Chapter X. Cryptogams

I. THEIR PLACE IN NATURE

=331. Order of development.=—All the forms that have hitherto claimed our attention belong to the great division of Spermatophytes, or seed-bearing plants, designated also as Phanerogams, or flowering plants. They comprise the higher forms of vegetable life, and because they are more conspicuous and better known than the other groups, they have been taken up first, since it is more convenient, for ordinary purposes, to work our way backward from the familiar to the less known, rather than in the reverse order.

But it must be understood that this is not the order of nature. The geological record shows that the simplest forms of life were the first to appear, and from these all the higher forms were gradually evolved. There is no sharp line of division between any of the orders and groups of plants, but the line of development can be traced through a succession of almost imperceptible changes from the lowest forms to the highest, and it is only by a study of the former that botanists have come to understand the true nature and structure of the latter.

=332. Basis of distinction.=—Cryptogams, or seedless plants as a whole, are distinguished from the phanerogams by their simpler structure and by their mode of propagation, which in the former is by means of spores, while in the phanerogams it is by seeds. A spore is a simple organic body, consisting usually of a single cell which separates from the parent plant at maturity and gives rise to a new individual. A seed is a complicated, many-celled structure, containing within itself the rudimentary structure of a new plant already organized.

Beginning with the simplest forms, cryptogams are grouped in three great orders:—

=333. I. Thallophytes=, or thallus plants.—This group takes its name from the thallus structure that characterizes its vegetation. In its typical form, a thallus is a more or less flat, expanded body, of which the lichens and liverworts offer familiar examples among land plants, and the kelps and laminarias among seaweeds. It may be of any size and shape, however, and sometimes consists of a mere filament, as in the common brook silk, or even of a single cell (Fig. 429). The term is applied in general to the simplest kinds of vegetable structure, in which there is no differentiation of tissues, and no true distinction of root, stem, and leaves. While it is not peculiar to the thallophytes, it has attained its most typical development among them, and the name is therefore retained as distinctive of that group. It embraces two great divisions, the Algæ and Fungi. The first includes seaweeds and the common freshwater brook silks and pond scums, besides numerous microscopic forms whose presence escapes the eye altogether, or is made known only by the discolorations and other changes caused by them in the water. To the fungi belong the mushrooms and puffballs, the molds, rusts, mildews, and the vast tribe of microscopic organisms called bacteria, which are so active in the production of fermentation, putrefaction, and disease.

=334. II. Bryophytes=, or moss plants.—This group likewise contains two main divisions, Mosses and Liverworts. Familiar examples of the latter are the flat, spreading green plants, bearing somewhat the aspect of lichens, met with everywhere on wet rocks and banks around shady watercourses. The name is a reminiscence of their former use in medicine as a specific for diseases of the liver, and not, as in the case of the liver leaf, of a fancied resemblance to that organ.

Mosses are one of the best defined of botanical orders, and are easily recognized by their slender, leafy fruiting stalks, growing usually in dense, spreading mats, and presenting every appearance of a highly organized structure, well differentiated into root, stem, and leaves.

The liverworts represent the more primitive division of the group, and in some of their forms approach so near the thallophytes that it is not difficult to recognize them as connecting links in the same chain of life. Their relationship to the next higher group is not clear, but while they represent a more primitive stage of evolution than the mosses, the development of the latter has followed a course divergent from the main line of evolutionary progress.

=335. III. Pteridophytes=, or fern plants, are classed roughly in the three divisions of ferns, horsetails, and club mosses. They differ greatly in structure, but all possess a vascular system, and a well-organized structure of root, stem, and leaves. They rank next to the spermatophytes in the order of development, and the group is of especial interest on account of its relationship to the higher plants. One of its divisions, the club mosses, has probably given rise to at least one section of the gymnosperms, while the ferns are regarded as the ancestors of the true flowering plants, which make up the great class of angiosperms, and represent the highest type of evolution yet attained in the vegetable kingdom.

II. THE ALGÆ

MATERIAL.—Simple forms of green algæ can be found on the shady side of tree trunks, damp walls, old fence palings, and the outside of flowerpots. Pleurococcus, one of the commonest kinds, occurs as a green, powdery mat or felt in damp places, and is often accompanied by protococcus, another good specimen for study. Spirogyra and other filamentous algæ can be found in stagnant pools and ditches and in old rain barrels.

APPLIANCES.—Eosin solution, nitric acid, alcohol, iodine solution; a white china plate; a hand lens; a compound microscope, and slides.

=336. Variety of forms.=—This group embraces plants of the greatest diversity of form and structure, from the minute volvox and desmids that hover near the uncertain boundaries dividing the vegetable from the animal world, to the giant kelps of the ocean, which sometimes attain a length of from six hundred to one thousand feet. They are usually classed according to their color, as green, brown, and red algæ, including various subdivisions of each group. They all contain chlorophyll, by means of which they manufacture their own food, though in the red and brown divisions it is masked by the presence of other pigments—an adaptation to the modified light that reaches them at various depths under water. With few exceptions they can live only in the water, and unlike any other form of plant life, attain their highest development in the salty depths of the ocean. The freshwater forms are small and inconspicuous, and generally of a more simple type than the seaweeds. The great majority of them belong to the two classes of green and blue-green algæ. The former is believed to have furnished the type from which the higher plants have been evolved.

=337. Study of a one-celled alga.=—Put a little of the green algæ in water on a glass slide. Hold up to the light, or over a sheet of white paper, and examine with a hand lens; then place under the microscope. It will probably be found to contain a number of minute organisms, but the pleurococci can be recognized as small round bodies of a bright green color, some of them separate, others adhering together in groups of two, four, or more, with the sides that are in contact slightly flattened. Each of these bodies is an individual plant consisting of a single cell, whence they are said to be unicellular. Draw one of the single cells and one of the groups, or colonies, as they appear under the microscope. Try to make out the cell wall and the nucleus, and label all the parts (see 7). If you have any difficulty in distinguishing the cell wall, drop a little glycerine or salt water on the slide. This will cause the cell contents to shrink by osmosis (56, 59). Can you make out the structure of the cell colonies? They have resulted from the peculiar mode of multiplication that prevails among this class of plants. A cell elongates, contracts in the middle, and divides into two parts, each of which becomes an independent plant like the mother cell. See if you can find one in the process of division. The daughter cells repeat the process, each one giving rise to two new individuals, and so on indefinitely. The new cells do not always separate immediately on their formation, but frequently adhere together for a time, in colonies, before falling away and beginning an independent existence.

=338. Reproduction by fission.=—This kind of reproduction is called fission, or cell division, and marks a very primitive stage of development. Under stress of adverse conditions the cells formed by division may remain inactive for a time. They are then called resting spores, and when more favorable circumstances arise, they begin again their work of reproduction and growth as actively as ever.

=339. Meaning of the name.=—The suffix coccus is a Latin noun (plural cocci) meaning a grain or berry, and is a general term applied to any small, round organism consisting of a single cell; hence, micrococcus, a minute round body; protococcus, a primitive form, or prototype of one-celled bodies; and pleurococcus, which may be freely translated “a one-sided little round body,” from the flattening of the adjacent sides during fission—pleuro meaning lateral, or pertaining to the side.

It is important to remember this definition, as the term coccus is of very frequent occurrence in works of biology, as a suffix for designating small round bodies of various kinds.

=340. Examination of a filamentous alga.=—Place on a white dish a few drops of water containing some of the green pond scum common in stagnant pools and ditches. Examine with a hand lens; of what does it appear to consist? Are the filaments all alike, or are they of different lengths and thickness? Soak a number of them in alcohol for half an hour and examine again; where has the green matter gone? Do these algæ contain chlorophyll? (336; Exp. 65.) This class are called filamentous algæ on account of their slender, threadlike thalli, which look like bits of fine floss floating about in the water. The bubbles of oxygen which they sometimes give off in great abundance cause the frothy appearance that has given rise to their popular name, “frog spit.”

=341. Spirogyra.=—The filamentous algæ are very numerous, and a drop of pond scum will probably contain several kinds. At least one of these, it is likely, will be a Spirogyra, as this is one of the commonest and most widely distributed of them all. Place a filament under the microscope and notice the spiral bands in which the chlorophyll is disposed within the cells. It is from this spiral arrangement that the species takes its name. Do you notice any roundish particles inclosed in the chlorophyll bands? Test with a little iodine solution and see what they contain. Each filament will be seen, when sufficiently magnified, to consist of a number of more or less cylindrical cells joined together in a vertical row, and thus forming the simple threadlike thallus which characterizes this class of algæ. Physiologically, each cell is an independent individual, and often exists as such. Can you see the cell nucleus? If not, place a few filaments in a solution of eosin and add a drop of acetic acid to give the solution a pale rose color. After twenty to thirty minutes, examine again; the nucleus will be stained a deep red. If you can find an unbroken filament, examine both ends to see whether there is any differentiation of base and apex.

=342. Conjugation.=—See if you can find two filaments sending out lateral protuberances toward each other. Watch and notice that after a time these projections come together and unite by breaking down the cell walls dividing them, the protoplasm in each contracts, the contents of one pass over into the other, and the two coalesce, forming a new cell but little, if any, larger than the original conjugating bodies. This cell germinates under favorable conditions and produces a new individual. This method of reproduction is known as conjugation. The cells thus produced by the union of the contents of two separate cells may either germinate at once, and give rise to new individuals, or remain quiescent for a time, as resting spores.

Practical Questions

1. Are any of the green algæ parasitic? How do you know? (186, 336.)

2. Why is their presence in water regarded as denoting unhygienic conditions?

3. Mention some of the ways in which their presence may contribute to the contamination of drinking water.

4. Refer to Exp. 66, and account for the bubbles and froth that usually accompany these plants in the water.

5. Can you suggest any other causes than the evolution of oxygen that might produce the same effect?

6. Is the presence of these gas bubbles of any use to floating plants?

III. FUNGI

=343. Classification.=—In the fungi the thallus structure is greatly modified, appearing usually as a network of fine threads called the mycelium (pl., mycelia), from a Greek word meaning “fungus” (369). These plants are all, with a few doubtful exceptions, parasites or saprophytes which contain no chlorophyll and are incapable of supporting an independent existence. Biologists are divided as to their position in the genealogical tree of life. The weight of authority at present inclines to the view that they are degenerate forms derived from the algæ, but they have been so modified by their parasitic habits as to render their position in the general scheme of life a doubtful one. They represent an offshoot, or side branch, as it were, of the great evolutionary line, and so may be considered for the present as standing apart in a class by themselves.

=344. Numbers and variety.=—Fungi exceed every other class of living organisms both in the number of species and of individuals composing them. They include such diverse forms as bacteria, molds, rusts, mildews, mushrooms, and the like, ranging in size all the way from the giant puffball, a foot or more in diameter, to the almost inconceivably minute influenza bacillus, of which nearly two thousand million can inhabit a single drop of water without inconvenient crowding!

=345. The parasitic habit.=—But while their life history is obscure and hard to trace, the fungi are, as a class, well differentiated by their parasitic habit. They contain no chlorophyll, can manufacture no food, and consequently have to obtain it ready-made from the tissues of living or dead animals and plants. On this account they are active agents in the production of disease and decay, especially certain of those manifold forms that have been grouped together under the general head of bacteria. While not responsible for all the disease known to be caused by living organisms,—some very serious ones, such as malaria and cattle fever, being due to animal parasites,—the majority of those that have been most carefully investigated are traced to the bacteria, or other fungi. After any of these parasites have found a lodgment in the body of an organism whose tissues furnish them a congenial habitat, they multiply with enormous rapidity, and through the action of certain poisons called toxins, which they excrete, give rise to the most destructive diseases in both animals and plants; and no rational sanitary science is possible without a knowledge of their habits and life history. Add to the vast amount of human suffering that is to be laid at their door the economic damage done by rust and smut fungi, by molds and blights and mildews, and we shall be tempted to conclude that the “battle of life” is largely a struggle against these invisible foes.

=346. Useful fungi.=—Not all fungi, however, are injurious. On the contrary, the great majority of them are harmless, and very many kinds are positively beneficial to man. Without the yeasts and bacteria of fermentation we could not have our bread and cheese. Other forms are active agents in the fertilization of soils, it having been estimated that there are 100,000 or more of these infinitesimal laborers at work in every cubic centimeter (about ¹⁄₁₆ of a cubic inch) of virgin soil! Even the bacteria of putrefaction, which we are accustomed to regard as the embodiment of all that is foul and loathesome, are engaged in an unceasing work as scavengers, without which life would no longer be possible on our globe, as will be shown in the following section.

A. BACTERIA

MATERIAL.—A vessel of water in which hay has been left to soak for several hours; a freshly boiled potato.

APPLIANCES.—A double boiler for sterilizing; a number of clean glass jars and bottles; cotton wool for stoppers; a compound microscope.

CULTURE MEDIUMS.—A freshly boiled potato answers very well for ordinary purposes. “Bread mash” can be made by drying some bread crumbs in an oven, then mashing and mixing them to a paste with boiling water; sterilize by three successive heatings in a double boiler. A sterilized preparation of gelatine solution is the medium most commonly used.

=347. How to obtain specimens for observation.=—While bacteria are plentiful almost everywhere, it is not always easy to capture them just when and where you want them. For this purpose, put some hay in water and leave in a warm place away from the light until the liquid becomes cloudy or a film forms on the surface. This will show that bacteria are present. If it is desired to study any particular kind of bacterium, inoculate one of the culture mediums described under “material,” or a few drops of sterilized extract of beef, with a small quantity of the substance to be examined, or with dust or scrapings from the locality under consideration.

EXPERIMENT 93. BY WHAT MEANS ARE BACTERIA COMMONLY DISTRIBUTED?—Put a slice of freshly boiled potato into each of three glass tumblers and cover with a filter of cotton wool held in place by tying tightly with a cord, or by an elastic band. Set them all in a vessel of water, bring it to a boil, and keep at that temperature for half an hour, to sterilize the air in the tumblers. When they have cooled, lift the cotton from (1) for a minute or two and then replace. Carefully pass the tip of a medicine dropper through the filter of (2) so as to prevent the entrance of unsterilized air, and put on the slice of potato a small quantity of the bacterial liquid prepared as directed in the last paragraph. Leave (3) unopened. Keep all together in a warm, dark place and observe at intervals of from 12 to 24 hours. Do any bacteria appear in (3)? Do any appear on the potato in (2), where the liquid was dropped? Are they more, or less abundant than in (1)? Since cotton wool is entirely impervious to the smallest microörganisms known, would you judge from this experiment that bacteria can get into any place unless carried there by the air, or by some other means?

