disseminated by offshoots is extremely slight, and the annual movement relatively insignificant. The certainty of migration and of ecesis, is, however, so great, and the presence of offshoots so generally the rule in terrestrial plants that growth plays an important part in migration, especially within formations.

8. Propulsion, bolochores. Like growth, dissemination by mechanical propulsion, though operating through insignificant distances, exerts an important effect in consequence of its cumulative action. The number of plants, however, with contrivances for propulsion is very much smaller than the number of blastochores. All bolochorous species agree in having modifications by means of which a tension is established. At maturity, this tension suddenly overcomes the resistance of sporangium or fruit, and throws the enclosed spores or seeds to some distance from the parent plant. In accordance with the manner in which the tension is produced, sling-fruits may be classified as follows:

(a) Hygroscopicity, pladoboles. These include the ferns with annulate sporangia, in which the expansion of the annulus by the absorption of moisture bursts the sporangium more or less suddenly, though the actual propulsion of the spores seems to come later as a result of dessication.

(b) Turgescence, edoboles. Dissemination by turgescence is highly developed in Pilobolus and in Discomycetes, though in the latter turgescence results rather in placing the spores in a position to be readily carried by the wind. Impatiens and Oxalis furnish familiar examples of fruits which dehisce in consequence of increased turgidity.

(c) Dessication, xerioboles. The number of fruits which dehisce upon drying is very large, but only a small portion of these expel their seeds forcibly. Geranium, Viola, Erysimum, and Lotus illustrate the different ways in which dessication effects the sudden splitting of fruits.

(d) Resilience, tonoboles. In some plants, especially composites, labiates, and borages, the achenes or nutlets are so placed in the persistent calyx or involucre that the latter serves as a sort of mortar for projection, when the stem of the plant is bent to one side by any force, such as the wind or an animal. It will be noticed that two separate agents are actually concerned in dissemination of this sort.

Frequently, two or more agents will act upon the same disseminule, usually in succession. The possibility of such combinations in nature is large, but actual cases seem to be infrequent, except where the activities of man enter into the question. Some parts, moreover, such as awned inflorescences, are carried almost equally well by wind or animals, and may often be disseminated by the cooperation of these two agents. The wind also often blows seeds and fruits into streams by which they are carried away, but here again, parts adapted to wind-dissemination are injured as a rule by immersion in water, and the number of plants capable of being scattered by the successive action of wind and water is small.

In the present state of our knowledge of migration, it is impossible to establish any definite correspondence between dissemination-contrivance, agent, and habitat. As a general rule, plants growing in or near the water, in so far as they are modified for this purpose at all, are adapted to water-carriage. Species which grow in exposed grassy or barren habitats are for the most part anemochores, while those that are found in the shelter of forests and thickets are usually zoochorous, though the taller trees and shrubs, being exposed to the upper air currents, are generally wind-distributed. There is then a fair degree of correspondence, inasmuch as most hydrophytes are hydrochorous, most hylophytes, zoochorous, and the majority of poophytes and xerophytes, anemochorous. Definite conclusions can be reached, however, only by the statistical study of representative formations.

With respect to their activity, agents may be distinguished as constant, as in the case of currents, streams, winds, slope, growth, and propulsion, or intermittent, animals and man. In the former, the direction is more or less determinate, and migration takes place year by year, i. e., it is continuous, while in the latter dissemination is largely an accidental affair, indeterminate in direction, and recurring only at indefinite intervals. The effective conversion of migration into invasion is greatest when the movement is continuous, and least when it is discontinuous, since, in the latter, species are usually carried not only out of their particular habitat but even far beyond their geographical area, and the migration, instead of being an annual one with the possibility of gradual adjustment, may not recur for several years, or may, indeed, never take place again. The rapidity of migration is greatest in the case of intermittent agents, while the distance of migration is variable, being great chiefly in the case of man, ocean currents, and wind, and slight when the movement is due to slope, growth, or propulsion. Disregarding the great distances over which artificial transport may operate, seeds may be carried half way across the continent in a week by strong-flying birds, while the possibilities of migration by growth or expulsion are limited to a few inches, or at most to a few feet per year. This slowness, however, is more than counterbalanced by the enormously greater number of disseminules, and their much greater chance of becoming established.

=268. The direction of migration= is determinate, except in the case of those distributive agents which act constantly in the same direction. The general tendency is, of course, forward, the lines of movement radiating in all directions from the parent area. This is well illustrated by the operation of winds which blow from any quarter. In the case of the constant winds, migration takes a more or less definite direction, the latter being determined to a large degree by the fruiting period of any particular species. In this connection, it must be kept clearly in mind that the position of new areas with reference to the original home of a species does not necessarily indicate the direction of migration, as the disseminules may have been carried to numerous other places in which ecesis was impossible. The local distribution of zoochorous species is of necessity indeterminate, though distant migration follows the pathways of migratory birds and animals. In so far as dissemination by man takes place along great commercial routes, or along highways, it is determinate. In ponds, lakes, and other bodies of standing water, migration may occur in all directions, but in ocean currents, streams, etc., the movement is determinate, except in the case of motile species. The dissemination of plants by slopes, glaciers, etc., is local and definite, while propulsion is in the highest degree indeterminate. Migration by growth is equally indefinite, with the exception that hydrotropism and chemotropism result in a radiate movement away from the mass, while propulsion throws seeds indifferently into or away from the species-mass. From the above it will be seen that distant migration may take place by means of water, wind, animals or man, and, since all these agents act in a more or less definite direction over great distances, that it will be in some degree determinate. On the other hand, local migration will as regularly be indeterminate, except in the case of streams and slopes. The direction of migration, then, is controlled by these distributive agents, and the limit of migration is determined by the intensity and duration of the agent, as well as by the character of the space through which the latter operates.

ECESIS

=269. Concept.= By the term ecesis is designated the series of phenomena exhibited by an invading disseminule from the time it enters a new formation until it becomes thoroughly established there. In a word, ecesis is the adjustment of a plant to a new habitat. It comprises the whole process covered more or less incompletely by acclimatization, naturalization, accommodation, etc. It is the decisive factor in invasion, inasmuch as migration is entirely ineffective without it, and is of great value in indicating the presence and direction of migration in a great number of species where the disseminule is too minute to be detected or too little specialized to be recognizable.

The relation of migration to ecesis is a most intimate one: the latter depends in a large measure upon the time, direction, rapidity, distance, and amount of migration. In addition, there is an essential alternation between the two, inasmuch as migration is followed by ecesis, and the latter then establishes a new center from which further migration is possible, and so on. The time of year in which fruits mature and distributive agents act has a marked influence upon the establishment of a species. Disseminules designed to pass through a resting period are often brought into conditions where they germinate at once, and in which they perish because of unfavorable physical factors, or because competing species are too far advanced. On the other hand, spores and propagules designed for immediate germination may be scattered abroad at a time when conditions make growth impossible. The direction of movement is decisive in that the seed or spore is carried into a habitat sufficiently like that of the parent to secure establishment, or into one so dissimilar that germination is impossible, or at least is not followed by growth and reproduction. The rapidity and distance of migration have little influence, except upon the less resistant disseminules, conidia, gemmae, etc. Finally, the amount of migration, i. e., the number of migrants, is of the very greatest importance, affecting directly the chances that vigorous disseminules will be carried into places where ecesis is possible.

Normally, ecesis consists of three essential processes, germination, growth, and reproduction. This is the rule among terrestrial plants, in which migration regularly takes place by means of a resting part. In free aquatic forms, however, the growing plant or part is usually disseminated, and ecesis consists merely in being able to continue growth and to insure reproduction. Here establishment is practically certain, on account of the slight differences in aquatic habitats, excepting of course the extremes, fresh water and salt water. The ease indeed with which migration and ecesis are effected in the water often makes it impossible to speak properly of invasion in this connection, since aquatics are to such a large extent cosmopolitan. In dissemination by offshoots, the conditions are somewhat similar. Here, also, ecesis comprises the sequence of growth and reproduction, and invasion, in the sense of passing from one habitat to another, is of rare occurrence, as the offshoot grows regularly under the same conditions as the parent plant. The adjustment of growing plants and parts is so slight, and their establishment so certain on account of their inability to migrate into very remote or different habitats, that they may be ignored in the following discussion.

In accordance with the above, it would be possible to distinguish three groups of terrestrial plants: (1) those migrants which germinate and disappear, (2) those which germinate and grow but never reproduce, (3) those which reproduce, either by propagation or generation, or both. Such a classification has little value, however, since the same species may behave in all three fashions, depending upon the habitat to which it has migrated, and since invasion does not occur unless the plant actually takes possession, i. e., reproduces. From the latter statement, it follows that invasion occurs only when a species migrates to a new place, in which it germinates, matures, and reproduces. Maintenance by annual invasion simply, in which the plants of each year disappear completely, can not then be regarded as invasion proper. On the other hand, though such instances are rare, it is not necessary that the invaders produce fruit, provided they are able to maintain themselves, or to increase by propagation. Furthermore, if a plant germinate, grow, and reproduce, it is relatively immaterial whether it persist for a few years or for many, since, as we shall see under Succession, the plants of one invasion are displaced by those of the next, the interval between invasions increasing with the stabilization.

=270. Germination of the seed.= The germination of seed or spore is determined by its viability and by the nature of the habitat. Viability depends upon the structural characters of fruit, seed-coat, and endosperm, and to a degree upon the nature of the protoplasm or embryo. The first three affect the last directly, by protecting the embryo against dryness, against injury due to carriage by water, or by deglutition, and probably in some cases against excessive heat or cold. Marloth has investigated the structure of seed coats, establishing the following groups, which are summarized here somewhat fully because of their bearing upon ecesis: (1) seed coats without protective elements, endosperm absent or rudimentary, Epilobium, Impatiens, Parnassia, Sagittaria, etc.; (2) protective elements lacking or few, endosperm highly developed with thick-walled cells, Liliaceae, Primulaceae, Rubiaceae, etc.; (3) protective cells present in the seed coats, endosperm little or none, Boraginaceae, Crassulaceae, Cruciferae, Labiatae, Papilionaceae, etc.; (4) protective elements present, Asclepias, Campanula, Gentiana, Silene, Saxifraga, etc.; (5) protective cells present, endosperm thick-walled, Euonymus, Helianthemum, Ribes. The protective cells are of various kinds: (1) epidermal cells strongly cuticularized, Caryophyllaceae, Crassulaceae, Fumariaceae, Saxifragaceae; (2) parenchyma thick-walled, several-layered, Aesculus, Castanea, Fagus; (3) parenchyma cells with the inner or radial walls thickened, Campanula, Erythraea, Gentiana; (4) epidermal cells cup-shaped, thick-walled, Cruciferae, Ribes, Vaccinium; (5) parenchyma with thickened, cellulose walls, Geranium, Viburnum; (6) a single row of stone-cells, Labiatae; (7) tissue of stone-cells, Hippuris, Naias, Potamogeton; (8) elongate stone-cells, Coniferae, Cupuliferae, Euphorbia, Linum, Malva, Viola; (9) short, columnar, thick-walled branched cells, Cucurbitaceae, Datura, Hypericum; (10) prosenchyma with cellulose walls, Clematis; (11) prosenchyma with lignified walls, Fraxinus, Rhamnus, Ranunculus. The seed coats have a certain influence in determining germination at the proper time, inasmuch as they make it difficult for the seed to germinate under the stimulus of a quantity of warmth and moisture insufficient to support the seedling. The effect of the endosperm, as well as that of other food supply in the seed, upon germination and the establishment of the seedling is obvious.

The behavior of seed or spore with respect to germination depends in a large degree upon the character of the protoplasm or embryo, though in just what way is at present a matter of conjecture. It is evident that many seeds are not viable because fertilization has not been effected, and in consequence no embryo has developed. This is the usual explanation of the low germinating power of the seeds of some species, especially polyspermatous ones. But even in viable seeds the behavior is always more or less irregular. The seeds of some species will grow immediately after ripening, while others germinate only after a resting period of uncertain duration. The same is true of spores. Even in the case of seeds from the same parent, under apparently similar conditions, while the majority will germinate the first year, some will lie dormant for one or more years. The precise reason why many seeds and spores germinate more readily after being frozen is equally obscure. The period of time for which disseminules may remain viable is extremely diverse, though, as would be expected, it is much longer as a rule for seeds than for spores. The greater vitality of seeds in the case of ruderal plants suggests that this diversity may be due simply to variation in the vigor of the embryos. It would seem that under proper conditions seeds may retain their viability for an indefinite period.

The influence of habitat upon germination is of primary importance, though the manner in which its influence is exerted is by no means as evident as might be supposed. In the case of seeds sown in the planthouse, it is almost universally the case that germination is less than in nature, notwithstanding the fact that temperature and moisture appear to be optimum. In nature, the seeds of the species may be carried into a number of different formations, any one or all of which may present conditions unfavorable to germination. With respect to probability of germination, habitats are of two sorts: those which are denuded and those which bear vegetation. It is impossible to lay down general propositions with respect to either group, since germination will vary with the character of the invading species, the annual distribution of heat and moisture in the habitat, etc. In a general way, however, it may be stated that the chances for germination are greater in vegetation than in denuded areas, chiefly because the latter are usually xerophytic. On the other hand, the lack of competition in the denuded area tends to make ultimate establishment much more certain. Here, as elsewhere when exact statistical results are desired, the use of the quadrat, and especially of the permanent quadrat, is necessary to determine the comparative germination of the invading species in relation to denudation and vegetation.

=271. Adjustment to the habitat.= The seedling once established by germination, the probability of its growing and maturing will depend upon its habitat form, plasticity, and vegetation form. Even though it may germinate under opposite conditions, a typical hylophyte, such as Impatiens for example, will not thrive in an open meadow, nor will characteristic poophytes, such as most grasses, grow in deep shade. In the same way, xerophytes do not adapt themselves to hydrophytic habitats, nor hydrophytes to xerophytic conditions. Many mesophytes, however, possess to a certain degree the ability to adjust themselves to somewhat xerophytic or hydrophytic situations, while woodland plants often invade either forest or meadow. This capability for adjustment, i. e., plasticity, is greatest in intermediate species, those that grow in habitats not characterized by great excess or deficiency of some factor, and it is least in forms highly specialized in respect to water-content, shade, etc. It may then be established as a fundamental rule that ecesis is determined very largely by the essential physical similarity of the old and the new habitat, except in the case of plastic forms, which admit of a wider range of accommodation. The plasticity of a plant is not necessarily indicated by structural modification, though such adjustment is usually typical of plastic species, but it may sometimes arise from a functional adaptation, which for some reason does not produce concomitant structural changes. The former explains such various habitat forms of the same species as are found in Galium boreale, Gentiana acuta, etc., and the latter the morphological constancy of plants like Chamaenerium, which grow in very diverse habitats.

The vegetation form of the invading species is often of the greatest importance in determining whether it will become established. The vegetation form represents those modifications which, produced in the original home by competition, i. e., the struggle for existence, are primarily of value in securing and maintaining a foothold. These comprise all structures by means of which the plant occupies a definite space in the air, through which the necessary light and heat reach it, and in the soil, from which it draws its food supply. These structures are all organs of duration or of perennation, such as root, rootstalk, bulb, tuber, woody stem, etc., which find their greatest development among trees and shrubs, and their least among annual herbs. But while the invaders are aided in securing possession by the proper vegetation form, the occupation of the plant already in possession is increased by the same means, and the outcome is then largely determined by other factors. To avoid repetition, the bearing of occupation upon invasion will be considered under succession.

BARRIERS

=272. Concept.= DeCandolle seems to have been the first to use the term barrier and to distinguish the various kinds, though Hedenberg clearly saw that stations of one kind were insurmountable obstacles to plants belonging to a very different type. De Candolle pointed out that the natural barriers to continuous invasion (“transport de proche en proche”) are: (1) seas, which decrease invasion almost in inverse proportion to their extent; (2) deserts; (3) mountain ranges, which are less absolute on account of passes, valleys, etc.; (4) vegetation, marshes being barriers to dry land plants, forests to those that fear the shade, etc. Grisebach, in discussing the effect of barriers upon the constitution of vegetation, laid down the fundamental rule that: “The supreme law which serves as the basis of the permanent establishment of natural floras is to be recognized in the barriers which have hindered or completely prevented invasion.”

Any feature of the topography, whether physical or biological, that restricts or prevents invasion, is a barrier. Such features are usually permanent and produce permanent barriers, though the latter may often be temporary, existing for a few years only, or even for a single season. In this last case, however, they are as a rule recurrent. Barriers may furthermore be distinguished as complete or incomplete with respect to the thoroughness with which they limit invasion. Finally, the consideration of this subject gains clearness if it be recognized that there are barriers to migration as well as to ecesis, and if we distinguish barriers as physical or biological with reference to the character of the feature concerned.

=273. Physical barriers= are those in which limitation is produced by some marked physiographic feature, such as the ocean or some other large body of water, large rivers, mountain ranges and deserts (including ice and snow fields). All of these are effective by virtue of their dominant physical factors; hence they are barriers to the ecesis of species coming from very different habitats, but they act as conductors for species from similar vegetation, especially in the case of water currents. A body of water, representing maximum water-content, is a barrier to mesophytic and xerophytic species, but a conductor for hydrophytic ones; deserts set a limit to the spread of mesophytic and hydrophytic plants, while they offer conditions favorable to the invasion of xerophytes; and a high mountain range, because of the reduction of temperature, restricts the extension of macrothermal and mesothermal plants. A mountain range, unlike other physical barriers, is also an obstacle to migration, inasmuch as natural distributive agents rarely act through it or over it.

=274. Biological barriers= include vegetation, man and animals, and plant parasites. The limiting effect of vegetation is exhibited in two ways. In the first place, a formation acts as a barrier to the ecesis of species invading it from the formations of another type, on account of the physical differences of the habitats. Whether such a barrier be complete or partial will depend upon the degree of dissimilarity existing between the formations. Hylophytes are unable to invade a prairie, though open thicket plants may do so to a certain degree. In the same way, a forest formation on account of its diffuse light is a barrier to poophytes; and a swamp, because of the amount and character of the water-content, sets a limit to both hylophytes and poophytes. Formations, such as forests, thickets, etc., sometimes act also as direct obstacles to migration, as in the case of tumbleweeds and other anemochores, clitochores, etc. A marked effect of vegetation in decreasing invasion arises from the closed association typical of stable formations and of social exclusive species. In these, the occupation is so thorough and the struggle for existence so intense that the invaders, though fitted to grow under the physical factors present, are unable to compete with the species in possession for the requisite amount of some necessary factor. Closed associations usually act as complete barriers, while open ones restrict invasion in direct proportion to the degree of occupation. To this fact may be traced a fundamental law of succession, viz., the number of stages in a succession is determined largely by the increasing difficulty of invasion as the habitat becomes stabilized. Man and animals affect migration directly, though not obviously, by the destruction of disseminules. They operate as a pronounced barrier to ecesis wherever they alter conditions in such a way as to make them unfavorable to invading species, or when, by direct action upon the latter, such as grazing, tramping, parasitism, etc., they turn the scale in the struggle for existence. The absence of insects adapted to insure fertilization is sometimes a serious barrier to the establishment of adventitious or introduced plants. The presence of parasitic fungi, in so far as they destroy the seeds of plants, acts as an obstacle to migration, and restricts or prevents ecesis in so far as the fungi destroy the invaders, or place them at a disadvantage in the struggle for existence.

=275. Influence of barriers.= Physical barriers are typically permanent in character, while biological ones are either permanent or temporary, depending upon the permanence of the formation and the constancy of the physical factors which determine it. A stable formation, such as a forest or meadow, which acts as a decided barrier to invasion from adjacent vegetation, may disappear completely, as a result of a landslide, flood, or burn, or through the activity of man, and may leave an area into which invaders crowd from every point. Often, without undergoing marked change, a formation which has presented conditions unfavorable to the ecesis of species of mesophytic character may, by reason of a temporary change in climate, become sufficiently modified to permit the invasion of mesophytes. On the other hand, a meadow ceases to be a barrier to prairie xerophytes during a period of unusually dry years. A peculiar example of the modification of a barrier is afforded by the defoliation of aspen forests in the mountains as a result of which poophytes have been enabled to invade them. Nearly all xerophytic stretches of sand and gravel, dunes, blowouts, gravel slides, etc., and even prairies to a certain degree, exhibit a recurrent seasonal change in spring, as a result of which the hot, dry surface becomes sufficiently moist to permit the germination and growth of invaders, which are entirely barred out during the remainder of the year. In an absolute sense, no barrier is complete, since the coldest as well as the driest portions of the earth’s surface are capable, at times at least, of supporting the lowest types of vegetation. Relatively, however, in connection with the natural spread of terrestrial plants, it is possible to distinguish partial barriers from complete ones. Such a distinction is of importance in the consideration of invasions from a definite region, as it is only in this restricted sense that complete barriers have produced endemism.

Distance, though hardly to be considered a barrier in the strict sense of the word, unquestionably plays an important part in determining the amount of invasion. The effect of distance is best seen in the case of migration, as it influences ecesis only in those rare cases where viability is affected. The importance of distance, or take the converse, of proximity, is readily ascertained by the study of any succession from denudation. It has been established that the contiguous vegetation furnishes 75–90 per cent of the constituent species of the initial formation, and in mountainous regions, where ruderal plants are extremely rare, the percentage is even higher. The reason for this is to be found not only in the fact that the adjacent species have a much shorter distance to go, and hence will be carried in much greater quantity, but also in that the species of the formations beyond must pass through or over the adjacent ones. In the latter case, the number of disseminules is relatively small on account of the distance, while invasion through the intermediate vegetation, if not entirely impossible, is extremely slow, so that plants coming in by this route reach the denuded area only to find it already occupied. It is as yet impossible to give a definite numerical value to proximity in the various invasions that mark any particular succession. This will not be feasible until a satisfactory method has been found for determining a coefficient of mobility, but, this once done, it will be a relatively simple matter, not merely to trace the exact evolution of any succession of formations, but actually to ascertain from the adjacent vegetation the probable constitution of a particular future stage.

From what has been said, it follows that the primary effect of barriers upon vegetation is obstruction. Where the barrier is in the pathway of migration, however, it causes deflection of the migrant as a rule, and sets up migration in a new direction. This is often the case when the strong winds of the plains carry disseminules towards the mountains and, being unable to cross the range, drop them at the base, or, being deflected, carry them away at right angles to the original direction. The same thing happens when resistant fruits and seeds borne by the wind fall into streams of water or into ocean currents. The direction of migration is changed, and what is normally a barrier serves as an agent of dissemination.

ENDEMISM

=276. Concept.= Since its first use by DeCandolle, the term endemic has been employed quite consistently by phytogeographers with the meaning of “peculiar to a certain region.” Some confusion, however, has arisen from the fact that a few authors have made it more or less synonymous with indigenous and autochthonous, while others have regarded it as an antonym of exotic. In its proper sense, endemic refers to distribution, and not to origin. Its exact opposite will be found then in Fenzl’s term polydemic, dwelling in several regions. Indigenous (autochthonous) and exotic, on the contrary, denote origin, and are antonyms, indigenous signifying native, and exotic foreign. As Drude has shown, endemic plants may be either indigenous, as in the case of those species that have never moved out of the original habitat, or exotic, as in the much rarer instances where a polydemic species has disappeared from its original home and from all regions into which it has migrated except one. It is understood that not all indigenous or exotic species are endemic. The proportion of endemic to polydemic species is a variable and somewhat artificial one, depending upon the size of the divisions employed.

=277. Causes.= The primary causes of endemism are two, lack of migration and presence of barriers. Since distributive agents are practically universal, lack of migration corresponds essentially to immobility, a fact which decreases the difficulty of ascertaining the immediate causes of endemism in any particular species. Either immobility or a barrier may produce endemism; extremely immobile plants, for example, liliaceous species propagating almost wholly by underground parts, are as a rule endemic, while alpine plants and those of oceanic islands are endemic in the highest degree, regardless of their mobility. When the two conditions act concomitantly upon a species, endemism is almost inevitable. It can not be supposed, however, that immobility or natural barriers alone, or the concomitance of the two, must invariably give rise to endemic species; the most immobile plant may be carried into another region by unusual or accidental agencies, or the most formidable barrier to migration may be overcome by the intensity of an agent or through the action of man. Endemism is also brought about by the modification of species; new or nascent species are as a rule endemic. Whether they will remain endemic or not will depend upon the perfection of their contrivances for dissemination and upon the presence of barriers to migration or ecesis. Finally, as Drude was the first to point out, the disappearance of a polydemic species in all regions but one, owing to the struggle for existence or to changed physical conditions, will result in endemism.

=278. Significance.= Endemism is readily recognized by methods of distributional statistics, applied to areas limited by natural barriers to migration or ecesis. For political areas, it has no significance whatever, unless the boundaries of these coincide with barriers. It determines in the first degree the validity of regions, though the latter are often recognized also by the presence of barriers and by the character of the vegetation. Endemism may occur in areas of vegetation of any rank from a formation to a zone. When the term is not qualified, however, it should be used of species with reference to formations alone. Comparisons to be of value, however, can be instituted only between areas of the same order, i. e., between two or more formations, two or more regions, provinces, etc. In the same way, taxonomic groups of the same rank should be used in such comparisons, i. e., species should be contrasted with species, genera with genera, and families with families, except when it is desired to obtain some measure of the age of the vegetation by the differentiation of the endemic phyla within it. There will be seen to exist a fundamental correspondence between the rank of the floral division and the taxonomic group, though the apparent exceptions to this are still too numerous to warrant its expression in a general law. As a rule, however, formations most frequently show endemic habitat forms and species, more rarely endemic genera; regions and provinces commonly exhibit endemic species and genera, rarely endemic families; while zones and hemispheres contain endemic orders as well as families. This correspondence is readily seen to depend primarily upon the fact that increased differentiation in the taxonomic sense is a concomitant of the increased invasion of endemic species, measured in terms of distance and difference in habitat.

It is too early to decide satisfactorily whether it is proper to speak of formations as endemic. At first thought it would seem that all formations, with the exception of ruderal ones, were endemic, but a study of almost any transition area between regions would seem to point to the opposite conclusion, viz., that no formations are properly endemic. It is equally impossible at present to distinguish different types of endemics, such as relictae, etc., as any such classification must await the elaboration of a method for determining the phylogeny of a natural group of species by an investigation of their comparative differentiation in connection with their migration in all directions from the vegetation center into new habitats. In short, it will not be possible to make a thorough study of endemism and to postulate its laws until modern methods of research have been extended to a much larger portion of the vegetation of the globe. The final task of phytogeography is the division of the earth’s vegetation into natural areas. It will be at once evident that most plants can not properly be called endemic until the natural regions in which they are found have been accurately defined, a work which has barely begun. In the much simpler matter of distribution, upon which the accuracy of statistical methods depends directly, there are few regions sufficiently well known at the present time to yield anything like permanent results.

