Читать книгу An Introduction to Nature-study - E. Stenhouse - Страница 5

If the seeds which were sown in damp sawdust and on flannel are kept warm they will soon be ready for study. You should remember that at present your object is not so much to rear the plants as to find out how they grow. As soon, therefore, as any sign of growth is to be seen when you take a seed out, you should begin to examine them at regular intervals, taking one or two out every day and leaving the rest to continue their development. Keep the sawdust damp, but not wet.

1. The pea and bean.—(a) General development.—Very soon the seed-coat splits at the micropyle-end of the stalk-scar, and the end of the radicle protrudes. Does the radicle grow upwards or downwards? Observe that even if the seed was so planted that the micropyle was at the top, the radicle turns over and grows straight down. Turn over a seedling and see if you can persuade the radicle to grow upwards. Open a seed when the radicle is about an inch long, and see what the plumule is doing. It is still enclosed in the seed-coat, and lies between the cotyledons, but is larger than at first. As the growth proceeds the cotyledons begin to separate near the top of the radicle, and you can get a glimpse of the plumule.

(b) The growth of the radicle.—Day by day the radicle becomes longer. Is it all growing longer, or does the increase in length take place more at one part than another? To answer this question, take five or six inches of cotton thread and moisten the middle part with Indian ink. Lay the seed on a flat ruler, so that the radicle lies over an inch divided into—say—tenths. Hold the thread tight, and press the inked part gently on the radicle, making about five marks at equal intervals from the point upwards. The ink will dry almost immediately. Then carefully replant the seed, taking care not to injure the radicle. After a few days take it out again, lay it once more on the ruler, and measure the distance between the marks.

The radicle is evidently the young root.

(c) The root-cap.—Hold up the radicle to the light, and examine its tip with a lens. Try to see that the tip is covered by a little cap, somewhat like a very small thimble. This is called the root-cap.

(d) The root-hairs.—Hold a seed, with the radicle about an inch long, against a dark surface. Is the surface of the radicle smooth, or can you see any fluffiness on it? Is all the radicle fluffy, or only a part? Which part? As you examine older and older seedlings notice how much of the radicle is fluffy, and where the fluffy part is. The fluffy appearance is caused by fine, closely-set hairs, called root-hairs.

(e) The plumule.—How soon after planting does the plumule become free? Does it grow upwards or downwards? The plumule is evidently the young stem.

As soon as the young stem is old enough mark it with Indian ink as you marked the young root, and replant it to find if there is any difference in the rates of growth of its different parts.

(f) The fate of the cotyledons.—From time to time examine the cotyledons and notice that as the seedling grows larger they become more and more shrunken. Something is evidently being taken from them, perhaps to feed the young plant. We shall inquire into this by further experiment (Chapter II.).

Do the cotyledons remain in their original position, or are they carried upwards with the growing stem or downwards with the growing root?

2. The yellow lupine.—In the same way observe how this seed grows. Do the cotyledons shift their position or change in colour? Do they become leaf-like? How do they differ from later-formed leaves? What becomes of them at last? What becomes of the seed-coat?

3. The mustard seed.—Notice that, soon after the radicle has come out of the seed-coat, a sort of hump forms at its upper end, and at length the cotyledons are pulled out of the seed-coat and turn up towards the light. What is their colour? Observe that the two cotyledons are soon raised on the end of a little stalk. Like the cotyledons of the yellow lupine they are plainly leaves. Notice their shape. Are they of equal size? Why not? When they are about three inches above the seed-coat gently separate them and notice the little bud between them. Draw the seedling. How large can you get a mustard seedling to grow on damp flannel? Plant a few mustard seeds on earth, and notice the difference between the shape of the cotyledons or seed-leaves and that of the leaves which appear later. What becomes of the cotyledons?

4. The vegetable marrow seed.—Make similar observations upon the vegetable marrow seeds, noticing particularly whether the cotyledons remain in their original positions and shrink up as the plant increases in size, or whether they are pulled out of the seed-coat by the elongating stem, and become green and leafy. How does the plant hold down its seed-coat whilst it pulls out its cotyledons?