EXPERIMENT 94. CAN BACTERIA BE CARRIED BY PURE AIR?—On a warm (and preferably cloudy) day, put a slice of potato on a plate, and leave uncovered in an unused room or closet, free from dust, and kept carefully closed. Put another slice arranged in exactly the same way in an open window on a dusty street, or in a room that is used and daily swept and dusted. Do bacteria appear in the first plate? In the second? Is air free from dust a good conveyor of bacteria?

EXPERIMENT 95. WHAT CONDITIONS ARE FAVORABLE TO BACTERIAL GROWTH?—Strain some of your culture liquid into half a dozen small bottles of the same size, filling each about half full. Put (1) in a dark, cool place—on ice, if the weather is warm; (2) in a dark, warm place; (3) in a warm, well-lighted place; into (4) put a drop of carbolic acid, formalin, corrosive sublimate, or boracic acid, and keep in a dark, warm place. Keep (5) in boiling water for half an hour or more, and then place beside (2). Keep (6) in a freezing mixture of salt and ice for several hours, then place with (2) and (5). Examine all at intervals of from 12 to 24 hours. In which bottles is the presence of bacteria indicated by cloudiness of the contained liquid, or the formation of a surface film? In which do they appear first? In which most abundantly? In which last, or not at all? What is the effect of light and darkness on their growth? Of heat and cold? Of disinfectants? Name the circumstances that tend to hinder their growth, in the order of their efficacy.

=348. Microscopic study of bacteria.=—Put a drop of hay infusion on a slide and examine with the highest power of the microscope. You will see a multitude of very small glistening bodies including different kinds of bacteria, a majority of which are probably the hay bacillus, B. subtilis, shown in Figs. 443, 444. Notice that some forms move about freely, while others are non-motile. Which kind are the more numerous? The motion may be either mechanical, resembling that of the small dust particles we see dancing about in the sunshine, or apparently voluntary, and caused by the vibration of little whiplike cilia. Can you distinguish the two kinds? Try to make out clearly the different shapes you see. Some appear as slender chains or filaments, but this is due to the individual cells’ adhering together for a time before breaking up and beginning an independent existence. The small, rounded bodies, like a period (Fig. 438), are cocci; the slender, rod-shaped ones—sometimes slightly curved (Fig. 440)—are bacilli (sing., bacillus); the comma-shaped ones, and those generally showing a slight spiral curvature, are vibrios (Fig. 441); the spirally twisted ones, like a corkscrew (Fig. 442), are spirilli (sing., spirillum). These are the principal forms which it is important to distinguish and remember. The names are applied very loosely, however, in practice, bacillus being often used as a general term applicable to almost any kind,—the spirillum of cholera, for instance, being commonly known as the cholera bacillus, while by some authors vibrios are ranked as a variety of spirillum.

=349. Life history of a typical bacterium.=—A pure culture of the Bacillus subtilis can easily be obtained by boiling some of the hay infusion for half an hour and then leaving in a warm place till the usual indications of the presence of bacteria appear (347). The spores of this micro-organism are so resistant that they can withstand the temperature of boiling water for several hours, while those of most other forms of bacteria are killed by a few minutes’ exposure to it; hence, the crop that develops after boiling will consist of a pure culture of the hay bacillus.

In their active state these organisms will be seen to consist of single-celled, rod-shaped bodies, about three or four times as long as broad, and generally cohering in bands or filaments, as shown in Fig. 444, c. The black dots within the cells are the spores. Each individual bacterium produces but a single spore, or rather becomes a spore itself, by the contraction of its contents and the formation around them of a strong inclosing membrane. On germinating, the spores give rise to little ciliated, one-celled organisms called “swarm spores,” that swim about freely in the containing medium and multiply rapidly for a time by cell division. After this they pass again into the quiescent state, ready, whenever favorable conditions arise, to begin anew the repetition of their life cycle, which is an irregular alternation of cell division and spore formation.

=350. Resistance of spores.=—Bacteriologists are not fully agreed as to the cause of spore formation, some holding that it takes place only when conditions are most favorable for bacterial growth, others claiming the reverse. The consensus of opinion at present is toward the view that the spores are a provision for tiding over periods of stress and difficulty. They are capable of retaining their vitality for a long time, and are much harder to kill than the bacterial cells in their ordinary vegetative state, as was seen in the case of the hay bacillus. The spores of one species of potato bacillus have retained their vitality after four hours of boiling, and those of the typhoid bacillus after continuous exposure to a freezing temperature for more than three months. The majority of bacteria, in their vegetative state, are, however, either killed or rendered inert by temperatures ranging below 10° or above 50° centigrade—equivalent to about 50° and 122° Fahrenheit, respectively. It is easy to see what important bearing these facts have on the process of disinfection.

=351. Reproduction and multiplication.=—The ordinary mode of reproduction in bacteria, as in other unicellular organisms, is by fission (337, 338). As each individual forms but a single spore, no increase in numbers could take place by this means alone. Hence, while the spores are an important factor in the preservation of the species by continuing its existence under conditions which the active organisms could not survive, their successful propagation depends on their power of rapid multiplication by division. If this process were to go on unchecked, every hour, in an unbroken geometrical progression, the progeny of a single bacterium would, in 24 hours, number nearly 17 million; in 25 hours, 34 million; in 26 hours, 68 million, and in five days they would cover the entire surface of the globe, land and sea, to a depth of 3 feet! In ordinary standard milk sold by dairymen, and containing, when examined, less than 10,000 microbes to the cubic centimeter,—about 20 drops,—the number was found to have increased after 24 hours to 600 million. It is comforting to know, however, that the majority of these are of the harmless kinds which are the active agents in the making of buttermilk and cheese.

The effects of their rapid multiplication will be better appreciated when we consider that bacteria are the smallest of known living creatures. If 1000 of the influenza bacilli were spread out in a single layer with their sides touching, but not overlapping, they would not take up more room than one of the periods used in punctuating this book; and a coccus concerned in a tubercular disease prevalent among cattle in South America has recently been discovered, of which double that number could be accommodated in the same space.

=352. Distribution of bacteria.=—Ordinary air, when free from dust, contains, on the average, not more than five germs to the liter—equal to about 1 for every 12 cubic inches. Pathogenic, or disease-producing, germs seldom occur in ordinary fresh air, and even when present are, under ordinary circumstances, harmful only to people whose bodies, by reason of physical weakness or unhygienic habits, offer a congenial soil for their multiplication. Numerous instances are known in which perfectly healthy persons have carried about infectious disease germs in their bodies and even transmitted them to others without experiencing any inconvenience, or even being aware of their presence. Among others, the germs of pneumonia, diphtheria, and tuberculosis are often found in the mouth, nose, and sputum of perfectly healthy persons. There are also a number of bacteria that are regular inhabitants of the mouth, some of which are the cause of decayed teeth and foul breath. One form of bacterium, concerned in the production of inflammation and abscesses (Staphylococcus) is so constantly present on the human epidermis that one authority has declared it impossible to sterilize the skin so thoroughly as to free it entirely of this microbe. It is ordinarily not harmful unless it comes in contact with open wounds and abrasions.

=353. The economic importance of bacteria.=—It is hard to say whether these organisms concern us most on account of the damages attributable to them on the one hand, or the benefits we owe them on the other. If they were all as harmful as the pathogenic kinds, life would hardly be possible on the globe, while without their presence life as we know it would have ceased to be possible long ago. They are nature’s great army of scavengers, the sole agents of decomposition, without which dead organic matter would be subject only to the slow changes by which the rocks and mineral matter of the earth’s crust are disintegrated, and the undecomposed bodies of the vast procession of plants and animals that have existed since life first began on our globe would long ago have cumbered its surface to such an extent as to render impossible the continued development of life such as we know.

=354. Sterilization= is the process of ridding a substance of living microörganisms. To do this effectively, the process must be repeated several times at intervals, so as to give any spores that may have survived previous applications time to pass into the vegetative state, when their power of resistance is diminished and they are more easily destroyed. The incubation period, as the time required for the germination of the spores is called, is different for different kinds of bacteria; hence the importance, from a sanitary point of view, of a thorough knowledge of their life history.

=355. Disinfection= is sterilization on a large scale, and the same principles apply to both. Heat is the safest disinfectant for objects that will bear it, if continued long enough and repeated often enough at a sufficiently high temperature. Freezing will destroy some kinds of germs and check or retard the development of nearly all, but is not to be relied on as a permanent germicide, since even among flowering plants there are many kinds, not only of seeds, but of perennial vegetative forms that are capable of enduring an arctic temperature of many degrees below freezing for long continued periods.

Chemical disinfectants act usually as microbe poisons, and are unsuitable as sterilizers for food, though valuable in the purification of houses, clothing, and utensils—especially the instruments employed in surgical operations.

The prevention of the growth of bacteria, especially in wounds and surgical incisions, whether by means of chemical or physical agencies, is known as antisepsis.

Practical Questions

1. Why should a person recovering from an ague continue for some time after to take quinine every third or every seventh day? (350, 354.)

2. Name some of the principal diseases produced by bacteria.

3. What is the principle to be acted on in the avoidance of such diseases? (Exps. 94, 95.)

4. Are the same means equally effective for prevention and for cure? (354, 355; Exps. 93-95.)

5. Why is “fresh air” beneficial in a sick room? (352; Exp. 94.)

6. Does it act as a disinfectant, or as a mere diluent of the infected air of the room? (352.)

7. Why ought preserved fruits and vegetables to be scalding hot when put into the can? (355.)

8. Why is it necessary to exclude the air from them? (Exps. 93, 94.)

9. Reconcile question 8 with question 5.

10. Why does the use, for drinking purposes, of water that has been boiled render a person less liable to infectious diseases? (355.)

11. Was the old-fashioned practice of handing the baby round to be promiscuously kissed by friends and neighbors a good one for the baby? (352.)

12. Why is the spitting habit to be condemned? The use of common drinking cups in schoolrooms and other public places? (352.)

13. Is it proper from a sanitary point of view that roommates at a boarding school, or even members of the same family, should use soap, towels, and other articles of the toilet in common? (352.)

B. YEASTS

MATERIAL.—A piece of fresh baker’s yeast, some warm water, and a little honey or sugar solution; a pipette, or a medicine dropper; three or four clean pint bottles or preserve jars.

To raise a crop of yeast fungi for observation, rub one fourth of a fresh yeast cake in water enough to make a paste; add one pint of water, with a tablespoonful of honey or sugar, and stir well.

EXPERIMENT 96. WHAT CONDITIONS FAVOR THE GROWTH OF YEAST?—Pour equal parts of the liquid made as directed (see Material) into each of three pint bottles, stopper lightly, and label. Put (1) in a warm, dark place; (2) in a cool, dark place; and (3) in a bright light in a warm place. Observe at intervals of a few hours the changes that occur in each. Notice the bubbles that rise from the liquid. In which bottle do they form most rapidly? Lower a lighted match into it, or transfer some of the gas with a pipette into a vessel containing limewater, and tell what it is. Taste some of the fermenting liquid. Is it sweet? What has become of the sugar that was put into it?

=356. Yeasts and ferments.=—Yeasts belong to a very different order of fungi from the bacteria, but on account of their simplicity of structure and the similarity of their action to that of some of the latter, it is usual to consider them together. They are the active agents of fermentation, and include a large number of species. The kind used for household purposes is the same as that employed in making beer. Of this species there are many varieties, each one of which gives a characteristic taste to the beer made from it; and brewers, by paying attention to the cultivation of yeasts, give their product the special flavors peculiar to the different brands. This kind of yeast is not known to exist except in a state of cultivation, and probably owes its survival and present condition of development to a symbiosis with man, on account of its usefulness in bread making, and still more, perhaps, to its part in the gratification of his bibulous propensities, for among savage tribes the manufacture of fermented liquors is practiced long before the wholesome art of bread making.

There are other yeasts existing in a state of nature, such as those on the surface of fruits, which cause the latter, under certain circumstances, to ferment and decay. For this reason artificial ferments are not needed in making wine and other alcoholic liquors from fruits. Fermentation is also caused by certain forms of bacteria, as in the formation of vinegar and the souring of milk. Such bacteria often contaminate the yeast ferments.

=357. Microscopic examination.=—Place a drop of the cultural liquid on a slide and examine under the highest power of the microscope. What do you see? These egg-shaped bodies are yeast plants, unicellular organisms like the pleurococcus. Do you see any chlorophyll? Are the yeasts parasitic? How do you know? What do they live on? (Suggestion: What food substance that has disappeared was put into the culture liquid?) In getting their nourishment from the sugar, these fungi disintegrate it into alcohol and carbon dioxide, which is a process of fermentation. It is the bubbles of gas that were seen rising in the liquid which cause beer to effervesce and bread to rise. They permeate the dough and by their expansion produce the sponginess peculiar to leavened bread. Look for a cell with a bud forming on it; from what part does it appear to grow? Where a number of buds remain for some time attached to the mother cell (Fig. 449), they form a colony. Make a sketch of a single cell and of a colony of two or more adherent ones, labeling all the parts. If the cell wall cannot be made out clearly, run a little glycerine, or salt water, under the cover glass with a medicine dropper. What causes the contents of the cell to contract and leave the wall? (56, 59.)

=358. Reproduction.=—From time to time buds break away from the mother cell and form new individuals or colonies of their own. This process is called multiplication by budding, and is only another form of cell division.

Whenever reproduction takes place by other means than seeds or spores, it is said to be vegetative. This sort of reproduction is not confined to unicellular plants, but exists also among the phanerogams, the propagation of species by means of buds, tubers, rootstocks, runners, grafting, and the like being variations of the same process. On the other hand, yeasts and bacteria and the unicellular algæ have the power, under extreme conditions, to form resting spores, which sometimes lie dormant for years and resume their activity when favorable conditions return.

Practical Questions

1. When is fermentation useful to man?

2. What is the effect on canned fruits and vegetables if yeast cells get into them?

3. Why does milk turn sour in warm weather? (350, 351; Exp. 96.)

4. Why do buttermilk and clabber spoil if left standing too long? (345, 356.)

5. What causes bread to be “heavy”? (356, 357.)

6. Why will dough not rise unless kept in a warm place? (Exp. 96.)

7. Why is beer kept cold during fermentation? (350, 356.)

C. RUSTS

MATERIAL.—A leaf of wheat affected with red rust; a leaf or a stalk with black rust. Some barberry leaves with yellowish pustules on the under side, which under the lens look like clusters of minute white corollas. These are popularly known as “cluster cups.” As the spots on barberry occur in spring, the red rust in summer, and the black rust in autumn, gather the specimens as they can be found, and preserve for use.