POLYPHYLESIS AND POLYGENESIS

=279. Concept.= The idea of polyphylesis, as advanced by Engler, contains two distinct concepts: (1) that a species may arise in two different places or at two different times from the same species, and (2) that a genus or higher group may arise at different places or times by the convergence of two or more lines of origin. It is here proposed to restrict polyphylesis, as its meaning would indicate, to the second concept, and to employ for the first the term polygenesis, first suggested by Huxley in the sense of polyphylesis. The term polyphylesis is extended, however, to cover the origin of those species which arise at different places or times from the convergence of two or more different species, a logical extension of the idea underlying polyphyletic genera, though it may seem at first thought to be absurd. Polygenesis may be formally defined as the origin of one species from another species at two or more distinct places on the earth’s surface, at the same time or at different times, or its origin in the same place at different times. Polyphylesis, on the contrary, is the origin of one species from two or more different species at different places, at the same time or at different times. It is evident that what is true of species in this connection will hold equally well of genera and higher groups. Opposed to polygenesis is monogenesis, in which a species arises but once from another species; with polyphylesis is to be contrasted monophylesis, in which the species arises from a single other species. It will be noticed at once that these two concepts are closely related. The following diagrams will serve to make the above distinctions more evident:

In I, a species A, becomes scattered over a large area in a series of places, m ... m^n, with the same physical factors, in any or all of which may arise the new species a. In II, a species with xerophytic tendency, A, and one with mesophytic tendency, B, in the course of migration find themselves respectively in a more mesophytic habitat, m, and a more xerophytic one, x, in which either may give rise to the new form, c, which is more or less intermediate between A and B. In III, the method of origin is of the simplest type, in which a species is modified directly into another one, or is split up into several.

=280. Proofs of polygenesis.= In affirming the probability of a polygenetic origin of species, there is no intention of asserting that all species originate in this way. It seems evident that a very large number of species of restricted range are certainly monogenetic, at least as far as origin in space is concerned. It is possible that any species may arise at two or more distinct times. Polygenesis can occur readily only in species of more or less extensive area, in which recur instances of the same or similar habitat. The relative frequence and importance of the two methods can hardly be conjectured as yet, but origin by monogenesis would seem to be the rule.

The arguments adduced by Engler in support of polygenesis are in themselves conclusive, but the investigations of the past decade have brought to light additional proofs, especially from the experimental side. In determining the physical factors of prairie and mountain formations, and especially by methods of experimental ecology, the author has found that habitats are much less complex than they are ordinarily thought to be, since water-content and humidity, and to a less degree light, constitute the only factors which produce direct modification. In addition, it has been ascertained that the minimum difference of water-content, humidity, or light, necessary to produce a distinguishable morphological adjustment is much greater than the unit differences recorded by the instruments. In short, the differences of habitats, as ascertained by thermograph, psychrometer and photometer, are much greater than their efficient differences, and, with respect to their ability to produce modification, habitats fall into relatively few categories. A striking illustration of this is seen in the superficially very different habitats, desert, strand, alkali plain, alpine moor, and arctic tundra, all of which are capable of producing the same type of xerophyte. It follows from this that many more or less plastic species of extensive geographical area will find themselves in similar or identical situations, measured in terms of efficient differences, and will be modified in the same way in two or more of these. In mountain regions, where interruption of the surface and consequent alternation are great, the mutual invasion of contiguous formations is of frequent occurrence, often resulting in habitat forms. The spots in which these nascent species, such as Galium boreale hylocolum, Aster levis lochmocolus, etc., are found, are often so related to the area of the parent species as to demonstrate conclusively that these forms are the result of polygenesis and not of migration. Naturally, what is true of a small area will hold equally well of a large region, and the recurrence of the same habitat form may be accepted as conclusive proof of polygenesis. The most convincing evidences of multiple origin, however, are to be found in what De Vries has called “mutations.” It makes little difference whether we accept mutations in the exact sense of this author, or regard them as forms characterized by latent variability. The evidence is conclusive that the same form may arise in nature or in cultivation, in Holland or in America, not merely once, but several or many times. In the presence of such confirmation, it is unnecessary to accumulate proofs. Polygenesis throws a new light upon many difficult problems of invasion and distribution, and, as a working principle, admits of repeated tests in the field. It obviates, moreover, the almost insuperable difficulties in the way of explaining the distribution of many polygenetic species on the basis of migration alone.

=281. Origin by polyphylesis.= In 1898, the author first advanced a tentative hypothesis to the effect that a species homogeneous morphologically may arise from two distinct though related species. During subsequent years of formational study, the conviction has grown in regard to the probability of such a method of origin. Since the appearance of Engler’s work, a polyphyletic origin for certain genera has been very generally accepted by botanists, but all have ignored the fact that the polyphylesis of genera carries with it the admission of such origin for species, since the former are merely groups of the latter. I can not, however, agree with Engler, that polyphyletic genera, and hence species also, are necessarily unnatural. If the convergence of the lines of polyphylesis has been great, resulting in essential morphological harmony, the genus is a natural one, even though the ancestral phyla may be recognizable. If, on the other hand, the convergence is more or less imperfect, resulting in subgroups of species more nearly related within the groups than between them, the genus can hardly be termed natural. This condition may, however, prevail in a monophyletic genus with manifest divergence and still not be an indication that it is artificial.

Darwin, in speaking of convergence, has said: “If two species, belonging to two distinct though allied genera, had both produced a large number of new and divergent forms, it is conceivable that these might approach each other so closely that they would have all to be classified under the same genus; and thus the descendants of two distinct genera would converge into one.” The application of this statement to species would at once show the possibility of polyphylesis in the latter, and a further examination of the matter will demonstrate its probability. It is perfectly evident that a species may be split into two or more forms by varying the conditions, let us say of water-content, and that the descendants of these forms may again be changed into the parent type by reversing the process. This has, in fact, been done experimentally. Since it is admittedly impossible to draw any absolute line between forms, varieties, and species, it is at once clear that two distinct though related species, especially if they are plastic, may be caused to converge in such a way that the variants may constitute a new and homogeneous species. This may be illustrated by a concrete case at present under investigation. Kuhnistera purpurea differs from K. candida in being smaller, in having fewer, smaller, and more narrow leaflets, and a globoid spike of purple flowers in place of an elongated one of white flowers; in a word, it is more xerophytic. This conclusion is completely corroborated by its occurrence. On dozens of slopes examined, Kuhnistera purpurea has never been found mingling with K. candida on lower slopes, except where an accident of the surface has resulted in a local decrease of water-content. The experiment as conducted is a simple one, consisting merely in sowing seed of each in the zone of the other, and in growing K. purpurea under controlled mesophytic conditions, and K. candida under similarly measured xerophytic conditions in the planthouse.

While the polyphyletic origin of species is in a fair way to be decided by experiment, it receives support from several well-known phenomena. The striking similarity in the plant body of families taxonomically so distinct as the Cactaceae, Stapeliaceae, and Euphorbiaceae, or Cyperaceae and Juncaceae, indicates that a vegetation form may be polyphyletic. On the other hand, the local appearance of zygomorphy, of symphysis, and of aphanisis in the floral types of phylogeneticallv distinct families is a proof of the operation of convergence in reproductive characters. To be sure, the convergence is never so great as to produce more than superficial similarity, but this is because the groups are markedly different in so many fundamental characters. The same tendency in closely related species would easily result in identity. As in the case of polygenesis, the relatively small number of typically distinct habitats makes it clear that two different species of wide distribution, bearing to each other the relations of xerophyte to mesophyte, of hydrophyte to mesophyte, or of poophyte to hylophyte, might often find themselves in reciprocal situations, with the result that they would give rise to the same new form. The final proof of the polyphylesis of species is afforded by the experiments of De Vries in mutation. De Vries found that Oenothera nanella arose from O. Lamarckiana, O. laevifolia, and O. scintillans; Oenothera scintillans arose from O. lata and O. Lamarckiana; Oenothera rubrinervis from O. Lamarckiana, O. laevifolia, O. lata, O. oblonga, O. nanella, and O. scintillans, etc. Whatever may be the rank assigned to these mutations, whether form, variety, or species, there can be no question of their polyphyletic origin, nor, in consequence of the connection of mutations with variations through such inconstant forms as O. scintillans, O. elliptica, and O. sublinearis, of the possibility of polyphylesis in any two distinct though related species or genera.

KINDS OF INVASION

=282. Continuous and intermittent invasion.= With respect to the frequency of migration, we may distinguish invasion as continuous, or intermittent. Continuous invasion, which is indeed usually mutual, occurs between contiguous formations of more or less similar character, in which there is an annual movement from one into the other, and at the same time a forward movement through each, resulting from the invaders established the preceding year. By far the greater amount of invasion is of this sort, as may readily be seen from the fact that migration varies inversely as the distance, and ecesis may decrease even more rapidly than the distance increases. The significant feature of continuous invasion is that an outpost may be reinforced every year, thus making probable the establishment of new outposts from this as a center, and the ultimate extension of the species over a wide area. The comparatively short distance and the regular alternation of migration and ecesis render invasion of this sort very effective. An excellent illustration of this is seen in transition areas and regions, which are due directly to continuous and usually to mutual invasion. Intermittent invasion results commonly from distant carriage, though it may occur very rarely between dissimilar adjacent formations, when a temporary swing in the physical factors makes ecesis possible for a time. It is characterized by the fact that the succession of factors which have brought about the invasion is more or less accidental and may never recur. Intermittent invasion is relatively rare, and from the small number of disseminules affected, it is of little importance in modifying vegetation quantitatively. On the other hand, since a species may often be carried far from its geographical area, it is frequently of great significance in distribution.

=283. Complete and partial invasion.= When the movement of invaders into a formation is so great that the original occupants are finally driven out, the invasion may be termed complete. Such invasion is found regularly in the case of many ruderal formations, and is typical of the later stages of many successions. It is ordinarily the result of continuous invasion. If the number of invaders is sufficiently small that they may be adopted into the formation without radically changing the latter, the invasion is partial. This is doubtless true of the greater number of invasions, though these are regularly much less striking and important than instances of complete invasion.

=284. Permanent and temporary invasion.= The permanence of invasion depends upon the success attending ecesis, and upon the stability of the formation. It has already been noticed that under certain conditions plants may germinate and grow, and if they are perennials, even become established, and still ecesis be so imperfect that reproduction is impossible. Others may find the conditions sufficiently favorable for propagation, but unfavorable for the formation of flowers and fruits. Finally there are plants which seem to be perfectly established for a few years, only to disappear completely. The latter are examples of temporary invasion. It is necessary to draw clearly the line between complete and partial invasion in this connection. The former is temporary in the initial or intermediate stages of nearly all successions, as compared with the ultimate stages, though it is in a large degree permanent in comparison with the partial invasion of species which are able to maintain themselves for a few years. In a sense, there is a real distinction between the two, inasmuch as a particular stage of succession is permanent as long as the habitat remains essentially the same. A critical study of the species of such stages shows, however, that they manifest very different degrees of permanence. Species which invade stable vegetation temporarily have been termed adventive by A. DeCandolle. Permanent invasion occurs when a species becomes permanently established in a more or less stable formation. It is characteristic of the great majority of invaders found in the grassland and forest stages of successions.

Plants which have arisen within a formation or have been a constituent part of it since its origin are indigenous. Contrasted with these are the species which have invaded the formation since it received its distinctive impress: these are derived. The determination of the indigenous and derived species of a formation or larger division is of the utmost importance, as it enables us to retrace the steps by which the formation has reached its present structure, and to reconstruct formations long since disappeared. To render it less difficult, it is necessary to scrutinize the derived elements closely, first, because it is easiest to recognize the indigenous species by eliminating the derived, and second, because this analysis will show that not all derived species have entered the formation at the same time and from the same sources. Derived species may be termed vicine, when they are fully established invaders from adjacent formations or regions, and adventitious, when they have come from distant formations and have succeeded in establishing themselves. Finally, those derived species which are unable to establish themselves permanently are adventive.

MANNER OF INVASION

=285. Entrance into the habitat.= Since the ecesis of invaders depends in large measure upon the occupation of the plants in possession, the method and degree of invasion will be determined by the presence or absence of vegetation. Areas without vegetation are either originally naked or denuded, while vegetation with respect to the degree of occupation is open (sporadophytia), or closed (pycnophytia). Each type of area presents different conditions to invaders, largely with respect to the factors determining ecesis. Naked habitats, rocks, talus, gravel slides, and dunes, while they offer ample opportunity for invasion on account of the lack of occupation, are really invaded with the greatest difficulty, not only because they contain originally few or no disseminules, but also because of their xerophytic character and the difficulty of obtaining a foothold, on account of the extreme density or instability of the soil. Denuded habitats, blowouts, sand draws, ponds, flood plains, wastes, fields, and burns, usually afford maximum opportunity for invasion. They invariably contain a large number of disseminules ready to spring up as soon as the original vegetation is destroyed. The surface, moreover, is usually such as to catch disseminules and to offer them optimum conditions of moisture and nutrition. Open formations are readily invaded, though the increased occupation renders entrance more difficult than it is in denuded areas. Closed formations, on the other hand, are characterized by a minimum of invasion, partly because invaders from different formations find unfavorable conditions in them, but chiefly because the occupation of the inhabitants is so complete that invaders are unable to establish themselves.

Invasion takes place by the penetration of single individuals or groups of individuals. This will depend in the first place upon the character of the disseminule. It is evident that, no matter how numerous the achenes may be, the invasion of those anemochorous species with comate or winged seeds or one-seeded fruits will be of the first type, while all species in which the disseminule is a several or many-seeded fruit or plant, as in hooked fruits, tumbleweeds, etc., will tend to produce a group of invaders. Occasionally of course, the accidents of migration will bring together a few one-seeded disseminules into a group, or will scatter the seeds of a many-seeded fruit, but these constitute relatively rare exceptions. This distinction in the matter of invasion is of value in studying the relative rapidity of the latter, and the establishment of new centers, but it is of greatest importance in explaining the historical arrangement of species in a formation, and hence has a direct bearing upon alternation. It is entirely independent of the number of invaders, which, as we have seen, depends upon seed-production, mobility, distance, occupation, etc., but is based solely upon mode of arrangement, and will be found to underlie the primary types of abundance, copious, and gregarious. In this connection, it should also be noted that the contingencies of migration, especially the concomitant action in the same direction of two or more distributive agencies, often results in the penetration of a group of individuals belonging to two or more species. This may well be termed mass invasion; it is characteristic of transition areas or regions, and along valleys or other natural routes for migration it gives rise to species guilds. The movement of species guilds constitutes one of the most complex and interesting problems in the whole field of invasion, the solution of which can be attempted only after the thorough analysis of the simpler invasions between formations. A better understanding of the meaning of invasion by species guilds is imperative for the natural limitation of regions, as at present such groups constitute alien associations in many regions otherwise homogeneous.

=286. Influence of levels.= The invasion of a formation may occur at three different levels: (1) at the level of the facies, (2) below the facies, (3) above the facies, depending directly upon the relative height of invaders and occupants. The invasion level is an extremely simple matter to determine, except in the case of woody plants, such as shrubs and trees, which attain their average height only after many years. Its importance is fundamental. The level at which invasion occurs not only determines the immediate constitution of the formation, whether its impress shall still be given by the occupants or by the invaders or by both together, but it also decides the whole future of the formation, i. e., whether the invaders or occupants shall persist unmodified or modified. The problem is an extremely complex one, but the careful analysis of invasion at each level throws a flood of light upon it. The entrance of invaders of the same general height as the facies of a formation results regularly in mixed formations. This is well illustrated by the structure of the transition areas between two formations of the same category, i. e., forests, meadows, etc. It is seldom, however, that the facies and invaders are so equally matched in height and other qualities that they remain in equilibrium for a long period. One or the other has a slight advantage in height, or the one suffers shading or crowding better than the other, is longer-lived or faster-growing, with the result that invader yields to occupant, or occupant to invader. It is a well-known fact that many mixed formations represent intermediate stages of development.

Invasion at a level different from that of the facies is inevitably followed by modification. If the invasion takes place below the facies, the invaders will be exterminated gradually, or slowly assimilated. In either case, there is little structural change in the formation, and its stability is affected slightly or not at all. If the invaders overtop the facies in any considerable number, the entire formation undergoes partial or complete modification, or in extreme cases it disappears, as is typically the case in succession. A peculiar variation of invasion at a level above the facies is seen where woody plants invade grassland, when the trees or shrubs become more or less uniformly scattered in an open woodland or open thicket. Here the grassland takes on an altogether different appearance superficially, though it is usually unchanged, except beneath and about the invaders, where either adaptation or extermination results. Finally, it should be borne in mind that the invasion of a particular formation, especially in the case of layered thickets and forests, often takes place at two levels, at the height of the facies and below the facies.

INVESTIGATION OF INVASION

=287.= The methods to be used in the study of invasion are those already described elsewhere. The migration circle is of the first importance because it makes it possible to secure an accurate record of actual movement. Quadrat and transect are valuable, but from their nature they are more serviceable for ecesis than for migration. All of these should be of the permanent type, in order that the fate of invaders may be followed for several years at least. Permanent areas furnish evidence of the changes wrought in the actual vegetation, while denuded ones can serve only to show the potential migration and ecesis of the constituent species. Transition zones and areas are special seats of invasion; they are best studied by means of the belt transect and the ecotone chart. The movement of a line of invaders or of scattered outposts is traced by the use of labeled stakes at the points concerned. It is clear that this method will yield conclusive data in regard to the great invasions between regions, such as the movement of species guilds, the advance of the forest frontier, etc. When invasion is scattered, factor instruments can not be used to advantage, but where the invading line is well marked, or where extra-formational areas occur, a knowledge of the physical factors is a great aid.

An invasion that has been completed can not be studied in the manner indicated. A method of comparison must be used, in order to determine the original home of the invaders. For this an exact knowledge of the contiguous formations and of the abundance of the species common to all is a prerequisite. With this as a basis, it is usually a simple task to refer all the species of the formation concerned to their proper place in the groups, indigenous, derived, and adventitious.

SUCCESSION

=288. Concept.= Succession is the phenomenon in which a series of invasions occurs in the same spot. It is important, however, to distinguish clearly between succession and invasion, for, while the one is the direct result of the other, not all invasion produces succession. The number of invaders must be large enough, or their effect must be sufficiently modifying or controlling to bring about the gradual decrease or disappearance of the original occupants, or a succession will not be established. Partial or temporary invasion can never initiate a succession unless the reaction of the invaders upon the habitat is very great. Complete and permanent invasion, on the other hand, regularly produces successions, except in the rare cases where a stable formation entirely replaces a less stable one without the intervention of other stages. Succession depends in the first degree upon invasion in such quantity and of such character that the reaction of the invaders upon the habitat will prepare the way for further invasion. The characteristic presence of stages in a succession, which normally correspond to formations, is due to the peculiar operation of invasion with reaction. In the case of a denuded habitat, for example, migration from adjacent formations is constantly taking place, but only a small number of migrants, especially adapted to somewhat extreme conditions, are able to become established in it. These reach a maximum development in size or number, and in so doing react upon the habitat in such a way that more and more of the dormant disseminules present, as well as those constantly coming into it, find the conditions favorable for germination and growth. The latter, as they in turn attain their maximum, cause the gradual disappearance of the species of the first stage, and at the same time prepare the way for the individuals of the succeeding formation. It is at present impossible to determine to what degree this substitution is due to the struggle for existence between the individuals of each species and between the somewhat similar species of each stage, and to what degree it arises out of the physical reaction.

It is evident that geological succession is but a larger expression of the same phenomenon, dealing with infinitely greater periods of time, and produced by physical changes of such intensity as to give each geological stage its peculiar stamp. If, however, the geological record were sufficiently complete, we should find unquestionably that these great successions merely represent the stable termini of many series of smaller changes, such as are found everywhere in recent or existing vegetation.

=289. Kinds of succession.= The fundamental causes of succession are invasion and reaction, but the initial causes of a particular succession are to be sought in the physical or biological disturbances of a habitat or formation. With reference to the initial cause, we may distinguish normal succession, which begins with nudation, and ends in stabilization, and anomalous succession, in which the facies of an ultimate stage of a normal succession are replaced by other species, or in which the direction of movement is radically changed. The former is of universal occurrence and recurrence; the latter operates upon relatively few ultimate formations. In the origin of normal successions, nudation may be brought about by the production of new soils or habitats, or by the destruction of the formation which already occupies a habitat. In a few cases, the way in which the habitat arises or becomes denuded is not decisive as to the vegetation that is developed upon it, but as a rule the cause of nudation plays as important a part in the development of a succession as does the reaction exerted by the invaders. The importance of this fact has been insisted upon under invasion. New soils present extreme conditions for ecesis, possess few or no dormant disseminules, and in consequence their successions take place slowly and exhibit many stages. Denuded soils as a rule offer optimum conditions for ecesis as a result of the action of the previous succession, dormant seeds and propagules are abundant, and the revegetation of such habitats takes place rapidly and shows few stages. The former may be termed primary succession, the latter secondary succession.

PRIMARY SUCCESSIONS

=290.= These arise on newly formed soils, or upon surfaces exposed for the first time, which have in consequence never borne vegetation before. In general they are characteristic of mountain regions, where weathering is the rule, and of lowlands and shores, where sedimentation or elevation constantly occur. The principal physical phenomena which bring about the formation of new soils are: (1) elevation, (2) volcanic action, (3) weathering, with or without transport.

=291. Succession through elevation.= Elevation was of very frequent occurrence during the earlier, more plastic conditions of the earth, and the successions arising as a result of it must have been important features of the vegetation of geological periods. To-day, elevation is of much less importance in changing physiography, and its operation is confined to volcanic islands, coral reefs, and islets, and to rare movements or displacements in seacoasts, lake beds, shore lines, etc. There has been no investigation of the development of vegetation on islands that are rising, or have recently been elevated, probably because of the slow growth of coral reefs and the rare appearance of volcanic islands. On coral reefs, the first vegetation is invariably marine, but as the reef rises higher above the surf line and the tide, the vegetation passes into a xerophytic terrestrial type adapted to an impervious rock soil, and ultimately becomes mesophytic. In volcanic islands, unless they are mere rocks over which the waves rush, the succession must always begin with a xerophytic rock formation. The best known example of a rising coast line is found in Norway and Sweden, where the southeastern coast is rising at the rate of five or six feet a century. There can be little question that such changes of level will produce marked changes in vegetation, but the modification will be so gradual as to be scarcely perceptible in a single generation. It is probable that the forests of the Atlantic coastal plains are the ultimate stages of successions initiated at the time of the final elevation of the sea bottom along the coast line.

=292. Succession through volcanic action.= The deposition of volcanic ashes and flows of lava are relatively infrequent at present, occurring only in the immediate vicinity of active volcanoes, chiefly in or near the tropics. Successions of this sort are in consequence not only rare, but they are also relatively inaccessible to investigators. They have been studied in a few cases, for example, those of Krakatoa by Treub, but this study has been confined to the general features of revegetation. Ash fields and lava beds are widely different in compactness, but they agree in having a low water- and nutrition-content. The pioneer plants in both will be intense xerophytes, but the soil differences will determine that these shall be sand-binders in the former, and rock-weathering plants in the latter.

=293. Weathering.= Practically all primary successions start on soils produced by weathering. This is also true of coral or volcanic islets and of lava beds, for no terrestrial vegetation can secure a foothold upon them until the surface of the rock has been to some extent decomposed or disintegrated. Weathering, as is well known, consists of two processes, disintegration and decomposition, which usually operate successively, though they are sometimes concomitant. Disintegration usually precedes, especially in rock masses, and unless it is soon followed by decomposition, results in dysgeogenous soils. Decomposition often goes hand in hand with disintegration, or it takes place so rapidly and perfectly that it alone seems to be present. In either case, the resulting soil is eugeogenous. The relation of decomposition to disintegration determines the size and compactness of the soil particles, and upon the latter depend the porosity, capillarity, and hygroscopicity of the soil. These control in large degree the character of the first vegetation to appear on the soil.

Another point of fundamental value in determining revegetation is the disposition of the weathered rock. If it remains in situ, it will evidently differ in respect to compactness, homogeneity, nutrition-content, water-content, disseminules, etc., from weathered material which has been transported. An essential difference also arises from the fact that a rock may be weathered a long distance from the place where the decomposed particles are finally deposited, and in the midst of a vegetation very different from that found in the region of deposit. The disposition of the weathered material affords in consequence a satisfactory basis for the arrangement of primary successions. The following classification is proposed, based upon the soil groups established by Merrill.

=294. Succession in residuary soils.= Residuary soils are always sedentary, i. e., they are formed in situ. They show certain differences dependent upon the rock from which they originate, which may be mixed crystalline shale, sandstone, or limestone, but the thoroughness of decomposition causes these differences to be comparatively small. Residuary soils are typically eugeogenous; their successions in consequence usually begin with mesophytes, and consist of a few stages. The soluble salt-content is comparatively low, since all soluble matters are readily leached out. Successions in these soils are especially characteristic of shale, sandstone, and limestone ledges or banks. Cumulose deposits, like residuary ones, are sedentary in character, but as they are produced by the accumulation of organic matter, they will be considered under reactions of vegetation upon habitat.

=295. Succession in colluvial soils.= Colluvial deposits owe their aggregation solely or chiefly to the action of gravity. They are the immediate result of the disintegration of cliffs, ledges, and mountain sides, decomposition appearing later as a secondary factor. The masses and particles arising from disintegration are extremely variable in size, but they agree as a rule in their angular shape. The typical example of the colluvial deposit is the talus, which may originate from any kind of rock, and contains pieces of all sizes. Gravel slides differ from ordinary talus in being composed of more uniform particles, which are worn round by slipping down the slope in response to gravity and surface wash. Boulder fields are to be regarded as talus produced by weathering under the influence of joints, resulting in huge boulders which become more and more rounded under the action of water and gravity. This statement applies to those fields which are in connection with some cliff that is weathering in this fashion; otherwise, boulder fields are of aqueous or glacial origin. The character of the successions in talus will depend upon the kind of rock in the latter. If the rock is igneous or metamorphic, decomposition will be slow, and the soil will be dysgeogenous. Successions on such talus consist of many stages, and the formations are for a long time open and xerophytic. In talus formed from sedimentary rocks, especially shales, limestones, and calcareous sandstones, decomposition is much more rapid, and the successions are simpler and more mesophytic.

=296. Succession in alluvial soils.= Alluvial soils are fluvial when laid down by streams and rivers, and litoral when washed up by the waves or tides. They are formed when any obstacle retards the movement of the water, decreasing its carrying power, and causing the deposit of part or all of its load. They consist of more or less rounded, finely comminuted particles, mingled with organic matter and detritus. Alluvial deposits are especially frequent at the mouth of streams and rivers, on their terraces and flood plains, and along silting banks as compared with the erosion banks of meanders. The filling of ponds by the erosion due to surface drainage, and of lakes by the deposition of the loads of streams that enter them, results in the formation of new alluvium. A similar phenomenon occurs along coasts, where bays and inlets are slowly converted into marshes in consequence of being shallowed by the material washed in by the waves and tides. Such paludal deposits are invariably salt water or brackish. Contrasted with these, which are uniformly black in consequence of the large amount of organic matter present, are the sandbars and beaches, which, though due to the same agents, are light grey or white in color, because of the constant leaching by the waves. Two kinds of alluvial deposits may accordingly be distinguished: (1) those black with organic matter, and little disturbed by water, and (2) those of a light color, which are constantly swept by the waves. The successions corresponding to these are radically different. In the first, the pioneer vegetation is hydrophytic, consisting largely of amphibious plants. The pioneer stages retard the movement of the water more and more, and correspondingly hasten the deposition of its load. The marsh bed slowly rises in consequence, and finally the marsh begins to dry out, passing first into a wet meadow, and then into a meadow of the normal type. A notable exception to this sequence occurs when the swamp contains organic matter or salts in excess, in which case the vegetation consists indefinitely of swamp xerophytes, or halophytes. The first vegetation on fresh water sandbars is xerophytic, or, properly, dissophytic, unless they remain water-swept, and the ultimate stages of their successions are mesophytic woodlands composed of water-loving genera, Populus, Salix, etc. It seems certain, however, that these will finally give way to longer-lived hardwoods. Maritime sandbars and beaches are always saline, and their successions run their short course of development entirely within the group of halophytes, unless the retreat of the sea or freshwater floods change the character of the soil. The chemical action of underground waters also produces new soils, which might be classed as alluvial. These soils are essentially rock deposits, travertine, silicious sinter, etc., made by iron and lime springs and by geysers, and they must be changed by decomposition into soils proper to be comparable with alluvial soils.

