Chapter V. Buds and Branches

I. MODES OF BRANCHING

MATERIAL.—For determinate growth, have twigs of an alternate and an opposite-leaved plant showing well-developed terminal buds: hickory, sweet gum, cottonwood, poplar, chestnut, are good examples of the first; maple, ash, horse-chestnut, viburnum, of the second; for the two-forked kind, mistletoe, buckeye, horse-chestnut, jimson weed, lilac. For showing indefinite growth: rose, willow, sumach, and ailanthus are good examples. Gummy buds, like horse-chestnut and poplar, should be soaked in warm water before dissecting, to soften the gum; the same treatment may be applied when the scales are too brittle to be handled without breaking. Buds with heavy fur on the scales cannot very well be studied in section; the parts must be taken out and examined separately.

=144. Modes of branching.=—Compare the arrangement of the boughs on a pine, cedar, magnolia, etc., with those of the elm, maple, apple, or any of our common deciduous trees. Draw a diagram of each, showing the two modes of growth. The first represents the excurrent kind, from the Latin excurrere, to run out; the second, in which the trunk seems to divide at a certain point and flow away, losing itself in the branches, is called deliquescent, from the Latin deliquescere, to melt or flow away. The great majority of stems, as a little observation will show, present a combination of the two modes.

=145. Terminal and axillary buds.=—Notice the large bud at the end of a twig of hickory, sweet gum, beech, cottonwood, etc. This is called the terminal bud because it terminates its branch. Notice the scars left by the leaves of the season as they fell away, and look for small buds just above them. These are lateral, or axillary, buds, so called because they spring from the axils of the leaves. How many leaves did your twig bear? What difference in size do you notice between the terminal and lateral buds?

=146. The leaf scars.=—Examine the leaf scars with a hand lens, and observe the number and position of the little dots in them. Ailanthus, varnish tree, sumach, and China tree show these very distinctly. They are called leaf traces, and mark the points where the fibrovascular bundles from the leaf veins passed into the stem. Look on the bark, or epidermis, for lenticels.

=147. Bud scales and scars.=—Notice the stout, hard scales by which the winter buds are covered in most of our hardy trees and shrubs. Remove these from the terminal one of your specimen, and notice the ring of scars left around the base. Look lower down on your twig for a ring of similar scars left from last year’s bud. Is there any difference in the appearance of the bark above and below this ring? If so, what is it, and how do you account for it? Is there more than one of these rings of scars on your twig, and if so, how many? How old is the twig and how much did it grow each year? Has its growth been uniform, or did it grow more in some years than in others?

=148. Arrangement and use of the scales.=—Notice the manner in which the scales overlap so as to “break joints,” like shingles on the roof of a house. Where the leaves are opposite, the manner of superposition is very simple. Remove the scales one by one, representing the number and position of the pairs by a diagram after the model given in Fig. 150. In the bud of an alternately branched twig the order will be different, and the diagram must be varied accordingly. Do you observe any difference as to size and texture between the outer and inner scales? Notice how the former inclose the tenderer parts within like a protecting wall. In cold climates the outer scales are frequently coated with gum, as in the horse-chestnut, for greater security against the weather. The hickory and various other trees have the inner scales covered with fur or down that envelops the tender bud like a warm blanket.

=149. Nature of the scales.=—The position of the scales shows that they occupy the place of leaves or of some part of a leaf. In expanding buds of the lilac and many other plants, they can be found in all stages of transition, from scales to true leaves. In the buckeye and horse-chestnut, they will easily be recognized as modified leaf stalks (Fig. 151). In the tulip tree, magnolia, India rubber tree, fig, elm, and many others, they represent appendages called stipules, often found at the bases of leaves. (See 165, 166.) In this case a pair of scales is attached with each separate leaflet, and as the growing axis lengthens in spring, they are carried apart by the elongation of the internodes so that the scars are separated, a pair at each node, making rings all along the stem, as shown in Fig. 152, instead of having them compacted into bands at the base of the bud. These scars are sometimes very persistent, and in the common fig and magnolia may often be traced on stems six to eight years old. Do they furnish any indication as to the relative age of the different parts of the stem, like the bands of scars on twigs of horse-chestnut and hickory? Give a reason for your answer. (Fig. 152.)