The orange leaf, or brown, rust (Puccinia rubigo-vera) is more common in some parts of the country than the ordinary wheat rust (Puccinia graminis), but the two are so much alike that the directions given will do for either. If the orange leaf-rust (so named from its color, and not from any connection with orange leaves, the logical relation of the words being orange leaf-rust, and not orange-leaf rust) is used, the cups and pustules should be looked for on plants of the borage family—comfrey, hound’s-tongue, etc. The orange leaf-rust of apple is caused by a fungus which will serve to illustrate the same class of parasites. The “teleuto” stage of this will be found on cedar trees, in the excrescences commonly known as “cedar apples”; the “cluster cups” on the leaves of apple and haw trees affected with the disease.

=359. Red rust.=—Uredo stage. Examine a leaf of “red rusted” wheat under the lens, and notice the little oblong brown dots that cover it. These are clusters of spore cases, and are the only part that appears above the surface. Viewed under the microscope, the red rust will be seen to consist of a mycelium (see Fig. 452), which ramifies through the tissues of the leaf and bears clusters of single-celled reddish spores that break through the epidermis and form the reddish brown spots and streaks from which the disease takes its name. These spores, falling upon other leaves, germinate in a few hours and form new mycelia, from which, in six to ten days, fresh spores arise. Formerly this was thought to complete the life history of the fungus, to which the name of Uredo was given. It is now known, however, that the red rust is merely a stage in the life cycle of the plant, and to this stage the old name uredo is applied, the spores being called uredospores.

=360. Black rust.=—Teleuto stage. Next examine with a lens a part of the plant attacked by black rust. Do you observe any difference except in the color? Do the two kinds of rust attack all parts of the plant equally? If not, what part does each seem to affect more particularly? At what season does the black rust appear most abundantly? Place a section of the diseased part under the microscope and notice that the difference in color is due to a preponderance of long, two-celled bodies with very thick, black walls (Fig. 453). These are called teleutospores, a word meaning “final spores,” because they are formed only toward the end of the season. They are developed from the same mycelium with the uredospores, and are not a product of the latter, but collateral with them and belong to the same stage in the life history of the fungus. After they appear, the uredospores cease to be developed at all, and only the dark teleutospores are produced. These remain on the culms in the stubble fields over winter, ready to begin the work of reproduction in spring. For this reason the teleutos are popularly known as “winter spores” in contradistinction to the uredos, or “summer spores,” whose activity is confined to the warm months.

It was formerly supposed that black rust was caused by a different fungus from that producing red rust, and to it the name Puccinia was given. This has been retained as a general designation for all fungi undergoing these two phases, and the particular form of fungus that we are now considering is known in all its stages as Puccinia graminis.

=361. The nonparasitic stage.=—The formation of teleutospores completes that portion of the life history of the fungus during which it is parasitic on wheat and grasses of different kinds. In spring they begin to germinate on the ground, each cell producing a small filament, from which arise in turn several small branches. Upon the tip of each of these branches is developed a tiny sporelike body called a sporidium (Fig. 454), which continues the generation of the rust fungus through the next stage of its existence. The filament which bears these sporidia is not parasitic, but when the sporidia ripen and the spores contained in them are scattered by the wind, there begins a second parasitic phase, which forms the most curious part of this strange life history.

=362. The æcidium.=—Examine next the under side of some barberry leaves (or comfrey, etc., if orange leaf-rust is used) for clusters of small whitish bodies that appear under the lens like little white corollas with yellow anthers in the center. Examine a section of one of these under the microscope and notice that the yellow substance is composed of regular layers of colored spores. The corolla-like receptacles containing them, popularly known as “cluster cups,” are borne on a mycelium produced from the spores described in the last paragraph. This mycelium is parasitic on barberry or other leaves, according to the kind of fungus, and was long believed to be a distinct plant, to which the name Æcidium (pl., Æcidia) was given. This term is now applied to the cluster cups, and those fungi which at any period of their life history produce them are called æcidium fungi.

=363. Spermogonia.=—On the upper surface of the leaves that bear the æcidia, notice some small black dots hardly larger than pin points, but which, when sufficiently magnified, appear as little flask-shaped bodies (Fig. 455) under the epidermis. These are known as spermogonia, or pycnidia. When mature, they break through the epidermis so that the necks protrude, and discharge a quantity of minute cells or spores, very like some that, later on, we shall find playing an important part in the reproductive processes of certain other fungi, and of mosses and liverworts. In the rust fungi, however, their function is not understood. They may possibly be survivals of organs which were once active in the life processes of the plant, but have become useless under changed conditions. Do such organs throw any light on the evolutionary history of plants?

=364. Connection between barberry and wheat rust.=—With the discharge of the æcidium spores, the part of the life cycle of the fungus spent on the barberry comes to an end, and it is ready to begin the uredo-teleuto stage over again as soon as it finds a suitable host. Where there are no barberries, it is capable of propagating without them, either by adapting itself to some other host plant, or by omitting the æcidium stage altogether. The parasitic habit being an acquired one, the fungus, like some animal organisms that we know of, can often be “educated” by force of circumstances into tolerating, and even thriving upon, foods which under other circumstances it would reject. The wheat rust is known to be capable of propagating year after year in the uredo stage, the spores surviving through the winter on volunteer grains and grasses; and in no other country in the world does rust do greater damage to the wheat crop than in Australia, where the barberry is practically unknown. This power of accommodation possessed by many parasites is one of the difficulties the agriculturist has to contend with in the development of rustproof varieties.

=365. Polymorphism.=—Plants that pass through different stages in their life history are said to be polymorphic, that is, of many forms. The habit is very common among the lower forms of vegetation. The fact that one or more of the phases are sometimes omitted, as the æcidium phase of wheat rust in warm climates, suggests the idea that it may be of use in helping the plant to tide over difficult conditions. Besides giving better chances of obtaining nourishment, it probably has the same effect as cross fertilization among flowering plants, in imparting increased strength and vitality to the succeeding generation. Wheat rust produced from barberry æcidia is said to be much more vigorous—and consequently more destructive—than when derived from a uredo that has reproduced itself for several generations.

=366. The damage done by rust= to the host is through the destruction of its tissues by the mycelium. The chlorophyll is destroyed so that the plant can no longer manufacture food, and is too starved to produce good grain. There are many varieties of wheat rust, which have been found on twenty-seven different kinds of grain. Most of them are specialized to a particular host plant and will not, ordinarily (364), infest any other. Has this fact any bearing upon the production of rustproof varieties?

Practical Questions

1. Is a farmer wise to leave scabby and mildewed weeds and bushes in the neighborhood of his grain fields? (364, 365.)

2. Are there any objections to the presence of volunteer grain stalks along roadsides and in fence corners during winter? (364.)

3. Should cedar trees be allowed to grow near an apple orchard? Give a reason for your answer. (p. 317, Material.)

4. Should diseased plants be plowed under? (361.)

5. What disposition should be made of them?

6. Ought diseased fruits to be left hanging on the tree?

7. Why is it necessary to pick over and discard from a crate or bin all decaying fruits and vegetables?

8. Does a rotation of crops tend to prevent the spread of disease in plants? Give reasons for your answer.

9. Are rustproof varieties to be relied on indefinitely? (364.)

D. MUSHROOMS

MATERIAL.—Any kind of gilled mushroom in different stages of development, with a portion of the substratum on which it grows, containing some of the so-called spawn. The common mushroom sold in the markets (Agaricus campestris) can usually be obtained without difficulty. Full directions for cultivating this fungus are given in Bulletin 53 of the U. S. Department of Agriculture. From 6 to 12 hours before the lesson is to begin, cut the stem from the cap of a mature specimen, close up to the gills, lay it, gills downward, on a piece of clean paper, cover with a bowl or pan to keep the spores from being blown about by the wind, and leave until a print (Fig. 466) has been formed.

=367. Mushrooms and toadstools.=—The popular distinction which limits the term “mushroom” to a single species, the Agaricus campestris, and classes all others as toadstools, has no sanction in botany. All mushrooms are toadstools and all toadstools are mushrooms, whether poisonous or edible. The real distinction is between mushrooms and puffballs, the former term being more properly applied to fungi which have the spore-bearing surface exposed.

=368. Examination of a typical specimen.=—The most highly specialized of the fungi, and the easiest to observe on account of their size and abundance, are the mushrooms that are such familiar objects after every summer shower. The gilled kind—those with the rayed laminæ under the cap—are usually the most easily obtained. Specimens should be examined as soon after gathering as possible, since they decay very quickly.

=369. The mycelium.=—Examine some of the white fibrous substance usually called spawn through a lens. Notice that it is made up of fine white threads interlacing with each other, and often forming webby mats that ramify to a considerable distance through the substratum of rotten wood or other material upon which the fungus grows. This webby structure, often mistaken for root fibers, is the thallus or true vegetative body of the plant, the part rising above ground, and usually regarded as the mushroom, being only the fruit, or reproductive organ. Place some of the mycelium under the microscope and notice that it is composed of delicate filaments made up of single cells placed end to end, as in Spirogyra (341). These filaments are called hyphæ.

=370. The button.=—Look on the mycelium for one of the small round bodies called buttons (Fig. 457). These are the beginning of the fruiting body popularly known as the mushroom, and are of various sizes, some of the youngest being barely visible to the naked eye. After a time they begin to elongate and make their way out of the substratum.

=371. The veil and the volva.=—Make a vertical section through the center of one of the larger buttons after it is well above ground, and sketch. Notice whether it is entirely enveloped from root to cap in a covering membrane—the volva (Fig. 458, a)—or whether the enveloping membrane extends only from the upper part of the stem to the margin of the cap—the veil (Fig. 458, d); whether it has both veil and volva, or finally, whether it is naked, that is, devoid of both.

=372. The stipe, or stalk.=—Notice this as to length, thickness, color, and position; that is, whether it is inserted in the center of the cap or to one side (excentric), or on one edge (lateral). Observe the base, whether bulbous, tapering, or straight, and whether surrounded by a cup, or merely by concentric rings or ragged bits of membrane (the remains of the volva). Look for the annulus or ring (remains of the veil) near the insertion of the stipe into the cap, and if there is one, notice whether it adheres to the stipe, or moves freely up and down (Fig. 459, a); whether it is thick and firm, or broad and membranous so that it hangs like a sort of curtain round the upper part of the stipe (Fig. 467, a). Break the stem and notice whether it is hollow or solid; observe also the texture, whether brittle, cartilaginous, fibrous, or fleshy.

=373. The pileus, or cap.=—Observe this as to color and surface, whether dry, or moist and sticky; smooth, or covered with scurf or scales left by the remains of the volva, as it was stretched and broken up by the expanding cap (Fig. 459, p, p). Note also the size and shape, whether conical, expanded, funnel-shaped (Fig. 460), or umbonate—having a protuberance at the apex (Fig. 459)—or whether the margin is turned up at the edge (revolute, Fig. 467), or under (involute, Fig. 459).

=374. The gills, or laminæ.=—Look at the under surface and notice whether the gills are broad or narrow, whether they extend straight from stem to margin, or are rounded at the ends, or curved, toothed, or lobed in any way. Notice their attachment to the stipe, whether free, not touching it at all; adnate, attached squarely to the stem at their anterior ends; or decurrent, running down on the stem for a greater or less distance (Fig. 460).

=375. The hymenium.=—Cut a tangential section through one side of the pileus and sketch the section of the gills as they appear under a lens, or a low power of the microscope. Notice that the blade consists of a central portion called the trama (tr, Fig. 462) and a somewhat thickened portion, h, constituting the hymenium, or spore-bearing surface. Now examine, under a high power, a small section from the edge of a gill, including a bit of the trama. Notice that this last consists of a tissue of mycelial cells (Fig. 463) covered by the hymenium, or spore-bearing membrane, which is thickly clothed with a layer of elongated, club-shaped cells (b, b and p, p, Fig. 463) set upon it at right angles to the surface. Some of these put out from two to four, or in some species as many as eight, little prongs, each bearing a spore (s, s, Fig. 463), while others remain sterile. The spore-bearing cells are called basidia; the sterile ones, paraphyses; and the whole spore-bearing surface together, the hymenium, from a Greek word meaning a membrane. It is from the presence of this expanded fruiting membrane that the class of mushrooms we are considering gets its botanical name, Hymenomycetes, membrane fungi. The hymenium is not always borne on gills, but is arranged in various ways which serve as a convenient basis for distinguishing the different orders. In the tube fungi, to which the edible boletus belongs (Figs. 464, 465), the basidia are placed along the inside of little tubes that line the under side of the pileus, giving it the appearance of a honeycomb. In another order, the porcupine fungi, they are arranged on the outside of projecting spines or teeth, while in the morelles they are held in little cups or basins.

=376. Spore prints.=—When the gills are ripe, they shed their spores in great abundance. Take up the pileus that was laid on paper, as directed under Material, on page 323, and examine the print made by the discharged spores; it will be found to give an exact representation of the under side of the pileus.

=377. The spores.=—Notice the color of the spores as shown in the print. This is a matter of importance in distinguishing gill-bearing fungi, which are divided into five sections according to the color of the spores. One source of danger, at least, to mushroom eaters would be avoided if this difference was always attended to, for the deadly amanita (Amanita phalloides) and the almost equally dangerous fly mushroom (A. muscaria) both have white spores, while the favorite edible kind (Agaricus campestris), though white-gilled when young, produces dark, purple-brown spores that cannot fail to distinguish it clearly for any one who will take the trouble to make a print.

=378. Economic properties.=—Most of the wood-destroying fungi belong to this and allied orders. They are among the worst enemies the forester has to deal with (140), and millions of feet of lumber are destroyed every year by them.

Over seven hundred kinds of fungi growing in the United States have been described as edible, but the evil repute into which the whole class has been brought by the poisonous qualities of a few species, and the difficulty, to any but an expert, of distinguishing between these and the harmless kinds, has caused them to be generally neglected as articles of diet. While they are pleasant relishes and furnish an agreeable variety to our daily fare, their food value has been greatly exaggerated. They contain a large proportion of water, often over 90 per cent, and the most valued of them, the Agaricus campestris, is about equivalent to cabbage in nutrient properties.