=297. Succession in aeolian soils.= The only wind-borne soils of geological importance at the present time are those which form dunes, both inland and coastal. Aeolian deposits consist largely of rounded sand particles, which are of almost uniform size in any particular dune, but vary greatly in dunes of different ages. The reaction of the pioneers on dunes plays an important part in building the latter, but the immense dunes of inland deserts, which are entirely destitute of vegetation, seem to indicate that its value has been overestimated. The first stages in dune successions are dissophytic, i. e., the plants grow in a soil of medium or high water-content, but in an atmosphere that is extremely xerophytic. The ultimate stages vary widely in accordance with the region in which they occur; they may be xerophytic heaths or mesophytic meadows and forests. Because of their striking character and economic significance, dunes have received much attention, with the result that their successions are the most thoroughly known of all. Prairie and steppe formations are probably to be regarded as the ultimate stages of successions established on wind-borne loess, and it is possible that the same is true of sand-hill vegetation in the prairie province.

=298. Succession in glacial soils.= The formation of glacial deposits is at present confined to alpine and arctic regions. Recent successions in such soils are localized in these regions, and are in consequence relatively unimportant. There can be little question, however, that the thorough investigation of succession in and near the moraines of existing glaciers will throw much light upon the successions of the glacial period. Moraines, drumlins, eskars, and alluvial cones represent the various kinds of glacial deposits. They agree in being heterogeneous in composition, and are covered to-day with ultimate stages of vegetation, except in the immediate vicinity of glaciers.

SECONDARY SUCCESSIONS

=299.= Generally speaking, all successions on denuded soils are secondary. When vegetation is completely removed by excessive erosion, it is an open question whether the resulting habitat is to be regarded as new or denuded. Erosion is rarely so extreme and so rapid, however, as to produce such a condition, even when it results from cultivation or deforestation. It is, moreover, especially characteristic of newly formed soils, and in studying succession in eroded habitats, it is fundamentally important to determine whether erosion has produced denudation, or has operated upon a new soil. The great majority of secondary successions owe their origin to floods, animals, or the activities of man, and they agree in occurring upon decomposed soils of medium water-content, which contain considerable organic matter, and a large number of dormant migrants. These successions consist of relatively few stages, and are rarely of extreme character.

=300. Succession in eroded soils.= Eroded soils show considerable differences, as they arise in consequence of erosion by water or by wind, though the initial stages of revegetation derive their character more from the aggregation of the soil than from the nature of the erosive agent. Eroded soils are as a rule xerophytic. In the case of erosion by water, dysgeogenous soils are readily worn away in consequence of their lack of cohesion, as in sand draws, etc., while eugeogenous soils are easily eroded only on slopes, as in the case of ravines, hillsides, etc. In the former, the extreme porosity and slight capillarity of the sand and gravel result in a low water-content. In the finer soils, the water-content is also low, on account of the excessive run-off, due to compactness of the particles and to the slope. The erosive action of winds upon soils bearing vegetation is not very general; it is found to some extent in more or less established dunes, and exists in a marked degree in buttes, mushroom rocks, and blowouts. The first two are regularly xerophytic, the last as a rule, dissophytic. The early stages of successions in eroded soils are composed of xerophytes. In loose soils, these are forms capable of binding the soil particles together, thus preventing wash, and increasing the accumulation of fine particles, especially of organic matter. In compact soils, the effect is much the same; the pioneers not only decrease erosion, but at the same time also increase the water-content by retarding the movement of the run-off.

=301. Succession in flooded soils.= The universal response of vegetation to floods is found in the amphibious plant, which is a plastic form capable of adjustment to very different water-contents. Floods are confined largely to river basins and coasts. In hilly and mountainous regions, where the slope is great, any considerable accumulation of flood waters is now impossible, although of frequent occurrence when land forms were more plastic.

In all streams that have become graded, the fall is insufficient to carry off the surplus water in the spring when snows are melting rapidly, or at times of unusual precipitation. These waters accumulate, and, overflowing the banks, spread out over the lowlands, resulting in the formation of a well-defined flood plain. This is a periodical occurrence with mature streams, and it occurs more or less regularly with all that are not torrent-like in character. The effect of the overflow is to destroy or to place at a disadvantage those plants of the flood plain that are not hydrophytes. At the same time, a thin layer of fresh silt is deposited upon the valley floor of sand or alluvium. Flooding is most frequent and of longest duration near the banks of the stream. It extends more or less uniformly over the flood plain, and disappears gradually or abruptly as the latter rises into the bench above. Floods destroy vegetation and make a place for secondary successions by drowning out mesophytic species, by washing away the aquatic forms of ponds and pools, and by the erosion of banks and sandbars. They affect the amphibious vegetation of swamp and shore to a certain extent, but, unless the period of flooding is long, they tend to emphasize such formations rather than to destroy them. The still-water formations of many cutoff and oxbow lakes owe their origin to a river which cuts across a meander in time of flood. This result is more often attained by the alternate silting and erosion of a meandering river by which it cuts across a bend in its channel. The usual successions in flooded lands are short as a rule; amphibious algae, liverworts, and mosses soon give way to ruderal plants, and these in turn to the original mesophytes of meadows, or dissophytes of sandbars. In the case of ponds and pools, the process of washing-out or silting up merely removes or destroys the vegetation, without effectively modifying the habitat, and the secondary successions that follow are extremely short.

=302. Succession by subsidence.= Subsidence is a factor of the most profound importance in changing vegetation. It operates over vast areas through immense periods of time. For these reasons, the changes are so slow as to be almost imperceptible, and the resulting successions can be studied only in the geological record. Extensive subsidence is confined to-day to coastal plains, as in Greenland, the south Atlantic coast, and the region of the Mississippi delta, where its effects are merged with the paludation of tidal rivers, and the wave and tide erosion of the sea shore. Such successions are unique, inasmuch as the denuding force operates very slowly instead of quickly, and the first pioneers of the new vegetation appear before the original formation has been destroyed. In all cases, the succession is from mesophytic or halophytic formations to paludose, and, finally, marine vegetation. In small areas of subsidence, such as shore slips along lakes and streams, sink holes, and sunken bogs, the succession is usually both short and simple, mesophytes giving place to amphibious and ultimately to aquatic forms.

=303. Successions in landslips.= Landslips occur only in montane and hilly regions, and here they are merely of local importance. In many respects, they are not unlike talus; they show essential differences, however, in that they are not sorted by gravity, and in that they destroy vegetation almost instantly. The succession arises as a rule, not upon the original soil, but upon that of the landslip, and, as pointed out elsewhere, might well be regarded as primary.

=304. Succession in drained, or dried soils.= In geological times, the subsidence of barriers must often have produced drainage and drying-out, just as elevation frequently resulted in flooding and lake formation. At the present time, the drying-out of lakes and ponds is the result of artificial drainage, or of climatic changes. The former will be considered under successions brought about by the agency of man. Climatic changes when general operate so slowly that the stages of such successions are perceptible only when recorded in strata. More locally, climate swings back and forth through a period of years, with the result that in dry years the swamps and ponds of wetter seasons are dried out, and the vegetation destroyed or changed. If the process be gradual, the succession passes from hydrophytic through amphibious to mesophytic, and, in dry regions, xerophytic conditions. When the process of drying-out occurs rapidly, as in a single summer, the original formation is destroyed, and the new vegetation consists largely of ruderal plants. A peculiar effect of climate occurs in regions with poor drainage, where the result of intense evaporation is to produce alkaline basins and salt lakes, in which the succession becomes more and more open, and is finally represented by a few stabilized halophytes, or disappears completely.

=305. Succession by animal agency.= Successions of this class are altogether of secondary importance, the instances in which animals produce denudation being relatively few. Such are the heaps of dirt thrown up by prairie dogs and other burrowing animals, upon which ruderal plants are first established, to be finally crowded out by the species of the original formation. Buffalo wallows furnish examples of similar successions in which the initial stages are subruderal, while overstocking and overgrazing frequently produce the same result with ruderal plants.

=306. Succession by human agency.= The activities of man in changing the surface of the earth are so diverse that it is impossible to fit the resulting successions in a natural system. While man does not exactly make new soils, he exposes soils in various operations: mining, irrigation, railroad building, etc. He destroys vegetation by fires, lumbering, cultivation, and drainage, and if he can not control climate, he at least modifies its natural effects by irrigation and the conservation of moisture. The operations of man extend from seacoasts and swampy lowlands through mesophytic forests and prairies to the driest uplands and inlands. Since the adjacent formations determine in large degree the course and constitution of a succession, it will be seen that the effects of any particular activity upon vegetation will differ greatly in different regions. For convenience, all classes of successions arising from the presence and activity of man will be considered in this place, though, as indicated above, some might well be regarded as producing primary successions, while others produce anomalous ones.

=307. Succession in burned areas.= It will suffice merely to point out that “burns” may arise naturally through lightning, volcanic cinders, lava flows, etc., but the chances are so slight that these causes may be ignored. The causes of fires are legion, and as they have little or no effect upon results, they need not be considered. From their nature, fires are of little significance in open vegetation, deserts, polar barrens, alpine fields, etc., since the area of the burn can never be large. In closed formations, the extent of fires is limited only by the area of the vegetation, and the effect of wind, rain, and other forces. Forest fires usually occur during the resting period, except in the case of coniferous forests. In grassland, the living parts are underground during autumn and winter, when prairie fires commonly occur. As a consequence, the repeated annual burning of meadow or prairie does not result in denudation and subsequent succession. On the contrary, it acts in part as a stabilizing agent, inasmuch as it injures the typical vegetation forms of grassland much less than it does the woody invaders. All formations with perennial parts above ground, viz., thicket, open woodland, and forest, are seriously injured by fire. A severe general fire destroys the vegetation completely; a local fire destroys the formation in restricted areas; while a slight or superficial burn removes the undergrowth and hastens the disappearance of the weaker trees. In the latter case, while the primary layer of the forest remains the same, succession takes place in the herbaceous and shrubby layers. These successions are peculiar in that they are composed almost wholly of the proper species of the forest, and that they are very short, showing only a few poorly defined stages. A local fire initiates a succession in which the pioneers are derived largely from the original formation, particularly when the latter encloses the burned area more or less completely. The constitution of the intermediate and ultimate stages will depend in a larger degree still upon the size and position of the burn. When a particular formation is destroyed wholly or in large part, the first stages of the new vegetation are made up by invaders from the adjacent formations. In the most perfect types of succession, this dissimilarity between the new and the old vegetation continues to the last stage, in which the reappearance of the facies precedes that of the subordinate layers. In many forest successions, however, the general physical similarity of the ultimate stages permits the early reappearance of the herbaceous and shrubby species, and the final stages affect the facies alone. Successions in burned areas operate usually within the water-content groups. The reconstruction of a mesophytic forest takes place by means of mesophytes; of the rarer xerophytic and hydrophytic forests, through xerophytes and hydrophytes respectively. This is due to the fact that the alteration of the soil is slight, except where the burning of the vegetation permits the entrance of erosion, as on mountain slopes.

=308. Succession in lumbered areas.= Commercial lumbering, especially where practiced for wood-pulp as well as for timber, results in complete or nearly complete destruction of the vegetation by removal and the change from diffuse light to sunlight, or by the action of erosion upon the exposed surface. In the first place, short mesophytic successions will result; in the second, the successions will be long and complex, passing through decreasingly xerophytic conditions to a stable mesophytic forest. Where a forest is cut over for certain species alone, the undisturbed trees soon take full possession, though the causes effective in the beginning will ultimately restore the original facies in many instances. Such successions are anomalous, and will be treated under that head.

=309. Succession by cultivation.= The clearing of forests and the “breaking” of grassland for cultivation destroy the original vegetation; the temporary or permanent abandonment of cultivated fields then permits the entrance of ruderal species, which are the pioneers of new successions. This phenomenon takes place annually in fields after harvest, resulting in the secondary formations of Warming, in which practically the same species reappear year after year. In fields that lie fallow for several years, or are permanently abandoned, the first ruderal plants are displaced by newcomers, or certain of them become dominant at the expense of others. In a few years, these are crowded out by invaders from the adjacent formations, and the field is ultimately reclaimed by the original vegetation, unless this has entirely disappeared from the region. The number of stages depends chiefly upon whether the final formation is to be grassland or woodland. Other activities of man, such as the construction of buildings, roads, railways, canals, etc., remove the native vegetation, and make room for the rapid development of ruderal formations. In and about cities, where the original formations have entirely disappeared, the chance for succession is remote, and the initial ruderal stages become more or less stabilized. Elsewhere the usual successions are established, and the ruderal formation finally gives way to the dominant type. In mountain and desert regions, where ruderal plants are rare or lacking, their place is taken by subruderal forms, species of the native vegetation capable of rapid movement in them. These, like ruderal plants, are gradually replaced by other native species of less mobility, but of greater persistence, resulting in a short succession operating often within a single formation. From the nature of cultivated plants, succession after cultivation generally operates within the mesophytic series.

=310. Succession by drainage.= Successions of this kind show much the same stages as are found in those due to flooding. They proceed from aquatic or swamp formations to mesophytic termini, either grassland or woodland. When drainage takes place rapidly and completely, the pioneer stages are usually xerophytic; cases of this sort, however, are infrequent.

=311. Succession by irrigation.= Irrigation produces short successions of peculiar stamp along the courses of irrigating canals and ditches, and in the vicinity of reservoirs. These are recent, as a rule, and are usually found in the midst of cultivated lands, so that their complete history is still a matter of conjecture. The original xerophytes are forced out not only by the disturbance of the soil, but also by its increased water-content. A few of them often thrive under the new conditions, and, together with the usual ruderal plants and a large number of lowland mesophytes and amphibious forms derived from the banks of the parent stream, constitute a heterogeneous association. This is doubtless to be regarded as an initial stage of a succession, but it is an open question whether the succession will early be stabilized as a new formation, or whether the original vegetation will sooner or later be reestablished under somewhat mesophytic conditions. From the number of mesophytes and from the behavior of valleys, it seems certain that the banks of such canals will ultimately be occupied by a formation more mesophytic than hydrophytic, into which some of the surrounding xerophytes of plastic nature have been adopted.

=312. Anomalous successions= are those in which the physical change in the habitat is relatively slight, resulting in a displacement of the ultimate stage, or the disturbance of the usual sequence, merely, instead of the destruction and reconstruction of a formation, or the gradual development of a new series of stages on new soil. In nature, the ultimate grass or forest stage of a normal succession is often replaced by a similar formation, especially if the facies be few or single. It is evident that certain trees naturally replace others in the last stages of a forest succession, without making the latter anomalous. The last occurs only when a normal stage is replaced by one belonging properly to an entirely different succession, as when a coniferous forest replaces a deciduous one in a hardwood region. The presence and development of such successions can be determined only after the normal types are known. The interpolation of a foreign stage in a natural succession, or a change of direction, by which a succession that is mesotropic again becomes hydrophytic, is easily explained when it is the result of artificial agents, as is often the case. In nature, anomalous successions are commonly the result of a slow backward and forward swing of climatic conditions.

=313. Perfect and imperfect successions.= A normal succession will regularly be perfect; it passes in the usual sequence from initial to ultimate conditions without interruption or omission. Imperfect succession results when one or more of the ordinary stages is omitted anywhere in the course, and a later stage appears before its turn. It will occur at any time when a new or denuded habitat becomes so surrounded by other vegetation that the formations which usually furnish the next invaders are unable to do so, or when the abundance and mobility of certain species enable them to take possession before their proper turn, and to the exclusion of the regular stage. Incomplete successions are of great significance, inasmuch as they indicate that the stages of a succession are often due more to biological than to physical causes, the proximity and mobility of the adjacent species being more determinative than the physical factors. Subalpine gravel slides regularly pass through the rosette, mat, turf, thicket, woodland, and forest stages; occasionally, however, they pass immediately from the rosette, or mat condition, to an aspen thicket which represents the next to the last stage. Such successions are by no means infrequent in hilly and montane regions; in regions physiographically more mature or stable, perfect successions are almost invariably the rule.

=314. Stabilization.= It may be stated as a general principle that vegetation moves constantly and gradually toward stabilization. Each successive stage modifies the physical factors, and dominates the habitat more and more, in such a way that the latter seems to respond to the formation rather than this to the habitat. The more advanced the succession, i. e., the degree of stabilization, the greater the climatic or physiographic change necessary to disturb it, with the result that such disturbances are much more frequent in the earlier stages than in the later development. Constant, gradual movement toward a stable formation is characteristic of continuous succession. Contrasted with this is intermittent succession, in which the succession swings for a time in one direction, from xerophytic to mesophytic for example, and then moves in the opposite direction, often passing through the same stages. This phenomenon usually is characteristic only of the less stable stages, and is generally produced by a climatic swing, in which a series of hot or dry years is followed by one of cold or wet years, or the reverse. The same effect upon a vast scale is produced by alternate elevation and subsidence, but these operate through such great periods of time that one can not trace, but can only conjecture their effects. A normal continuous succession frequently changes its direction of movement, or its type, in transition regions or in areas where the outposts of a new flora are rapidly advancing, as in wide mesophytic valleys that run down into or traverse plains. Here the change is often sudden, and grass and desert formations are replaced by thickets and forests, resulting in abrupt succession. Species guilds are typical examples of this. More rarely, a stage foreign to the succession will be interpolated, replacing a normal stage, or slipping in between two such, though finally disappearing before the next regular formation. This may be distinguished as interpolated succession.

The apparent terminus of all stabilization is the forest, on account of the thoroughness with which it controls the habitat. A close examination of vegetation, however, will show that its stable terms are dependent in the first degree upon the character of the region in which the formation is indigenous. It is obviously impossible that successions in desert lands, in polar barrens, or upon alpine stretches should terminate in forest stages. In these, grassland must be the ultimate condition, except in those extreme habitats, alpine and polar, where mosses and lichens represent the highest type of existing vegetation. Forests are ultimate for all successions in habitats belonging to a region generally wooded, while grassland represents the terminus of prairie and plains successions as well as of many arctic-alpine ones.

CAUSES AND REACTIONS

=315.= The initial cause of a succession must be sought in a physical change in the habitat; its continuance depends upon the reaction which each stage of vegetation exerts upon the physical factors which constitute the habitat. A single exception to this is found in anomalous successions, where the change of formation often hinges upon the appearance of remote or foreign disseminules. The causes which initiate successions have already been considered; they may be summarized as follows: (1) weathering, (2) erosion, (3) elevation, (4) subsidence, (5) climatic changes, (6) artificial changes. The effect of succeeding stages of vegetation upon a new or denuded habitat usually finds expression in a change of the habitat with respect to a particular factor, and in a definite direction. Often, there is a primary reaction, and one or more secondary ones, which are corollaries of it. Rarely, there are two or more coordinate reactions. The general ways in which vegetation reacts upon the habitat are the following: (1) by preventing weathering, (2) by binding aeolian soils, (3) by reducing run-off and preventing erosion, (4) by filling with silt and plant remains, (5) by enriching the soil, (6) by exhausting the soil, (7) by accumulating humus, (8) by modifying atmospheric factors. The direction of the movement of a succession is the immediate result of its reaction. From the fundamental nature of vegetation, it must be expressed in terms of water-content. The reaction is often so great that the habitat undergoes a profound change in the course of the succession, changing from hydrophytic to mesophytic or xerophytic, or the reverse. This is characteristic of newly formed or exposed soils. Such successions are xerotropic, mesotropic, or hydrotropic, according to the ultimate condition of the habitat. When the reaction is less marked, the type of habitat does not change materially, and the successions are xerostatic, mesostatic, or hydrostatic, depending upon the water-content. Such conditions obtain for the most part only in denuded habitats.

=316. Succession by preventing weathering.= Reactions of this nature occur especially in alpine and boreal regions, in the earlier stages of lichen-moss successions. They are typical of igneous and metamorphic rocks in which disintegration regularly precedes decomposition. The influence of the vegetation is best seen in the lichen stages, where the crustose forms make a compact layer, which diminishes the effect of the atmospheric factors producing disintegration. In alpine regions especially, this protection is so perfect that the crustose lichens may almost be regarded as the last stage of a succession. There are no recorded observations which bear upon this point, but it seems certain that the pioneer rock lichens, Lecanora, Lecidea, Biatora, Buellia, and Acarospora, cover alpine rocks for decades, if not for centuries. Ultimately, however, the slow decomposition of the rock surface beneath the thallus has its effect. Tiny furrows and pockets are formed, in which water accumulates to carry on its ceaseless work, and the compact crustose covering is finally ruptured, permitting the entrance of foliose forms. The latter, like the mosses, doubtless protect rock surfaces, especially those of the softer rocks, in a slight degree against the influence of weathering, but this is more than offset by their activity in hastening decomposition, and thus preparing a field for invasion. Rocks and boulders (petria, petrodia, phellia) furnish the best examples of this reaction; cliffs (cremnia) usually have a lichen covering on their faces, while the forces which produce disintegration operate from above or below.

=317. Succession by binding aeolian soils.= Dunes (thinia) are classic examples of the reaction of pioneer vegetation upon habitats of wind-borne sand. The initial formations in such places consist exclusively of sand-binders, plants with masses of fibrous roots, and usually also with strong rootstalks, long, erect leaves, and a vigorous apical growth. They are almost exclusively perennial grasses and sedges, possessing the unique property of pushing up rapidly through a covering of sand. They react by fixing the sand with their roots, thus preventing its blowing about, and also by catching the shifting particles among their culms and leaves, forming a tiny area of stabilization, in which the next generation can establish a foothold. The gradual accumulation of vegetable detritus serves also to enrich the soil, and makes possible the advent of species requiring better nourishment. Blowouts (anemia) are almost exact duplicates of dunes in so far as the steps of revegetation are concerned; while one is a hollow, and the other a hill, in both the reaction operates upon a wind-swept slope. Sand-hills (amathia) and deserts (eremia) show similar though less marked reactions, except where they exhibit typical inland dunes. Sand-binders, while usually classed as xerophytic or halophytic, are in reality dissophytes. Their roots grow more or less superficially in moist sand, and are morphologically mesophytic while their leaves bear the stamp of xerophytes. The direction of movement in successions of this kind is normally from xerophytes to mesophytes, i. e., it is mesotropic. In sand-hills and deserts, the succession operates wholly within the xerophytic (dissophytic) series. Along seacoasts, the mesophytic terminus is regularly forest, except where forests are remote, when it is grassland.

=318. Succession by reducing run-off and erosion.= All bare or denuded habitats that have an appreciable slope are subject to erosion by surface water. The rapidity and degree of erosion depend upon the amount of rainfall, the inclination of the slope, and the structure of the surface soil. Regions of excessive rainfall, even where the slope is slight, show great, though somewhat uniform erosion; hill and mountain are deeply eroded even when the rainfall is small. Slopes consisting of compact eugeogenous soils, notwithstanding the marked adhesion of the particles, are much eroded where the rainfall is great, on account of the excessive run-off. Porous dysgeogenous soils, on the contrary, absorb most of the rainfall; the run-off is small and erosion slight, except where the slope is great, a rare condition on account of the imperfect cohesion of the particles. In compact soils, the plants of the initial formations not merely break the impact of the raindrops, but, what is much more important, they delay the downward movement of the water, and produce numberless tiny streams. The delayed water is largely absorbed by the soil, and the reduction of the run-off prevents the formation of rills of sufficient size to cause erosion. As in dunes, such plants are usually perennial grasses, though composites are frequent; the root system is, however, more deeply seated, and a main or tap root is often present. On sand and gravel slopes, the loose texture of the soil results generally in the production of sand-binders with fibrous roots. Unlike dunes, such slopes exhibit a large number of mats and rosettes with tap-roots, which are effective in preventing the slipping or washing of the sand, and run little danger of being covered, as is the case with duneformers. In both instances, each pioneer plant serves as a center of comparative stabilization for the establishment of its own offspring, and of such invaders as find their way in. From the nature of these, slopes almost invariably pass through grassland stages before finding their termini in thickets or forests. Bad lands (tiria) furnish the most striking examples of eroded habitats. The rainfall in the bad lands of Nebraska and South Dakota is small (300 mm.); yet the steepness of the slope and the compactness of the soil render erosion so extreme that it is all but impossible for plants to obtain a foothold. Their reaction is practically negligible, and the vegetation passes the pioneer stages only in the relatively stable valleys. Mountain slopes (ancia), and ridges and hills (lophia) are readily eroded in new or denuded areas. This is especially true of hill and mountain regions which have been stripped of their forest or thicket cover by fires, lumbering, cultivation, or grazing. Where the erosion is slight, the resulting succession may show initial xerophytic stages, or it may be completely mesostatic. Excessively eroded habitats are xerostatic, as in the case of bad lands, or, more frequently, they are mesotropic, passing first through a long series of xerophytic formations. Sandbars (cheradia, syrtidia) should be considered here, though they are eroded by currents and waves, and not by run-off. They are fixed and built up by sand-binding grasses and sedges, usually of a hydrophytic nature, and pass ultimately into mesophytic forest.

=319. Succession by filling with silt and plant remains.= All aquatic habitats into which silt, wash, or other detritus is borne by streams, currents, floods, waves, or tides are slowly shallowed by the action of the water plants present. These not only check the movement of the water, thus greatly decreasing its carrying power, and causing the deposition of a part or all of its load, but they also retain and fix the particles deposited. In accordance with the rule, each plant becomes the center of a stabilizing area, which rises faster than the rest of the floor, producing the well-known hummocks of lagoons and swamps. All aquatics produce this reaction. It is more pronounced in submerged and amphibious forms than in floating ones, and it takes place more rapidly with greatly branched or dissected plants than with others. In pools (tiphia) and lakes (limnia), debouching streams and surface waters deposit their loads in consequence of the check exerted by the still water and the marginal vegetation, and delta-like marshes are quickly built up by filling. Springs (crenia) likewise form marshes where they gush forth in sands, the removal of which is impeded by vegetation. The flood plains and deltas of rivers show a similar reaction. The heavily laden flood waters are checked by the vegetation of meadows and marshes, and deposit most of their load. The banks of streams (ochthia) and of ditches (taphria) are often built up in the same fashion by the action of the marginal vegetation upon the current. The presence of marginal vegetation often determines the checking or deflecting of the current in such a way as to initiate meanders, while natural levees owe their origin to it, in part at least. Along low seacoasts, waves and tides hasten the deposit of river-borne detritus, causing the water to spread over the lowlands and form swamps. They often throw back also the sediment that has been deposited in the sea, the marsh vegetation acting as a filter in both cases. Successions of the kind indicated above are regularly mesotropic. Where the soil is sandy, and the filling-up process sufficiently great, or where salts or humus occur in excess, xerophytic formations result. In certain cases, these successions appear to be permanently hydrostatic, changing merely from floating or submerged to amphibious conditions, but this is probably due to the slowness of the reaction. As a rule, the accumulation of plant remains is relatively slight, and plays an unimportant part in the reaction. In peat bogs and other extensive swamps, the amount of organic matter is excessive, and plays an important role in the building up of the swamp bed.