=150. Different rates of growth.=—Notice the very great difference between branches in this respect. Sometimes the main stem will have lengthened from twenty to fifty centimeters or more in a single season, while some of the lateral ones will have grown but an inch or two in four or five seasons. One reason for this is because the terminal bud, being on the great trunk line of sap movement, gets a larger share of nourishment than the others, and being stronger and better developed to begin with, starts out in life with better chances of success.

Make a drawing of your specimen, showing all the points brought out in the examination just made. Cut sections above and below a set of bud scars and count the rings of annual growth in each section. What is the age of each? How does this agree with your calculation from the number of scar clusters left by the bud scales?

=151. Irregularities.=—Take a larger bough of the same kind that you have been studying, and observe whether the arrangement of branches on it corresponds with the arrangement of buds on the twig. Did all the buds develop into branches? Do those that did develop all correspond in size and vigor? If all the buds developed, how many branches would a tree produce every year?

In the elm, linden, beech, hornbeam, hazelnut, willow, and various other plants, the terminal bud always dies and the one next in order takes its place, giving rise to the more or less zigzag axis that generally characterizes trees of these species. (Fig. 153.)

=152. Forked stems.=—Take a twig of buckeye, horse-chestnut, or lilac, and make a careful sketch of it, showing all the points that were brought out in the examination of your previous specimen. Which is the larger, the lateral or the terminal bud? Is their arrangement alternate or opposite? What was the leaf arrangement? Count the leaf traces in the scars; are they the same in all? If all the buds had developed into branches, how many would spring from a node? Look for the rings of scars left by the last season’s bud scales. Do you find any twig of more than one year’s growth, as measured by the scar rings?

Look down between the forks of a branched stem for a round scar. This is not a leaf scar, as we can see by its shape, but one left by the last season’s flower cluster. The flower, as we know, dies after perfecting its fruit, and so a flower bud cannot continue the growth of its axis as other buds do, but has just the opposite effect and stops all further growth in that direction. Hence, stems and branches that end in a flower bud cannot continue to develop their main axis, but their growth is usually carried on, in alternate-leaved stems, by the nearest lateral bud, or in opposite-leaved ones, by the nearest pair of buds. In the first case there results the zigzag spray characteristic of such trees as the beech and elm (Fig. 155, B); in the second, the two-forked, or dichotomous branching, exemplified by the buckeye, horse-chestnut, jimson weed, mistletoe, and dogwood (Fig. 155, A).

Draw a diagram of the buckeye, or other dichotomous stem, as it would be if all the buds developed into branches, and compare it with your diagrams of excurrent and deliquescent growth. Draw diagrams to illustrate the branching of the elm, beech, lilac, linden, rose, maple, or their equivalents.

=153. Definite and indefinite annual growth.=—The presence or absence of terminal buds gives rise to another important distinction in plant development—that of definite and indefinite annual growth. Compare with any of the twigs just examined, a branch of rose, honey locust, sumac, mulberry, etc., and note the difference in their modes of termination. The first kind, where the bough completes its season’s increase in a definite time and then devotes its energies to developing a strong terminal bud to begin the next year’s work with, are said to make a definite or determinate annual growth. Those plants, on the other hand, which make no provision for the future, but continue to grow till the cold comes and literally nips them in the bud, are indefinite, or indeterminate annual growers. Notice the effect of this habit upon their mode of branching. The buds toward the end of each shoot, being the youngest and tenderest, are most readily killed off by frost or other accident, and hence new branches spring mostly from the older and stronger buds near the base of the stem. It is their mode of branching that gives to plants of this class their peculiar bushy aspect. Such shrubs generally make good hedges on account of their thick undergrowth. The same effect can be produced artificially by pruning.