Practical Questions

1. Why are mushrooms generally grown in cellars? (186, 343.)

2. Name any fungi you know of that are good for food or medicine or any other purpose.

3. Name the most dangerous ones you know of.

4. Do you find fungi most abundant on young and healthy trees, or on old, decrepit ones? Account for the difference. (141, 343, 378.)

5. Do you ever find them growing on perfectly sound wood anywhere?

6. Are they ever beneficial to a tree? (86.)

7. Is it wise to leave old, unhealthy trees and decaying trunks in a timber lot?

IV. LICHENS

MATERIAL.—Specimens can be found almost everywhere, growing on rocks, walls, logs, stumps, and trees. Some of the more common kind are: Parmelia, recognizable by the shallow spore cups borne on the upper surface of the thallus; Cladonia, by the little stalked receptacles, like goblets, in which its spores are held; Physcia, by its bright orange color. Where practicable, it is well to have several different kinds for comparison. Iceland moss (Cetraria islandica) can generally be obtained from the grocers, and is a good example of an intermediate form between foliaceous and fruticose lichens.

If the specimens are very dry, they will be too brittle to handle conveniently, and should be moistened by soaking a short time in water. This will render them quite flexible and also bring out the green color more clearly.

=379. Examination of a typical specimen.=—The commonest kind of lichens, and generally the most easily obtained, are those that grow on rocks and tree trunks in flat, spreading patches. Their margins are much dented and curled, giving them a somewhat leaflike appearance, whence they are called “foliaceous” lichens. This broad, expanded body is the thallus, or vegetative part, as distinguished from its reproductive part. Examine carefully the thallus of your specimen. Note the size and shape of the indentations. Is there any order or regularity about them, such as was observed in the lobing of leaves? Is there any difference in color between the upper and under sides? What other differences do you notice? Do you see anything like hairs, or rootlets, on the under side? Mount one of them in water and place under the microscope. What does it look like? Compare with one of the hairs from a leaf of mullein, gromwell, blueweed, or other hairy plant, with the hypha of a fungus mycelium, and with your study of the root hair in 67 (a). Is it a hair or a root? These rootlike hairs are called rhizoids, and serve to anchor the lichen to its substratum. Look on the upper side for little cup-shaped or saucer-shaped receptacles. On what part of the thallus are they situated? Examine with a lens and see if you can make out what they contain. These cups are the spore cases. The lichen fungus belongs to the division of sac fungi, which produce their spores in closed sacs, or cups.

=380. Structure of the thallus.=—Make a thin section through a thallus and place under the microscope. Notice the small green bodies enveloped in the hyphæ of the fungus. Are they most abundant near the upper or the lower epidermis? Has their green color anything to do with this, and with the difference in color between the two surfaces of the thallus? (184.) Do they look like chlorophyll granules? Can you tell what they are? Compare with your study of the unicellular algæ (337) and with Fig. 429. Does this throw any light on their real nature?

=381. The lichen thallus a composite body.=—You will probably have no difficulty in making out that these small round bodies are green algæ of some kind, but of what species will depend upon the kind of lichen with which it is associated. In Cladonia and the bearded lichen (Fig. 473), it is a protococcus; in other forms, a pleurococcus or a nostoc—and so on, each species of lichen fungus being specialized to a certain form of alga. The great botanist, De Bary, showed that it is even possible to produce a lichen thallus artificially by sowing the spores of a fungus among the cells of the particular alga with which it is able to unite. The spores will germinate without the alga, but soon perish unless they come in contact with the right one. It is thus made clear that the lichen plant as a whole is a combination of elements belonging to two distinct orders, the algæ and fungi, but so closely associated as to constitute practically a single individual.

=382. Slavery, or partnership?=—Now, what can be the object of this peculiar association? Is it a symbiosis, or a case of enslavement? The fungi, as we know, are all parasites, unable to manufacture their own food or to exist at all except at the expense of other organisms, living or dead. But the lichens have refined upon the gross rapacity of their order, and instead of indiscriminately destroying the hosts that furnish their nourishment, have used their victims to better purpose by converting them into contented, well-fed slaves! The imprisoned algæ perform for them the same service that the chlorophyll bodies do for the higher plants, and so the lichen fungi have the advantage of other parasites in getting their food manufactured at home, so to speak. And while the algæ have to do double work in order to feed both themselves and their masters, the fungus, in return, shelters them against cold and drought, and prolongs their growing period by giving them a more continuous supply of moisture and food materials, which it draws from the substratum by means of its rhizoids. In this way both plants are enabled to live in situations that neither could occupy without the other.

=383. Reproduction.=—The multiplication of the lichen algæ is exclusively vegetative. The fungus, on the other hand, reproduces normally by spores, and the fruiting bodies found on the thallus originate from the fungus mycelium.

=384. Classification.=—To be strictly accurate, the two kinds of vegetable bodies that make up the lichen thallus would probably have to be classified separately, as algæ or fungi, respectively, but as fructification is the generally accepted basis of classification, and the plant body is too intimately permeated with both kinds of tissue to be divided, each lichen body as a whole is classed with its particular kind of fungus. The entire group, on account of the distinctive characters that mark it, is placed in a separate order of its own. This includes three principal divisions, distributed according to the shape of the thallus, and its habit of growth: (1) Crustaceous, those that adhere closely to the substratum, as if glued or inscribed on it; (2) Foliaceous, with a broad, more or less lobed and leaflike thallus that adheres loosely to the substratum by means of rhizoids springing from its under surface; (3) Fruticose, with branching, stemlike thallus attached at the base like a regularly rooting plant (Figs. 473, 474). Among these are the Iceland moss, used as an article of food by man, and the reindeer moss (Cladonia rangiferina), which is the chief sustenance of the reindeer.

Practical Questions

1. Have lichens any economic value? (384.)

2. In what way are they most useful? (320.)

3. Do you find them, as a general thing, on healthy young trees and boughs, or on old ones, and those showing signs of decay?

4. Do you ever find them growing on trees or other objects in densely inhabited areas,—cities, large towns, and manufacturing centers?

5. Do they grow more thickly on the shady (northern) side of rocks, walls, and trees growing in the open, than on the sunny and (presumably) warmer sides?

6. Mention some ways in which a growth of lichens might be beneficial to a tree.

7. In what ways could it be harmful?

V. LIVERWORTS

MATERIAL.—Liverworts can generally be found growing with mosses in damp, shady places, and are easily recognized by their flat, spreading habit, which gives them the appearance of green lichens. Marchantia polymorpha (Fig. 475), one of the largest and best specimens for study, is common in shady, damp ground throughout the states. It is diœcious, and specimens bearing both male and female organs should be provided. Lunularia, a smaller species that can be recognized by the little crescent-shaped receptacles on some of the divisions of the thallus, is abundant in greenhouses on the floor, or on the sides of pots and boxes kept in damp places; but the spore-bearing receptacles are seldom or never present, the species being an introduced one and possibly rendered sterile by changed conditions. Anthoceros (Fig. 426) and leafy liverworts, such as that shown in Fig. 484, also make good examples for study.

EXPERIMENT 97. WHY ARE THE UPPER AND UNDER SIDES OF A LIVERWORT DIFFERENT?—Plant a growing branch of marchantia, or of any flat, spreading liverwort, in moist earth so that the upper side will lie next the soil, and watch for a week or two, noting the changes that take place. What would you infer from these as to the cause of any differences that may have been observed between the two surfaces?

=385. Examination of a typical liverwort=—The thallus.—The broad, flat, branching organ that forms the body of the plant is the thallus. Examine the end of each branch; what do you find there? Are the two forks into which the apex of the branches divides equal or unequal? Compare the growing end with the distal one; does it proceed from a true root? Notice that as the lower end dies, the growing branches go on increasing and reproducing the thallus.

Do you find anything like a midrib? If so, trace it through the branches and body of the thallus; where does it end? Does it seem to be formed like the midrib of a leaf? Hold a piece of the thallus up to the light and see if you can detect any veins. Is it of the same color in all parts, and if there is a difference, can you give a reason for it? Examine the upper surface with a lens. Peel off a piece of the epidermis, place it under a low power of the microscope, or between two moistened bits of glass, and hold up to the light, keeping the upper surface toward you; what is its appearance? Observe a tiny dot near the center of the rhomboidal areas into which the epidermis is divided and compare it with your drawings of stomata (181, 183). What would you judge that these dots are for? While differing in structure from the stomata of leaves, they serve the same purposes and may be regarded a more rudimentary form of the same organ.

=386. Rhizoids.=—Wash the dirt from the under side of a thallus and examine with a lens; how does it differ from the upper surface? Do you see anything like roots? Place one in a drop of water under the microscope. Compare with similar organs found on the lichen (379). What are they? Would rhizoids be of any use on the upper side? stomata on the under side?

=387. Gemmæ.=—Look along the upper surface for little saucer-shaped (in lunularia, crescent-shaped) cupules (g, g, Fig. 476). Notice their shape and position, whether on a midrib or near the margin. Examine the contents with a lens and see if you can tell what they are. These little bodies, called gemmæ, are of the nature of buds, by which the plant propagates itself vegetatively somewhat as the onion and the tiger lily do by means of bulblets. Sow some of the gemmæ on moist sand, cover them with a tumbler to prevent evaporation, and watch them develop the thalloid structure.

=388. The fruiting receptacles.=—Procure, if possible, thalli with upright pedicels bearing flattened enlargements at the top (Figs. 475, 476). These are thallus branches modified into receptacles containing the reproductive organs, which, in marchantia, are diœcious, the two kinds growing on separate thalli. Notice their difference in shape, one kind being slightly lobed or scalloped, the other rayed like the spokes of a wheel. The first kind are known as antheridial, or male, receptacles; the second as archegonial, or female.

=389. The antheridia.=—Examine one of the male receptacles on both surfaces and in vertical section. Notice the tiny egg-shaped bodies sunk in little cavities between the lobes just under the upper epidermis (Fig. 478). These are antheridia. When mature, they rupture at the apex, and multitudes of extremely small bodies, called antherozoids, or spermatozoids, are discharged from them.

=390. Archegonia.=—Next examine one of the female receptacles. Look on the under surface, between the narrow divisions of the receptacle, for radiating rows of flask-shaped bodies with their necks turned downward, and all surrounded by a toothed sheath or involucre (Fig. 479). These bodies are the archegonia, or female organs, and correspond, loosely speaking, to the ovaries of flowering plants. If the receptacle is a mature one, the archegonia will be replaced by the ripe spore cases (sporangia), as at f, Fig. 479.

Make enlarged drawings of the upper surface of a male and a female receptacle, and of a vertical section of each, passing through an antheridium in the male, and an archegonial row in the female receptacle. Label the parts observed in each.

=391. Minute study of an archegonium.=—Place under the microscope a very thin, longitudinal section through a ray of a receptacle containing a young archegonium, and observe that the latter consists of a lower portion, the venter, v, Fig. 480, and an upper part, the neck, which is perforated by the neck canal, ca. The venter contains the egg cell, o, and the ventral canal cell, vc. The neck canal is filled with small cells which, at maturity, dissolve into a mucilaginous substance that swells on being wet and discharges itself through the top of the neck, leaving an open passage to the venter, where the egg cell is ready to be fertilized.

Make a drawing of the section as seen under the microscope, labeling all the parts.

=392. Fertilization.=—In the liverworts, and in cryptogams generally, this process has to take place under water, as the antherozoids are motile only in a liquid, but the amount required is so small that a few drops of rain or dew will enable them to make their journey to the archegonium. The mucilaginous substances discharged from the neck canal attract them to the mouth of the opening, one or more of them penetrates to the egg cell, and fertilization is accomplished. Do you see any analogies between this and the same function among flowering plants? (250, 251.)

=393. The spore case.=—After fertilization the egg becomes an oöspore, capable of producing a new plant. Instead, however, of separating from the mother plant and giving rise to an independent growth, it germinates within the archegonium and produces there a small, stalked body, called a sporogonium, or sporophyte, which at length ripens into a spore case, as shown at f, Fig. 479. At maturity this capsule-like sporophyte ruptures at the apex, and discharges a mass of spores, mingled with elongated filaments called elators, which, by their elastic movements, assist in disseminating the spores. These latter, on germinating, produce, not a simple sporophyte like that which bore them, but the thallus of the liverwort with all its complicated arrangement of antheridia and archegonia and vegetative organs that seem to foreshadow, by the analogies they suggest, the coming of the higher plants.

=394. Sexual and asexual reproduction.=—We find here a very marked change from the simple reproductive processes observed in the algæ and fungi. In the forms thus far considered, this function was carried on mainly by simple vegetative fission or budding, with a more or less irregular intervention of resting spores. If only one kind of spore is concerned, reproduction is said to be asexual. When two different kinds of cells, the egg and sperm cell, unite to form an oöspore, as in the liverworts, reproduction is said to be sexual. While sexual reproduction takes place to some extent among both algæ and fungi, the prevailing method among thallophytes is asexual, and may be carried on in three different ways: by fission (and budding), by resting spores, and by conjugation.

Representing the plant body by A and the resting spores by a, the primitive asexual processes may be expressed to the eye by the accompanying formulas:—

(1) Fission and budding: A → A → A → A → (2) Resting spores: A a → A a → A a → (3) Conjugation: A + A → a → A + A → a →

In (3), as was seen in the conjugating cells of the spirogyra (342), the method is a little more complicated, showing an approach toward the sexual process. In each of these cases, however, there is only one kind of cell concerned, while in the liverworts there are not only different kinds, technically known as gametes, but specialized organs, archegonia and antheridia, for producing them. The thallus body bearing these organs is termed the gametophyte, because it bears the gametes, or sexual organs,—the suffix phyte meaning a plant; for example, epiphyte, on or upon plants; spermophyte, or spermatophyte, seed plant; sporophyte, spore plant. The sporophyte, produced within the archegonium, bears simple nonsexual spores that are capable of germinating independently. Structurally it is a separate, individual organism, though it does not appear as such in this class, but lives inclosed in the archegonium, as a parasite on the mother plant.