=320. Succession by enriching the soil.= This reaction occurs to some degree in the great majority of all successions. The relatively insignificant lichens and mosses produce this result upon the most barren rocks, while the higher forms of later stages, grasses, herbs, shrubs, and trees, exhibit it in marked progression. The reaction consists chiefly in the incorporation of the decomposed remains of each generation and each stage in the soil. A very important part is played by the mechanical and chemical action of the roots in breaking up the soil particles, and in changing them into soluble substances. Mycorrhizae, bacterial nodules, and especially soil bacteria play a large part in increasing the nutrition-content of the soil, but the extent to which they are effective in succession is completely unknown. The changes in the color, texture, and food value of the soil in passing from the initial to ultimate stages of a normal succession are well known, and have led many to think them the efficient reactions of such successions. It seems almost certain, however, that this is merely a concomitant, and that, even in anomalous successions where facies replace each other without obvious reasons, the reactions are concerned more with water-content, light, and humidity than with the food-content of the soil.

=321. Succession by exhausting the soil.= This is a reaction not at all understood as yet in nature. A number of phenomena, such as the “fairy rings” of mushrooms and other fungi, the peripheral growth and central decay of lichens, Lecanora, Placodium, Parmelia, and of matforming grasses, such as Muhlenbergia, and the circular advance of the rootstalk plants, indicate that certain plants at least withdraw much of the available supply of some essential soil element, and are forced to move away from the exhausted area. It is probable that the constant shifting of the individuals of a formation year after year, a phenomenon to be discussed under alternation, has some connection with this. It will be impossible to establish such a relation, however, until the facts are exactly determined by the method of quadrat statistics. So far as native formations are concerned, there can not be the slightest question that prairies and forests have existed over the same area for centuries without impoverishing the soil in the least degree, a conclusion which is even more certain for the open vegetation of deserts and plains. With culture formations, the case is quite different. The exhaustion of the soil by continuous or intensive cultivation is a matter of common experience in all lands settled for a long period. Calcium, phosphorus, and nitrogen compounds especially are used up by crops, and must be supplied artificially. The reason for this difference in reaction between native and culture formations seems evident. In harvesting, not merely the grain, but the stems and leaves, and in gardening often the root also, are removed, so that the plant makes little or no return to the soil. In nature, annual plants return to the ground every year all the solid matter of roots, stems, leaves, and fruits, with the exception of the relatively small number of seeds that germinate. Perennial herbs return everything but the persistent underground parts. Shrubs and trees replace annually an immense amount of material used in leaves and fruits, and sooner or later, by the gradual decay of the individuals or by the destruction of the whole formation, they restore all that they have taken from the soil. This balance is further maintained to an important degree by the activity of the roots, which take from the deep-seated layers of the soil the crude materials necessary for the formation of leaves and fruits. Upon the fall and decay of these, their materials are incorporated with the upper layers of the formation floor, from which they may be absorbed by the undergrowth, or find their way again into the layers permeated by the tree roots. From the universal occurrence of weeds in cultivated regions, the pioneers in impoverished or exhausted fields are uniformly ruderal plants. As is well known, the seed production and ecesis of these forms are such that they take possession quickly and completely, while their demands upon the soil are of such a nature that the most sterile field can rapidly be covered by a vigorous growth of weeds. As indicated elsewhere, ruderal formations ultimately yield to the native vegetation, though in regions so completely given over to culture that native formations are lacking or remote, it is probable that successions reach their final stage within the group of ruderal plants.

=322. Succession by the accumulation of humus.= This is the characteristic reaction of peat bogs and cypress swamps (oxodia), in which the accumulation of vegetable matter is enormous. The plant remains decompose slowly and incompletely under the water, giving rise to the various humic acids. These possess remarkable antiseptic qualities, and have an injurious effect upon protoplasm. They affect the absorption of water by the root-hairs, though this is also influenced by poor aeration. The same acids are found in practically all inland marshes and swamps, but the quantity of decomposing vegetation in many is not great enough to produce an efficient reaction. Formations of this type usually start as freshwater swamps. The succession is apparently hydrostatic, but no thorough study of its stages has as yet been made.

=323. Succession by modifying atmospheric factors.= All layered formations, forests, thickets, many meadows and wastes, etc., show reactions of this nature, and are in fact largely or exclusively determined by them. The reaction is a complex one, though it is clear that light is the most efficient of the modified factors, and that humidity, temperature, and wind, while strongly affected, play subordinate parts. In normal successions, the effect of shade, i. e., diffuse light, enters with the appearance of bushes or shrubs, and becomes more and more pronounced in the ultimate forest stages. The reaction is exerted chiefly by the facies, but the effect of this is to cause increasing diffuseness in each successively lower layer, in direct ratio with the increased branching and leaf expansion of the plants in the layer just above. In the ultimate stage of many forests, especially where the facies are reduced to one, the reaction of the primary layer is so intense as to preclude all undergrowth. Anomalous successions often owe their origin to the fact that certain trees react in such a way as to cause conditions in which they produce seedlings with increasing difficulty, and thus offer a field favorable to the ecesis of those species capable of enduring the dense shade. Successions of this kind are almost invariably mesostatic, as it is altogether exceptional that layered formations are either xerophytic or hydrophytic.

LAWS OF SUCCESSION

=324.= The investigation of succession has so far been neither sufficiently thorough nor systematic to permit the postulation of definite laws. Enough has been done, however, to warrant the formulation of a number of rules, which apply to the successions studied, and afford a convenient method for the critical investigation of all successions upon the basis of initial causes, and reactions. Warming has already brought together a few such rules, and an attempt is here made to reduce the phenomena of succession, including its causes and effects, to a tentative system. At present it is difficult to make a thoroughly satisfactory classification of such rules, and they are here arranged in general conformity with the procedure in succession.

I. Causation. The initial cause of a succession is the formation or appearance of a new habitat, or the efficient change of an existing one.

II. Reaction. Each stage reacts upon the habitat in such a way as to produce physical conditions more or less unfavorable to its permanence, but advantageous to the invaders of the next stage.

III. Proximity and mobility.

(1) The pioneers of a succession are those species nearest at hand that are the most mobile.

(2) The number of migrants from any formation into a habitat varies inversely as the square of the distance.

(3) The pioneer species are regularly derived from different formations, as the latter nearly always contain permobile species capable of effective ecesis.

(4) The plants of the initial stages are normally algae and fungi, with minute spores, composites, and grasses, which possess permobile fruits, or ruderal plants, on account of their great seed production.

IV. Ecesis.

(1) All the migrants into a new, denuded, or greatly modified habitat are sorted by ecesis into three groups: (1) those that are unable to germinate or grow, and soon die; (2) those that grow normally under the conditions present; (3) those that pass through one or more of the earlier stages in a dormant state to appear at a later stage of the succession.

(2) Wherever ruderal vegetation is present, it contributes a large number of the pioneer species of each succession, on account of the thorough ecesis. In other regions this part is played by subruderal native species.

(3) Annuals and biennials are characteristic of the early stages of secondary successions, on account of their great seed production and ready ecesis.

(4) In layered formations, heliophytes appear before sciophytes; they ultimately yield to the latter, except where they are able to maintain a position in the primary layer.

(5) Excessive seed production and slight mobility lead to the imperfect ecesis of individuals in dense stands, and in consequence usually produce great instability.

(6) Each pioneer produces about itself a tiny area of ecesis and stabilization for its own offspring, for the disseminules of its fellows, or of invaders.

(7) Species propagating by offshoots, or producing relatively immobile disseminules in small number, usually show effective ecesis, as the offspring appear within the area of the reaction of the parent forms.

V. Stabilization.

(1) Stabilization is the universal tendency of vegetation.

(2) The ultimate stage of a succession is determined by the dominant vegetation of the region. Lichen formations are often ultimate in polar and niveal zones; grassland is the final vegetation for plains and alpine stretches, and for much prairie, while forest is the last stage for mesophytic midlands and lowlands, as well as for subalpine regions.

(3) Grassland or forest is the usual terminus of a succession; they predominate in lands physiographically mature.

(4) The limit of a succession is determined in large part by the progressive increase in occupation, which makes the entrance of invaders more and more difficult.

(5) Stabilization proceeds radiately from the pioneer plants or masses. The movement of offshoots is away from the parent mass, and the chances of ecesis are greatest near its edges, in a narrow area in which the reaction is still felt, and the occupation is not exclusive.

VI. General laws.

(1) The stages, or formations, of a succession are distinguished as initial (prodophytia), intermediate (ptenophytia), and ultimate (aiphytia).

(2) Initial formations are open, ultimate formations are closed.

(3) The number of species is small in the initial stages; it attains a maximum in intermediate stages; and again decreases in the ultimate formation, on account of the dominance of a few species.

(4) The normal sequence of vegetation forms in succession is: (1) algae, fungi, mosses; (2) annuals and biennials; (3) perennial herbs; (4) bushes and shrubs; (5) trees.

(5) The number of species and of individuals in each stage increases constantly up to a maximum, after which it gradually decreases before the forms of the next stage. The interval between two maxima is occupied by a mixed formation.

(6) A secondary succession does not begin with the initial stage of the primary one which it replaces, but usually at a much later stage.

(7) At present, successions are generally mesotropic, grassland and forest being the ultimate stages, though many are xerostatic or hydrostatic. If erosion continue until the sea level is reached, the ultimate vegetation of the globe will be hydrophytic. Should the heat of the sun decrease greatly before this time, the last vegetation will be xerophytic, i. e., crymophytic.

(8) The operation of succession was essentially the same during the geological past as it is to-day. From the nature of their vegetation forms, the record deals largely with the ultimate stages of such successions.

CLASSIFICATION AND NOMENCLATURE

=325. Basis.= New or denuded habitats arise the world over by the operation of the same or similar causes, and they are revegetated in consequence of the same reactions. Similar habitats produce similar successions. The vegetation forms and their sequence are usually identical, and the genera are frequently the same, or corresponding in regions not entirely unrelated. The species are derived from the adjacent vegetation, and, except in alpine and coast regions, are normally different. The primary groups of successions are determined by essential identity of habitat or cause, e. g., aeolian successions, erosion successions, burn successions, etc. When they have been more generally investigated, it will be possible to distinguish subordinate groups of successions, in which the degree of relationship is indicated by the similarity of vegetation forms, the number of common genera, etc. For example, burn successions in the Ural and in the Rocky mountains show almost complete similarity in the matter of vegetation forms and their sequence, and have the majority of their genera in common. A natural classification of successions will divide them first of all into normal and anomalous. The former fall into two classes, primary and secondary, and these are subdivided into a number of groups, based upon the cause which initiates the succession.

=326. Nomenclature.= The need of short distinctive names of international value for plant formations is obvious; it has become imperative that successions also should be distinguished critically and designated clearly. From the very nature of the case, it is impossible to designate each formation or succession by a single Greek or Latin term, as habitats of the same character will show in different parts of the world a vegetation taxonomically very different. It may some day be possible to use a binomial or trinomial for this purpose, somewhat after the fashion of taxonomy, in which the habitat name will represent the generic idea as applied to formations, and a term drawn from the floristic impress the specific idea. Such an attempt would be futile or valueless at the present time; it could not possibly meet with success until there is more uniformity in the concept of the formation, and until there has been much accurate and thorough investigation of actual formations, a task as yet barely begun. At present, it seems most feasible as well as scientific to designate all formations occupying similar habitats by a name drawn from the character of the latter, such as a meadow formation, poium, a forest formation, hylium, a desert formation, eremium, etc. A particular formation is best designated by using the generic name of one or two of its most important species in conjunction with its habitat term, as Spartina-Elymus-poium, Picea-Pinus-hylium, Cereus-Yucca-eremium, etc. Apparently a somewhat similar nomenclature is adapted to successions. The cause which produces a new habitat may well furnish the basis for the name of the general groups of successions, as pyrium (literally, a place or a habitat burned over), a burn succession, tribium, an erosion succession, etc. A burn succession consists of a sequence of certain formations in one part of the world, and of a series of quite different ones, floristically, in another. A particular burn succession should be designated by using the names of a characteristic facies of the initial and ultimate stages in connection with the general term, e. g., Bryum-Picea-pyrium, etc. A trinomial constructed in this way represents the desirable mean between definition and brevity. Greater definiteness is possible only at the expense of brevity, while to shorten the name would entirely destroy its precision. The following classification of successions is proposed, based upon the plan outlined above. The termination -ium (εῖον) has been used throughout in the construction of names for successions, largely for reasons of euphony. If it should become desirable to distinguish the names of formations and successions by the termination, the locative suffix -on (-ών) should be used for the latter. The terms given below would then be hypson, rhyson, hedon, sphyron, prochoson, pnoon, pagon, tribon, clyson, repon, olisthon, xerasion, theron, broton, pyron, ecballon, camnon, ocheton, ardon.

I. Normal successions: cyriodochae (κύριος, regular, δοχή, ἡ, succession)

a. Primary successions: protodochae (πρῶτος, first, primary)

1. By elevation: hypsium (ὔψος, το, height, elevation, -εῖον, place)

2. By volcanic action: rhysium (ῥυσίς ἡ, flowing, especially of fire)

3. In residuary soils: hedium (ἔδος, τό, a sitting base)

4. In colluvial soils: sphyrium (σφύρον, τό, ankle, talus)

5. In alluvial soils: prochosium (πρόχωσις, ἡ, a deposition of mud)

6. In aeolian soils: pnoium (πνοή, ἡ, blowing, blast)

7. In glacial soils: pagium (πάγος, ὁ, that which becomes solid, i. e., a glacier)

b. Secondary successions: hepodochae (ἕπω, to follow)

8. In eroded soils: tribium (τρίβω, wear or rub away)

9. In flooded soils: clysium (κλύσις, ὁ, a drenching, flooding)

10. By subsidence: repium (ῥέπω, incline downwards, sink)

11. In landslips: olisthium (ὄλισθος, ὁ, slip)

12. In drained and dried out soils: xerasium (ξηρασία, ἡ, drought)

13. By animal agencies: therium (θήρ, ὁ, wild animal)

14. By human agency: brotium (βροτός, ὁ, a mortal)

a. Burns: pyrium (πῦρ, τό, fire)

b. Lumbering: ecballium (ἐκβάλλω, cut down forests)

c. Cultivation: camnium (κάμνω, cultivate)

d. Drainage: ochetium (ὀχετός, ὁ, drain)

e. Irrigation: ardium (ἄρδω, irrigate)

II. Anomalous successions: xenodochae (ξένος, strange, unusual)

=327. Illustrations.= The following series will illustrate the application of this system of nomenclature to particular successions, and their stages, or formations.

Thlaspi-Picea-sphyrium: pennycress-spruce talus succession Thlaspi-Eriogonum-chalicium: pennycress-eriogonum gravel slide formation Elymus-Gilia-chalicium: wildrye-gilia half gravel slide formation Quercus-Holodiscus-driodium: oak-fringewood dry thicket formation Pinus-xerohylium: pine dry forest formation Picea-Pseudotsuga-hylium: spruce-balsam forest formation

Bryum-Picea-pyrium: moss-spruce burn succession Bryum-telmatium: moss meadow formation Aster-Chamaenerium-poium: aster-fireweed meadow formation Deschampsia-Carex-poium: hairgrass-sedge meadow formation Salix-Betula-helodrium: willow-birch meadow thicket formation Populus-hylium: aspen forest formation Picea-hylium: spruce forest formation

Lecanora-Carex-hedium: lichen-carex residuary succession Lecanora-Gyrophora-petrium: crustose lichen rock formation Parmelia-Cetraria-chalicium: foliose lichen gravel slide formation Paronychia-Silene-chalicium: nailwort-campion gravel slide formation Carex-Campanula-coryphium: sedge-bluebell alpine meadow formation

Eragrostis-Helianthus-xerasium: eragrostis-sunflower drainage succession Eragrostis-Polygonum-telmatium: eragrostis-heartsease wet meadow formation Helianthus-Ambrosia-chledium: sunflower-ragweed waste formation

INVESTIGATION OF SUCCESSION

=328. General rules.= The study of succession must proceed along two fundamental lines of inquiry: it is necessary to investigate quantitatively the physical factors of the initial stages and the reactions produced by the subsequent stages. This should be done by automatic instruments for humidity, light, temperature, and wind, in order that a continuous record may be obtained. Water-content is taken daily or even less frequently, while soil properties, and physiographic factors, altitude, slope, surface, and exposure are determined once for all. It is equally needful to determine the development and structure of each stage with particular reference to the adjacent formations, to the stage that has just preceded, and the one that is to follow. For this, the use of the permanent quadrat is imperative, as the sequence and structure of the stages can be understood only by a minute study of the shifting and rearrangement of the individuals. Permanent migration circles are indispensable for tracing movement away from the pioneer areas by which each stage reaches its maximum. Denuded quadrats are a material aid in that they furnish important evidence with respect to migration and ecesis, By means of them, it is possible to determine the probable development of stages which reach back a decade or more into the past. In the examination of successions, since cause and effect are so intimately connected in each reaction, it is especially important that general and superficial observations upon structure and sequence be replaced by precise records, and that vague conjectures as to causes and reactions be supplanted by the accurate determination of the physical factors which underlie them.

=329. Method of alternating stages.= The period of time through which a primary succession operates is usually too great to make a complete study possible within a single lifetime. Secondary successions run their course much more quickly, and a decade will sometimes suffice for stabilization, though even here the period is normally longer. The longest and most complex succession, however, may be accurately studied in a region, where several examples of the same succession occur in different stages of development. In the same region, the physical factors of one example of a particular succession are essentially identical with those of another example in the same stage. If one is in an initial stage, and the other in an intermediate condition, the development of the former makes it possible to reestablish more or less completely the life history of the latter. The same connection may be made between intermediate and ultimate stages, and it is thus possible to determine with considerable accuracy and within a few years the sequence of stages in a succession that requires a century or more for its complete development. In the Rocky mountains, gravel slides (talus slopes) are remarkably frequent. They occur in all stages of development, and the alternating slides of different ages furnish an almost perfect record of this succession. This method lacks the absolute finality which can be obtained by following a succession in one spot from its inception to final stabilization, but it is alone feasible for long successions, i. e., those extending over a score or more of years. When it comes to be universally recognized as a plain duty for each investigator to leave an exact and complete record in quadrat maps and quadrat photographs of the stages studied by him, it will be a simple task for the botanists of one generation to finish the investigations of succession begun by their predecessors.

=330. The relict method= of studying succession is next in importance to the method of alternating areas. The two in fact are supplementary, and should be used together whenever relicts are present. This method is based upon the law of successive maxima, viz., the number of species and of individuals in each stage constantly increases up to a certain maximum, after which it gradually decreases before the forms of the next stage. In accordance with this, secondary species usually disappear first, principal species next, and facies last of all. There are notable exceptions to this, however, and the safest plan is to use the relict method only when principal species or facies are left as evidence. An additional reason for this is that secondary species are more likely to be common to two or more formations. In the majority of cases, the relict is not modified, and is readily recognized as belonging properly to a previous stage. This is true of herbs in all the stages of grassland, and in the initial ones of forest succession. The herbs and shrubs of earlier stages, which persist in the final forest stages, are necessarily modified, often in such a degree as to become distinct ecads, or species. The facies of the stages which precede the ultimate forest are rarely modified. The application of the relict method, together with the modification just described, is nicely illustrated by the balsam-spruce formation at Minnehaha. Of the initial gravel slide stage, the relicts are Vagnera stellata and Galium boreale, the one modified into Vagnera leptopetala, and the other into G. boreale hylocolum. The thicket stage is represented by Holodiscus dumosa, greatly changed in form and branching, and in the shape and structure of the leaf. The most striking relict of the aspen formation is the facies itself, Populus tremuloides. The tall slender trunks of dead aspens are found in practically every balsam-spruce forest. In many places, living trees are still found, with small, straggling crowns, which are vainly trying to outgrow the surrounding conifers. Of the aspen undergrowth, Rosa sayii, Helianthella parryi, Frasera speciosa, Zygadenus elegans, Castilleia confusa, Gentiana acuta, and Solidago orophila remain more or less modified by the diffuse light. It is still a question whether the aspen stage passes directly into the balsam-spruce forest, or whether a pine forest intervenes. The presence of both Pinus ponderosa and P. flexilis, which are scattered more or less uniformly through the formation, furnishes strong evidence for the latter view.

The lifetime of forest and thicket stages of successions is ascertained by counting the annual rings of the stumps of facies. This is a perfectly feasible method for many woodland formations where stumps already abound or where a fire has occurred, and it is but rarely necessary to cut down trees for this purpose. When trees or shrubs are present as relicts, the same method is used to determine the length of time taken by the development of the corresponding stages.

THE STRUCTURE OF THE FORMATION

=331.= Since all the structures exhibited by formations, such as zones, layers, consocies, etc., are to be referred to zonation or alternation, these principles are first considered in detail. This, then, constitutes the basis for a consideration of the structure of a normal formation, with special reference to the different parts that compose it. The investigation of formational structure, since the latter is the result of aggregation, invasion, and succession, is accomplished by instruments, quadrats, etc., in the manner already indicated under development, and no further discussion of it is necessary here.

ZONATION

=332. Concept.= The recognition of vegetation zones dates from Tournefort, who found that, while the plants of Armenia occupied the foot of Mount Ararat, the vegetation of the slopes above contained many species of southern Europe. Still higher appeared a flora similar to that of Sweden, and on the summit grew arctic plants, such as those of Lapland.

As the historical summary shows, the concept of zonation is the oldest in phytogeography. Notwithstanding this, it has never been clearly defined, nor has there been any detailed investigation of the phenomenon itself, or of the causes which produce it. Zones are so common, and often so clearly marked, that they invite study, but no serious attempt has heretofore been made to analyze zonation, or to formulate a definite method of investigating it. Zonation is the practically universal response of plants to the quantitative distribution of physical factors in nature. In almost all habitats, one or more of the physical factors present decreases gradually in passing away from the point of greatest intensity. The result is that the plants of the habitat arrange themselves in belts about this point, their position being determined by their relation to the factor concerned. Close investigation will show that there is hardly a formation that is entirely without zonation, though in many cases the zones are incomplete or obscure for various reasons. Zonation is as characteristic of vegetation as a whole as it is of its unit, the formation, a fact long ago recognized in temperature zones. A continental climate, however, often results in the interruption of these, with the consequence that these belts of vegetation are not always continuous.

CAUSES OF ZONATION

=333. Growth.= The causes that produce zones are either biological or physical: the first have to do with some characteristic of the plant, the second with the physical features of the habitat. Biological causes arise from the method of growth, from the manner of dissemination, or from the reaction of the species upon the habitat. The formation of circles as a result of radial growth is a well-known occurrence with certain plants, but it is much more common than is supposed. In the case of agarics, this phenomenon has long been known under the name of “fairy-rings.” It is found in a large number of moulds, and is characteristic of early stages of the mycelium of the powdery mildews. It occurs in nearly all maculicole fungi, and is exhibited by certain xylogenous fungi, such as Hysterographium. Among the foliose lichens, it is a common occurrence with the rock forms of Parmelia, Placodium, Physcia, and Lecanora, and with the earth forms of Parmelia and Peltigera. The thalloid liverworts show a similar radial growth. The flowering plants, and many mosses also, furnish good examples of this sort of growth in those species which simulate the form of the mycelium or thallus. These are the species that form mats, turfs, or carpets. Alpine mat formers, such as Silene acaulis, Paronychia pulvinata, Arenaria sajanesis, etc., are typical examples. Xerophytic, turf-forming species of Muhlenbergia, Sporobolus, Bouteloua, Festuca, Poa, and other grasses form striking ring-like mats, while creeping species of Euphorbia, Portulaca, Amarantus, etc., produce circular areas. Rosettes, bunch-grasses, and many ordinary rootstalk plants spread rapidly by runners and rhizomes. The direction of growth is often indeterminate in these also, and is in consequence more or less bilateral or unilateral. Growth results in zonation only when the older central portions of the individual or mass die away, leaving an ever-widening belt of younger plants or parts. This phenomenon is doubtless due in part to the greater age of the central portion, but seems to arise chiefly from the demands made by the young and actively growing parts upon the water of the soil. There may possibly be an exhaustion of nutritive content, as in the case of the fungi, but this seems improbable for the reason that young plants of the same and other species thrive in these areas. It must not be inferred that these miniature growth zones increase in size until they pass into zones of formations. Growth contributes its share to the production of these, but there is no genetic connection between a tiny plant zone and a zone of vegetation.

Radial and bilateral growth play an important part in formational zones in so far as they are related to migration. The growth of the runner or rhizome itself is a very effective means of dissemination, while the seeding of the plants thus carried away from the central mass is most effective at the edge of the newly occupied area. This holds with equal force for plants with a mycelium or a thallus. The circular area becomes larger year by year. Sooner or later, the younger, more vigorous, and more completely occupied circumference passes into a more or less complete zone. This will result from the reaction of the central individuals upon the habitat, so that they are readily displaced by invaders, or from their increasing senility and dying out, or from the invasion of forms which seed more abundantly and successfully. This result will only be the more marked if the radiating migrants reach a belt of ground especially favorable to their ecesis. In this connection it must be carefully noted that vegetation pressure, before which weaker plants are generally supposed to flee, or by which they are thought to be forced out into less desirable situations, is little more than a fanciful term for radial growth and migration. It has been shown under invasion that disseminules move into vegetation masses, as well as away from them, the outward movement alone being conspicuous, because it is only at the margin and beyond that they find the necessary water and light for growth.

=334. Reactions.= Certain reactions of plants upon habitats produce zonation. The zones of fungi are doubtless caused by the exhaustion of the organic matter present, while in lichens and mosses the decrease in nutritive content has something to do with the disappearance of the central mass. In the mats of flowering plants, the connection is much less certain. The reaction of a forest or thicket, or even of a tall herbaceous layer, is an extremely important factor in the production of zonation. The factor chiefly concerned here is light. Its intensity is greatest at the edge of the formation and just below the primary layer; the light becomes increasingly diffuse toward the center of the forest, and toward the ground. In response to this, both lateral and vertical zones appear. The former are more or less incomplete, and are only in part due to differences in illumination. The vertical zones or layers are characteristic of forest and thickets, and are caused directly by differences in light intensity.

