Outlines of Lessons in Botany Part 9

The buds are terminal, and axillary, from the axils of the leaves of the preceding year, usually from those at the ends of the branchlets. They are covered with brown scales and contain many leaves.

[Ill.u.s.tration: FIG. 18.--Branch of Cherry in winter state: _a_, leaf-scar; _b_, bud-scar; _c_, flower-scar.]

[Ill.u.s.tration: FIG. 19.--Branch of Red Maple in winter state (reduced). 2.

Flower-buds]

The leaves are needle-shaped and short.[1] They are arranged densely on the branches, alternately on the 8/21 plan (see section on phyllotaxy).

When they drop off they leave a hard, blunt projection which makes the stem very rough. As the terminal bud always develops unless injured, the tree is excurrent, forming a straight trunk, throwing out branches on every side. The axillary buds develop near the ends of the branchlets, forming apparent whorls of branches around the trunk. In the smaller branches, as the tree grows older, the tendency is for only two buds to develop nearly opposite each other, forming a symmetrical branch.

[Footnote 1: The pupils should observe how much more crowded the leaves are than in the other trees they have studied. The leaves being smaller, it is necessary to have more of them. Large-leaved trees have longer internodes than those with small leaves.]

The bud-scales are persistent on the branches and the growth from year to year can be traced a long way back.

The cones hang on the ends of the upper branches. They are much larger than in our native species of Black and White Spruce.

The Evergreens are a very interesting study and an excellent exercise in morphology for the older scholars.

2. _Vernation_. This term signifies the disposition of leaves in the bud, either in respect to the way in which each leaf is folded, or to the manner in which the leaves are arranged with reference to each other.

The pupils have described the folding of the leaves in some of their specimens.

In the Beech, the leaf is _plicate_, or plaited on the veins. In the Elm, Magnolia, and Tulip-tree, it is _conduplicate_, that is, folded on the midrib with the inner face within. In the Tulip-tree, it is also _inflexed_, the blade bent forwards on the petiole. In the Balm of Gilead, the leaf is _involute_, rolled towards the midrib on the upper face.

Other kinds of vernation are _revolute_, the opposite of involute, where the leaf is rolled backwards towards the midrib; _circinate_, rolled from the apex downwards, as we see in ferns; and _corrugate_, when the leaf is crumpled in the bud.

[Ill.u.s.tration: FIG. 20.--Branch of Norway Spruce.]

In all the trees we have studied, the leaves simply succeed each other, each leaf, or pair of leaves, overlapping the next in order. The names of the overlapping of the leaves among themselves, _imbricated, convolute, etc_., will not be treated here, as they are not needed. They will come under _aestivation_, the term used to describe the overlapping of the modified leaves, which make up the flower.[1]

[Footnote 1: Reader in Botany. VIII. Young and Old Leaves.]

3. _Phyllotaxy_. The subject of leaf-arrangement is an extremely difficult one, and it is best, even with the older pupils, to touch it lightly. The point to be especially brought out is the disposition of the leaves so that each can get the benefit of the light. This can be seen in any plant and there are many ways in which the desired result is brought about. The chief way is the distribution of the leaves about the stem, and this is well studied from the leaf-scars.

The scholars should keep the branches they have studied. It is well to have them marked with the respective names, that the teacher may examine and return them without fear of mistakes.

In the various branches that the pupils have studied, they have seen that the arrangement of the leaves differs greatly. The arrangement of leaves is usually cla.s.sed under three modes: the _alternate_, the _opposite_, and the _whorled_; but the opposite is the simplest form of the whorled arrangement, the leaves being in circles of two. In this arrangement, the leaves of each whorl stand over the s.p.a.ces of the whorl just below. The pupils have observed and noted this in Horsechestnut and Lilac. In these there are four vertical rows or ranks of leaves. In whorls of three leaves there would be six ranks, in whorls of four, eight, and so on.

When the leaves are alternate, or single at each node of the stem, they are arranged in many different ways. Ask the pupils to look at all the branches with alternate leaves that they have studied, and determine in each case what leaves stand directly over each other. That is, beginning with any leaf, count the number of leaves pa.s.sed on the stem, till one is reached that stands directly over the first.[1] In the Beech and the Elm the leaves are on opposite sides of the stem, so that the third stands directly over the first. This makes two vertical ranks, or rows, of leaves, dividing the circle into halves. It is, therefore, called the 1/2 arrangement. Another way of expressing it is to say that the angular divergence between the leaves is 180, or one-half the circ.u.mference.