=395. Alternation of generations.=—If we represent the sporophyte by S, the thallus, or gametophyte, by G, the female gamete, or egg cell, by fg, the antherozoids (male gametes) by mg, the fertilized egg cell, or oöspore, resulting from their union by oös, and the asexual spores discharged from the sporophyte by o, this complicated mode of reproduction may be expressed diagrammatically as follows:—

fg fg ╱ ╲ ╱ ╲ G oös→s→o→G oös→s→o→G→etc. ╲ ╱ ╲ ╱ mg mg

A glance at the diagram will show a continual interchange of the sexual and asexual modes of reproduction, in which each generation gives rise to its opposite, the asexual sporophyte producing the sexual gametophyte, and this in turn, through its gametes, giving rise to the asexual sporophyte. This regular recurrence in genealogical succession of two differing forms is what is meant by the expression “alternation of generations.” Analogous processes occur also among some of the thallophytes, but as there is no well-defined differentiation of sporophyte and gametophyte, alternation proper may be regarded as beginning with the bryophytes. The subject is a complicated one and somewhat difficult to grasp, but it is important to form a correct idea of it and to fix clearly in mind the different modes of reproduction as we proceed from the lower to the higher forms of vegetation, since in this way alone can their biological relationships and their order of succession in the evolutionary scale be made intelligible.

VI. MOSSES

MATERIAL.—One of the most widely distributed of mosses is the Sphagnum, or peat moss, so generally used by florists in packing plants for shipment, and it can be obtained from them at almost all times. It is rather difficult, however, to find specimens with the fruiting organs, since they are rarely to be met with except in late autumn or early spring. Other common forms are Polytrichum, Funaria, and Mnium, any of which will meet all essential conditions of the study outlined in the text.

=396. The protonema or thallus stage.=—In mosses the sexual, or gametophyte generation differs from that of liverworts in undergoing two phases. The germinating cells of the sporophyte do not develop immediately into the leafy stem, which is the typical gametophyte of true mosses, but produce first a filamentous, creeping structure called the protonema (Fig. 483), that spreads over the ground and forms the tangled green felt usually observed where mosses are growing. Place a few of these filaments on a slide in water, and examine under the microscope. Do they remind you of any of the forms of algæ? Look near the base of the branches for knots or enlargements, like those seen at kn, Fig. 483. These are buds from which the leafy moss stems will develop. Do they correspond to anything observed among the thallophytes? Notice the rootlike filaments that extend under ground; how do they differ from the ones above ground? Why are they colorless? How do you know that they are not true roots? [67 (a), 379.] Sketch one of each kind of filament sufficiently enlarged to show the cells composing it.

A protonema that arises directly from the spore is said to be primary, while those which sometimes spring from rhizoids above ground, or from stems or leaves, are secondary. The fact that a protonema can bud from parts of the fruiting stems shows that the two do not belong to different generations, but are merely successive stages of a single generation, and both together compose the gametophyte.

=397. The leafy stage.=—In their fully developed state the true mosses show a marked advance in organization over the liverworts. There is a distinct differentiation of the growing axis into stem and leaves, though no true roots are formed. The leaves are arranged spirally, on upright stems, while in the liverworts the vegetative body is either a flat, spreading thallus, or the leaves are arranged horizontally on opposite sides of a prostrate, or more or less inclined, axis. Sometimes a second set occurs, on the upper side of the axis, but in this case the leaves are usually much smaller and inclined to the horizontal arrangement, as shown in Fig. 484.

=398. The reproductive organs.=—The antheridia and archegonia are borne in groups at the end either of the main axes, or of lateral branches (Figs. 485, 486), but as a rule only one archegonium is fertilized, so the mature sporogonia are solitary. The plants may be either diœcious or monœcious, as in Fig. 485; and in the latter case, the reproductive organs may be borne on the same, or on different, receptacles. The antheridia and the archegonia are both mixed with club-shaped hairs called paraphyses (Fig. 485).

=399. The sporophyte.=—An examination of the fruiting capsule of any of the true mosses will show that it consists of a long footstalk, the seta, s, Fig. 486, bearing a capsule, or ripened sporogonium, f, which is at first surmounted by a cap or hood, known as the calyptra, c. The hood represents the excessively developed and often highly specialized wall of the archegonium. It falls away at maturity, and the spores are discharged through an opening made by the removal of the operculum, or lid, d. The spores and the capsule are both developed from the fertilized egg (oöspore), within the archegonium, in much the same manner as in the liverworts, and together constitute the sporophyte, or asexual generation. It never leads a completely independent existence, but remains a partial parasite on the mother plant, though the lower part of the young sporogonium is usually provided with stomata and chlorophyll so that it is capable of manufacturing food. In this respect it shows a distinct advance on the corresponding phase of the liverworts—if we except the single genus Anthoceros, which alone among the liverworts has the cells of the sporogonium provided with chlorophyll.

=400. Alternation of generations.=—The process of reproduction in mosses is so closely similar to that of liverworts that it is unnecessary to repeat the details. There are some minor variations, but in all essentials the processes are the same and may be represented to the eye by the same formula.

=401. Relative position of mosses and liverworts in the line of evolution.=—Though mosses, as a rule, show a higher degree of organization than liverworts, in both generations, their development has been away from the general course of evolution followed by the higher plants. This, as will be seen later, tends towards a decreasing complexity of the gametophyte with increasing complexity of the sporophyte, while the mosses show increasing complexity of both. Like the order of birds in the animal kingdom, they form a highly specialized and somewhat isolated group. While they may be regarded as descendants from a common ancestral stock with the ferns and club mosses, they have been switched off, so to speak, on a side track of the great evolutionary trunk line, and their advance on this side track has carried them to a point more remote from the course along which the higher forms of plant life have traveled than the distant junction at which they branched off from their less progressive kindred, the humble liverworts.

VII. FERN PLANTS

MATERIAL.—Any kind of fern in the fruiting stage. Several different varieties should be cultivated in the schoolroom for observation. While gathering specimens, look along the ground under the fronds, or in greenhouses where ferns are cultivated, among the pots and on the floor, for a small, heart-shaped body like that represented in Figs. 501, 502, called a prothallium. It is found only in moist and shady places, and care should be taken in collecting specimens, as in their early stages the prothallia bear a strong resemblance to certain liverworts found in the same situations. The best way is for each class to raise its own specimens by scattering the spores of a fern in a glass jar, on the bottom of which is a bed of moist sand or blotting paper. Cover the jar loosely with a sheet of glass and keep it moist and warm, and not in too bright a light. Spores of the sensitive ferns (Onoclea) will germinate in from two to ten days, according to the temperature. Those of the royal fern (Osmunda) germinate promptly if sown as soon as ripe, but if kept even for a few weeks are apt to lose their vitality. The spores of sensitive fern can be kept for six months or longer, while those of the bracken (Pteris) and various other species require a rest before germinating, so that in these cases it is better to use spores of the previous season.

=402. Study of a typical fern.=—Observe the size and general outline of the fronds, and note whether those of the same plant are all alike, or if they differ in any way, and how. Observe the shape and texture of the divisions or pinnæ composing the frond, their mode of attachment to the rachis, and whether they are simple, or notched, or branched in any way. Hold a pinna up to the light and notice the veining. Is it like any of the kinds described in 171, 172? In what respect is it different? This forked venation is a very general characteristic of ferns. When the forks do not reticulate or intercross, the veins are said to be free; are they free in your specimen, or reticulated? Make a sketch, labeling the primary branches of the frond, pinnæ (sing., pinna), the secondary ones, if any, pinnules, and the common stalk that supports them, stipe. Note the color, texture, and surface of the stipe. If any appendages are present, such as hairs, chaff, or scales (in Pteris, nectar glands), notice whether they are equally distributed. If not, where are they most abundant?

Examine the mode of attachment of the stipes to their underground axis. Break one away and examine the scar. Compare with your drawings of leaf scars and with Fig. 105. Do the stipes grow from a root or a rhizome? How do you know? Do you find any remains of leafstalks of previous years? How does the rootstock increase in length? Measure some of the internodes; how much did it increase each year? Cut a cross section and look for the ends of the fibrovascular bundles. Trace their course through several internodes. Do they run straight, or do they turn or bend in any way at the nodes? If so, where do they go? Do you see anything like roots? Where do they originate? Put one of them under the microscope and find out whether they are roots or hairs.

True roots are first developed in the pteridophytes. Since those of the fern spring from an underground stem, to what class of roots do they belong? (83.)

=403. Minute study of a fern stem.=—Place a very thin section of a fern rhizoma, or of the stipe of a frond, under the microscope. Except in very young stems the vascular bundles are arranged in a ring, or sometimes in two or more rings (Fig. 492), with plates of strengthening tissue, l, l, between the inner and outer rings. Notice the inner epidermal layer of hard brown tissue, and within that, the soft parenchyma, which fills the rest of the interior. Test it with iodine and observe how rich in starch it is. If the section of a petiole is under observation, the details will be somewhat different; would you expect to find as much starch in the stipe as in the rootstock? Why, or why not?

Make a longitudinal section of a rhizome through the point where a leafstalk is attached and trace the course of the bundles. This will be facilitated if the specimen has stood in eosin solution a few hours. Make enlarged drawings of both sections, labeling all the parts.

Clearly differentiated conducting bundles occur in the mosses, but they are of much simpler structure than in the pteridophytes, consisting usually of a single central strand, and are found more frequently in the leaves than in the stems. A true vascular structure appears first in the pteridophytes, whence these plants are distinguished as vascular cryptogams.

=404. Fructification.=—Examine the back of a fruiting frond; what do you find there? These dots are the sori (sing., sorus), or spore clusters, and the fronds or pinnæ bearing them are said to be fertile. Are there any differences of size, shape, etc., between the fertile and the sterile fronds of your specimen? between the fertile and the sterile pinnæ? On what part of the frond are the fertile pinnæ borne? Notice the shape and position of the sori, and their relation to the veins, whether borne at the tips, in the forks, on the upper side (toward the margin), or the lower (toward the midrib). Look for a delicate membrane (indusium) covering the sori, and observe its shape and mode of attachment. If the specimen under examination is a polypodium, there will be no indusium; if a maidenhair, or a bracken, it will be formed of the revolute margin of the pinna. In lady fern and Christmas fern (Aspidium), the sori frequently become confluent, that is, so close together as to appear like a solid mass. Sketch a fertile pinna as it appears under the lens, bringing out all the points noted.

=405. The spore cases.=—Look under the indusium at the cluster of little stalked circular appendages (Fig. 490). These are the sporangia, or spore cases, in which the reproductive bodies are borne. Place one of them under the microscope, and it will be found to consist of a little stalked circular body like a tennis racket (Fig. 491), surrounded by a jointed ring called the annulus. Watch a few moments and see if you can find out the use of the annulus. If not, warm the slide and you will probably see the ring straighten itself with a sudden jerk, rupturing the wall of the sporangium and discharging the spores with considerable force. If this does not happen, add a drop of strong glycerine to a specimen mounted in water; the rupture will be apt to follow quickly. What causes it, in either case? [56, (1); Exp. 19.]

=406. The sporophyte.=—The spores found in such abundance on the fertile pinnæ; are all alike, and each one is capable of germinating and continuing the work of reproduction as effectually as the sexual spores of the bryophytes. The fertile frond, or part of a frond, on which they are borne is called a sporophyll (spore-bearing leaf), and the entire plant is the sporophyte, which, with its crop of spores, makes up one generation.

It is important to observe that in the ferns and in all pteridophytes the sporophyte is the conspicuous and highly organized body that is commonly recognized as the normal growing plant; while with the bryophytes just the reverse holds true,—the sexual generation, or gametophyte, represents the normal plant structure, while the sporophyte is an insignificant appendage which never attains an independent existence. Broadly speaking, in bryophytes, it is a spore fruit; in the pteridophytes and spermatophytes a highly developed plant.

=407. The gametophyte.=—When one of these asexual spores germinates, it produces, not a fern plant like the one that bore it, but a small, heart-shaped body like that shown in Fig. 501. Examine one of these bodies carefully with a lens. Observe that there are no veins nor fibrovascular bundles, and the whole body of the plant seems to consist of one uniform tissue. Compare it with the forked apex of a branching thallus of a liverwort. Do you perceive any points of similarity? The two are, in fact, morphologically the same. This heart-shaped body is called a prothallium, and is the gametophyte of the fern. It may be of different shapes, and in some species is branching and filamentous, like the protonema of a moss. Generally, however, it is flat and more or less two-lobed, as shown in Fig. 501. It is small and inconspicuous and very short-lived, being of importance only in connection with the work of reproduction.

Look with your lens for a cluster of small, bottle-shaped bodies just below the deep cleft in the heart. If you cannot make out what they are, put a thin section through a part of the prothallium containing one under the microscope, and you will see that they are the archegonia. Lower down among the rhizoids, near the pointed base, will be found the antheridia. In some species the prothalli are diœcious, one kind bearing antheridia, the other archegonia, but this is rare among the true ferns.

=408. Fertilization.=—This process is the same in all essentials as in the bryophytes. As in other cryptogams, it can take place only under water,—a circumstance which points to an aquatic origin for this sub-kingdom, and through them to the entire flora of the globe. The archegonia differ somewhat in shape from those of the liverworts and mosses, but a section under the microscope will show that they consist of essentially the same parts. On account of the similarity of these organs, the pteridophytes and bryophytes are often classed together as Archegoniates.

=409. Alternation of generations.=—Among the section of ferns that we have been considering, the order of alternation corresponds in all essentials to that prevailing among the bryophytes, and may be represented by the same formula. The chief difference is in the relatively much greater importance of the sporophyte, which may be expressed by putting it first:—

fg fg ╱ ╲ ╱ ╲ S→o→G oös→S→o→G oös→S→o→G etc. ╲ ╱ ╲ ╱ mg mg

But some of the pteridophytes—of which the Selaginella offers a conspicuous example—have differentiated their asexual spores (o of the formula) into two kinds, large and small, known respectively as megaspores and microspores. The prothallia developed by the former bear archegonia containing female gametes only; those by the latter, antheridia containing male gametes—while in the diœcious bryophytes, the archegonial and antheridial thalli are produced by spores of the same kind.

The differentiation of the asexual spores in the higher pteridophytes gives rise to corresponding changes in the sporangia that bear them, and even in the sporophylls themselves, one kind bearing microsporangia only, the other megasporangia. In this way the differentiation of sex is pushed back, step by step, until it virtually begins with the sporophyte, or asexual generation.