=335. Physical factors.= The physical causes of zonation are by far the most important. They arise from differences in temperature, water, and light. In the large, temperature differences are the most important, producing the great zones of vegetation. In a particular region or habitat, variations of water-content and humidity are controlling, while light, as shown above, is important in the reactions of forest and thicket. Physical factors produce zonation in a habitat or a series of habitats, when there is either a gradual and cumulative, or an abrupt change in their intensity. Gradual, slight changes are typical of single habitats; abrupt, marked changes of a series of habitats. This modification of a decisive factor tends to operate in all directions from the place of greatest intensity, producing a characteristic symmetry of the habitat with reference to the factor concerned. If the area of greatest amount is linear, the shading-out will take place in two directions, and the symmetry will be bilateral, a condition well illustrated by rivers. On the other hand, a central intense area will shade out in all directions, giving rise to radial symmetry, as in ponds, lakes, etc. The essential connection between these is evident where a stream broadens into a lake, or the latter is the source of a stream, where a mountain ridge breaks up into isolated peaks, or where a peninsula or landspit is cut into islands. The line that connects the points of accumulated or abrupt change in the symmetry is a stress line or ecotone. Ecotones are well-marked between formations, particularly where the medium changes; they are less distinct within formations. It is obvious that an ecotone separates two different series of zones in the one case, and merely two distinct zones in the other.

=336. Physiographic symmetry.= The physical symmetry of a habitat depends upon the distribution of water in it, and this is profoundly affected by the soil and the physiography. The influence of precipitation is slight or lacking, as it is nearly uniform throughout the habitat; the effects of wind and humidity are more localized. Differences of soil rarely obtain within a single habitat, though often occurring in a zoned series. The strikingly zonal structure or arrangement of habitats is nearly always due to differences in water-content produced by physiographic factors, slope, exposure, surface, and altitude. The effect of these upon water-content and humidity is obvious. Wherever appreciable physiographic differences occur, there will be central areas of excess and deficiency in water-content, between which there is a symmetrical modification of this factor. Peaks are typical examples of areas of deficiency, lakes and oceans of areas of excess. When these areas are extreme and close to each other, the resulting zonation will be marked; when they are moderate, particularly if they are widely separated, the zones produced are obscure. Asymmetry of a habitat or a region practically does not exist. Central areas of excess and deficiency may be very large and in consequence fail to seem symmetrical, or the space between them so great that the symmetry is not conspicuous, but they are everywhere present, acting as foci for the intervening areas.

The response of vegetation to habitat is so intimate that physiographic symmetry everywhere produces vegetational symmetry, which finds its ready expression in plant zones. The reaction of vegetation upon habitat causes biological symmetry, typical of growth zones and light zones. From these facts it is clear that zonation will be regularly characteristic of the vegetative covering. The zonal arrangement of formations is usually very evident; the zones of a formation are often obscured, or, where the latter occupies a uniform central area of excess or deficiency, they are rudimentary or lacking, as in shallow ponds. Zones are frequently imperfect, though rarely entirely absent in new soils, such as talus. They are rendered obscure in several ways. In the initial stages of a succession, as well as in the transitions between the various stages, the plant population is so scattered, so transient, or so dense as to respond not at all to a degree of symmetry which produces marked zonation in later formations. The alternation of conspicuous species not only causes great interruption of zones, but often also completely conceals the zonation of other species, such as the grasses, which, though of more importance in the formation, have a lower habit of growth. Furthermore, the ecotones of one factor may run at right angles to those of another, and the resulting series of zones mutually obscure each other. Finally, such a physiographic feature as a hill may have its symmetry interrupted by ridges or ravines, which deflect the zones downward or upward, or cause them to disappear altogether, while the shallows or depths of a pond or lake may have the same effect. An entire absence of zones, i. e., azonation, is exceptional in vegetation. Almost all cases that seem to exhibit it may be shown by careful examination to arise in one of the several ways indicated above.

KINDS OF ZONATION

=337.= Two kinds of zonation are distinguished with reference to the direction in which the controlling factor changes. When this is horizontal, as with water-content and temperature, zonation will be lateral; when it is vertical, as in the case of light, the zonation is vertical. There exists an intimate connection between the two in forests, where the secondary layer of small trees and shrubs is continuous with a belt of trees and shrubs around the central nucleus, and the lower layers of bushes and herbaceous plants with similar zones still further out. This connection doubtless arises from the fact that conditions are unfavorable to the facies, outside of the nucleus as well as beneath it. Floristically, each layer and its corresponding zone are distinct, as the one consists of shade, the other of sun species. Lateral zonation is radial when the habitat or physiographic feature is more or less circular in form, and it is bilateral, when the latter is elongated or linear. Vertical zonation is unilateral.

=338. Radial zonation= is regularly characteristic of elevations and depressions. From the form of the earth, it reaches its larger expression in the girdles of vegetation corresponding to the zones of temperature. The zones of mountain peaks are likewise due largely to temperature, though humidity is a very important factor also. Mountain zones are normally quite perfect. The zonation of islands, hills, etc., is due to water-content. In the former, the zones are usually quite regular and complete; in the latter, they are often incomplete or obscured. Prairies and steppes are not zoned as units, but are complexes of more or less zonal hills and ridges. Ponds, lakes, and seas regularly exhibit complete zones, except in those shallow ponds where the depth is so slight that what is ordinarily a marginal zone is able to extend over the entire bottom. The line between an elevation and a depression, i. e., the edge of the water level, is the most sharply defined of all ecotones. It separates two series of zones, each of which constitutes a formation. One of these is regularly hydrophytic, the other is usually mesophytic. The line between the two can rarely be drawn at the water’s edge, as this is not a constant, owing to waves, tides, or periodical rise and fall. There is in consequence a more or less variable transition zone of amphibious plants, which are, however, to be referred to the hydrophytic formation. Nearly all forest formations serve as a center about which are arranged several somewhat complete zones. As a rule, these merge into a single heterogeneous zone of thickets.

=339. Bilateral zonation= differs from radial only in as much as it deals with linear elevations and depressions instead of circular ones. With this difference, the zones of ranges and ridges correspond exactly to those of peaks and hills, while the same relation is evident between the zones of streams, and of lakes and ponds. The ecotones are identical except as to form; they are linear in the one and circular in the other. Incompleteness is more frequently found in bilateral zonation, though this is a question of distance or extent, rather than one of symmetry.

=340. Vertical zonation= is peculiar in that there is no primary ecotone present, on either side of which zones arrange themselves with reference to the factor concerned. This arises from the fact that the controlling factor is light, which impinges upon the habitat in such manner as to shade out in but one direction, i. e., downward. Vertical zones appear in bodies of water, on account of the absorption of light by the water. In a general way, it is possible to distinguish bottom, plancton, and surface zones, consisting almost wholly of algae. There is little question that minor zones exist, especially in lakes and seas, but these await further investigation. The most characteristic vertical zones occur in forests, where the primary layer of trees acts as a screen. The density of this screen determines the number of zones found beneath it. In extreme cases the foliage is so dense that the light beneath is insufficient even for mosses and lichens. As a rule, however, there will be one or more zones present. In an ordinary deciduous forest, the layers below the facies are five or six in number: (1) a secondary layer of small trees and shrubs, (2) a tertiary layer of bushes, (3) an upper herbaceous layer of tall herbs, (4) a middle herbaceous layer, (5) a lower herbaceous layer, (6) a ground layer of mosses, lichens, other fungi, and algae. The upper layers are often discontinuous, the lower ones are more and more continuous. As a forest becomes denser, its layers disappear from the upper downward, the ground layer always being the last to disappear because of its ability to grow in very diffuse light. A vertically zoned formation shows a complex series of reactions. The primary layer determines the amount of heat, light, water, wind, etc., for the subordinate layers in general. Each of these layers then further determines the amount for those below it, the ground layer being subject in some degree to the control of every layer above it. This accounts probably for the definiteness and permanence of this layer. The degree to which the lower layers influence the upper by reacting upon the habitat is not known. It is evident that this influence must be considerable by virtue of their control of the water supply in the upper soil strata, by virtue of their transpiration, their decomposition, etc.

The ecotone between two formations is never a sharp line, but it is an area of varying width. The edge of this area which is contiguous to one formation marks the limit for species of the other. Both formations disappear in this transition zone, but in opposite directions. The overlapping which produces such zones arises from the fact that the physical factors tend to approach each other at the line of contact between formations, and that many species are more or less adjustable to conditions not too dissimilar.

=341. Vegetation zones.= As a fundamental expression of progressive change in the amount of heat and water, zonation is the most important feature of vegetation. It constitutes the sole basis for the division of continental as well as insular vegetation. The continent of North America furnishes striking proof of the truth of this. Conforming to the gradual decrease of temperature and water-content northward, three primary belts of vegetation stretch across the continent from east to west. These are forest, grassland, and polar desert. The first is further divided into the secondary zones of broad-leaved evergreen, deciduous, and needle-leaved forests. At right angles to this temperature-water symmetry lies a symmetry due to water alone, in accordance with which forest belts touch the oceans, but give way in the interior to grasslands, and these to deserts. It is at once evident that the mutual interruption of these two series of zones has produced the primary features of North America vegetation, i. e., tropical forests where heat and water are excessive, deserts where either is unusually deficient, grassland when one is low, the other moderate, and deciduous and coniferous forests, where the water-content is as least moderate and the temperature not too low. Such a simple yet fundamental division has been modified, however, by the disturbing effect which three continental mountain systems have had upon humidity and upon temperature symmetry. The two are intimately interwoven. The lowering of temperature due to altitude produces the precipitation of the wind-borne moisture upon those slopes which look toward the quarter from which the prevailing winds blow. A mountain range thus makes an abrupt change in the symmetry, and renders impossible the gradual change from forest to grassland and desert. The Appalachian system is not sufficiently high to produce a pronounced effect, and forests extend far beyond it into the interior before passing into prairies and plains. On the other hand, the influence of the Rocky mountains and the Sierra Nevada is very marked. The latter rise to a great height relatively near the coast, and condense upon their western slopes nearly all of the moisture brought from the Pacific. The Rocky mountains have the same effect upon the much drier winds that blow from the east, and the two systems in consequence enclose a parched desert. This series of major zones thus becomes, starting at the east, forest, grassland, desert, and forest, instead of the more symmetrical series, forest, grassland, desert, grassland, forest, which would prevail were it not for these barriers. This actual series of major zones undergoes further interruption by the action of these mountain systems in deflecting northern isotherms far to the south. This action is greatest in the high ranges, the Rocky mountains and the Sierras, and least in the lower Appalachians. Its result is to carry the polar deserts of the north far southward along the crests of the mountains, and to extend the boreal coniferous forests much further south along their slopes. In the Appalachians, this means no more than the extension of a long tongue of conifers into the mass of deciduous forests, and the occasional appearance of an isolated peak. In the western ranges, it produces two symmetrical series of minor mountain zones, forest, alpine grassland or desert, and forest, to say nothing of the foot-hill and timber-line zones of thicket.

There seems to be no good reason for distinguishing the zones of mountains as regions. The term itself is inapplicable, as it has no reference to zonation, and is used much more frequently as a term of general application. Its use tends to obscure also the essential identity of the so-called vertical zones of mountains with the major continental zones, an identity which can not be insisted upon too strongly. For the sake of clearness, it is important to distinguish all belts of vegetation as zones, though it is evident that these are not all of the same rank. The following division of the vegetation of North America is based upon the fundamental principles of continental symmetry and the community of continental and mountain zones.

I. Polar-niveal zone—zona polari-nivalis

II. Arctic-alpine zone—zona arctici-alpina

Arctic province—provincia arctica

Alpine province—provincia alpina

III. Boreal-subalpine zone—zona boreali-subalpina

Alaska province—provincia alaskana

Cordilleran province—provincia cordillerana

Ontario province—provincia ontariensis

IV. Temperate zone—zona temperata

Atlantic province—provincia atlantica

Appalachian province—provincia appalachiana

Nebraska province—provincia nebraskensis

Utah province—provincia utahensis

Coast province—provincia litoralis

Pacific province—provincia pacifica

V. Subtropical zone—zona subtropicalis

Florida province—provincia floridana

Mexican province—provincia mexicana

VI. Tropical zone—zona tropicalis

Antilles province—provincia antilleana

Andean province—provincia andeana

ALTERNATION

=342. Concept.= The term alternation is used to designate that phenomenon of vegetation, in which a formation recurs at different places in a region, or a species at separate points in a formation. Although it is a fundamental feature of vegetation, it has been recognized but recently.

Alternation is the response of vegetation to the heterogeneity of the surface of the earth. It is in sharp contrast to zonation, inasmuch as it is directly caused by asymmetry in the topography. In consequence, it deals with the subdivisions of zones, arising from physical differences within the symmetrical area. It deals with vegetation areas of every rank below that of major zone, with the habitat and geographical areas of species, and, in a certain way, with the correspondence of vicarious genera. The breaking up of vegetation into formations is a striking example of alternation. The same phenomenon occurs in every formation, producing consocies and minor plant groups, and everywhere giving variation to its surface and structure. The essential idea involved in this principle is the recurrence of like formations, consocies, or groups, which are more or less separated by formations, consocies, or groups differing from them. It is an exact expression of the primary law of association that heterogeneity of structure varies directly as the extent and complexity of the habitat, or the series of habitats. Vegetation is made up of what are superficially homogeneous formations, but upon analysis these are seen to contain consocies. The latter, though more uniform than formations, break up into groups, each of which still shows a characteristic heterogeneity arising from the varying number and arrangement of its constituent species.

=343. Causes.= The primary cause of alternation is physical asymmetry, which is everywhere present within the symmetrical areas which produce zones. This is influenced so strongly, however, by migration and plant competition (phyteris) that the consideration of this subject will gain in clearness if these are treated as separate causes. The essential relation between them must not be lost sight of, however. Migration carries disseminules into all, or only some of the different areas of a formation, or into different formations, with little respect to the physical nature of these. The physical character of these asymmetrical areas determines that some of these plants shall be established in one series of places, and some in another, while the competition between the individuals in the various areas determines the numerical value of each species as well as its persistence. These three causes are invariably present in the production of alternating areas, and originally, i. e., in new or denuded soils, the sequence is constant, viz., migration, ecesis in asymmetrical areas, and competition.

With respect to the different portions of an asymmetrical area, migration will have one of three effects: (1) it will carry disseminules into both favorable and unfavorable areas, (2) into favorable ones only, or (3) into unfavorable ones alone. From the radial nature of migration, the first case is far the most frequent; it is typical of sporostrotes, and the highly specialized spermatostrotes and carpostrotes. The effect of migration is uniform here, and alternation arises in consequence of the selective power of ecesis. It is evident that migration does not have an even indirect effect, when the disseminules are carried into none but unfavorable situations. Where the movement is into favorable places alone, alternation is the immediate result. The intermittent operation of migration and the presence of barriers are responsible for the absence of plants in situations favorable to them, and in consequence bring about a certain alternation between corresponding species.

The selective operation of physical factors upon the disseminules carried into the different parts of an asymmetrical area is the usual cause of alternation. Asymmetry alone is universal within the more conspicuous structures termed zones, down to the smallest areas which a group of plants can occupy. The difference between contiguous areas, particularly within the same habitat, is often small. It sometimes seems inefficient in the initial stages of a succession when a single species is present, but even in extreme cases its effect will be recognizable in the size and density of the individuals. Asymmetry is clearly evident in vegetation where two symmetrical series cross each other, or when a symmetry is interrupted by barrier-like elevations or depressions. Within formations, it arises from differences, often very slight, in slope, exposure, elevation, from irregularities of surface, differences in soil structure, or composition, in the amount of cover, and in the reactions of the living plants. At the last point, it is in direct connection with plant competition.

=344. Competition.= Much uncertainty, as well as diversity of opinion, seems still to exist in regard to the precise nature of the competition between plants that occupy the same area. It has long been admitted that the phrase, “struggle for existence,” is true of this relation only in the most figurative sense, but the feeling still prevails that, since plants live in associations, there must be something mysterious and vitalistic in their relation. No one has been able to discover anything of this nature, but nevertheless the impression remains. Such a direct relation exists only between parasites, epiphytes, and lianes, and the plants which serve to nourish or support them. In the case of plants growing on the same stratum, actual competition between plant and plant does not occur. One individual can affect another only in as much as it changes the physical factors that influence the latter. Competition is a question of the reaction of a plant upon the physical factors which encompass it, and of the effect of these modified factors upon the adjacent plants. In the exact sense, two plants do not compete with each other as long as the water-content and nutrition, the heat and light are in excess of the needs of both. The moment, however, that the roots of one enter the area from which the other draws its water supply, or the foliage of one begins to overshade the leaves of the other, the reaction of the former modifies unfavorably the factors controlling the latter, and competition is at once initiated. The same relation exists throughout the process; the stronger, taller, the more branched, or the better rooted plant reacts upon the habitat, and the latter immediately exerts an unfavorable effect upon the weaker, shorter, less branched, or more poorly rooted plant. This action of plant upon habitat and of habitat upon plant is cumulative, however. An increase in the leaf surface of a plant not merely reduces the amount of light and heat available for the plant near it or beneath it, but it also renders necessary the absorption of more water and other nutritive material, and correspondingly decreases the amount available. The inevitable result is that the successful individual prospers more and more, while the less successful one loses ground in the same degree. As a consequence, the latter disappears entirely, or it is handicapped to such an extent that it fails to produce seeds, or these are reduced in number or vitality.

Competition in vegetation furnishes few instances as simple as the above, but this will serve to make clear the simplest case of ordinary competition, i. e., that in which the individuals belong to a single species. The various individuals of one species which grow together in a patch show relatively slight differences, in height, width, leaf expanse, or root surface. Still, some will have the largest surfaces for the impact of water, heat, and light, while others will have the smallest; the majority, perhaps, will occupy different places between the extremes. The former will receive more than their share of one or more factors. The reaction thus produced will operate upon the plants subject to it inversely as the amount of surface impinged upon. The usual expression of such competition is seen in the great variation in height, branching, etc., of the different individuals, and in the inability of many to produce flowers. This is particularly true of annuals, and of perennials of the same generation. In the competition between parents and offspring of the same perennial species, the former usually have so much the advantage that the younger plants are often unable to thrive or even germinate, and disappear, leaving a free space beneath and about the stronger parents. This illustrates the primary law of competition, viz., that this is closest when the individuals are most similar. Similar individuals make nearly the same demands upon the habitat, and adjust themselves least readily to their mutual reactions. The more unlike plants are, the greater the difference in their needs, and some are able to adjust themselves to the reactions of others with little or no disadvantage.

In accordance with the above principles, the competition is closer between species of like form than between those that are dissimilar. This similarity must be one of vegetation or habitat form, not one of systematic position. The latter is in fact of no significance, except where there is a certain correspondence between the two. Leaf, stem, and root characters determine the outcome, and those species most alike in these features will be in close competition, regardless of their taxonomic similarity or dissimilarity. This is as conclusive of the competition between the species of the same genus as it is between those belonging to genera of widely separated families. From this may be deduced a second principle of competition, viz., the closeness of the competition between the individuals of different species varies directly with their similarity in vegetation or habitat form. This principle is of primary importance in the competition which arises between occupants and invaders in the different stages of succession. Those invading species that show the greatest resemblance to occupants in leaf, stem, and root form experience the greatest difficulty in establishing themselves. The species, on the contrary, which are so unlike the occupants that they come in at a clear advantage or disadvantage, establish themselves readily, in the one case as a result of the reaction, in the other by taking a subordinate position. This principle lies at the base of the changes in succession which give a peculiar stamp to each stage. A reaction sufficient to bring about the disappearance of one stage can be produced only by the entrance of invaders so different in form as to materially or entirely change the impress of the formation. Stabilization results when the entrance of invaders of such form as to exert an efficient reaction is no longer possible. In forests, while many vegetation forms can still enter, none of these produce a reaction sufficient to place the trees at a disadvantage, and the ultimate forest stage, though it may change in composition, can not be displaced by another.

It is obvious that the vegetation forms and habitat forms of associated species are of fundamental importance in determining the course and result of competition. Identity of vegetation form regularly produces close competition, and the consequent numerical reduction or disappearance of one or more species. Dissimilarity, on the other hand, tends to eliminate competition, and to preserve the advantage of the superior form. Species of trees compete sharply with each other when found together; the same is true of shrubs, or rosettes, etc. The relation of the shrubs to the trees, or of the rosettes to the shrubs of a formation is one of subordination rather than of competition. The matter of height and width often enters here also to such a degree that the tallest herbs compete with the bushes and shrubs, and rosettes with mats or grasses. The amount and disposition of the leaf surface are decisive factors in the competition between species of the same vegetation form, in so far as this is governed by light. In those plants in which the leaves are usually erect, notably the grasses and sedges, the competition between the aerial parts is relatively slight, and the result is determined by the reactions of the underground stems and roots.

The position of the competing individuals is of the greatest importance. The distance between the plants affects directly the degree of competition, while their arrangement, whether in groups according to species or singly, exerts a marked influence by determining that the contest shall be between like forms, or unlike forms. Position is controlled primarily by the relation existing between seed-production and dissemination. It is of course influenced in large measure by the initial position taken by the invaders into a nudate area, but this is itself a result of the same phenomena. The individuals of species with great seed-production and little or no mobility usually occur in dense stands. In these, the competition is fierce, for the two reasons of similarity and density, and the result is that the plants fall far below the normal in height and width. This is an extreme example of the group arrangement. When the seed-production is small, the mobility may be great or little without seriously affecting the result. The individuals of a species of this kind will be scattered among those of other species, and the closeness of competition will depend largely upon the similarity existing between the two. The arrangement in such cases is sparse. A species with great seed-production and great mobility usually shows both kinds of arrangement, the position of the individuals and the competition between them varying accordingly. This is due to the intermittent action of distributing agents, making it possible for the seeds to fall directly to the ground during the times that winds, etc., are absent. The three types of arrangement indicated above are termed gregarious, copious, and gregario-copious. They furnish the basis for the investigation of abundance which deals essentially with the number and arrangement of the individuals of competing species. The effect of distance, i. e., the interval between individuals, upon competition is fundamental. The competition increases as the interval diminishes, and the reverse.

The view here advanced, i. e., that competition is purely physical in nature, renders untenable the current conceptions of vegetation pressure, occupation, etc. Masses of vegetation are thought to force the weaker species toward the edge, thus initiating an outward or forward pressure. As has been shown above, no such phenomenon occurs in vegetation. This movement is nothing but simple migration, followed by ecesis, and has no connection with “weaker” species, or the development of a vital pressure. The direction taken by the migrating disseminules is essentially indeterminate. Migration seems to be outward, or away from the mass, merely because the ecesis is greater at the edge, where the increased dissimilarity between plant forms diminishes the competition. The actual movement is outward, but it takes place through the normal operation of competition. In this connection, it should be pointed out that the common view that plants require room is inexact, if not erroneous. This is difficult of proof, as it is impossible to distinguish room as such from the factors normally present, light, heat, water, and nutrient salts, but it seems obvious that the available amounts of these will determine the space occupied by a plant, irrespective of the room adjacent plants may allow it. The explanation of competition upon physical grounds likewise invalidates the view that plants possess spheres of influence other than the areas within which they exert a demonstrable reaction upon the physical factors present.

Competition plays a very important role in alternation. It produces minor examples of alternation in the physical units of an asymmetrical series. Its greatest influence, however, is exerted in modifying the effects of asymmetry. The reaction of occupants emphasizes or reduces the effect of asymmetry, and has a corresponding action upon alternation. This result of competition is typical of succession, in which the sequence of stages arises from the interaction of occupant and invader.

=345. Kinds of alternation.= Alternation involves two ideas, viz., the alternation of different species or formations with each other, and the alternation of the same species or formation in similar but separate situations. This is the evident result of asymmetry, in response to which contiguous areas are dissimilar and remote ones often similar. Individuals of the same species or examples of the same formation may be said to alternate between two or more similar situations, while different species or formations are said to alternate with each other, occurring usually in situations different in character. From the nature of alternation, the two phenomena are invariably found together.

It is possible to distinguish three kinds of alternation: (1) of a formation, consocies, layer, facies, or species in similar situations; (2) of similar or corresponding formations, species, etc., in similar situations; (3) of facies and other species with respect to number. The last two are merely variations of the first, arising out of slight differences in the physical factors of the alternating areas, the adjacent flora, or the course of competition. The alternation of different examples of the same formation is a significant feature of greatly diversified areas, such as mountains. It is naturally much less characteristic of lands physiographically more uniform. A xerophytic formation will alternate from ridge to ridge, a mesophytic formation between the intermediate valleys; aquatic vegetation will alternate from pond to pond, or stream to stream. The appearance of new or denuded soils upon which successions establish themselves is the most important cause of the alternation of formations. The weathering of rocks in different areas of the same region produces in each a sequence of similar or identical formations. The same statement is true in general of other causes of succession, such as erosion, flooding, burning, cultivation, etc., wherever they operate upon areas physically similar and surrounded by the same type of vegetation. The areas of more or less heterogeneous formations characterized by major physical differences are occupied by consocies. In an extensive formation, the same consocies alternates from one to another of these areas that are similar. When the formation is interrupted and occurs here and there in separate examples, a consocies often alternates from one to another of these. A consocies regularly derives its character from the fact that one or more of the facies of the formation is more intimately connected with certain areas of the latter than with others. This explains why the alternations of consocies and facies are usually identical. Layers sometimes alternate between different examples of the same forest or thicket formation, when they are suppressed in some by the diffuseness of the light.

The alternation of species is a typical feature of formations; it is absent only in those rare cases where the latter consist of a single species. The areas of a habitat which show minor physical or historical (i. e., competitive) differences are occupied by groups of individuals belonging to one or more species responsive to these differences. Each of these groups will recur in all areas essentially similar, the intervals being occupied of course by slightly different groups. Such groups are constituted by gregarious or copious species of restricted adjustability. Sparse plants likewise alternate, but they necessarily play a much less conspicuous part. In habitats not too heterogeneous, a large number of species are sufficiently adjustable to the slight differences so that they occur throughout the formation. Often, to be sure, they show a characteristic response, expressed in the size or number. This is illustrated by the facies and many of the principal species of the prairie formation. Festuca, Koelera, Panicum, and Andropogon occur throughout, except in the moist ravines which are practically meadows. Astragalus, Psoralea, Erigeron, and Aster grow everywhere on slopes and crests, but they are much more abundant in certain situations. Other plants, Lomatium, Meriolix, Anemone, Pentstemon, etc., recur in similar or identical situations upon different hills. Lomatium alternates between sandy or sandstone crests, Meriolix and Pentstemon occur together upon dry upper slopes, while Anemone alternates between dry slopes and crests.

Owing to the accidents of migration and competition, similar areas within a habitat are not occupied by the same species, or group of species. A species found in one area will be replaced in another by a different one of the same or a different genus. The controlling factors of the area render imperative an essential identity of vegetation and habitat form, though in systematic position the plants may be very diverse. Such genera and species may be termed corresponding. The relation between such plants is essentially alternation; it should, perhaps, be distinguished from alternation proper as corresponsive. The prairie formation furnishes a good example of this on exposed sandy crests, upon which Lomatium, Comandra, and Pentstemon alternate. Formations exhibit a similar correspondence.

All species that alternate show a variation in abundance from one area to another. Frequently, the difference is slight, and may be ignored, except in determining abundance. Very often, however, the variation is so great that a facies may be reduced, numerically, to the rank of a principal species, or one of the latter to a secondary species. This phenomenon is distinguished as numerical alternation. It arises from the fact that the similar areas are sufficiently different to affect the abundance, without producing complete suppression. It is probable that this result is due almost entirely to competition. Astragalus crassicarpus grows on all the slopes of the prairie formation, but on some it has the abundance of a facies, while on others it is represented by a few scattered individuals. This difference is much more striking in separate examples of the same formation, particularly when a normal facies is reduced to the numerical value of a secondary species. This is a matter of great importance in the study of formations, for it has doubtless often resulted in mistaking a consocies for a formation.