[Footnote 1: The pupils must be careful not to pa.s.s the bud-rings when they are counting the leaves.]

The 1/3 arrangement, with the leaves in three vertical ranks, is not very common. It may be seen in Sedges, in the Orange-tree, and in Black Alder _(Ilex verticillata)_. In this arrangement, there are three ranks of leaves, and each leaf diverges from the next at an angle of 120, or one-third of the circ.u.mference.

By far the commonest arrangement is with the leaves in five vertical ranks. The Cherry, the Poplar, the Larch, the Oak, and many other trees exhibit this. In this arrangement there are five leaves necessary to complete the circle. We might expect, then, that each leaf would occupy one-fifth of the circle. This would be the case were it not for the fact that we have to pa.s.s twice around the stem in counting them, so that each leaf has twice as much room, or two-fifths of the circle, to itself. This is, therefore, the 2/5 arrangement. This can be shown by winding a thread around the stem, pa.s.sing it over each leaf-scar. In the Beech we make one turn of the stem before reaching the third leaf which stands over the first. In the Apple the thread will wind twice about the stem, before coming to the sixth leaf, which is over the first.

Another arrangement, not very common, is found in the Magnolia, the Holly, and the radical leaves of the common Plantain and Tobacco. The thread makes three turns of the stem before reaching the eighth leaf which stands over the first. This is the 3/8 arrangement. It is well seen in the Marguerite, a greenhouse plant which is very easily grown in the house.

Look now at these fractions, 1/2, 1/3, 2/5, and 3/8. The numerator of the third is the sum of the numerators of the first and second, its denominator, the sum of the two denominators. The same is true of the fourth fraction and the two immediately preceding it. Continuing the series, we get the fractions 5/13, 8/21, 13/34. These arrangements can be found in nature in cones, the scales of which are modified leaves and follow the laws of leaf-arrangement.[1]

[Footnote 1: See the uses and origin of the arrangement of leaves in plants. By Chauncey Wright. Memoirs Amer. Acad., IX, p. 389. This essay is an abstruse mathematical treatise on the theory of phyllotaxy. The fractions are treated as successive approximations to a theoretical angle, which represents the best possible exposure to air and light.

Modern authors, however, do not generally accept this mathematical view of leaf-arrangement.]

[1]"It is to be noted that the distichous or 1/2 variety gives the maximum divergence, namely 180, and that the tristichous, or 1/3, gives the least, or 120; that the pentastichous, or 2/5, is nearly the mean between the first two; that of the 3/8, nearly the mean between the two preceding, etc. The disadvantage of the two-ranked arrangement is that the leaves are soon superposed and so overshadow each other. This is commonly obviated by the length of the internodes, which is apt to be much greater in this than in the more complex arrangements, therefore placing them vertically further apart; or else, as in Elms, Beeches, and the like, the branchlets take a horizontal position and the petioles a quarter twist, which gives full exposure of the upper face of all the leaves to the light. The 1/3 and 2/5, with diminished divergence, increase the number of ranks; the 3/8 and all beyond, with mean divergence of successive leaves, effect a more thorough distribution, but with less and less angular distance between the vertical ranks."

[Footnote 1: Gray's Structural Botany, Chap, iv, p. 126.]

For directions for finding the arrangement of cones, see Gray's Structural Botany, Chap. IV, Sect. 1.

The subject appears easy when stated in a text-book, but, practically, it is often exceedingly difficult to determine the arrangement. Stems often twist so as to alter entirely the apparent disposition of the leaves. The general principle, however, that the leaves are disposed so as to get the best exposure to air and light is clear. This cannot be shown by the study of the naked branches merely, because these do not show the beautiful result of the distribution.[1] Many house plants can be found, which will afford excellent ill.u.s.trations (Fig. 21). The Marguerite and Tobacco, both easily grown in the house, are on the 3/8 plan. The latter shows the eight ranks most plainly in the rosette of its lower leaves. The distribution is often brought about by differences in the lengths of the petioles, as in a Horsechestnut branch (Fig. 22) where the lower, larger leaves stand out further from the branch than the upper ones; or by a twist in the petioles, so that the upper faces of the leaves are turned up to the light, as in Beech (Fig. 23). If it is springtime when the lessons are given, endless adaptations can be found.

[Footnote 1: Reader in Botany. IX. Leaf-Arrangement.]

[Ill.u.s.tration: FIG. 21. Branch of Geranium, viewed from above.]

[Ill.u.s.tration: FIG. 22.]