=154. Differences in the branching of trees.=—We are now prepared to understand something about the causes of that endless variety in the spread of bough and sweep of woody spray that makes the winter woods so beautiful. Where the terminal bud is undisputed monarch of the bough, as in the pine and fir, or where it is so strong and vigorous as to overpower its weaker brethren and keep the lead, as in the magnolia, tulip tree, and holly, we have excurrent growth. In plants like the oak and apple, where all the buds have a more nearly equal chance, the lateral branches show more vigor, and the result is either deliquescent growth, or a mixture of the two kinds. In the elm and beech, where the usurping pseudo-terminal bud keeps the mastery, but does not completely overpower its fellows, we find the long, sweeping, delicate spray characteristic of those species. Examine a sprig of elm, and notice further that the flower buds are all down near the base of the stem, while the leaf buds are near the tip. The chief development of the season’s growth is thus thrown toward the end of the branch, giving rise to that fine, feathery spray which makes the elm an even more beautiful object in winter than in summer (Fig. 158).

An examination of the twigs of other trees will bring out the various peculiarities that affect their mode of branching. The angle, for instance, which a twig makes with its bough has a great effect in shaping the contour of the tree. Compare in this respect the elm and hackberry; the tulip tree and willow; ash and hickory. As a general thing, acute angles produce slender, flowing effects; right or obtuse angles, more bold and rugged outlines.

Practical Questions

1. Has the arrangement of leaves on a twig anything to do with the way a tree is branched? (145, 151, 152.)

2. Why do most large trees tend to assume the excurrent, or axial, mode of growth if let alone? (150, 154.)

3. If you wished to alter the mode of growth, or to produce what nurserymen call a low-headed tree, how would you prune it? (152, 153.)

4. Would you top a timber tree? (152, 153.)

5. Are low-headed or tall trees best for an orchard? Why?

6. Why is the growth of annuals generally indefinite?

7. Name some trees of your neighborhood that are conspicuous for their graceful winter spray.

8. Name some that are characterized by sharpness and boldness of outline.

9. Account for the peculiarities in each case.

II. BUDS

MATERIAL.—Expanding leaf and flower buds in different stages of development; large ones show the parts best and should be used where attainable. Some good examples for the opposite arrangement are horse-chestnut, maple, lilac, ash; for the alternate: hickory, sweet gum, balsam poplar, beech, elm. Where material is scarce, the twigs used in the last section may be placed in water and kept till the buds begin to expand.

=155. Folding of the leaves.=—Remove the scales from a bud of horse-chestnut nearly ready to open, and notice the manner in which the young leaves are folded. This is called vernation, or prefoliation, words meaning respectively “spring condition” and “condition preceding the leaf.” Leaves are packed in the bud so as to occupy the least space possible, and in different plants they will be found folded in a great many different ways, according to the shape and texture of the leaf and the space available for it in the bud. When doubled back and forth like a fan, or crumpled and folded as in the buckeye, horse-chestnut, and maple, the vernation is plicate (Figs. 160, 162).

=156. Position of the flower cluster.=—What do you find within the circle of leaves? Examine one of the smaller axillary buds, and see if you find the same object within it. If you are in any doubt as to what this object is, examine a bud that is more expanded, and you will have no difficulty in recognizing it as a rudimentary flower cluster. Notice its position with reference to the scales and leaves. If at the center of the bud, it will, of course, terminate its axis when the bud expands, and the growth of the branch will culminate in the flower. The branching of any kind of stem that bears a central flower cluster must, then, be of what order? Compare your drawings with the section of a hyacinth bulb or jonquil, and note the similarity in position of the flower clusters. In a bud of the hickory, walnut, oak, etc., the position of the flower clusters is different from that of flowers in the buds of lilac and horse-chestnut. Look for a bud containing them, and find out where they occur. Can the axis continue to grow after flowering, in this kind of stem? Give a reason for your answer. Make sketches in transverse and longitudinal section (see Figs. 162, 163) of two different kinds of buds, illustrating the terminal and axillary position of the flower cluster.