Using the same terms as before, and representing the microspores by the abbreviation mo, the megaspores by Mo, the archegonial gametophyte by arG, the antheridial by anG, the formula may be modified to express this more complicated process of alternation, as follows:—

Mo→arG→fg Mo→arG→fg ╱ ╲ ╱ ╲ s oös→S oös→S etc. ╲ ╱ ╲ ╱ mo→anG→mg mo→anG→mg

Comparing this formula with the preceding, it will be seen that the increased complexity affects the sporophyte at the expense of the gametophyte, which has now become a mere dependent on the former.

=410. Advantages of alternation.=—This roundabout mode of reproduction would hardly have been developed unless it had been of some benefit to the plants in which it occurs. The chief advantage seems to be in more rapid multiplication and consequently better chance to propagate the species, as compared with the slow process of sexual reproduction were the plant confined to that method alone. Only one plant is produced by each oöspore, and if this were a gametophyte with its limited number of archegonia, multiplication would be slow; but the sporophyte with its millions of spores, each capable of producing a new individual, enables the species to multiply indefinitely. At the same time the interposition of a gametophyte, or sexual generation, secures the introduction of a new strain with effects analogous to those of cross fertilization.

=411. Classification of pteridophytes.=—In our study of this group, the ferns have been taken as the type because they are the most familiar and most widely distributed of all the vascular cryptogams. But while they exceed in numbers, both of individuals and species, all the other orders combined, they form only one division of three great groups that make up the class Pteridophyta. These groups are: (1) ferns, under which are included, besides the true ferns, two widely differing orders, with the grape ferns and adder’s-tongue in one, and the water ferns in the other; (2) the club mosses, embracing the two subdivisions of Lycopodium and Selaginella; (3) the horsetail family, including horsetails and scouring rushes. Orders (2) and (3) are grouped together as cone-bearing (strobilaceous) pteridophytes, because their sporangia are clustered in oblong heads, or strobiles (Fig. 509), somewhat like the cones of the pine. The orders of pteridophytes differ greatly among themselves, but agree in possessing certain characteristics that point to their derivation from a common ancestry.

=412. Distinction between pteridophytes and bryophytes.=—In passing from the Thallophytes and Bryophytes to the vascular cryptogams, we cross the widest chasm in the vegetable kingdom—a gap relatively as great as that between vertebrates and invertebrates among animals. The most important modifications that discriminate the two groups are: (1) the presence in Pteridophytes of a highly organized vascular system accompanied by a well-marked differentiation of the plant body into root and stem; (2) increased importance and complexity of the sporophyte with proportionate diminution of the gametophyte.

While vessels for conducting water occur in some of the bryophytes (403), a well-defined vascular system and true roots are met with first in the Pteridophytes. The change in the relative importance of sporophyte and gametophyte is so marked that in Selaginella, the genus which approaches nearest in structure to the seed-bearing plants, the suppression of the gametophyte has proceeded so far that it never leads an independent existence at all and is difficult even to recognize as a distinct individual.

Practical Questions

1. Have ferns any economic use—that is, are they good for food, medicines, etc.?

2. What is their chief value?

3. Under what ecological conditions do they grow?

4. Are they often attacked by insects, or by blights and disease of any kind?

5. Of what advantage is it to ferns to have their stems underground, in the form of rootstocks? (321.)

6. What causes the young frond of ferns to unroll? (54, 98.)

7. Name the ferns indigenous to your neighborhood.

8. Which of these are most ornamental, and to what peculiarities of structure do they owe that quality?

9. Are cultivated ferns usually raised from the spores or in some other way? Why?

10. After the great eruption of Krakatoa in 1883, by which the vegetation of the island was completely destroyed, ferns were the first plants to reappear. Explain why. (19; Exp. 17.)

VIII. THE RELATION BETWEEN CRYPTOGAMS AND SEED PLANTS

=413. No break in the chain of life.=—The great gap that was once supposed to exist between the cryptogams and phanerogams has been bridged over by the discovery of analogies in the reproductive processes of the two groups that connect them together as successive links in one continuous chain of vegetable life. It is therefore very important to have a clear understanding of the nature and meaning of these processes, for the chief turning points in the life history of the different groups of plants are connected with them, their natural relationships to each other, and their distribution according to their respective places in the evolutionary scale, being determined largely by a comparison of their modes of continuing the life of the group.

=414. Alternation of generations in seed plants.=—This process, so conspicuous among Bryophytes and Pteridophytes, and not unknown among Thallophytes, is universal among seed plants (Spermatophytes) also, though in so masked a form that it is not easy to recognize without a more detailed study than would be practicable within the limits of a book like this. Briefly, we may say that the stamens of spermatophytes, and the pistils, or rather the carpels, which we have seen to be transformed leaves (298), represent the sporophylls (406) of the higher pteridophytes. The pollen sacs and ovules are sporangia, bearing microspores and megaspores (409), represented respectively by the pollen grains in the anther and the embryo sac in the ovule. These go through a series of microscopic changes in the body of the ovule analogous to the production of the oöspore in the archegonia of ferns and liverworts, but the process is so obscure that to an ordinary observer the pollen grains and the ovule appear to be the real gametes, and were long supposed to be such. The fertilized germ cell in the embryo sac (251) corresponds to an oöspore; the embryo sac with the endosperm found in all seeds (previous to its absorption by the cotyledons) is a rudimentary gametophyte; and the embryo in the matured seed is the undeveloped sporophyte, destined, after germination and further growth, to produce a new generation with its recurrent cycle of alternating phases.

In the gymnosperms,—pines, yews, cycads, etc.,—which represent the most ancient and primitive type of existing seed-bearing plants, the similarity of these processes to those of certain of the pteridophytes is very striking, and it was through the study of these that the sequences of the process were traced in the much more obscure form in which they occur among the angiosperms. From the endosperm in the seeds of gymnosperms archegonia were found to be developed (Fig. 510) in much the same way as in Selaginella, from the prothallium, thus showing the endosperm to be a modified and greatly reduced gametophyte. In some cases, it has even been found to protrude a little way out of the embryo sac and to take on a slightly greenish tinge—another reminiscence of its origin. Fertilization, too, takes place in precisely the same manner as in the pteridophytes, except that in all but the ginkgo and the cycads, the fertilizing cells in the pollen grains are non-motile, and find their way to the ovule by growing down into the embryo sac with the pollen tube, instead of swimming to it—an adaptation probably brought about in response to changed condition during the course of evolution from aquatic to terrestrial life.

The analogies between the sequence of alternations in the two classes will be made clearer by a comparison of the accompanying diagrams. The corresponding terms applied to the various organs stand in the same vertical row. Diagram (1) shows the process as it takes place in the more highly developed Pteridophytes; diagram (2) the corresponding phases in angiosperms.

PTERIDOPHYTES

mospl→mic→mo→anG→ant→mg→ ╱ ╲ (1) S öos→S ╲ ╱ Mospl→Mgc→Mo→arG→arc→fg→

mospl, microsporophyll; mic, microsporangium; mo, microspores; anG, male gametophyte; ant, antheridia; mg, antherozoids. The letters in the lower line stand for the corresponding female organs.

SPERMATOPHYTES

st→an→pol→fc→ not →gc→ ╱ developed ╲ (2) S öos→S ╲ developed ╱ p →ov→em→end→ only in →ec→ gymnosperms

st, stamen; an, anther; pol, pollen; fc, food cells in pollen grain; gc, generative cell; p, pistil; ov, ovules; em, embryo sac; end, endosperm; ec, egg cell.

=415. Disappearance of the gametophyte.=—The seed is a comparatively recent development in plant evolution. It has no counterpart anywhere among the cryptogams, but is strictly characteristic of the three great orders of Spermophytes: Monocotyl, Dicotyl, and Gymnosperms, which compose the greater part of the vegetation of the globe. Structurally, it is a matured sporangium containing a rudimentary sporophyte (the embryo), and a reduced gametophyte (the embryo sac), which, under the form of endosperm, has dwindled to an insignificance that makes it difficult to recognize it as a phase in an alternation of generations.

=416. Significance of the sporophyte.=—The gametophyte is obviously a more ancient and primitive structure than the sporophyte, which first becomes prominent in the ferns and their allies. The sudden and violent break in the succession of vegetable life that accompanies the appearance of the pteridophytes (412) is probably to be explained by the development of a land flora and the necessity of adaptation to life in a new medium. The fact that no living cell, whether vegetable or animal, can absorb nourishment except in a liquid form, seems to point to an aquatic origin more or less remote for all life. This inference is further strengthened, in the case of plants, by the fact that even in so highly organized a group as the pteridophytes, fertilization cannot take place except in water. Such a requirement would manifestly be a great disadvantage to land plants, and one of the first steps in response to the demands of a new habitat would be to get rid, as far as possible, of the primitive gametophyte with its outgrown adaptations to a liquid medium, and to transfer the greater part of the work of reproduction to the asexual generation, in which the problem of fertilization did not have to be directly met, the asexual spores germinating without it. The greater the number of these produced, the better the chance that at least some of the gametes developed from them would meet the difficult conditions of fertilization, and the survival of the species be assured. At the same time, in order to meet the requirements of terrestrial life successfully, and to provide for continuing the sexual generation, correlative changes would have to take place in the gametophyte by which the increasing uncertainty of fertilization due to structural changes in the sporophyte, and the absence of a liquid medium for the conveyance of free swimming antherozoids would be avoided. This necessity has been met by the development of the pollen tube, which bores its way to the egg cell, carrying with it the generative cells, which in seed plants have taken the place of the more primitive antherozoids. With the concomitant reduction of the gametophyte and development of the seed habit, the adaptation to land conditions has been made complete.

Roughly speaking, it may be said: (1) that Thallophytes are predominantly aquatic; (2) Archegoniates (Bryophytes and Pteridophytes), amphibious; (3) Spermophytes, terrestrial; (4) that the seed habit is a response to terrestrial conditions; and (5) that the increased development of the sporophyte was a necessary adaptation to meet those conditions.

IX. THE COURSE OF PLANT EVOLUTION

=417. Plant genealogy.=—It has been shown by a study of existing forms of plant life that there is no hard and fast line of division anywhere between the different groups, but that they are all connected by ties of kinship more or less defined, according to their distance from a common ancestral stock. The geological record points to the same conclusion, and our classification of them into families, orders, and species is merely a very imperfect genealogical table of their supposed pedigrees. This does not mean, however, that we can assert positively that such and such a species is derived from such or such another, but that both are descended from some common intermediate form more or less remote. While we have reason to believe that the flowering plants are derived through pteridophyte and bryophyte types from some of the green algæ, no direct connection has ever been traced between any particular kind of flowering plant and any particular kind of alga,—or between a liverwort and an alga, for that matter,—and probably never will be, because the intermediate forms die out, or pass on by variation into other lines of development. But while this is true, all the evidence we possess does go to show that, since the beginning of life on the globe, there has been a general progressive evolution from lower and simpler to higher and more complex forms.

=418. Retrogressive evolution.=—While the general course of evolution has been upward and onward, the movement has not always followed a straight line, but, like a mountain road, shows many windings and deviations from the direct route. The monocotyls furnish a conspicuous example of this departure from the general law of progression. It was formerly supposed, on account of their greater simplicity of structure, that they were a more ancient type than dicotyls, but recent investigations point to the conclusion that they are a later offshoot, derived from some primitive form of aquatic dicotyl, and represent, not an ancient and primitive stock, but a case of retrogressive evolution from a higher type. Strong presumptions in favor of this view are: (1) that various species of dicotyls show an unequal development of the seed leaves, amounting, in the bryony, to complete abortion of one of them, while some monocotyl seeds show morphological characters that can best be explained as survivals, or inheritances, from a dicotyl ancestor; (2) the structural resemblances between gymnosperms and dicotyls are closer than between gymnosperms and monocotyls, which could hardly be the case if the latter were the more ancient; (3) the geological record does not show them to have appeared before dicotyls; (4) the number of cotyledons furnishes no criterion as to the relative age of any plant group, since all three types are represented among the pteridophytes, where plants are found bearing one, two, or more cotyledons.

The theory of their comparatively recent origin from an aquatic ancestor is further borne out by the many points of similarity between their internal structure and that of hydrophytes (318), and also by the great proportion of aquatic plants among them, amounting to thirty-three per cent, while in dicotyls the proportion is only four per cent. Can you give any reasons, from your examination of their internal structure (113, 114), for believing that the line of development which they have followed is a less effective one for meeting conditions now existing on the globe than that attained by dicotyls?

We should remember, too, that while progressive evolution implies successful adjustment to surroundings, it is possible to conceive of a state, as our planet approaches the period of cosmic debility and decay, when the conditions of existence may become progressively more and more unfavorable. In this case the course of evolution would be reversed, the higher types gradually dying out as the struggle for life became more severe, and the tendency would be constantly toward lower and simpler forms, until finally all life would become extinct on our planet. We have no right, however, to assume that during such a course of retrogressive evolution the same forms would be repeated in reverse order as have already appeared, because there is no reason to believe that the conditions brought about by planetary decline and “old age” would be the same as those attending planetary birth and adolescence.

=419. Explanation of the diagram.=—An attempt to show the general course of plant evolution up to the present time is made in the accompanying diagram. The four great divisions, Thallophytes, Bryophytes, Pteridophytes, and Spermatophytes, are represented by spaces between four horizontal lines arranged one above the other in the order of their succession in time and complexity of organization. It should be borne in mind that these dividing lines are not sharply defined in nature, but overlap or indent the territory between them with varying degrees of irregularity, like the coast line on a map. The relative positions of the different orders we have been considering are represented by upright and diagonal lines, the general course of which, as indicated by the arrows, is intended to give an idea of the trend of evolutionary progress in the particular group represented by each line. No one of these lines is made to originate directly in any other, because, with the possible exception of the monocotyls, we have no authority for asserting that any such direct connection exists between plants as we know them, but only that certain types give evidence of descent from a common ancestry. This lack of certainty is expressed by placing the point of origin for any given line in more or less close proximity to the one which is supposed to be the nearest living representative of the common ancestor. The line of ferns, for instance, is depicted as originating in the region of the bryophytes, somewhere in the neighborhood of the liverworts, but the two lines nowhere come in contact, because there is no evidence that any fern, living or fossil, is directly descended from any particular kind of liverwort known to us. With these explanations, the diagram shows, in a rough way, the generally accepted view of plant relationships as based on the evidence at present before us. But in questions of this sort it is wise to keep in mind the blunt remark of a famous old American statesman, that “only fools and dead people never change their opinions.”