[Ill.u.s.tration: FIG. 23.]

_Gray's First Lessons_. Sect. IV. VII, --4. _How Plants Grow_. Chap. I, 51-62; I, 153.

V.

STEMS.

The stem, as the scholars have already learned, is the axis of the plant.

The leaves are produced at certain definite points called nodes, and the portions of stem between these points are internodes. The internode, node, and leaf make a single plant-part, and the plant is made up of a succession of such parts.

The stem, as well as the root and leaves, may bear plant-hairs. The accepted theory of plant structure a.s.sumes that these four parts, root, stem, leaves, and plant-hairs, are the only members of a flowering plant, and that all other forms, as flowers, tendrils, etc., are modified from these. While this idea is at the foundation of all our teaching, causing us to lead the pupil to recognize as modified leaves the cotyledons of a seedling and the scales of a bud, it is difficult to state it directly so as to be understood, except by mature minds. I have been frequently surprised at the failure of even bright and advanced pupils to grasp this idea, and believe it is better to let them first imbibe it unconsciously in their study. Whenever their minds are ready for it, it will be readily understood. The chief difficulty is that they imagine that there is a direct metamorphosis of a leaf to a petal or a stamen.

Briefly, the theory is this: the beginnings of leaf, petal, tendril, etc., are the same. At an early stage of their growth it is impossible to tell what they are to become. They develop into the organ needed for the particular work required of them to do. The organ, that under other circ.u.mstances might develop into a leaf, is capable of developing into a petal, a stamen, or a pistil, according to the requirements of the plant, but no actual metamorphosis takes place. Sometimes, instead of developing into the form we should normally find, the organ develops into another form, as when a petal stands in the place of a stamen, or the pistil reverts to a leafy branch. This will be more fully treated under flowers.

The study of the different forms in which an organ may appear is the study of _morphology_.

1. _Forms of Stems_.--Stems may grow in many ways. Let the pupils compare the habits of growth of the seedlings they have studied. The Sunflower and Corn are _erect_. This is the most usual habit, as with our common trees.

The Morning Glory is _twining_, the stem itself twists about a support.

The Bean, Pea and Nasturtium are _climbing_. The stems are weak, and are held up, in the first two by tendrils, in the last by the twining leaf-stalks. The English Ivy, as we have seen, is also climbing, by means of its aerial roots. The Red Clover is _ascending_, the branches rising obliquely from the base. Some kinds of Clover, as the White Clover, are _creeping_, that is, with prostrate branches rooting at the nodes and forming new plants. Such rooting branches are called _stolons_, or when the stem runs underground, _suckers_. The gardener imitates them in the process called layering, that is, bending down an erect branch and covering it with soil, causing it to strike root. When the connecting stem is cut, a new plant is formed. Long and leafless stolons, like those of the Strawberry are called _runners_. Stems creep below the ground as well as above. Probably the pupil will think of some examples. The pretty little Gold Thread is so named from the yellow running stems, which grow beneath the ground and send up shoots, or suckers, which make new plants.

Many gra.s.ses propagate themselves in this way. Such stems are called _rootstocks_. "That these are really stems, and not roots, is evident from the way in which they grow; from their consisting of a succession of joints; and from the leaves which they bear on each node, in the form of small scales, just like the lowest ones on the upright stem next the ground. They also produce buds in the axils of these scales, showing the scales to be leaves; whereas real roots bear neither leaves nor axillary buds."[1] Rootstocks are often stored with nourishment. We have already taken up this subject in the potato, but it is well to repeat the distinction between stems and roots. A thick, short rootstock provided with buds, like the potato, is called a _tuber_. Compare again the corm of Crocus and the bulb of Onion to find the stem in each. In the former, it makes the bulk of the whole; in the latter, it is a mere plate holding the fleshy bases of the leaves.

[Footnote 1: Gray's First Lessons, revised edition, 1887, page 42.]

2. _Movements of Stems.--_Let a gla.s.s thread, no larger than a coa.r.s.e hair, be affixed by means of some quickly drying varnish to the tip of the laterally inclined stem of one of the young Morning-Glory plants in the schoolroom. Stand a piece of cardboard beside the pot, at right angles to the stem, so that the end of the gla.s.s will be near the surface of the card. Make a dot upon the card opposite the tip of the filament, taking care not to disturb the position of either. In a few minutes observe that the filament is no longer opposite the dot. Mark its position anew, and continue thus until a circle is completed on the cardboard. This is a rough way of conducting the experiment. Darwin's method will be found in the footnote.[1]