Alternation furnishes the logical basis for what may be called comparative phytogeography. The latter is of much broader scope than the old subject of geographical distribution, for it treats not only of the distribution of formations and associations as well as of species, but it also seeks to explain this by means of principles drawn from the relation between habitat and vegetation. When the latter come to be fully based upon physical factor investigations, and upon the effects of migration and competition as shown in alternation, the comparative study of formations will represent the highest type of phytogeographical activity.

THE FORMATION IN DETAIL

=346. The rank of the formation.= There have been as many different opinions in regard to the application of the term formation as there are concerning the group which is to be called a species. In taxonomy, however, the concept of the species is purely arbitrary, and agreement can not be hoped for. In vegetation, on the contrary, the connection between formation and habitat is so close that any application of the term to a division greater or smaller than the habitat is both illogical and unfortunate. As effect and cause, it is inevitable that the unit of the vegetative covering, the formation, should correspond to the unit of the earth’s surface, the habitat. This places the formation upon a basis which can be accurately determined. It is imperative, however, to have a clear understanding of what constitutes the difference between habitats. A society is in entire correspondence with the physical factors of its area, and the same is true of the vegetation of a province. Nevertheless, many societies usually occur in a single habitat, and a province contains many habitats. The final test of a habitat is an efficient difference in one or more of the direct factors, water-content, humidity, and light, by virtue of which the plant covering differs in structure and in species from the areas contiguous to it. A balsam-spruce forest shows within itself certain differences of physical factors and of structure. The water-content will range from 20–25 per cent, and the light from .02–.003. One portion may consist chiefly of Pseudotsuga mucronata, another of Picea engelmannii, and a third of Picea parryana, or these species may be intermingled. If, however, this forest is compared with the gravel slide, which touches it on one side, and the meadow thicket, which meets it on another, the physical factors and the species both demonstrate that it is the forest, and not its parts, which corresponds to a distinct physical entity, the habitat. This test of a formation is superfluous in a great many cases, where the physiognomy of the contiguous areas is conclusive evidence of their difference. It is evident also that remote regions which are floristically distinct, such as the prairies and the steppes, may possess areas physically almost identical and yet be covered by different formations. This point is further discussed under classification.

The existing confusion in the matter of formations is due to two causes. The first arises from the fact that much ecological work has been hasty. Little or no attention has been given to development, and in consequence rudimentary and transitory stages of succession have often been described as formations. Mixed areas in particular have caused trouble. In the second place, there has been a marked tendency to minimize the need of thoroughness and training by calling every slightly different area a formation. A failure to recognize the primary value of alternation has also contributed materially to this. Alternating facies, and principal species, when separated from each other, have often been mistaken for formations. This is a danger that must be fully appreciated and guarded against. In practically all regions, the same formation is represented by numerous scattered areas, all showing greater or less differences arising from alternation. This is especially true of thickly populated regions where virgin areas are rare. The fact that twenty-five miles intervene to-day between two small stretches of primitive prairie is permitted to unduly emphasize their differences. It requires the study of a number of such examples to counteract this tendency, and to cause one to see clearly that they must have been at one time merely so many bits of the prairie formation.

In this connection, the lichen and moss groups which are found on rocks constitute an interesting problem. It is clear that Peltigera and Cladonia, which grow on the forest floor, and Evernia, Ramalina, and Physcia, which are found on the trees, are merely constituent species of the forest formation. The same is true of Cladonia, Urceolaria, and Parmelia, which are found among the sedges and grasses of alpine meadows. The physical conditions are essentially those of the formation, and the lichens themselves are more or less peculiar to it. This is particularly true of the forest, in which the two strata, bark and moist shaded soil, are present because of the trees. In the case of granitic rocks, the circumstances are very different. The species of lichens found on the rocks are not peculiar to the formation, but they also occur elsewhere. In the forest, Parmelia, Placodium, Physcia, Rinodina, Urceolaria, Lecanora, Lecidea, etc., occur on the rocks. In the alpine meadows, the rock groups are composed of Parmelia, Gyrophora, Cetraria, Acarospora, Lecanora, Lecidea, Buellia, etc. The stratum itself is physically very different and constitutes a distinct habitat. These groups are really small formations, which are quite distinct from the surrounding forest or meadow. This is proven conclusively in many places in the mountains where areas of the characteristic lichen formations of cliffs are carried by the fall of rock fragments into forest and meadow, where they persist without modification. This also shows clearly that the groups on scattered rocks in the same area are to be regarded as examples of the same cliff formation, except where the differences are evidently to be ascribed to development and not to alternation. Where these rock formations can not be traced to cliffs or magmata with certainty, they must be considered as antedating the vegetation in which they occur. Often, indeed, especially in igneous areas, they are relicts of the initial stage of a primary succession. Finally, they prove their independence of the forest or meadow formation by initiating a distinct succession within these. Crustaceous groups or formations yield to foliose ones, and these in turn give way to formations of mosses, particularly in the forest where the effect of the diffuse light is felt. From the above, the following rule of formational limitation is obtained: any area, which shows an essential difference in physical character, composition, or development from the surrounding formation is a distinct formation.

=347. The parts of a formation.= All the parts which make up the structure of a formation are directly referable to zonation and alternation, alone or together, or to the interaction of the two. The principles which underlie this have already been discussed under the phenomena concerned. It is necessary to point out further that the structure may be produced in several ways: (1) by zonation alone, (2) by alternation alone, (3) by zonation as primary and alternation as secondary, (4) by primary alternation and secondary zonation, (5) by the interaction of the two, as in layered formations. Though all these methods occur, the first two are relatively rare, and the resulting structure comparatively imperfect. The typical structure of formations can best be made clear by the consideration of a prairie which belongs to the fourth group, and a forest which represents the last.

The major divisions of prairie and forest formations are regularly due to alternation. There is an inherent tendency to the segregation of facies, arising out of physical or historical reasons, or from a combination of both. Not all formations show this, but it is characteristic of the great majority of them. The primary areas which thus arise have been called associations: they are naturally subordinate to the formation. To avoid the confusion which inevitably results from using the word association in two different senses, it is proposed to term this primary division of the formation, a consociation, or better, a consocies. This term is applied only to an area characterized by a facies, or less frequently, by two or more facies uniformly commingled. The consocies of grassland are determined by grasses, those of forests by trees, etc. From the different position of the facies in these two types of vegetation such areas are readily seen at all times in the forest, but they are often concealed in grassland by the tall-growing principal species of the various aspects. When definite consocies are present, they are often found to mingle where they touch, producing miniature transition areas, and, very rarely, they sometimes leave gaps in which no facies appears.

The seasonal changes of a formation, which are called aspects, are indicated by changes in composition or structure, which ordinarily correspond to the three seasons, spring, summer, and autumn. The latter affect the facies relatively little, especially those of woody vegetation, but they influence the principal species profoundly, causing a grouping typical of each aspect. For these areas controlled by principal species, but changing from aspect to aspect, the term society is proposed. They are prominent features of the majority of herbaceous formations, where they are often more striking than the facies. In forests, they occur in the shrubby and herbaceous layers, and are consequently much less conspicuous than the facies. A close inspection of the societies formed by principal species shows that they are far from uniform. Since they usually fail to exhibit distinct parts, it becomes necessary to approach the question of their structure from a new standpoint. Such is afforded by aggregation, which yields the simplest group in vegetation, i. e., that of parent and offspring. This is so exactly a family in the ordinary sense that there seems to be ample warrant for violating a canon of terminology by using the word for this group, in spite of its very different application in taxonomy. It has already been shown that aggregation further produces a grouping of families, which may properly be called a community. As they are used here, family and community become equally applicable to the association of plants, animals, or man. Both families and communities occur regularly in each society of the formation, and they represent its two structures. In some cases, all the families are grouped in communities, two or more of which then form the society. Very frequently, however, families occur singly, without reference to a community, and the two then constitute independent parts of the same area. This is typically the case wherever gregarious species are present, since these are merely family groups produced by aggregation.

Objection may be made that this analysis of formational structure has been carried too far, and that some of the structures recognized are mere interpretations, and not actual facts. Such a criticism will not come from one who has got beyond the superficial study of formations, for he will at once recognize that certain probable features of structure have not been considered. On the other hand, the ecologist or the botanist who has not made a careful investigation from the standpoints of development and structure will naturally refrain from expressing an opinion, until he has obtained an acquaintance at first hand with the facts. Over-refinement is the usual penalty of intensive work. The unbiased investigator, however, will not be misled by the suddenness with which new concepts appear. It seems plausible that the structure of a formation, if not as definite, is at least nearly as complex as that of an individual plant. Few botanists will insist that the refinement of tissues and tissue systems has been carried further than the differentiation of the plant warrants. Yet, if these had been defined within a period of a few years rather than slowly recognized during more than a century, they would have been called seriously in question. As a matter of fact, the consocies, under the term association, and the society, under various names, have been recognized by ecologists for several years. They are definite phenomena of alternation which can be found anywhere. The family and the community, though the latter is less distinct in outline, are equally valid structures, the proof of which anyone can obtain by thorough methods of study.

=348. Nomenclature of the divisions.= The suffix -etum is used to designate a consocies of a formation, e. g., Picetum, Caricetum, etc. When two or more species characterize the area, the most important, or more rarely, the two are used. The termination used to designate a society is -ile, as Asterile, Sedile, Rosile. The suffix which denotes the community is -are, and for the family, it is -on, viz., Giliare, Bromare, Bidenton, Helianthon, etc. Layers are indicated by the affix -anum, as Opulasteranum, Verbesina-Rudbeckianum, etc. It is evident that these suffixes, like the terms to which they refer, must be used always for the proper divisions if they are to have any value at all. There has been a marked tendency, for example, to use -etum in connection with the names of groups of very different rank. It is hardly necessary to point out that such a practice does not promote clearness. The following tabular statement will illustrate the application of both terms and suffixes:

Picea-Pseudotsuga-hylium formation (-ium) Paronychia-Silene-chalicium Picetum consocies (-etum) Paronychietum Opulaster-Ribesanum layer (-anum) Opulasterile society (-ile) Androsacile Thalictrare community (-are) Festucare Pirolon family (-on) Arenarion

=349. The investigation of a particular formation.= A comprehensive and thorough study of a formation should be based upon as many examples of it as are accessible. The example which is at once the most typical and the most accessible is made the base area. This plan saves time and energy, reduces the number of instruments that are absolutely necessary, and establishes a common basis for comparison. The inquiry should be made along four lines, all fundamental to a proper knowledge of the formation. These lines are: (1) the determination of the factors of the habitat, (2) a quadrat and a transect study of the structure of the formation, (3) a similar investigation of development, (4) a floristic study of the contiguous formation, with special reference to migration. The sequence indicated has proven to be the most satisfactory, and is to be regarded as all but absolutely essential. Naturally, this applies only to the order in which the various lines are to be taken up, as they are carried on together when the work is fully under way. Since instrument and quadrat methods have already been given in detail, it is unnecessary that they be repeated. Similarly, the questions which pertain to structure and development and to the surrounding vegetation are considered in detail in the pages which precede.

CLASSIFICATION AND RELATIONSHIP

=350. Bases.= Formations may be grouped with reference to habitat or kind, development or position. Classification upon the basis of habitat places together formations which are similar in physiognomy and structure. Developmental classification is based upon the fact that the stages of a particular succession are organically connected or related, though they are normally different in both physiognomy and structure. Grouping with respect to position is made solely upon occurrence in the same division of vegetation. The formations thus brought together usually possess neither similarity of kind or structure, nor do they have any necessary developmental connection. Habitat and developmental classification are of fundamental value; regional arrangement is more superficial in character. All serve, however, to emphasize different relations, and, while the developmental system expresses the most, they should all be used to exhibit the vegetation of a region, province, or zone.

=351. Habitat classification.= In arranging formations with reference to habitats, the direct factors, water and light, can alone be used to advantage. Such a system is fundamental, because it is founded upon similarity of habitat and of structure. Proposed groupings based upon nutrition-content, or upon the division of factors into climatic and edaphic, have elsewhere been shown to be altogether of secondary importance, if not actually erroneous. The basis of the habitat grouping is water-content, which is supplemented by light whenever the factor is decisive. The primary divisions thus obtained are water, forest, grassland, and desert, which are characterized respectively by associations of hydrophytes, mesophytes, hylophytes, poophytes, and xerophytes respectively. Within these, formations are arranged according to the type of habitat, i. e., pond, meadow, forest, dune, etc. These divisions comprise all formations which belong to the type by virtue of their physiognomy and structure. Such formations differ from each other very considerably or completely in the matter of floristic, i. e., component species, but they still belong to the same type. A dune formation in the interior and one on the coast may not have a single species in common, and yet they are essentially alike in habitat, development, and structure.

=352. Nomenclature.= The names of formations are taken from the habitats which they occupy. Each formation should have a vernacular and a scientific name. The latter is especially important since it ensures brevity and uniformity, and obviates the obscurity and confusion that arise from vernacular terms in many tongues. Scientific names have been made uniformly from Greek words of proper meaning by the addition of the suffix -ium (εῖον), which denotes place. The following list gives the English and the scientific name of the various habitats, and their corresponding formations, and indicates the primary divisions into which these fall.

I. Hydrophytia: water plant formations

1. ocean: oceanium: oceanad, oceanophilous, etc.

2. sea: thalassium

surface of the sea: pelagium

deep sea: pontium

3. lake: limnium, limnad

4. pond, pool, tiphium, tiphad

5. stagnant water: stasium: stasad

6. salt marsh: limnodium, limnodad

7. fresh marsh: helium

8. wet meadow: telmatium

9. river: potamium

10. creek: rhoium

11. brook: namatium

12. torrent: rhyacium

13. spring: crenium

14. warm spring: thermium

15. ditch: taphrium

16. sewer: laurium

17. swamp forest: helohylium

18. swamp open woodland: helodium

19. meadow thicket: helodrium

20. bank: ochthium

rock bank: petrochthium

sand bank: ammochthium

mud bank: pelochthium

21. rocky seashore: actium

22. sandy seashore: agium

23. sandbar: cheradium

24. tank: phretium

II. Mesophytia: middle plant formations

a. Sciophytia: shade plant formations

26. forest: hylium

27. grove: alsium

28. orchard: dendrium

29. canyon: ancium

30. open woodland: orgadium

31. thicket: lochmium

b. Heliophytia: sun plant formations

32. meadow: poium

33. pasture: nomium

34. culture land: agrium

35. waste place: chledium

III. Xerophytia: dry plant formations

36. desert: eremium

37. sand-hills, sandy plain: amathium

38. prairie, plains: psilium

39. dry, open woodland: hylodium

40. dry thicket: driodium

41. dry forest: xerohylium

42. gravel slide: chalicium

43. sandbar: syrtidium

44. sand draw: enaulium

45. blowout: anemium

46. strand: psamathium

47. dune: thinium

48. badlands: tirium

49. hill, ridge: lophium

50. cliff: cremnium

51. rock field: phellium

52. boulder field: petrodium

53. rock, stone: petrium

54. humus marsh: oxodium

55. alkali area: drimium

56. heath, dry meadow: xeropoium

57. moor: sterrhium

58. alpine meadow: coryphium

59. polar barrens: crymium

60. snow: chionium

61. wastes: chersium

Particular formations are indicated by means of floristic distinctions. Thus, Populus-hylium is the aspen forest as distinguished from the Picea-Pseudotsuga-hylium, or the balsam-spruce forest; and the Bulbilis-psilium, or buffalo-grass prairie, from the Bouteloua-Andropogon-psilium, or grama-bluestem prairie. Similarly, the aspen formation of the Old World and of the New may be distinguished as Populus-tremula-hylium and Populus-tremuloides-hylium, respectively. In all formational names, the facies alone should be used. Frequently, a single facies will suffice for clearness. As a rule, however, the two most important facies should be employed; in rare cases only is it necessary to use the names of three. When it is desirable to refer to two or more examples of the same formation, a geographical term is added, e. g., (1) Populus-hylium (Crystal Park), (2) Populus-hylium (Cabin Canyon).

=353. Developmental classification.= This is based upon succession as the record of development. Upon the basis of development, all the formations which belong to the same succession are classed together. They are arranged within each group in the sequence found in the particular succession. From its nature, developmental classification is of primary importance in exhibiting the history of vegetational changes. It has less value than the habitat system for summarizing the essential structure of a vegetation, inasmuch as it places the emphasis upon historical rather than structural features. It is evident that both deal with the same formations, and that the difference is merely one of viewpoint. The habitat classification is simpler in that it considers only those formations actually on the ground, while development has regularly to take into account stages which have disappeared. The groups of the developmental system, and the arrangement of formations within them have already been indicated under the nomenclature of succession (sections 326 and 327).

=354. Regional classification.= The grouping of formations with respect to the divisions of vegetations is chiefly of geographical value. It indicates a certain general relationship, but its principal use is to summarize the structure of the vegetative covering of a region. The arrangement of formations in the various divisions is made with reference to the outline of North American vegetation (section 341). This is naturally based upon the identity of altitude and latitude zones. In the study of mountain countries, it is often desirable to group formations with reference to altitude alone. In this case, the grouping is based upon the following divisions: (1) bathyphytia, lowland plant formations; (2) mesiophytia, midland formations; (3) pediophytia, upland formations; (4) pagophytia, foot-hill formations; (5) orophytia, subalpine formations; (6) acrophytia, alpine formations; (7) chionophytia, niveal formations.

=355. Mixed formations.= These are mixtures of two, rarely more, adjacent formations, or of two consecutive stages of the same succession. Mixed formations are really transitions in space or in time between two distinct formations. Theoretically, they are to be referred to one or the other, according to the preponderance of species. Actually, however, they often persist in an intermediate condition for many years, and it becomes necessary to devote considerable attention to them. In some cases, there is good reason to think that the species of two contiguous formations have become permanently associated, and thus constitute a new formation. This is often apparently true in succession, when the change from one stage to the next requires a long term of years, but it is really true only of the very rare cases in which a succession becomes stabilized in a transition stage. When the mixture is due to development, the formations concerned are often quite dissimilar, e. g., grassland and thicket, thicket and forest. If it is the result of position, the formations are usually similar, i. e., both are grassland, thicket, or forest, since the plants of the lower level are regularly assimilated or destroyed, when invasion occurs at two levels. The term mictium (μικτόν, mixture) is here proposed for the designation of all mixed formations, whether they arise from succession or from juxtaposition. Thus, the Mentzelia-Elymus-mictium is the transition between the Mentzelia-Pseudocymopterus-chalicium and the Elymus-Muhlenbergia-chalicium. Similarly, the Populus-Picea-mictium and the Pinus-Pseudotsuga-mictium are transition stages in the development of the Picea-hylium. On the other hand, the Andropogon-Bulbilis-mictium is a mixture produced by the mingling of two contiguous prairie formations. In the future development of this subject, it will probably become desirable to name mixed formations on the basis of origin, but at present this is unnecessary. Both in classification and in description they should be considered between the formations which give rise to them, and this will at once indicate their origin.

Puzzling cases of mixture resulting from position occur toward the limits of facies which occupy extensive areas. Bouteloua oligostachya, and Andropogon scoparius extend from the prairies through the sand-hills and plains, and into the foot-hills of the Rocky mountains. Their abundance at once raises a question as to the validity of the prairie, sand-hill, plain, and foot-hill formations. If these two grasses were controlling, and equally characteristic throughout, then the entire stretch would have to be regarded as a single formation. Since they are often absent, or mixed with other facies of greater importance, they can not be considered the sole tests of the formation. This view is reinforced by the fact that prairie, sand-hill, plains, and foot-hill all have their characteristic principal and secondary species, in addition to facies that are more or less typical. In certain formations, doubtless, Bouteloua and Andropogon are relicts, in others invaders, while in the formations actually constituted by them they are dominant. The final solution of such problems is quite impossible, however, until the comparative study of large areas can be based upon the accurate detailed investigation of the component formations.

EXPERIMENTAL VEGETATION

=356. Scope and methods.= The experimental study of the formation as a complex organism rests upon methods essentially similar to those discussed under experimental evolution. The scope of the two fields is practically the same, moreover, in that both deal with the experimental development of an organism and the structures that result. The actual problems are naturally very different, since the formation is a complex of individual plants, but the fundamental basis of habitat, function, and structure is common to both. However, the functions now to be considered are aggregation, invasion, competition, etc., and the structures, zones, consocies, societies, communities, and families. The latter may properly be regarded as adaptations called forth by the adjustment, i. e., aggregation, migration, ecesis, etc., of the formation to the physical factors of the habitat. As consequences of measured factors, formational adjustment and adaptation must themselves be carefully measured and recorded. For these purposes, the methods of quadrat and transect, of chart, photograph, and formation herbarium are used. Invaluable as they are for any scientific inquiry into vegetation, such methods form the very foundation of experimental study in which accuracy is the first desideratum.

It has already been shown that nature’s own experiments in the production of new forms furnish the best material for experimental evolution. This statement is equally true of experimental vegetation. The formation of new habitats by weathering and transport, and the denuding of old ones, yield experimental plots of the greatest value. This is likewise the case in the great majority of formations, where invasion or competition is active. These are the phenomena that must be considered in any careful study of vegetation, but in taking them up from the experimental standpoint, greater attention must be paid to detail, and the changes must be followed closely for a longer time. The method that makes use of existing changes in vegetation is designated the method of natural habitats. In contrast with this is the method of artificial habitats, in which the habitat itself is definitely modified, or a group of species actually transferred to a different habitat. Many problems of vegetation can be attacked with greater success under control than in the field. This is particularly true of competition, in which results can be obtained most readily by means of the method of control habitats, as carried on in the plant house.

METHOD OF NATURAL HABITATS

=357. Natural experiments.= Every family as well as every community constitutes an experiment in competition; the same statement necessarily holds for the larger groups, society, consocies, and formation, which are composed of families and communities. The last also make it possible to study competition in two typical instances, viz., in the family, where the individuals are of one kind, and in the community, where they belong to two or more different species. The community, moreover, is a product of invasion, and it furnishes material for the study of this function, as well as for that of aggregation and competition. Practically every formation shows some invasion, but as a rule stable formations contain so few invaders that they are relatively unimportant in this connection. Invasion is most active in transition areas and in mixed formations, whether produced by juxtaposition or by succession, and its study in these places yields by far the largest number of valuable results.

As typical complete invasion, a succession is the best of all natural experiments in aggregation, migration, ecesis, and competition. This is especially true of the initial stages in which changes in the number and position are most readily followed. The methods used in studying successions have been given elsewhere. In addition, it should be pointed out that one of the first tasks in taking up the ecological investigation of a region is to make a careful search for all new and denuded areas, as well as for those in which succession is taking place. The phenomena in these areas can not be explained until the habitats and formations have been worked over critically, but the facts must be collected at the earliest possible moment, since the stages of the succession are constantly changing, while the stable formations are not.

METHOD OF ARTIFICIAL HABITATS

=358. Modification of habitat.= As the final factors in ecesis and competition, water, light, and temperature control the grouping of plants into vegetation. An efficient change in one of these, or in all of them, brings about a visible adjustment in the structure of the plant group concerned. Modifications of water-content and light are readily produced in the field by drainage, irrigation, shading, clearing, etc. In fact, all the changes of habitat indicated under experimental evolution serve equally well to initiate experiments in experimental vegetation; indeed, the same experiment covers both fields. It is impracticable, however, to modify the temperature of a habitat without changing its water-content or light, and consequently the influence of temperature can not be determined through experiment by modification. The extent of the area modified should be as large as convenience will permit, in order that the number of individuals may be large enough to indicate clearly the resulting adjustment in position and arrangement. The best results can be obtained where a small separate area of a formation can be modified, e. g., where a small swamp can be drained, or a depression flooded. In the case of light, however, it is usually impossible to clear or to shade a large area, and the study must be restricted to a relatively small group of plants. In regions where lumbering is actively carried on, the consequent clearing initiates invaluable experiments over large areas, and this is likewise true of forest plantations. Modification of a large area has decided advantages in bringing out the changes in the more prominent structural features, but the causes and the details of the adjustment can be worked out much more satisfactorily in a small area.

=359. Denuding.= The modification of the habitat by denuding is the sole method of initiating succession by experiment. It is consequently of the most fundamental importance in investigating aggregation, ecesis, and competition, as well as the reactions exerted by the invaders of the different stages. The possibilities of denuding an entire habitat or an extensive area are not great, and the investigator must content himself with denuded quadrats, transects, and migration circles, which are small enough to permit a critical study of all the factors in succession. It is of course unnecessary that the denuding be done by the ecologist himself, provided he is able to follow the succession from the very beginning. Accordingly, it becomes possible for him to make the very best use of all those changes wrought by man in which the vegetation is destroyed over considerable areas. These are essentially natural experiments, and at this point the methods of natural and artificial habitats merge.

The manner of denuding depends in a degree upon the nature of vegetation, but, when time, convenience, and safety are all taken into account, the actual removal of the vegetation as indicated under the denuded quadrat is by far the most satisfactory. Under certain conditions, flooding or burning can be used to advantage, but cases of this kind are infrequent. The purpose of the experiment determines the kind of area to be denuded. Quadrat, transact, and migration circle are equally valuable for ecesis and competition. The quadrat is best adapted to work in a homogeneous area, while the transect is suited to a heterogeneous one characterized by zones, societies, or communities. It is an advantage to replace the denuded transect by a series of denuded quadrats, one for each zone or society, when the transect would be too long for convenience. The denuded migration circle is invaluable for aggregation and ecesis, since it makes possible the study of migration as a distinct function. A series of denuded quadrats, consisting of one or more in the different stages of a succession, furnishes important evidence concerning the development of each stage. By far the best method, however, for making a comparative study of the stages of a succession is the quadrat sequence. A quadrat is denuded each year, thus yielding a complete sequence of miniature stages through the whole course of succession. This method is especially valuable when a succession is represented by a single example, and there is no opportunity of reconstructing it by the comparison of various stages. A quadrat sequence is naturally of the greatest value if begun at the time when the first invaders appear.

=360. Modification of the formation by transfer.= The study of partial and intermittent invasion into an established vegetation is made through the transfer of a species or group of species by means of seeding or planting. The process differs in no way from that described for experimental evolution, except in so far that an endeavor is made to establish a family or a community, and not merely a few individuals. Transfer makes possible the critical investigation of ecesis under conditions of intense competition, as well as the study of aggregation and the origin of plant groups under these conditions. Perhaps its greatest value is in the experimental study of alternation and zonation, especially the former. It is practically impossible to determine whether alternation, especially when corresponsive, is due to physical or historical causes, i. e., migration and competition, except by means of the reciprocal transfer of the species concerned.

Field cultures for the careful study of ecesis and competition are made by transferring seeds or plants to new or denuded soils. This is practically a combination of the methods of modification and transfer. It has a unique value in making it possible to initiate artificial successions of almost any character that is desired, and to carry them out with the reactions more or less under control. This opens up an extremely important field of experimental inquiry, which promises to put the study of succession upon a much more exact basis. Competition cultures in the field are not essentially different from those under control, and they will be considered under the next method.