Field Work

1. If you live in the country, study the appearance of plants affected with blights, smuts, rusts, and mildews, and learn to recognize the different kinds of disease by their signs. Notice which kinds are most prevalent in your neighborhood, and what plants are most affected by them.

2. Notice the different kinds of mushrooms you find growing wild. Observe the difference between those that grow on the ground and those that grow on logs, stumps, and trees; between those found in the woods and those in open ground. Find out how those on the ground get their nourishment. Uncover the mycelium, and notice the extent of its surface. Examine the soil and find out if it contains anything upon which they could feed. Note the prevalence of shelf fungi on trees. Examine the condition of the wood where they grow, and decide in what ways they injure their hosts. Notice whether they abound most on healthy or on decaying trunks and boughs, and decide whether this is because the fungus prefers that kind of host, or whether the injury it does causes the decay, or whether both causes operate together. Notice what fungi grow on different trees, and study their preferences in this respect.

3. Observe the different kinds of lichens found in your walks and try to distinguish the three classes. Which kind are most abundant in your neighborhood? Which least so? Note the situations in which you find each kind growing, whether on stumps, trees, rocks, or the ground. Consider how the algæ and fungi aid each other in the different positions; could either, for instance, exist independently on bald rocks? Notice on what kind of trees the different lichens seem to thrive best and on which poorly or not at all, and whether the character of the bark—rough, smooth, scaly—has anything to do with their choice of a habitat.

APPENDIX

SYSTEMATIC BOTANY

=Taxonomy, or systematic botany=, deals with the family relationships of plants in the order of their nearness or remoteness with regard to a common line of descent. Its chief value is the insight it gives into the course of plant evolution and into the nature of the modifications that differentiate each group from the ancestral type. While it is not advisable to spend too much time in the mere identification of species, a sufficient number should be examined and described to familiarize the student with the distinctive characteristics of the principal botanical orders.

=Principles of classification.=—All the known plants in the world, numbering not less than one hundred and twenty thousand species of the seed-bearing kind alone, are ranged according to certain resemblances of structure, into a number of great groups known as families or orders. The names of these families are distinguished by the ending aceæ; the rose family, for instance, are the Rosaceæ; the pink family, Caryophyllaceæ; the walnut family, Juglandaceæ, etc. Each of these families is divided into lesser groups called genera (singular, genus), characterized by similarities showing a still greater degree of affinity than that which marks the larger groups or orders; and finally, when the differences between the individual plants of a kind are so small as to be disregarded, they are considered to form one species; all the common morning-glories, for instance, of whatever shade or color, belong to the species Ipomea purpurea. The small differences that arise within a species as to the color and size of flowers, and other minor points, constitute mere varieties, and have no special names applied to them. The line between varieties and species is not clearly defined, and in the nature of things can never be, since progressive development, through unceasing change, is the law of all life.

In botanical descriptions, the name both of the species and the genus is given, just as in designating a person, like Mary Jones or John Robinson, we give both the surname and the Christian name. The genus, or generic name, answers to the surname, and that of the species to the Christian name—except that in botanical nomenclature the order is reversed, the generic, or surname, coming first, and the specific or individual name last; for example, Ipomea is the generic, or surname, of the morning-glories, and purpurea the specific one.

=How to use the key.=—Any good manual will answer the purpose. Gray’s “School and Field Book” is, perhaps, the best available at present for the states east of the Mississippi. Reference to the floral analyses in sections =I-IV= of Chapter VII will make its use clear. Suppose, for instance, we want to find out to what botanical species the morning-glory or the sweet potato belongs. Turning to the key, we find the sub-kingdom of Phænerogams—flowering or seed-bearing plants—divided into two great classes, Angiosperms and Gymnosperms, as explained in 18. A glance will show that our specimen belongs to the former class. Angiosperms, again, are divided into the two subclasses of Dicotyledons and Monocotyledons (18, 171). We at once recognize our plant, by its net-veined leaves and pentamerous flowers, as a dicotyledon (171, 229), and turning again to the key, we find this subclass divided into three great groups: Sympetalous (211), called also Monopetalous and Gamopetalous; Apopetalous, or Polypetalous (211), and Apetalous—having no petals or corolla. A glance will refer our blossom to the sympetalous or monopetalous group, which we find divided into two sections, characterized by the superior or inferior ovary (218, 225). Further examination will show that the morning-glory belongs to the former class, which is in turn divided into two sections, according as the corolla is regular, or more or less irregular. We see at once that we must look for our specimen in the group having regular corollas. This we find again subdivided into four sections, according to the number and position of the stamens, and we find that the morning-glory falls under the last of these,—“Stamens as many as the lobes or parts of the corolla and alternate with them.” A very little further search brings us to the family Convolvulaceæ, and turning to that title in the descriptive analysis, we find under the genus, Ipomea, a full description of the common morning-glory, in the species Ipomea purpurea, and of the sweet potato in the species Ipomea batatas.

=Making collections.=—Mere labeled aggregations of species are not recommended, but the collection of examples illustrating special points in morphology and plant variation may be made with profit; such, for instance, as the adaptations observed in tendrils and stipular appendages, the various modifications of leaves and stems to serve other than their normal purposes, or the different forms of leaves and flowers on the same stem, or on different plants of the same species. A collection made with some specific object in view would also be instructive, and might prove of great value; for instance, to get together examples of all the troublesome weeds of a locality for the purpose of studying their habits and devising means for their eradication; or of all the native useful plants, with detailed analyses of their economic properties, and observations on their habits and the practicability of further developing them. In short, wherever collecting is carried on, it should be done with some object other than the mere identification of species, which often results in greater detriment to the wild plants of a neighborhood than profit to the collector.

WEIGHTS, MEASURES, AND TEMPERATURES

As the metric system of weights and measures and the Centigrade appraisement of temperatures are universally employed in scientific works, the following tables showing the equivalents in our common English standards of those in most frequent use, are given for the convenience of students who have not already familiarized themselves with the subject. The values given are approximate only, but will answer for all practical purposes, except in cases where very great exactitude is required. The micron, or micrometer, is used principally by scientific investigators for measuring extremely minute objects seen under the microscope.

MEASURES OF LENGTH

================================+===================================== METRIC | ENGLISH EQUIVALENTS -----------------+--------------+------------------------------------- Kilometer | km. | ⅔ of a mile. -----------------+--------------+------------------------------------- Meter | m. | 39 inches. -----------------+--------------+------------------------------------- Decimeter | dm. | 4 inches. -----------------+--------------+------------------------------------- Centimeter | cm. | ⅖ of an inch. -----------------+--------------+------------------------------------- Millimeter | mm. | ¹⁄₂₅ of an inch. -----------------+--------------+------------------------------------- Micron | µ | ¹⁄₂₅₀₀₀ of an inch. -----------------+--------------+-------------------------------------

CAPACITY

-----------------+---------+------------------------------------------ Liter | l. | 61 cubic inches, or 1 quart, U.S. measure -----------------+---------+------------------------------------------ Cubic centimeter | cc. | ¹⁄₁₆ of a cubic inch. -----------------+---------+------------------------------------------

WEIGHT

-----------------+---------------+------------------------------------ Kilogram | kg., or kilo. | 2⅕ pounds. -----------------+---------------+------------------------------------ Gram | gm. | 15½ grains avoirdupois. | | ¹⁄₂₈ of an ounce avoirdupois. =================+===============+====================================

METRIC AND ENGLISH SCALES

100 MILLIMETERS

4 INCHES]

TEMPERATURE EQUIVALENTS

The next table gives the Fahrenheit equivalent, in round numbers, for every tenth degree Centigrade from absolute zero to the boiling point of water. To find the corresponding F. for any degree C., multiply the given C. temperature by nine, divide by five, and add thirty-two. Conversely, to change F. to C. equivalent, subtract thirty-two, multiply by five, and divide by nine.

Cent. Fahr. ---------------- 100 212 90 194 80 176 70 158 60 140 50 122 40 104 30 86 20 68 10 50 0 32 −10 14 −20 −4 −30 −22 −40 −40 −50 −58 −100 −148 ———————————————— Absolute zero. -273 -459

FOOTNOTES:

Vines, “Lectures on the Physiology of Plants,” p. 282. See also Sachs, “Physiology of Plants.”

Marshall Ward, “The Oak.”

INDEX

(The numbers, unless otherwise designated, refer to paragraphs.)

Aborted, 220, 291.

Absorption, 58, 71, 72; Exp. 39. selective, 60.

Accessory buds, 158.

Accessory fruits, 302.

Adaptation, 206, 237.

Adhesive fruits, 20; Exp. 20.

Adjustment of leaves, 196-202.

Adnate, 374.

Adventitious buds, 65, 158.

Adventitious roots, 37, 83.

Æcidium, 362.

Aëration, 319.

Aërial roots, 88.

Aggregate fruits, 301, 303.

Air space, 114, 116, 184.

Akene, 234, 296, 302, 305.

Albumin, 3.

Albuminous, 56.

Albuminous seed, i.e., containing endosperm; Field work, p. 28.

Aleurone, 3.

Algæ, 333, 336-342.

Alternate leaves, 168.

Alternation of generations, 395, 400, 409, 414.

Analogous, 108.

Anatropous, Fig. 26.

Angiosperms, 15, 18; Fig. 511.

Annuals, 91.

Annulus, 372, 405.

Anther, 213, 235; Figs. 270-274.

Antheridia, 389, 394, 398, 407.

Antheridial, 388.

Antherozoids, 389, 392, 395, 416.

Antisepsis, 355.

Arch of the hypocotyl, 42, 44.

Archegonia, 390, 394, 407, 408.

Archegonial, 388.

Archegoniates, 408, 416.

Archegonium, 391, 394, 398.

Asexual generation, 395, 399, 409, 416.

Asexual reproduction, 394, 395.

Asexual spore, 395, 407, 409, 410, 416.

Assurgent, 95.

Axial placenta, 216, 300.

Axil, 100, 166.

Axillary buds, 145.

Axis, 64, 65, 79, 152, 156, 159, 161.

Bacillus, 348, 349.

Bacteria, 333, 345, 347-353.

Bark, 118, 119, 122, p. 128, (3).

Basidia, 375.

Bast, 116, 119, 122.

Berry, 291.

Biennial, 92.

Bilabiate, 237, 243.

Bilateral regularity, 219.

Bilateral zonation, 326.

Black rust, 360.

Blade of leaf, 165.

Biogenetic law, 253.

Biological factors, 309.

Bordered pits, 114, 117; Fig. 123.

Boreal, 329.

Bract, 161.

Bryophytes, 334, 385-401.

Bud scales, 147-149.

Buds, 145, 155-158.

Bulb, 107.

Button (of mushroom), 370.

Calyptra, 399.

Calyx, 211.

Cambium, 115, 116, 120, 123.

Cap, 372, 373.

Capillarity, 136; Exp. 53.

Capitate, 220.

Caprification, 279, 305.

Caprifig, 279.

Capsule, 298.

Carbon, 27, 28, 62.

Carbon dioxide, 29, 63, 185, 186, 187, 189; Exps. 23, 25.

Carpels, 216, 288.

Caruncle, 13.

Catkin, 161.

Caulicle, 46.

Cedar apples, Fig. 456.

Cell, 6, 7. collecting, 184. companion, 114.

Cell sap, 7, 110.

Cell wall, 7, 183.

Central cylinder, 67.

Central placenta, 216, 300.

Chalaza, 13.

Chlorophyll, 186, 341, 366.

Chlorophyll bodies, 184, 186, 382.

Cion, 65.

Classification, 90, 252, 283, 343, 384, 411, 417.

Cleistogamic flowers, 272.

Climatic zones, 329.

Climbing stems, 96-98.

Clipped seed, p. 12 (material).

Closed bundle, 114.

Close-fertilized, 272.

Cluster cups, 362.

Coccus (pl. cocci), 339, 348.

Coiled inflorescence, 162.

Collective fruits, 304.

Colony, 316, 337, 357.

Color of flowers, 276.

Compass plants, 199.

Complete flower, 219.

Composite, 235, 381.

Composite flower, 236.

Compound leaf, 178.

Conduplicate, Figs. 159, 160.

Confluent, 404.

Conifers, 117, 327.

Conjugation, 342, 394.

Corolla, 211.

Cortex, 64, 115, 122.

Corymb, 161.

Cotyledon, 11, 12, 18.

Cross cut, 133.

Cross fertilization, 255.

Cross pollination, 255.

Crustaceous lichen, 384.

Cryptogam, 332.

Crystalloids, 60.

Culture medium, 347; p. 306 (material).

Cycle, 217, 219, 229.

Cycle of growth, 50.

Cyme, 162.

Cymose inflorescence, 162.

Cypress knees, 319.

Deciduous, 203.

Declined, 95.

Decurrent, 374.

Definite annual growth, 153.

Definite inflorescence, 160, 162.

Dehiscent fruits, 283, 298.

Deliquescent, 144.

Determinate growth, 153.

Determinate inflorescence, 160, 162.

Diadelphous, 239.

Diastase, 9.

Dichogamy, 269.

Dichotomous, 152; Fig. 155.

Dicotyl, 42, 115, 116, 171, 220.

Dicotyledonous, 12.

Differentiate, 245, 345, 409.

Diffusion, 9, 57.

Digestion, 9.

Dimorphic, 270.

Dimorphism, 270.

Dimorphous, 270.

Diœcious, 268.

Disinfection, 355.

Disk flower, 233.

Dispersal of seed, 19-25.

Dominant, 257, 258.

Dormant buds, 157.

Dorsal; Figs. 390, 391.

Drupe, 292.

Dry fruits, 283, 293-300.

Duct, 67, 111, 114.

Ecological factors, 310.

Ecology, 266, 308, 310.

Edgings, 134.

Egg cell, 251, 391.

Elators, 393.

Embryo, 11.

Embryology, 253.

Embryo sac, 251.

Endodermis, 67 (b).

Endosperm, 11, 13, 14, 16, 17, 414.

Epicotyl, 45, 46, 47.

Epidermis, 64, 115, 122, 183.

Epigynous, 225, 230.

Epiphyte, 87, 394.

Essential constituents, 62.

Essential organs, 212.

Evolution, 242, 245, 265, 334, 335, 401, 414, 415, 417, 418, 419.

Evolutionary, 253, 413.

Excentric attachment, 372.

Excurrent, 144, 154.

Factors, 54, 265, 310.

Fall of the leaf, 203.

Fascicled roots, 80, 81.

Fats, 1, 3, 4.