=157. Dormant buds.=—A bud may often lie dormant for months or even years, and then, through the injury or destruction of its stronger rivals, or some other favoring cause, develop into a branch. Such buds are said to be latent or dormant. The sprouts that often put up from the stumps of felled trees originate from this source.

=158. Supernumerary buds.=—Where more than one bud develops at a node, as is so often the case in the oak, maple, honey locust, etc., all except the normal one in the axil are supernumerary or accessory. These must not be confounded with adventitious buds—those that occur elsewhere than at a node.

Practical Questions

1. Would protected buds be of any use to annuals? Why, or why not?

2. Of what use is the gummy coating found on the buds of the horse-chestnut and balm of Gilead? (148.)

3. Can you name any plants the buds of which serve as food for man?

4. How do flower buds differ in shape from leaf buds?

5. At what season can the leaf bud and the flower bud first be distinguished? Is it the same for all flowering plants?

6. Watch the different trees about your home, and see when the buds that are to develop into leaves and flowers the next season are formed in each species.

III. THE BRANCHING OF FLOWER STEMS

MATERIAL.—Typical flower clusters illustrating the definite and indefinite modes of inflorescence. Some of those mentioned in the text are:—

Indefinite: hyacinth, shepherd’s purse, wallflower, carrot, lilac, blue grass, smartweed (Polygonum), wheat, oak, willow, clover.

Definite: chickweed, spurge (Euphorbia), comfrey, dead nettle, etc. Any examples illustrating the principal kinds of cluster will answer.

=159. Inflorescence= is a term used to denote the position and arrangement of flowers on the stem. It is merely a mode of branching, and follows the same laws that govern the branching of ordinary stems.

The stalk that bears a flower is called the peduncle. In a cluster the main axis is the common peduncle, and the separate flower stalks are pedicels. A simple leafless flower stalk that rises directly from the ground, like those of the dandelion and daffodil, is called a scape (Fig. 165).

=160. Two kinds of inflorescence.=—The growth of flower stems, like that of leaf stems, is of two principal kinds, definite and indefinite, or, as it is frequently expressed, determinate and indeterminate. The simplest kind of each is the solitary, a single flower either terminating the main axis, as the tulip, daffodil, trillium, magnolia, etc., or springing singly from the axils, as the running periwinkle, moneywort, and cotton.

=161. Indeterminate inflorescence= is always axillary, since the production of a terminal flower would stop further growth in that direction and thus terminate the development of the axis. The raceme is the typical flower cluster of the indefinite sort. In such an arrangement the oldest flowers are at the lower nodes, new ones appearing only as the axis lengthens and produces new internodes. The little scale or bract usually found at the base of the pedicel in flower clusters of this sort is a reduced leaf, and the fact that the flower stalk springs from the axil shows it to be of the essential nature of a branch. When the flowers are sessile and crowded on the axis in various degrees, the cluster produced may be a spike, as seen in the plantain, knotweed, etc., or a head, like that of the clover, buttonwood, and sycamore. The catkins that form the characteristic inflorescence of most of our forest trees are merely pendant spikes. The corymb is a modification of the raceme in which the lower pedicels are elongated so as to place their flowers on a level with those of the upper nodes, making a convex, or more or less flat-topped cluster, as in the wallflower and hawthorn. The umbel differs from the corymb in having the pedicels with their bracts all gathered at the top of the peduncle, from which they spread in every direction like the rays of an umbrella, as the name implies. This is the prevalent type of flower cluster in the parsley family, which takes its botanical name, Umbelliferæ, from its characteristic form of inflorescence. The pedicels of an umbel are called rays, and the circle of bracts at the base of the cluster is an involucre.