METHOD OF CONTROL HABITATS

=361. Competition cultures.= Although it is quite possible to carry on experiments in invasion and succession in the planthouse, the limited space usually available makes this undesirable, except in a few problems where control is necessary. Competition cultures, on the other hand, yield better results in the planthouse than in the field, since the physical factors and the appearance of unwelcome migrants are much more easily controlled. The possibilities of the culture method in the study of competition seem inexhaustible, and the author has found it necessary to confine his own investigations to a few of the fundamental problems. In this work, he has distinguished several kinds of cultures, based chiefly upon the species concerned and the arrangement of the individuals. Simple cultures are those in which a single species is used. The resulting group is a family, and the competition is between like individuals. In such cultures, the problem of the factors in competition is reduced to its simplest terms. Mixed cultures are based upon two or more species, and the problem is correspondingly complicated. As a rule, all the seeds have been sown at the same time in both simple and mixed cultures, but it has been found desirable to make some heterochronous cultures, in which seeds are also sown after the plants have appeared. Mixed cultures are distinguished as layered cultures, when the species are of very different height. Thus, rosettes have been grown with stemmed plants, tall slender forms with low branching ones, erect plants with twining and climbing plants, etc. Further evidence as to the nature of competition has been sought by means of ecad cultures, and factor cultures. In the former, plants of different response to water and light are grown together under the same conditions, in order to evaluate the part played by the nature of the plant. In a factor culture, the area is divided into two or more parts which are given different amounts of water or of light, in order to determine the influence of slight variations upon the same competitors. In somewhat similar fashion, an attempt has been made to ascertain the bearing of biotic factors upon competition. Cultures are easily made in which Cuscuta or parasitic fungi are used to place certain species at a disadvantage. Permanent cultures are obtained by allowing the plants to ripen and drop their seeds for several generations, just as in nature. They are indispensable for determining the final outcome of the competition between different species.

=362. Details of culture methods.= All competition cultures have been made 1 meter square. In other words, they are quadrats, and they are treated exactly as denuded quadrats in the field with respect to factor readings, charts, and photographs. In the writer’s studies, germination tests were made of a large number of species, and those selected which showed a high per cent of germinability. Since this was the first experimental study of competition, this test was deemed necessary, but it is quite evident that no such selection is made in nature. Consequently, when the seeds used are known to be fresh, a germination test is usually superfluous. Considerable care was taken also to select species known to be vigorous growers, with the result that practically all the species used for experiment were ruderal or subruderal. The species employed, and the kinds of cultures in which they were grouped were as follows:

1. Simple culture of Helianthus annuus. The culture plot was divided into four equal parts; 12 seeds were planted in one, 25 in another, 50 in the third, and 100 in the fourth.

2. Mixed culture of Helianthus annuus, Panicum virgatum, and Elymus canadensis. Twenty-five seeds each of Helianthus and Panicum were planted alternately at equal distances in one-half of the plot, while the other half was planted similarly with Helianthus and Elymus.

3. Mixed culture of Solidago rigida and Onagra biennis. Over one-half of the plot were scattered 50 seeds of Solidago and 100 of Onagra; over the other, 100 and 200 seeds respectively.

4. Layered culture of Laciniaria punctata, Bidens frondosa, Salvia pitcheri, Cassia chamaecrista and Kuhnia glutinosa. Fifty seeds of each species were scattered more or less uniformly over the entire plot.

5. Layered culture of Silphium laciniatum, Datura stramonium and Lactuca ludoviciana. Fifty seeds of Datura and Lactuca, and 25 of Silphium were sown uniformly in one-half of the plot. In the other half, 25 holes were made at equal intervals, and one seed of each of the three planted in each hole.

6. Ecad culture of Oenothera rhombipetala (xerophytic), Verbascum thapsus (mesophytic), and Penthorum sedoides (hydrophytic). One hundred seeds of Oenothera and 200 each of Verbascum and Penthorum were scattered over the plot.

7. Heterochronous culture of Helianthus annuus and Datura stramonium. One hundred seeds of Helianthus were scattered over one half, and the same number of Datura seeds over the other half of the plot. In both, also, 50 seeds were sown in one 4–inch circle, and 25 seeds in a second circle at some distance. A month later, 100 seeds of Helianthus were sown in the Datura plot, and vice versa.

8. Family culture of Helianthus, Kuhnia, Panicum, Bidens, Onagra, Datura, Penthorum, Solidago and Verbascum. The plot was divided into 9 squares and in each were sown 50 seeds of one of these plants.

9. Community culture. The sowing was made exactly as for the family culture, except that 20 seeds of each plant were used. In the middle of each square, 5 seeds of a different species were planted. For the Helianthus, Kuhnia, and Panicum groups, Onagra was used; for Bidens, Onagra, and Datura, Helianthus was used, and for Penthorum, Solidago, and Verbascum, Panicum.

At the time the cultures were started, check plants were sown in pots. The most vigorous seedlings were transplanted singly to large pots, and grown under conditions of water, light, and soil as similar as possible to those of the competition plots. Photographs of check plants and plots were made at the proper intervals, and the plots were charted in quadrats to show the course of competition. The factors which control competition were sought in a critical study of water-content and light values, which is still in process. This work has gone far enough to indicate the correctness of the view that competition is purely physical in character. It has, moreover, been demonstrated that “room” in competition is merely a loose expression for the relation between the number of individuals in a given space, and the amount of water, light, and temperature available in the same space.

GLOSSARY

NOTE: Last terms frequent in compounds are found in their proper place alphabetically. The accent is indicated only in those words accented on the penult; all others are accented on the antepenult, or recessively.

=abundance=, the total number of individuals in an area.

=acospore= (ἀκή, point), a plant with awned disseminules.

=acrophyti´um= (ἄκρον, peak), an alpine plant formation.

=acti´um= (ἀκτή, rocky coast), a rocky seashore formation; =actad=, plant of a rocky seashore.

=-ad= (-αδης, patronymic suffix), suffix for denoting an ecad.

=adaptable=, able to originate ecads; =adaptation=, the structural response to stimuli.

=adjustment=, the functional response to stimuli.

=adventicious= (adventicius, foreign), invading from distant formations.

=adventive= (adventivus, accidental), established temporarily.

=aggregation=, the coming together of plants into groups.

=agi´um= (ἀγή, beach), a beach formation; =agad=, a beach plant.

=agri´um= (ἀγρός, field), a culture formation; =agrad=, a cultivated plant.

=aiphyti´um= (ἀεί, permanent), an ultimate formation.

=alsi´um= (ἄλσος, grove), a grove formation; =alsad=, a grove plant.

=alternation=, the heterogeneous arrangement of plant groups and formations universally present in vegetation.

=amathi´um= (ἄμαθος, sand of the plain), a sand-hill or sandplain formation; =amathad=, a sand-hill plant.

=ammochthi´um= (ἄμμος, sand, ὄχθη, bank), a sand bank formation; =ammochthad=, a sand bank plant.

=anci´um= (ἄγκος, mountain glen), a canyon formation; =ancad=, a canyon plant.

=anemi´um= (ἄνεμος, wind), a blowout formation; =anemad=, a blowout plant; =anemochore=, a plant distributed by wind.

=-anum= (locative suffix), a suffix denoting a layer.

=apostrophe= (ἀπό, away from, στροφή, a turning), the arrangement of the row of chloroplasts parallel to the rays of light.

=apparent noon=, the time when the sun crosses the meridian, i. e., sun noon as distinguished from noon, standard time.

=-ard= (ἄρδον, water of the land), combining term for water-content; =ardium=, a succession due to irrigation.

=ardesiacus=, slate colored.

=-are= (locative suffix), suffix denoting a community.

=aspect= (aspectus, appearance), the seasonal impress of a formation, e.g., the spring aspect.

=association=, the arrangement of individuals in vegetation.

=atmometer= (ἀτμός, vapor), an instrument for measuring evaporation.

=atropurpureus=, dark purple.

=atrovirens=, dark green.

=autochore= (αὐτός, self), motile plants, or those with motile spores; =autochthonous= (χθών, ground), native.

=avellaneus=, drab.

=barrier=, a physical or biological obstacle to migration or ecesis.

=bathyphyti´um= (βαθύς, low), a lowland plant formation.

=blastochore= (βλάστη, growth), a plant distributed by offshoots.

=-bole= (βολή, a throw), combining term for propulsion; =bolochore=, a plant distributed by propulsion.

=broti´um= (βροτός, mortal), a succession caused by man; =brotochore=, a plant distributed by man.

=caeruleus=, pale blue.

=caesius=, eye-blue.

=camni´um= (κάμνω, cultivate), a succession due to cultivation.

=carphospore= (κάρφος, scale), a plant with disseminules possessing a scaly or chaffy pappus.

=carpostrote= (καρπός, fruit), a plant migrating by means of fruits.

=centrospore= (κέντρον, spur), a plant with spiny disseminules.

=chalici´um= (χάλιξ, gravel), a gravel slide formation; =chalicad=, a gravel slide plant.

=cheradi´um= (χέραδος, a sandbar), a wet sandbar formation; =cheradad=, a wet sandbar plant.

=chersi´um= (χέρσος, dry barren waste), a dry waste formation; =chersad=, plant of a dry waste.

=chioni´um= (χιών, όνος, snow), a snow formation; =chionad=, a snow plant; =chionophyti´um=, a niveal plant formation.

=chledi´um= (χλῆδος, rubbish), a ruderal formation; =chledad=, a ruderal plant.

=chlorenchym= (χλωρός, greenish yellow, ἐνχύμα, infusion), the chlorophyll tissue of the leaf.

=-chore= (χωρέω, to spread abroad), combining term to denote agent of migration.

=chresard= (χρῆσις, use), the available water of the soil, the physiological water-content.

=clitochore= (κλίτος, slope), a plant distributed by gravity.

=clysi´um= (κλύσις, a flooding), a succession in a flooded soil.

=-colus= (κόλος, dwelling in), combining term for habitat forms.

=community=, a mixture of the individuals of two or more species, a group of families.

=comospore= (κόμη, hair) a plant with hairy or silky disseminules.

=competition=, the relation between plants occupying the same area, and dependent upon the same supply of physical factors.

=consocies=, that subdivision of a formation controlled by a facies.

=copious=, used of species in which the individuals are arranged closely but uniformly.

=coryphi´um= (κορυφή, peak), an alpine meadow formation; =coryphad=, an alpine meadow plant.

=creatospore= (κρέας, ατος, meat), a plant with nut fruits.

=cremni´um= (κρημνός, crag, cliff), a cliff formation; =cremnad=, a cliff plant.

=creni´um= (κρήνη, spring), a spring formation; =crenad=, a spring plant.

=crymi´um= (κρυμός, frost), a polar barren formation; =crymad=, a polar plant; =crymophytic=, pertaining to polar plants.

=crystallochore= (κρύσταλλος, ice), a plant distributed by glaciers.

=cyaneus=, azure.

=cyriodoche= (κύριος, regular), a normal succession.

=dendri´um= (δένδρα, fruit trees), an orchard formation; =dendrad=, an orchard plant.

=derived=, coming from other formations or regions, not native.

=diphotic= (δι-, two), the two surfaces unequally lighted; =diphotophyll=, a leaf differentiated into palisade and sponge tissues owing to unequal illumination.

=diplophyll= (διπλόος, twofold), an isophotic leaf with water-storage cells in the middle.

=disseminule= (semen, seed), a seed fruit modified for migration.

=dissophyte= (δισσός, double), a plant with xerophytic leaves and stems, and mesophytic roots.

=-doche= (δοχή, succession), succession.

=drimi´um= (δριμύς, biting, pungent), an alkaline habitat, and the corresponding formation; =drimad=, a plant of such a formation.

=driodi´um= (δρίος, thicket), a dry thicket formation; =driodad=, plant of a dry thicket.

=dysgeogenous= (δυς-, bad, γῆ, soil), weathering with difficulty to form soil.

=ecad= (οἶκος, home), a habitat form due to origin by adaptation; =ece´sis= (οἰκῆσις, act of coming to be at home), the germination and establishment of invaders; =ecograph=, an instrument for measuring a physical factor of a habitat; =ecotone= (τόνος, tension), the tension line between two zones, formations, consocies, etc.

=ecballi´um=, (ἐκβάλλω, cut down forests), a succession due to lumbering.

=echard= (ἔχω, to withhold), the non-available water of the soil.

=edobole= (οἶδος, swelling), a plant whose seeds are scattered by propulsion through turgescence.

=efficient difference=, the amount of a physical factor necessary to produce a change in the response.

=enauli´um= (ἔναυλος, hollow channel), a sanddraw formation; =enaulad=, a sanddraw plant.

=ende´mic= (ἐν, within, δῆμος, district), occurring in a single formation, or natural region; =ende´mism=, the condition of growing in but one natural area.

=epistrophe= (ἐπί, towards, στροφή, a turning), the arrangement of the row of chloroplasts at right angles to the incident light.

=eremi´um= (ἔρημος, desert), a desert formation; =eremad=, a desert plant.

=estival=, pertaining to summer.

=-etum= (locative suffix), suffix used to denote a consocies.

=eugeogenous= (εὖ-, well, γῆ, soil), weathering readily to form soil.

=facies=, a dominant species of a formation: a distinct area controlled by it is a consocies.

=family=, a group of individuals belonging to one species.

=fixity=, the condition characterized by little or no response to stimuli.

=flavovirens=, yellow green.

=forewold=, equivalent to the German “vorwald,” the thicket zone bordering a forest.

=-genous= (γένω, to produce), producing.

=geotome= (γῆ, earth, τομή, edge), an instrument for obtaining soil samples.

=gloeospore= (γλοιός, sticky stuff), a plant with viscid disseminules.

=-graph= (γραφή, a writing), combining term for a recording instrument.

=gregarious= (gregarius, grouped in herds), used of species in which the individuals occur in groups.

=habitat=, a definite physical area characterized by a formation; =habitat form=, the impress given the plant by the habitat.

=harmosis= (ἅρμοσις, an adapting), response to stimuli, comprising both adjustment and adaptation.

=hedi´um= (ἕδος, a sitting, base), a succession in a residuary soil.

=heliad= (ἥλιος, sun), a heliophyte; =heliophyll=, the leaf of a sun plant; =heliophyte=, a sun plant; =heliophyti´um=, a sun plant formation; =heliophilous=, sun-loving.

=heli´um= (ἕλος, marsh), a marsh formation; =helad=, a marsh plant; =helodi´um= (ἑλώδης, marshy), a swampy open woodland formation; =helodad=, a marsh plant; =helodrium= (δρίος, thicket), a thicket formation; =helodrad=, a plant of a marshy thicket; =helohyli´um= (ὕλη, forest) a marsh forest formation; =helohylad=, a marsh forest plant.

=hepodoche= (ἕπω, follow), a secondary succession.

=hizometer= (ἵζω, to sink), an instrument for measuring gravitation water.

=holard= (ὅλος, whole), the total water-content of the soil.

=hydrad= (ὑδρο-, water), a hydrophyte; =hydrochore=, a plant distributed by water; =hydroharmose=, response to water stimuli; =hydrophyll=, the leaf of a hydrophyte; =hydrophyte=, a water plant; =hydrophyti´um=, a water plant formation; =hydrophilous=, water-loving; =hydrosta´tic= (στατικός, standing), completing the succession under hydrophytic conditions; =hydrotropic= (τροπικός, turning), applied to successions which become mesophytic.

=hygrome´tric= (ὑγρίς, wet), measuring or absorbing water; =hygroscopic= (σκοπέω, look), measurable only by a hygroscope; able to absorb moisture.

=hyli´um= (ὕλη, forest), a forest formation; =hylad=, a forest plant; =hylocolum=, dwelling in a forest; =hylodi´um= (ὑλώδης, wooded), a dry open woodland formation; =hylodad=, a plant of this formation; =hylophyte=, a forest plant.

=hypsi´um= (ὕψος, elevation), a succession caused by elevation.

=-ile= (locative affix), suffix denoting a society.

=immobile=, without effective devices for migration.

=indigenous= (indigena, sprung from the land), native.

=insolation=, exposure to intense heat and light.

=isabellinus=, leather-colored.

=isolation=, separation by barriers.

=isopho´tic= (ἴσος, equal), equally illuminated; =isophotophyll=, a leaf in which both halves of the chlorenchym are alike, due to equal illumination.

=-ium= (-εῖον, locative affix), suffix denoting a formation.

=labile=, plastic, easily modified.

=lauri´um= (λαύρα, drain), a drain formation; =laurad=, a drain plant.

=limni´um= (λίμνη, lake), a lake formation; =limnad=, a lake plant; =limnodium= (λιμνῶδες, marshy ground), a salt marsh formation; =limnodad=, a plant of a salt marsh.

=lochmi´um= (λόχμη, thicket), a thicket formation; =lochmad=, a thicket plant.

=lophi´um= (λόφος, crest, hill), a hill formation; =lophad=, a hill plant; =lophospore=, a plant with plumose disseminules.

=mastigospore= (μάστιξ, ιγος, lash), a plant with ciliate or flagellate disseminules.

=melleus=, honey-colored.

=meridian=, used chiefly as a synonym for apparent noon; also an imaginary line of longitude.

=mesad= (μέσος, middle), a mesophyte; =mesophilous=, growing in moist soils; =mesophyll=, the leaf of a mesophyte; =mesophyte=, a plant of moist soils; =mesophyti´um=, a mesophytic formation; =mesosta´tic= (στατικίς, standing), completing the succession under mesophytic conditions; =mesotro´pic= (τροπικός, turning), applied to successions which become mesophytic.

=-meter= (μέτρον, measure), combining term for instrument.

=micti´um= (μικτόν, mixture), a mixed formation.

=migrant=, a plant that is migrating or invading.

=migration= (migratio, removal), the movement of plants into new areas; =migration circle=, a circle employed to measure migration.

=mobile=, able to be moved, i. e., modified for migration.

=monochronic= (μόνος, single, χρόνος, time), arising but once; =monogenesis= (γένεσις, origin), the origin of a new form at a single place or time; =monophyle´sis= (φῦλον, race), origin from a single ancestral type; =monoto´pic= (τόπος, place), arising at one place only.

=motile=, able to move by growth, by means of cilia, etc.

=mutable=, able to produce mutants; =mutant=, a form arising by mutation; =mutation=, the sudden appearance of new forms.

=namati´um= (νάμα, ατος, brook), a brook formation; =namatad=, a brook plant.

=nomi´um= (νομός, pasture), a pasture formation; =nomad=, a pasture plant.

=occupation=, possession of the ground by plants.

=oceani´um= (ὠκεανός, ocean), an ocean formation; =oceanad=, an ocean plant; =oceanophyte=, an ocean plant; =oceanophilous=, ocean-dwelling.

=ocheti´um= (ὀχετός, drain), a succession due to artificial drainage.

=ochroleucus=, yellowish white.

=ochthi´um= (ὄχθη, bank), a bank formation; =ochthad=, a bank plant.

=oligope´lic= (ὀλίγος, little, πηλός clay), containing little clay; =oligopsam´mic= (ψάμμος, sand), containing little sand.

=olisthi´um= (ὄλισθος, slip), a succession in a landslip.

=ombrometer= (ὄμβρος, a rainstorm), a rain gauge.

=-on= (-ών, locative suffix), suffix used to denote a family.

=oncospore= (ὄγκος, hook), a plant with hooked disseminules.

=orgadi´um= (ὀργάς, άδος, meadowland partially wooded), an open woodland formation; =orgadad=, an open woodland plant.

=orophyti´um= (ὄρος, mountain), a subalpine plant formation.

=oxodi´um= (ὀξώδης, sour), a humus marsh formation; =oxodad=, a plant of a humus marsh.

=pagi´um= (πάγος, rocky hill, glacier), a succession in a glacial soil; =pagophyti´um=, a foot-hill plant formation.

=pediophyti´um= (πεδίον, plain), an upland plant formation.

=pelagi´um= (πέλαγος, surface of the sea), a surface sea formation; =pelagad=, a plant of the sea surface.

=pelochthi´um= (πηλός, mud, ὄχθη, bank), a mud bank form; =pelogenous=, producing clay; =pelopsammic= (ψάμμος, sand), composed of mixed clay and sand; =pelopsammogenous=, producing clay and sand.

=permobile=, extremely mobile.

=perquadrat=, a quadrat of 16 square meters or more.

=petasospore= (πέτασος, sunshade), a plant with parachute-like disseminules.

=petri´um= (πέτρα, rock, stone), a rock formation; =petrad=, a rock plant; =petrochthi´um= (ὄχθη, bank), a rock bank formation.

=petrodi´um= (πετρώδης, abounding in boulders), a boulder field formation; =petrodad=, a plant of a boulder field.

=phelli´um= (φελλεύς, stony ground), a rock field formation; =phellad=, a rock field plant.

=-philous= (φίλος), loving, dwelling in.

=-photic= (φῶς, φωτός, light), pertaining to light; =photoharmose=, response to light stimuli; =photometer=, an instrument for measuring light.

=phreti´um= (φρητός, tank), a tank formation; =phretad=, a tank plant.

=phyad= (φυή, form of growth), a vegetation form, e. g., tree, shrub, etc.

=-phyll= (φύλλον, leaf), combining term for leaf.

=-phyte= (φυτόν, plant), combining term denoting plant; =phyteris= (ἔρις, strife), plant competition; =-phyti´um= (φυτεῖον, place covered with plants), combining term for formation; =phytostrote=, a species migrating by means of the plant body.

=pladobole= (πλάδος, moisture), a plant whose seeds are scattered by propulsion due to moisture.

=plasticity=, the condition characterized by ready response to stimuli.

=pnoi´um= (πνοή, blast), a succession in an aeolian soil.

=poi´um= (πόα, meadow), meadow formation; =poad=, a meadow plant; =poophyte=, a meadow plant.

=polyan´thous= (πολύς, many, ἄνθος, flower), producing many flowers; =polychro´nic= (χρόνος, time), arising at two or more times; =polyde´mic= (δῆμος, district), occurring in two or more formations or natural regions; =polygenesis= (γένεσις, origin), the origin of a new form at two or more places or times; =polyphyle´sis= (φῦλον, race), the origin of a form, species, or genus from two or more ancestral types; =polyspermatous= (σπέρμα, seed), producing many seeds in each flower; =polyto´pic= (τόπος, place), arising at two or more distinct places.

=ponti´um= (πόντος, deep sea), a deep sea formation.

=potami´um= (ποταμός, river), a river formation; =potamad=, a river plant.

=potometer= (ποτόν, drink), an instrument for measuring absorption.

=prevernal=, pertaining to early spring.

=prior=, earlier, used of alpine aspects.

=prochosi´um= (πρόχωσις, a deposition of mud), a succession in an alluvial soil.

=prodophyti´um= (πρόοδος, pioneer), an initial formation.

=protodoche= (πρῶτος, first), a primary succession.

=proximity= (proximitas, nearness), nearness to the area invaded.

=psamathi´um= (ψάμαθος, sand of the seashore), a strand formation; =psamathad=, a strand plant; =psammogenous= (ψάμμος, sand), producing a sandy soil.

=psili´um= (ψιλά, land without trees), a prairie formation; =psilad=, a prairie plant.

=psychrometer= (ψυχρός, chill), an instrument that measures humidity by means of a fall in temperature; =psychrograph=, a psychrometer that records automatically.

=ptenophyti´um= (πτηνός, passing), an intermediate formation.

=pterospore= (πτερόν, wing), a plant with winged disseminules.

=purpureus=, purple.

=pycnophyti´um= (πυκνός, thick), a closed formation.

=pyri´um= (πῦρ, fire), a burn succession.

=quadrat= (quadratum, a square), a square meter of vegetation marked off for counting, mapping, etc.; =major=, a quadrat of 2–14 square meters.

=reaction=, the effect of the formation upon the habitat.

=relict= (relictus, left), a species belonging properly to an earlier type of succession than the one in which it is found.

=repi´um= (ῥέπω, sink), a succession due to subsidence.

=rhoi´um= (ῥόος, stream), a creek formation; =rhoad=, a creek plant.

=rhoptometer= (ῥοπτόν, something absorbed), an instrument to measure absorption of water by the soil.

=rhyaci´um= (ῥύαξ, ακος, mountain torrent), a torrent formation; =rhyacad=, a torrent plant.

=rhysi´um= (ῥυσίς, a flowing of fire), a succession due to volcanic action.

=ruber=, red.

=saccospore= (σάκκος, sack), a plant with sack-like disseminules.

=sarcospore= (σάρξ, σαρκός, flesh), a plant with fleshy disseminules.

=sciad= (σκιά, shade), a sciophyte; =sciophyll=, the leaf of a shade plant; =sciophyte=, a shade plant; =sciophyti´um=, a shade plant formation; =sciophilous=, shade-loving.

=selagraph= (σέλας, light), an instrument for recording light values automatically.

=serotinal=, late, pertaining to autumn.

=social=, used of plants in which the individuals are compactly grouped; =exclusive=, excluding individuals of other species; =inclusive=, permitting the entrance of individuals of other species.

=society=, a subdivision of the formation, characterized by a principal species.

=sparse=, scattered singly.

=spermatostrote= (σπέρμα, ατος, seed), a plant migrating by means of seeds.

=sphyri´um= (σφύρον, ankle, talus), a succession in a talus soil.

=spongophyll= (σπόγγος, a sponge), a leaf consisting of sponge tissue.

=sporadophyti´um= (σποράς, άδος, scattered), an open formation.

=-spore= (σπορά, seed, fruit), combining term for migration contrivance; =sporostrote=, a plant migrating by means of spores.

=stability=, the condition in which the plant makes little or no response.

=stabilization=, the tendency typical of succession, in which the successive stages become more stable.

=stasi´um= (στάσις, a standing), a stagnant pool formation; =stasad=, a plant of stagnant water.

=staurophyll= (σταυρός, a pale), a leaf consisting of palisade tissue.

=sterrhi´um= (στερρός, barren), a moor formation; =sterrhad=, a moor plant.

=-strote= (στρώτος, strewn), combining term for means of migration.

=subcopious=, scattered somewhat loosely.

=subgregarious=, arranged in loose groups.

=subquadrat=, a quadrat of 1–8 decimeters.

=succession=, complete and continuous or repeated invasion, in consequence of which formations succeed each other.

=symmetry=, used of topography when it shows uniform changes; =radial=, a condition in which the different areas are concentric; =bilateral=, where the areas occur in two similar rows.

=syrtidi´um= (σύρτις, ιδος, sandbar), a dry sandbar formation; =syrtidad=, a plant of a dry sandbar.

=taphri´um= (τάφρος, ditch), a ditch formation; =taphrad=, a ditch plant.

=telmati´um= (τέλμα, ατος, water meads), a wet meadow formation; =telmatad=, a wet meadow plant.

=testaceus=, pale brick colored.

=thalassi´um= (θάλασσα, sea), a sea formation; =thalassad=, a sea plant.

=thallostrote= (θαλλός, shoot), a species migrating by means of offshoots.

=theri´um= (θήρ, wild animal), a succession due to animals.

=thermi´um= (θέρμη, hot spring), a hot spring formation; =thermad=, a hot spring plant.

=thini´um= (θίς, θινός, a dune), a dune formation; =thinad=, a dune plant.

=tiphi´um= (τῖφος, pool), a pool formation; =tiphad=, a pond plant.

