Читать книгу The Elements of Botany, For Beginners and For Schools - Gray Asa - Страница 7

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8. Growth from the Seed. Phanerogamous plants grow from seed, and their flowers are destined to the production of seeds. A seed has a rudimentary plant ready formed in it—sometimes with the two most essential parts, i.e. stem and leaf, plainly discernible; sometimes with no obvious distinction of organs until germination begins. This incipient plant is called an Embryo.

9. In this section the Flax-plant is taken as a specimen, or type, and the development and history of common plants in general is illustrated by it. In flax-seed the embryo nearly fills the coats, but not quite. There is a small deposit of nourishment between the seed-coat and the embryo: this may for the present be left out of the account. This embryo consists of a pair of leaves, pressed together face to face, and attached to an extremely short stem. (Fig. 2–4.) In this rudimentary condition the real nature of the parts is not at once apparent; but when the seed grows they promptly reveal their character—as the accompanying figures (Fig. 5–7) show.

Fig. 1. Pod of Flax. 2. Section lengthwise, showing two of the seeds; one whole, the other cut half away, bringing contained embryo into view. 3. Similar section of a flax-seed more magnified and divided flatwise; turned round, so that the stem-end (caulicle) of the embryo is below: the whole broad upper part is the inner face of one of the cotyledons; the minute nick at its base is the plumule. 4. Similar section through a seed turned edgewise, showing the thickness of the cotyledons, and the minute plumule between them, i.e. the minute bud on the upper end of the caulicle.

10. Before the nature of these parts in the seed was altogether understood, technical names were given to them, which are still in use. These initial leaves were named Cotyledons. The initial stem on which they stand was called the Radicle. That was because it gives rise to the first root; but, as it is really the beginning of the stem, and because it is the stem that produces the root and not the root that produces the stem, it is better to name it the Caulicle. Recently it has been named Hypocotyle; which signifies something below the cotyledons, without pronouncing what its nature is.

Fig. 5. Early Flax seedling; stem (caulicle), root at lower end, expanded seed-leaves (cotyledons) at the other: minute bud (plumule) between these. 6. Same later; the bud developed into second pair of leaves, with hardly any stem-part below them; then into a third pair of leaves, raised on a short joint of stem; and a fifth leaf also showing. 7. Same still older, with more leaves developed, but these singly (one after another), and with joints of stem between them.

11. On committing these seeds to moist and warm soil they soon sprout, i.e. germinate. The very short stem-part of the embryo is the first to grow. It lengthens, protrudes its root-end; this turns downward, if not already pointing in that direction, and while it is lengthening a root forms at its point and grows downward into the ground. This root continues to grow on from its lower end, and thus insinuates itself and penetrates into the soil. The stem meanwhile is adding to its length throughout; it erects itself, and, seeking the light, brings the seed up out of the ground. The materials for this growth have been supplied by the cotyledons or seed-leaves, still in the seed: it was the store of nourishing material they held which gave them their thickish shape, so unlike that of ordinary leaves. Now, relieved of a part of this store of food, which has formed the growth by which they have been raised into the air and light, they appropriate the remainder to their own growth. In enlarging they open and throw off the seed-husk; they expand, diverge into a horizontal position, turn green, and thus become a pair of evident leaves, the first foliage of a tiny plant. This seedling, although diminutive and most simple, possesses and puts into use, all the Organs of Vegetation, namely, root, stem, and leaves, each in its proper element—the root in the soil, the stem rising out of it, the leaves in the light and open air. It now draws in moisture and some food-materials from the soil by its root, conveys this through the stem into the leaves, where these materials, along with other crude food which these imbibe from the air, are assimilated into vegetable matter, i.e. into the material for further growth.

12. Further Growth soon proceeds to the formation of new parts—downward in the production of more root, or of branches of the main root, upward in the development of more stem and leaves. That from which a stem with its leaves is continued, or a new stem (i.e. branch) originated, is a Bud. The most conspicuous and familiar buds are those of most shrubs and trees, bearing buds formed in summer or autumn, to grow the following spring. But every such point for new growth may equally bear the name. When there is such a bud between the cotyledons in the seed or seedling it is called the Plumule. This is conspicuous enough in a bean (Fig. 29.), where the young leaf of the new growth looks like a little plume, whence the name, plumule. In flax-seed this is very minute indeed, but is discernible with a magnifier, and in the seedling it shows itself distinctly (Fig. 5, 6, 7).

13. As it grows it shapes itself into a second pair of leaves, which of course rests on a second joint of stem, although in this instance that remains too short to be well seen. Upon its summit appears the third pair of leaves, soon to be raised upon its proper joint of stem; the next leaf is single, and is carried up still further upon its supporting joint of stem; and so on. The root, meanwhile, continues to grow underground, not joint after joint, but continuously, from its lower end; and commonly it before long multiplies itself by branches, which lengthen by the same continuous growth. But stems are built up by a succession of leaf-bearing growths, such as are strongly marked in a reed or corn-stalk, and less so in such an herb as Flax. The word "joint" is ambiguous: it may mean either the portion between successive leaves, or their junction, where the leaves are attached. For precision, therefore, the place where the leaf or leaves are borne is called a Node, and the naked interval between two nodes, an Internode.

Fig. 8. Upper part of Flax-plant in blossom.

14. In this way a simple stem with its garniture of leaves is developed from the seed. But besides this direct continuation, buds may form and develop into lateral stems, that is, into branches, from any node. The proper origin of branches is from the Axil of a leaf, i.e. the angle between leaf and stem on the upper side; and branches may again branch, so building up the herb, shrub, or tree. But sooner or later, and without long delay in an annual like Flax, instead of this continuance of mere vegetation, reproduction is prepared for by

15. Blossoming. In Flax the flowers make their appearance at the end of the stem and branches. The growth, which otherwise might continue them farther or indefinitely, now takes the form of blossom, and is subservient to the production of seed.

Fig. 9. Flax-flowers about natural size. 10. Section of a flower moderately enlarged, showing a part of the petals and stamens, all five styles, and a section of ovary with two ovules or rudimentary seeds.

16. The Flower of Flax consists, first, of five small green leaves, crowded into a circle: this is the Calyx, or flower-cup. When its separate leaves are referred to they are called Sepals, a name which distinguishes them from foliage-leaves on the one hand, and from petals on the other. Then come five delicate and colored leaves (in the Flax, blue), which form the Corolla, and its leaves are Petals; then a circle of organs, in which all likeness to leaves is lost, consisting of slender stalks with a knob at summit, the Stamens; and lastly, in the centre, the rounded body, which becomes a pod, surmounted by five slender or stalk-like bodies. This, all together, is the Pistil. The lower part of it, which is to contain the seeds, is the Ovary; the slender organs surmounting this are Styles; the knob borne on the apex of each style is a Stigma. Going back to the stamens, these are of two parts, viz. the stalk, called Filament, and the body it bears, the Anther. Anthers are filled with Pollen, a powdery substance made up of minute grains.

17. The pollen shed from the anthers when they open falls upon or is conveyed to the stigmas; then the pollen-grains set up a kind of growth (to be discerned only by aid of a good microscope), which penetrates the style: this growth takes the form of a thread more delicate than the finest spider's web, and reaches the bodies which are to become seeds (Ovules they are called until this change occurs); these, touched by this influence, are incited to a new growth within, which becomes an embryo. So, as the ovary ripens into the seed-pod or capsule (Fig. 1, etc.) containing seeds, each seed enclosing a rudimentary new plantlet, the round of this vegetable existence is completed.