=162. Determinate, or cymose, inflorescence.=—In the cyme, the typical cluster of the determinate kind, the older blossoms in the center, being terminal, stop the axis of growth in that direction and force the stem, in continuing its growth, to send out side branches from the axils of the topmost leaves, in a manner precisely similar to the two-forked branching of stems like the horse-chestnut and jimson weed. When the older peduncles are lengthened as described in 161, a flat-topped cyme is produced, which is distinguished from the corymb by its order of flowering, the oldest blossoms being at the center, while in the corymb they appear in the reverse order. A peculiar form of cyme is found in the scorpioid or coiled inflorescence of the pink-root (Spigelia), heliotrope, comfrey, etc. Its structure will be made clear by an inspection of Figs. 174-176.

=163. The nature of flower stems.=—A comparison of the types of inflorescence with the modes of branching in ordinary stems (144, 152, 153) will show a strict correspondence between them. Both bear leaves and buds, and the individual flowers of a cluster usually spring from the axils of leaves or from bracts, which are merely reduced leaves. What, then, is the essential nature of flower stems?

=164. Significance of the clustered arrangement.=—As a general thing the clustered arrangement marks a higher stage of development than the solitary, just as in human life the rudest social state is a distinct advance upon the isolated condition of the savage. In plant life it is the beginning of a system of coöperation and division of labor among the associated members of the flower cluster, as will be seen later when we take up the study of the flower.

Practical Questions

1. Name as many solitary flowers as you can think of.

2. Do you, as a rule, find very small flowers solitary, or in clusters?

3. Would the separate flowers of the clover, parsley, or grape be readily distinguished by the eye among a mass of foliage?

4. Should you judge from these facts that it is, in general, advantageous to plants for their flowers to be conspicuous?

Field Work

(1) In connection with 144-154, the characteristic modes of branching among the common trees and shrubs of each neighborhood should be observed and accounted for. The naked branches of the winter woods afford exceptional opportunities for studies of this kind, which cannot well be carried on except out of doors. Note the effect of the mode of branching upon the general outline of the tree; compare the direction and mode of growth of the larger boughs with that of small twigs in the same species, and see if there is any general correspondence between them; note the absence of fine spray on the boughs of large-leaved trees, and account for it. Account for the flat sprays of trees like the elm, beech, hackberry, etc.; the irregular stumpy branches of the oak and walnut; the stiff straight twigs of the ash; the zigzag switches of the black locust, Osage orange, elm, and linden. Measure the twigs on various species, and see if there is any relation between the length and thickness of branches. Notice the different trend of the upper, middle, and lower boughs in most trees, and account for it. Observe the mode of branching of as many different species as possible of some of the great botanical groups of trees; the oaks, hickories, hawthorns, and pines, for instance, and notice whether it is, as a general thing, uniform among the species of the same group, and how it differs from that of other groups.

(2) In connection with 155-158, buds of as many different kinds as possible should be examined with reference to their means of protection, their vernation and leaf arrangement, and the resulting modes of growth. Compare the folding of the cotyledons in the seed with the vernation of the same plants, and observe whether the folding is the same throughout a whole group of related plants, or only for the same species. Notice which modes seem to be most prevalent. Select a twig on some tree near your home or your schoolhouse, and keep a record of its daily growth from the first sign of the unfolding of its principal bud to the full development of its leaves. Any study of buds should include an observation of them in all stages of development.

(3) With 160-165, study the inflorescence of the common plants and weeds that happen to be in season, until you have no difficulty in distinguishing between the definite and indefinite sorts, and can refer any ordinary cluster to its proper form. Notice whether there is any tendency to uniformity in the mode of inflorescence among flowers of the same family. Consider how each kind is adapted to the shape and habit of the flowers composing it, and what particular advantage each of the specimens examined derives from the way its flowers are clustered. In cases of mixed inflorescence, see if you can discover any reason for the change from one form to the other.