=tiri´um= (τείρω, rub away), a bad land formation; =tirad=, a bad land plant.

=tonobole= (τόνος, tension), a plant whose seeds are scattered by projection from calyx or involucre.

=transect= (transectus, cut through), a cross section of vegetation.

=trechometer= (τρέχω, to run off), an instrument for measuring run-off.

=tribi´um= (τρίβω, wear or rub away), a succession in an eroded soil.

=umbrinus=, umber.

=variable=, able to produce variants; =variant=, a form arising from origin by variation; =variation=, the origin of new forms by the action of selection upon minute differences.

=vegetation form=, a characteristic plant form, e. g., tree, rosette, etc.

=vernal=, pertaining to spring.

=vicine= (vicinus, neighboring), invading from adjacent formations.

=viridis=, green.

=vixgregarious=, arranged in small or indistinct groups.

=water-content=, the water of the soil or habitat; =physiological=, the available soil water; =physical=, the total amount of soil water.

=xenodoche= (ξένος, strange), an anomalous succession.

=xerad= (ξηρός, dry), a xerophyte; =xerasi´um= (ξηρασία, drought), a succession due to drainage or drought; =xeriobole= (ξηρία, dryness), a plant whose seeds are scattered by dehiscence due to dryness; =xerohyli´um= (ὕλη, forest), a dry forest formation; =xerohylad=, a dry forest plant; =xerophyll=, the leaf of a xerophyte; =xerophyte=, a dry soil plant; =xerophyti´um=, a xerophytic formation; =xerophilous=, dwelling in a dry habitat; =xeropoi´um=, a heath formation; =xeropoad=, a heath plant; =xerosta´tic= (στατικός, standing), used of successions which are completed under xerophytic conditions; =xerotro´pic= (τροπικός, turning), applied to successions which become xerophytic.

=zonation=, that condition in which plant groups or formations appear in belts or zones.

=zone=, a belt of more or less uniform vegetation.

=zoochore= (ζῶον, animal), a plant distributed by animals.

BIBLIOGRAPHY

ASCHERSON, P.

Von der Verbreitung der Pflanzen. Leunis Synopsis, 1:724. 1883.

BACHMANN, E. T.

Darstellung der Entwicklungsgeschichte und des Baues der Samenschale der Scrofularineen. Just ’81:494. 1881.

BERG, E. V.

Das Verdrängen der Laubwälder durch die Fichte und Kiefer. Grisebach Berichte, 1844: 15. 1844.

BIBERG, I. J.

Oeconomia Naturae. Amoen. Acad., 4:1. 1749.

BLYTT, A.

Ueber zwei Kalktuffbildungen in Gudbrandsdalen. Engler Jahrb., 16:b1. 1892.

Essay on the Immigration of the Norwegian Flora. Just ’76:693. 1876.

BRISSEAU-MIRBEL, C. F.

Elémens de Physiologie Végétale et de Botanique. 1815.

BUCH, C. L. V.

Allgemeine Uebersicht der Flora auf den Canarischen Inseln. 1819.

BUCHENAU, F.

Monographie der Juncaceen. Engler Jahrb., 1:124. 1881.

BUCHWALD, J.

Die Verbreitungsmittel der Leguminosen der tropischen Afrika. Engler Jahrb., 19:494. 1893.

BURGERSTEIN, A.

Die Transpiration der Pflanzen. 1904.

CALLMÉ, A.

Om de nybildade Hjelmaröarnes vegetation. Bihang Sv. Vet. Akad. Handl., XII. 1887.

CLEMENTS, E. S.

The Relation of Leaf Structure to Physical Factors. Trans. Am. Micro. Soc., 25. 1905.

CLEMENTS, F. E.

Peculiar Zonal Formations of the Great Plains. Am. Nat., 31:968. 1897.

A System of Nomenclature for Phytogeography. Engler Jahrb., 31:b1. 1902.

Greek and Latin in Biological Nomenclature. Uni. Nebr. Studies, 3:1. 1902.

The Development and Structure of Vegetation. Rep. Bot. Surv. Nebr., 7. 1904.

Formation and Succession Herbaria. Uni. Nebr. Studies, 4:329. 1904.

CLEMENTS, F. E. AND E. S.

Herbaria Formationum Coloradensium. 1902.

CONTEJEAN, C.

De l’influence du terrain sur la végétation. Ann. Nat. Sci., 5:20:266. 1874.

COSTANTIN, J.

Accommodation des plantes aux climats froids et chauds. Bull. Soc. Bot. Fr., 31:490. 1897.

COVILLE, F. V.

Botany of the Death Valley Expedition. 16. 1893.

COWLES, H. C.

The Ecological Relations of the Sanddunes of Lake Michigan. Bot. Gaz., 27:95. 1899.

The Physiographic Ecology of Chicago and Vicinity. Bot. Gaz., 31:73. 1901.

DAMMER, U.

Die Verbreitungsausrüstungen der Polygonaceen. Engler Jahrb., 15:260. 1893.

DARWIN, C.

Origin of Species. 1859.

DECANDOLLE, A.

Géographie Botanique Raisonnée. 1855.

Constitution dans le regne végétal de groupes physiologiques applicables à la géographie ancienne et moderne. Arch. Sci. Phys. Nat., 1874:5. 1874.z

Sur les causes de l’inegale distribution des plantes rares dans la chaîne des Alpes. Just 3:663. 1875.

DECANDOLLE, A. P.

Essai Elémentaire de Geographie Botanique. 1820.

Physiologie. 1832.

DELPINO, F.

Studi di geografia botanica secondo un nuovo indirizzo. Mem. R. Acc. Sci., 5:7:329. 1898.

DE VRIES, H.

Die Mutationstheorie. 1901.

DINGLER, H.

Die Bewegung der pflanzlichen Flugorgane. 1889.

DOUGLAS, R.

Succession of Forest Growth. Gard. Chron. ’89:10. Just ’89:172. 1889.

DRUDE, O.

Anwendung physiologischer Gesetze zur Erklärung Vegetationslinien. Just 4:674. 1876.

Die Vertheilung und Zusammensetzung östlicher Pflanzengenossenschaften in der Umgebung von Dresden. Isis. 1885.

Die systematische und geographische Anordnung der Phanerogamen. Schenck Handbuch. 3:487. 1886.

Atlas der Pflanzenverbreitung. 1887.

Ueber die Principien in der Unterscheidung von Vegetationsformationen. Engler Jahrb., 11:21. 1889.

Handbuch der Pflanzengeographie. 1890.

Deutschlands Pflanzengeographie. 1896.

Der Hercynische Florenbezirk. 1902.

ELMORE, C. J.

Flora of a Dried-up Millpond. Proc. Nebr. Acad. Sci., 7:29:69. 1901.

ENGLER, A.

Versuch einer Entwicklungsgeschichte der Pflanzenwelt. 1879.

ENGLER & DRUDE

Die Vegetation der Erde. Engler Jahrb., 17:b55. 1893.

EWART, A. J.

On the Physics and Physiology of Protoplasmic Streaming in Plants 1903.

FENZL, E.

Versuch einer Darstellung der geographischen Verbreitungs- und Vertheilungsverhältnisse der natürlichen Familien der Alsineen. 1833.

FLAHAULT, C.

Les zones botaniques dans le Bas-Languedoc et les pays voisins. Bull. Bot. Soc. Fr., 40:36. 1893.

FLAHAULT, C., & COMBRES, P.

Sur la flore de la Camargue. Bull. Soc. Bot. Fr., 41:37. 1894.

FLICHE, M. P.

Un reboisement, étude botanique et forestière. Engler Jahrb., 11:104. 1888.

GAIN, E.

Recherches sur le role physiologique de l’eau dans la végétation. Ann. Nat. Sci., 7:20:63. 1895.

GERHARDT, J.

Handbuch des deutschen Dünenbaues. 1900.

GOEBEL, K.

Pflanzenbiologische Schilderungen II. 1892.

Organography of Plants. English edition. 1900.

GOEZE, E.

Pflanzengeographie. 1882.

GRAEBNER, P.

Studien über die norddeutschen Heide. Engler Jahrb., 30:500. 1895.

Ueber die Bildung natürlichen Vegetationsformationen in norddeutschen Flachlande. Bot. Cent., 77:212. 1898.

Gliederung der Westpreussischen Vegetationsformationen. Schrift. Nat. Ges. Danzig, 9:43. 1898.

Die Heide Norddeutschlands. 1902.

GREVILLIUS, A. Y.

Om vegetationens utveckling pa de nybildade Hjelmaröarne. Bot. Cent. Beih., 5:36. 1892.

Studier öfver vegetationens sammansättning. Sver. Geol. Unders., 144. 1895.

GRISEBACH, A.

Bericht über die Leistungen in der Pflanzengeographie. 1843.

Die Vegetation der Erde. 1872.

GROENLUND, C.

Planteväxten paa Island. Nat. For. Festschr. 1884–90.

HABERLANDT, G.

Physiologische Pflanzenanatomie. 1904.

HANSGIRG, A.

Phyllobiologie. 1903.

HEDENBERG, A.

Stationes Plantarum. Amoen. Acad., 4:64. 1754.

HEGELMEYER, F.

Ueber Bau und Entwickelung einiger Cuticulargebilde. Pringsheim Jahrb., 9:286. 1873.

HENFREY, A.

The Vegetation of Europe. 1852.

HENSEL, E. P.

An Investigation of the Movements of Petals. Uni. Nebr. Studies, 5. 1905.

HESSELMANN, H.

Zur Kenntnis des Pflanzenlebens schwedischer Laubwiesen. Beih. Bot. Cent., 18:311. 1904.

HILDEBRAND, F.

Die Schleuderfrüchte und ihr in anatomischen Bau begründeter Mechanismus. Pringsheim Jahrb., 9:235. 1873.

Der Verbreitungsmittel der Pflanzen. Just ’73:224. 1873.

Die Lebensdauer und Vegetationsweise. Engler Jahrb., 2:51. Just ’81:302. 1881.

HILDEBRANDT, H.

Einige Beobachtungen über den Witterungs-Einfluss auf die Lebensdauer und Vegetationsweise der Pflanzen. Engler Jahrb., 4:1. 1883.

HILL

Reise in Sibirien. 1858.

HOFFMANN, H.

Ueber Accommodation. Just ’76:952. 1876.

Der nördliche Ural und das Küstengebirge Pae-Choi. 1856.

HULT, R.

Försok till analytisk behandlung af formationerna. Medd. Soc. Fenn., 8. 1881.

Blekinges vegetation. Just ’85:312. 1885.

Die alpinen Pflanzenformationen des nördlichstens Finnlands. Engler Jahrb., 11:b61. 1887.

Moosfloran i traktan mellan Aavasaksa och Pallastunturi. Engler Jahrb., 8:b1. 1887.

HUMBOLDT, A. V.

Essai sur la Géographie des Plantes. 1805.

Views of Nature. 1848.

HUMBOLDT, A. V., & BONPLAND, A.

Tableau Physique des Régions Equatoriales. 1805.

HUTH, E.

Die Klettpflanzen mit besonderer Berüchsichtigung ihrer Verbreitung durch Tiere. Just ’87:432. 1887.

JACCARD, P.

Étude Comparative de la Distribution Florale dans une portion des Alpes et du Jura. Bull Soc. Vaud., 37:547. 1901.

Gesetze der Pflanzenvertheilung in der alpinen Region. Flora, 90:349. 1902.

KABSCH, W.

Das Pflanzenleben der Erde. 1865.

KERNER, A.

Das Pflanzenleben der Donauländer. 1863.

Pflanzenleben. 1888, 1891.

KIHLMANN, A. O.

Pflanzenbiologische Studien aus Russisch-Lappland. 1890.

KLINGE, J.

Ueber den Einfluss der mittleren Windrichtung auf das Verwachsen der Gewässer. Engler Jahrb., 11:264. 1890.

KOEPPEN, W.

Die Wärmezonen der Erde. 1:215. 1884.

KORSCHINSKY, S.

Ueber die Entstehung und das Schicksal der Eichenwälder in mittleren Russland. Engler Jahrb., 13:471. 1891.

KRASAN, F.

Ueber die Bedeutung der gegenwärtigen Verticalzonen der Pflanzen, etc. Engler Jahrb., 4:266. 1883.

KRASSNOFF, A.

Bemerkungen über die Vegetation des Altai. Engler Jahrb., 9:53. 1888.

KRAUSE, E. H. L.

Beitrag zur Geschichte der Wiesenflora in Norddeutschland. Engler Jahrb., 15:387. 1892.

Die Heide. Engler Jahrb., 14:515. 1892.

KURTZ, F.

Die Flora des Chilcatgebietes im südostlichen Alaska. Engler Jahrb., 19:327. 1895.

LEGRAND, A.

Statistique Botanique de Ferez. Ann. Soc. Agr. Loire, 17:290. 1873.

LINK, H. F.

Die Urwelt und das Alterthum. 1820.

LINNÉ, C. V.

Flora Suecica. 1745.

Philosophia Botanica. 1751.

LOHDE, G.

Ueber die Entwicklungsgeschichte und den Bau einiger Samenschalen. Just 2:441. 1874.

MACMILLAN, C.

Metaspermae of the Minnesota Valley. 1892.

On the Occurrence of Sphagnum Atolls in Central Minnesota. Minn. Bot. Studies, 1:1. 1894.

On the Formation of Circular Muskeag in Tamarack Swamps. Torr. Bull., 23:500. 1896.

Observations on the Distribution of Plants along Shore at Lake of the Woods. Minn. Bot. Studies, 1:968. 1897.

MAGNIN, A.

Recherches sur la végétation des lacs du Jura. Rev. Gen. Bot., 5:241, 303, 515. 1893.

MARLOTH, R.

Ueber mechanische Schutzmittel der Samen gegen schädliche Einflüsse von aussen. Engler Jahrb., 4:247. 1883.

MEIGEN, F.

Beobachtungen über Formationsfolge im Kaiserstuhl. Deut. Bot. Monat., 18:145. 1900.

Beobachtungen über Formationsfolge bei Freyburg a. d. Unstrut. Deut. Bot. Monat., 13. 1895.

Die erste Pflanzenansiedelung auf den Reblausherde, ib.

Die Besiedlung der Reblausherde in der Provinz Sachsen. Engler Jahrb., 21:212. 1896.

MERRILL, G. P.

Rocks, Rock-weathering and Soils. 1897.

MEYEN, F. J. F.

Grundriss der Pflanzengeographie. 1836.

MIDDENDORFF, A.

Die Gewächse Sibiriens. 1864.

MUELLER, P. E.

Bjärgfyrren (Pinus montana) Tidskr. pop. Fremst. 1871.

NAEGELI, C.

Das gesellschaftliches Entstehen neuer Species. Bot Mitth., 3:365. 1873.

Verdrängung der Pflanzenformen durch ihrer Mitbewerber. Bot. Mitth., 3:205. 1874.

NILSSON, A.

Om örtrika barrskogar. Tids. Skog., 93. 1896.

Nagra drag ur de svenska växtamhällenas utvecklingshistoria. 1899. Bot. Cent. Beih., 9:370. 1900.

NILSSON, N. H.

Einiges über die Biologie der schwedischen Sumpfpflanzen. Bot. Cent., 76:9. 1898.

PALACKY, J.

Pflanzengeographische Studien. Just ’89:169. 1864.

PFEFFER, W.

Pflanzenphysiologie. I, 1897; II, 1904.

PIETERS A. J.

The Plants of Lake St. Clair. Bull. Mich. Fish Com. 1894.

POUND, R., & CLEMENTS, F. E.

The Phytogeography of Nebraska. 1898. 2d ed. 1900.

A Method of Determining the Abundance of Secondary Species. Minn. Bot. Studies, 2:19. 1898.

The Vegetation Regions of the Prairie Province. Bot. Gaz., 25:381. 1898.

RAMOND, L. F. E.

Voyages au Mont-Perdu et dans la partie adjacente des Haut-Pyrénées. 1801.

RAUNKIAR, C.

Vesterhavets Ost- og Sydkysts Vegetation. Bot. Cent., 41:361. 1889.

SACCARDO, P. A.

Chromotaxia seu Nomenclator Colorum. 1891.

SACHS, J.

Abhandlungen über Pflanzenphysiologie. 1892.

SCHIMPER, A. F. W.

Ueber Schutzmittel des Laubes gegen Transpiration. Monatsb. Akad. Berl., 7. 1890.

Die indo-malayische Strandflora. 1891.

Pflanzengeographie auf physiologischer Grundlage. 1898.

SCHOUW, J. P.

Grundzüge einer Allgemeinen Pflanzengeographie. 1823.

SENDTNER, O.

Die Vegetationsverhältnisse des Bayerischen Waldes. 1860.

SENFT, F.

Der Erdboden. Just ’88:45. 1888.

SERNANDER, R.

Die Einwanderung der Fichte in Skandinavien. Engler Jahrb., 15:1. 1893.

SERNANDER, R., & KJELLMARK, K.

Eine Torfmoor Untersuchung aus dem nördlichen Nerike. Bull. Geol. Inst. Ups., 2:4. 1895.

SOLEREDER, H.

Systematische Anatomie der Dicotyledonen. 1899.

STEBLER, F. G., & SCHROETER, C.

Beiträge zur Kenntniss der Matten und Weiden der Schweiz. Engler Jahrb., 17:b17. 1893.

STREENSTRUP, J.

Geognostisk-geologisk-Undersögelse i det nordlige Själland. 1842.

STOSSNER, A.

Vegetationsverhältnisse von Annaberg und Umgebung. 1859.

SWARTZ, O.

Observationes Botanicæ. 1791.

TANFILJEW, G.

Ueber die Sümpfe des Gouv. Petersburgs. Engler Jahrb., 24:68. 1889.

THORNBER, J. J.

The Prairiegrass Formation in Region I. Rep. Bot. Surv. Nebr., 5:29. 1901.

THURMANN, J.

Essai de phytostatique appliquée à la chaine du Jura et aux contrées voisines. 1849.

TOURNEFORT, J. P.

Relation d’un Voyage du Levant. 1717.

TREUB, M.

Notice sur la nouvelle flore de Krakatau. Ann. Jard. Buit., 7. 1888.

TREVIRANUS, G. R.

Biologie. 1802.

VAUPELL, C.

De Nordsjaellandske Skovmoser. 1851.

VINES, S. H.

Lectures on the Physiology of Plants. 1886.

WABLENBERG, G.

De Vegetatione et Climate Helvetiæ Septentrionalis. 1813.

WARMING, E.

Ueber Sprossbau, Ueberwinterung und Verjüngung. Engler Jahrb., 5:56. 1884.

Fra Vesterhavskystens Marskegne. Vid. Medd. Foren. 1890.

De psammofile Vegetationer in Danmark. Vid. Medd. Foren. 1891.

Plantesamfund, Grundtrok af den oekologiske Plantegeografi. 1895.

WATSON, H. L.

Cybele Britannica: or, British plants and their geographical relations. 1847.

WEBER, C.

Ueber die Zusammensetzung des natürlichen Graslandes. Engler Jahrb., 16:b56. 1892.

Ueber Veränderungen in der Vegetation der Hochmoore unter dem Einflusse der Cultur. Engler Jahrb., 23:b15. 1894.

Ueber die Erhaltung von Mooren und Heiden Norddeutschlands in Naturzustande. Abh. Nat. Ver. Bremen, 15:263. 1901.

Ueber die Vegetation und Entstehung des Hochmoores von Augstumal. 1902.

WHITFORD, H. N.

The Genetic Development of the Forests of Northern Michigan. Bot. Gaz., 31:289. 1901.

WHITNEY, M. D.

Instructions to Field Parties and Descriptions of Soil Types. 1903.

WIESNER, J.

Biologie der Pflanzen. 1889.

WILLDENOW, K. L.

Allgemeine Bemerkungen über den Unterschied der Vegetation auf der nördlichen und südlichen Halbkugel der Erde. 1811.

WILLKOMM, M.

Grundzüge der Pflanzenverbreitung auf der Iberischen Halbinsel. 1896.

WOJEIKOW, A.

Die Einwirkung des Menschen auf die Natur. Engler Jahrb., 24:69. 1894.

ZIMMER.

Ueber Pflanzenwanderungen. 1871.

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Footnote 1:

BONNIER, G.

Les Plantes Arctiques Comparées aux Mêmes Espèces des Alps et des Pyrénées. Rev. Gen. Bot. 6:505. 1894.

Cultures Expérimentales dans les Alps et les Pyrénées. Rev. Gen. Bot. 2:514. 1890.

Recherches Expérimentales sur l’Adaptation des Plantes au Climat Alpin. Ann. Nat. Sci. 7:20:218. 1895.

BONNIER, G., ET CH. FLAHAULT

Modifications des végétaux sur l’influence des conditions physiques du milieu. Ann. Nat. Sci. 6:7:93. 1878.

Footnote 2:

GARDNER, F. D. The Electrical Methods of Moisture Determination in Soils. Bull. Div. Soils, 12:12. 1898.

Footnote 3:

HEDGCOCK, G. G. The Relation of the Water-Content of the Soil to Certain Plants, Principally Mesophytes. Rep. Bot. Surv. Nebr., 6:48. 1902.

Footnote 4:

BUNSEN, R., AND ROSCOE, H. Photometrische Untersuchungen. Poggendorff’s Annalen., 117:529. 1862.

WIESNER, J.

Photometrische Untersuchungen auf pflanzenphysiologischen Gebiete. Sitzb. Akad. Wiss. Wien., I, 1893. II, 1895.

Untersuchungen über das photochemische Klima von Wien, Cairo, und Buitenzorg (Java) Denksch. Kais. Akad. Wien., 64. 1896.

Untersuchungen über den Lichtgenuss der Pflanzen im arktischen Gebiete. Sitzb. Kais. Akad. Wien., 109. 1900.

Footnote 5:

REINKE, J. Bot. Zeit., 41:713. 1883.

Footnote 6:

MEYEN, F. J. F. Grundriss der Pflanzengeographie, 12. 1836.

Footnote 7:

Instructions to Field Parties and Descriptions of Soil Types, 35. 1903.

Footnote 8:

PFEFFER-EWART. Physiology of Plants, 1:13. 1900.

Footnote 9:

The Relation of Leaf Structure to Physical Factors. 1905.

Footnote 10:

JEVONS, W. A. The Principles of Science, 2:137. 1874.

Footnote 11:

Die Transpiration der Pflanzen, 14. 1904.

Footnote 12:

Recherches sur le Rôle Physiologique de l’Eau dans la Végétation. Ann. Nat. Sci., 7:20:65. 1895.

Footnote 13:

Observations on Stomata by a New Method. Proc. Camb. Phil. Soc., 9:303. 1897.

Footnote 14:

Zur Kenntnis des Pflanzenlebens schwedischer Laubwiesen. Beih. Bot. Cent., 18:311. 1904.

Footnote 15:

Lehrbuch der Oekologischen Pflanzengeographie. 2d ed., 196. 1902.

Footnote 16:

Physiologische Pflanzenanatomie. 3d ed., 537. 1904.

Footnote 17:

SACHS, J. Ein Beitrag zur Kenntniss der Ernährungsthätigkeit der Blätter. Gesammelte Abhandlungen über Pflanzenphysiologie. 1:355. 1892.

Footnote 18:

CLEMENTS, E. S. The Relation of Leaf Structure to Physical Factors. 1905.

Footnote 19:

DE VRIES, H. Die Mutationstheorie, 1:6. 1901.

Footnote 20:

POUND AND CLEMENTS. A Method of Determining the Abundance of Secondary Species. Minn. Bot. Studies, 2:19. 1898.

Footnote 21:

The Development and Structure of Vegetation, 84. 1904.

THORNBER, J. J. The Prairiegrass Formation in Region I. Rep. Bot. Surv. Neb., 5:29. 1901.

Footnote 22:

Pound and Clements. The Vegetation Regions of the Prairie Province. Bot. Gaz., 25:381. 1898.

Footnote 23:

Clements, F. E. and E. S. Herbaria Formationum Coloradensium. 1902.

Footnote 24:

Lehrbuch der Ökologischen Pflanzengeographie, 97. 1896.

Footnote 25:

Phytogeography of Nebraska, 1st ed., 101. 1898.

Footnote 26:

Pflanzengeographie auf physiologischer Grundlage, 208. 1898.

Footnote 27:

Die Vegetation der Erde. Engler Bot. Jahrb., 17:b55. 1893.

Footnote 28:

Thornber, J. J. The Prairiegrass Formation in Region I. Rep. Bot. Surv. Nebr., 5:36, 46. 1901.

Footnote 29:

Grundzüge einer allgemeinen Pflanzengeographie, 157. 1823.

Footnote 30:

Essai de phytostatique, etc. 1849.

Footnote 31:

l. c., 116. 1896.

Footnote 32:

l. c., 3. 1898.

Footnote 33:

Uber mechanische Shutzmittel der Samen gegen schädliche Einflüsse von aussen. Engler Bot. Jahrb., 5:56. 1883.

Footnote 34:

Essai Elémentaire de Géographie Botanique, 45. 1820.

Footnote 35:

Stationes Plantarum Amoen. Acad., 4:64. 1754.

Footnote 36:

Die Vegetation der Erde, 4. 1872.

Footnote 37:

When this word was first proposed, the author did not know that Briquet had already applied the term polytopism to this concept (Ann. Conserv. Bot. Gen., 5:73. 1901). Since polygenesis expresses the idea of origin, and applies to multiple origin in time as well as in space, it is retained as the name of this concept. Polytopic and monotopic are adopted for multiple and single origin in space respectively, and polychronic and monochronic are proposed for similar origin in time.

Footnote 38:

The Origin of Species, 186. 1859.

Footnote 39:

Rocks, Rock-weathering, and Soils, 300. 1897.

Footnote 40:

Relation d’un Voyage du Levant. 1717.

Footnote 41:

Clements, F. E. The Development and Structure of Vegetation. Rep. Bot. Surv. Nebr., 7:163. 1904.

Footnote 42:

Clements, F. E. The Development and Structure of Vegetation, 24, 27. 1904.

Footnote 43:

Clements, F. E. A System of Nomenclature for Phytogeography. Engler Jahrb., 31:b70:1. 1902.

Footnote 44:

The terms, oceanad, hylad, poad, eremad, etc., are proposed in place of oceanophyte, hylophyte, etc. They are much shorter and make consistent groups under the general term, ecad., i. e., habitat form.

Footnote 45:

CLEMENTS, F. E. The Development and Structure of Vegetation, 166. 1904.

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TRANSCRIBER’S NOTES

Page Changed from Changed to

84 144 square inches of water, with 144 cubic inches of water, with an opening ¼ × 12 inches at the an opening ¼ × 12 inches at the base base

234 as many readily be seen from the as may readily be seen from the fact that migration varies fact that migration varies inversely as the inversely as the

319 ochthi´um (ὄχφη, bank), a bank ochthi´um (ὄχθη, bank), a bank formation; ochthad, a bank plant formation; ochthad, a bank plant

1. Typos fixed; non-standard spelling and dialect retained. 2. Enclosed italics font in underscores. 3. Enclosed bold or blackletter font in =equals=. 4. The caret (^) serves as a superscript indicator, applicable to individual characters (like 2^d) and even entire phrases (like 1^{st}). 5. Subscripts are shown using an underscore () with curly braces { }, as in H{2}O.