Feather-veined, 172.

Ferments, 9, 356.

Fertile, 404.

Fertile flower, 267.

Fertilization, 247, 251, 252, 392, 408, 416.

Fibrous roots, 37, 78, 80, 81.

Fibrovascular bundle, 67, 114, 116, 176, 288.

Fig wasp, 279.

Filament of the stamen, 213; a hairlike appendage, 341, 361, 369, 393, 396.

Filamentous algæ, 340, 341.

Fission, 338, 394.

Fleshy fruits, 283, 288-292.

Floral envelopes, 211.

Foliaceous lichen, 379, 384.

Follicle, 298.

Forestry, 139-142.

Forked stems, 152.

Formation, 316.

Free, 218, 374.

Free central placenta, 216.

Free gills, 374.

Free ovary, 218.

Free veining, 402.

Freezing, 33.

Frog’s spit, 340.

Frond, 402.

Fruit, 282.

Fruticose lichen, 384.

Function, 41.

Fungi, 333, 343, 344, 345, 346, 378.

Fungus, 86, 364.

Gametes, 394.

Gametophyte, 394, 395, 396, 406, 407, 410, 412, 414, 415, 416.

Gemmæ, 387.

Generative cell, 249, 416.

Geophilous, 321.

Geotropism, 51, 52, 53.

Germ, 2, 11.

Germ cell, 251, 414.

Germination, 32, 35; Exps. 25, 26-29.

Germs, 352, 355.

Gills (of mushroom), 374.

Girdling, 131.

Glutin, 3.

Gourd, 14, 290.

Grain, 11, 297.

Grain of timber, 133, 134, 135.

Gravity, 52.

Growth, 48-52, 179.

Guard cell, 183.

Gymnosperms, 15, 18, 117, 414.

Gymnosporangium, Fig. 456.

Halophyte, 317, 323.

Haustoria, 85.

Hay bacillus, 348, 349.

Head, 161.

Heartwood, 131.

Heliotropic, 200.

Heliotropism, 198.

Herbaceous, 90, 94, 115, 116.

Heredity, 264, 265.

Hilum, 12, 13, 14.

Homologous, 108.

Host plant, 85.

Humus, 75, 86.

Hybrid, 256.

Hybridization, 256, 257, 263.

Hydrophytes, 317, 318, 319.

Hymenium, 375.

Hymenomycetes, 375.

Hyphæ (sing. hypha), 369, 380.

Hypocotyl, 11, 12, 14, 46. arched, 42, 44. straight, 44.

Hypogynous, 218, 225.

Imbibition, 136.

Imperfect flower, 219, 231, 267.

Impure hybrid, 258, 259.

In-breeding, 254.

Incomplete flower, 219.

Incubation, 354.

Indefinite annual growth, 153.

Indefinite inflorescence, 160, 161.

Indefinite number of parts, 229.

Indehiscent fruit, 283, 294.

Indeterminate growth, 153.

Indeterminate inflorescence, 160, 161.

Indusium, 404.

Inferior ovary, 221, 225.

Inflorescence, 159.

Insectivorous plants, 208-210.

Internode, 46, 110; Exp. 35.

Invasion, 328.

Inverted seed, 14.

Involucre, 161, 232.

Involute, 373; Fig. 251.

Iodine solution, Exp. 3.

Irregular flower, 219, 237.

Irritability, 201.

Joint, 110, 113.

Keel, 238.

Knots, 137.

Lamina, 209.

Laminæ, 368, 374.

Lateral, 372, 398.

Lateral buds, 145.

Leaf attachment, 167.

Leaf cups, 202.

Leaf scars, 146.

Leaf traces, 146.

Legume, 299.

Lenticels, 106, 118, 288.

Lichen, 379.

Life cycle, 359, 364.

Loam, 75.

Lobing, 177; Figs. 210-212.

Locule, 216.

Loment, Fig. 394.

Lyrate, Fig. 197.

Medulla, 119, 122.

Medullary rays, 64, 116, 121, 122, 134, 135.

Megasporangia, 409.

Megaspore, 409, 414.

Mendel’s law, 258.

Mesophyte, 317, 324.

Metabolism, 193.

Microbe, 351, 355.

Micrococcus, 339.

Micropyle, 12, 13, 14, 15, 45.

Microsporangia, 409.

Microspore, 409, 414.

Midrib, 172.

Mixed forest, 139, 324.

Modification, 100-108, 206, 207, 289.

Molecule, 136.

Monadelphous, 239.

Monocotyl, 110, 112, 171, 217, 221, 418.

Monocotyledonous, 11.

Monœcious, 268.

Monopetalous, 211.

Monosepalous, 211.

Morphology, 108. of the flower, 244.

Mosaic (leaf), 197.

Mosses, 334, 396-401.

Muck, 75.

Multiple fruit, 304, 305.

Mushroom, 333, 367.

Mutation, 264.

Mycelium, 343, 359, 369.

Mychorrhiza, 86.

Neck canal, 391.

Net-veined, 171.

Neuter, 267.

Neutral flower, 231, 267.

Nitrogen, 62, 63, 188.

Nitrogenous food, 188.

Node, 46, 65, 110, 113.

Nucleus, 7, 341.

Numerical plan, 217, 229.

Nut, 295.

Nutriment, 3, 186.

Nutrition, 50, 54, 179, 193.

Nyctitropic, 200.

Obsolete, 220.

Oil, 1, 3, 8.

Oöspore, 393, 394, 395.

Open bundle, 116.

Operculum, 399.

Opposite leaves, 168.

Organ, 41.

Organic foods, 4.

Organs of reproduction, 40. of vegetation, 40.

Osmosis, 56, 57.

Ovary, 214, 216, 223.

Ovule, 216.

Oxidation, 27; Exps. 21, 22.

Oxygen, 62, 63, 186, 187; Exps. 22, 66.

Palisade cells, 184.

Palmate veining, 172.

Panicle, Fig. 171.

Papilionaceous, 237, 238.

Pappus, 234.

Parallel veining, 171.

Paraphyses, 375, 398.

Parasitic, 5, 345, 364.

Parasitic plants, 85, 343, 382.

Parenchyma, 110, 114, 115.

Parietal, 216.

Pathogenic, 352, 353.

Pedicel, 159.

Peduncle, 159, 288.

Pentamerous, 229.

Pepo, 290.

Perennial, 93.

Perfect flower, 219.

Perianth, 211.

Pericarp, 288.

Perigynous, Figs. 301, 302.

Persistent, 166.

Petals, 211.

Petiole, 165.

Phanerogams, 331, 332.

Phloem, 114, 116.

Photosynthesis, 186, 192, 193.

Phototropism, 195.

Phyllotaxy, 168, 169.

Pileus, 373.

Pinna, 402.

Pinnate veining, 172.

Pinnule, 402.

Pioneer plant, 316, 319, 320.

Pistil, 212, 214, 223, 228, 240.

Pistillate, 267.

Pitcher plant, 209.

Pith, 110, 115, 116, 119, 121, 122.

Pitted ducts, 114.

Placenta, 216, 288, 298, 300.

Plant society, 316.

Plasmolysis, 59.

Pleurococcus, 337.

Plicate, 155.

Plumule, 11, 12, 14, 45, 46.

Pod, 298.

Pollen, 213.

Pollen grains, 213.

Pollen sac, 213.

Pollen tubes, 249, 250.

Pollination, 215, 247.

Polycotyledons, 15, 45.

Polymorphic, 365.

Polymorphism, 365.

Polypetalous, 211.

Polysepalous, 211.

Pome, 288.

Prefoliation, 155.

Primary, 396.

Primary root, 42, 79.

Pronuba, 278.

Prostate, 95.

Protection, 199, 204, 207, 280, 287.

Proteins, 3, 8, 33, 188, 204.

Prothallium, 407.

Protonema, 396.

Protoplasm, 6, 7, 57, 58, 67, 110, 116.

Pteridophytes, 335, 411, 412.

Puccinia, 360.

Pure dominant, 258, 259.

Pure forest, 139, 324.

Pure recessive, 258, 259.

Pycnidia, 363.

Quartered cut, 135.

Raceme, 161.

Rhachis, 178.

Radial section, 132, 135.

Radicle, 46.

Rhaphe, 13.

Ray, 161, 391.

Ray flowers, 231.

Receptacle, 211, 288, 289, 388, 390, 398.

Recessive, 257, 258.

Red rust, 359.

Regular flower, 219.

Reproduction, 338, 351, 358, 383.

Respiration, 30, 31, 191, 192.

Resting spore, 338, 342, 358, 394.

Reticulation, 172, 402.

Retrogressive evolution, 418.

Revolute, 373, 404.

Rhizoids, 379, 386.

Rhizome, 105.

Ringing, 127.

Rings of growth, 122, 123, 134, 135.

Rogue, 260.

Root cap, 39.

Root hairs, 38, 67.

Root pressure, Exp. 49.

Root pull, 69.

Rootstock, 105.

Root system, 89.

Root tubercles, 63, 300.

Rosette, 197.

Rotation of crops, 24, 327.

Runner, 95.

Samara, 296.

Sap movement, 125, 126, 128, 129.

Saprophyte, 86.

Sapwood, 131.

Scale leaves, 101, 106, 107, 147-149, 207.

Scape, 107, 159.

Scorpioid inflorescence, 162; Figs. 173-176.

Screenings, 20; p. 28, Qn. 22.

Secondary roots, 37, 42, 79.

Seed, 11-18, 332, 415.

Seed coat, 12, 14, 15, 43.

Seedless fruits, 285, 286.

Seedlings, 36, 42, 43, 45.

Seed plants, 331, 414.

Seed vessel, 282.

Selection, 260, 265, 286. artificial, 262. natural, 261.

Self-fertilization, 254, 271.

Sepals, 211.

Sessile, 167, 214.

Seta, 399.

Sexual generation, 395, 396, 406, 410, 416.

Sexual reproduction, 394, 395, 410.

Sheath, 67, 116.

Shrinking of timber, 136.

Sieve tube, 114.

Slabs, 134.

Sleep movements, 200.

Soils, 75, 77.

Sori, 404.

Spathe, 221.

Specialization, 237.

Spermatophytes, 331, 335, 394, 414.

Spermatozoid, 389.

Spermogonia, 363.

Spike, 161.

Spirillum, 348.

Spirogyra, 341.

Sporangia, 390, 405.

Spore, 332, 349, 350, 377, 406, 410.

Spore case, 390, 393, 405.

Spore print, 376.

Sporidium, 361.

Sporogonium, 393, 399.

Sporophyll, 406, 414.

Sporophyte, 393-395, 399, 406, 410, 412, 414, 416.

Sport, 264.

Stamen, 212, 213.

Staminate, 267, 268.

Staminodia, 244.

Standard, 238.

Starch, 3, 4, 187, 204, 288; Exps. 69, 70.

Stems, 90-99.

Sterile flower, 267.

Sterilization, 354.

Stigma, 214.

Stigmatic surface, 223.

Stimulus, 98, 186, 201.

Stipe, 240, 372, 402.

Stipule, 149, 165, 166.

Stolon, 95.

Stoma, 181, 182, 183.

Stomata, 181, 182.

Stone fruit, 292.

Storage of food, 2, 3, 4, 17, 70, 103, 104-107, 287.

Strangling fig, 88.

Strobile, 411.

Strobiliaceous, 411.

Style, 214.

Succession, 327.

Sugars, 3, 4, 204, 288.

Summer spores, 360.

Sundew, 210.

Superior ovary, 218, 221, 225.

Supernumerary buds, 158.

Suppressed, 220.

Survival of the fittest, 261.

Suture, 216, 298, 299.

Swarm spore, 349.

Swelling of timber, 136.

Symbiosis, 309, 382.

Symmetrical flower, 219.

Sympetalous, 211.

Syncarpous, 300.

Synsepalous, 211.

Systematic botany, see Appendix.

Tangential cut, 132, 134.

Tap root, 79.

Teleutospore, 360.

Tendril, 96, 97.

Terminal bud, 145, 154.

Testa, 14.

Thallophytes, 333.

Thallus, 333, 341, 343, 379, 380, 381, 385.

Tillage, 76.

Tissue, 60, 61.

Toadstools, 367.

Toxins, 345.

Tracheids, 114, 117.

Trailing, 95.

Trama, 375.

Transpiration, 179, 180.

Trifoliolate, Figs. 215, 216.

Trimerous, 217.

Trimorphic, 270.

Tuber, 106.

Tumbleweeds, 23.

Turgidity, 7.

Turgor, 179.

Twining, cause of, 98; Exp. 55.

Twining stems, 96; Exp. 54.

Type, 18, 260, 263, 265, 336, 411.

Umbel, 161.

Umbonate, 373.

Underground stems, 104-107.

Unicellular, 337.

Unisexual, 267.

Uredo, 359.

Uredospore, 359, 360.

Variation, 263, 264, 265.

Vascular bundles, 111.

Vascular cryptogams, 403, 411, 412.

Vascular cylinder, 64.

Vascular system, 111, 113, 335.

Vegetative reproduction, 358.

Veil, 371.

Veins, 173-176.

Venter, 391.

Ventral, Figs. 390, 391.

Vernation, 155.

Vessels, 111.

Vexillum, 238, 239.

Vibrio, 348.

Vitality of seeds, 34; Exp. 30.

Volva, 371.

Water roots, 39, 84.

Whorled leaves, 168.

Wind pollination, 274, 275.

Wings, 238.

Winter spores, 360.

Xerophyte, 317.

Xerophyte societies, 317, 320-322.

Xylem, 114, 116.

Yeast, 356.

Yeast colony, 357.

Yellow trumpets, 209.

Yucca, 278.

Yucca moth, 278.

Zonation, 325, 327. bilateral, 326. concentric, 326. horizontal, 326. vertical, 326.

Zones of vegetation, 325.

TRANSCRIBER’S NOTE

Obvious typographical errors and punctuation errors have been corrected after careful comparison with other occurrences within the text and consultation of external sources.

Some hyphens in words have been silently removed, some added, when a predominant preference was found in the original book.

Except for those changes noted below, all misspellings in the text, and inconsistent or archaic usage, have been retained.

p. 45: ‘many of them has’ amended to ‘many of them have’ p. 281: ‘are adpated’ amended to ‘are adapted’ p. 291: ‘and as it can, moveover’ amended to ‘and as it can, moreover’ p. 354: ‘eruption of Krakatao’ amended to ‘eruption of Krakatoa’