5. The sycamore seed.—From what you have seen of the cotyledons of the sycamore seed, will you expect them to behave like those of the mustard seed, or like those of the pea and bean? Even in the seed they are green, and plainly leaves. How do they escape from the seed-coat? What is their shape? Do they come out before or after the radicle? Do they get any larger as the stem grows? How large can you get a sycamore seedling to grow in damp sawdust? As large as a seedling of pea or bean? Plant some sycamore seeds on earth and compare the shape of the cotyledons with that of the next-formed leaves. How soon do the “true” leaves appear after the cotyledons have escaped from the seed? Do any “true” leaves grow on the plants in sawdust? What becomes of the cotyledons at last?

The embryo.—The plumule, radicle, and cotyledons, which have now been seen in the seed, form the embryo of the plant. The adult plant will be wholly formed by the growth and development of these parts, and we must now follow carefully the changes which take place when the seed germinates, and try to find out what becomes of each part. It is better to put the seeds at first in damp sawdust rather than in earth, as the young roots can then be more readily cleaned and observed. With small seeds the early stages of growth are better seen if damp flannel is used.

Germination.—Under the influence of moisture and warmth the embryo in the seed begins to swell and unfold its parts. The radicle makes its appearance first (Fig. 5), breaking through the seed-coat at the micropyle; it is the young root. The radicle always grows downwards, that is, toward the centre of the earth. If the seed lies in such a position that the micropyle is directed upwards, the point of the radicle turns over and grows downwards as soon as it escapes from the seed-coat. As the young root becomes longer and thicker (Fig. 6) the seed-coat opens more and more, showing the cotyledons beneath, and these, too, are gradually forced apart.

Fig. 5.—An early stage in the germination of a Broad-Bean seed. R, radicle; s.c., seed-coat. (×⅔.)

Fig. 6.—A slightly later stage in the germination of a Broad-Bean seed. cot., cotyledon; pl, plumule; R, radicle; s.c., seed-coat. (×⅔.)

Fig. 7.

—Mustard

Seedling, showing

root-hairs and

cotyledons. (×½.)

The cotyledons.—During the germination of various seeds, a very marked difference in the behaviour of the cotyledons is to be seen. In the case of the broad bean and pea the cotyledons remain in their original positions, partially enclosed by the split seed-coat. Presently a hump (Figs. 6 and 11) forms at the upper end of the radicle, as if the plant were making an effort to pull its plumule out of the seed. It soon succeeds (Fig. 12), and the plumule turns up to the light. It is the young stem. At its end is a little bud, formed by a number of small, overlapping, green leaves which surround the growing point. Henceforth the stem grows upwards, that is in a direction precisely opposite to that of the root’s growth. Both stem and root are attached to the cotyledons, which gradually shrivel up as the stem and root become larger and larger.

When, however, the seed of the mustard, or sycamore, germinates the cotyledons behave very differently (Figs. 7 and 8). Soon after the root has become well established the cotyledons come quite out of the seed-coat and unfold themselves. Instead of remaining on or under the surface of the ground they are carried upwards at the end of a stalk toward the light, and for some time the little plant appears to consist of root, stalk, and cotyledons only. If, however, the cotyledons are gently pressed apart, a tiny bud is seen between them. This evidently corresponds to the bud at the end of the stem of the bean or pea.

In the case of the lupine (Fig. 9) or vegetable marrow (Fig. 10) the cotyledons appear to combine these two conditions. They are swollen and contain stored food; yet they come out of the seed-coat early, become green, and open out to the light. They are evidently leaves, though their shape differs from that of the later leaves.

Fig. 8.—Three stages in the growth of a Sycamore Seedling. cot., cotyledons; fol., first pair of foliage leaves. (Slightly reduced.)

The germinating vegetable-marrow seed possesses a curious contrivance for pulling its cotyledons out of the seed-coat. This is a peg (p, Fig. 10) which develops at the top of the radicle, and holds down the lower half of the seed-coat whilst the other half is forced upwards to allow the cotyledons to be withdrawn.

Fig. 9.—Three stages in the growth of the Yellow Lupine. On the right the cotyledons are still enclosed in the mottled seed-coat. In the middle plant the cotyledons are spreading out; the first foliage leaves have not yet unfolded. On the left, the first two foliage leaves are unfolding, and the cotyledons have spread out flat. (Slightly reduced.)

After a little thought a possible explanation of these differences in the cotyledons suggests itself. It may be that, in the case of the mustard and sycamore, leaves are required as early as possible, while the bean and pea have no immediate need for leaves because their cotyledons contain so much stored food. The cotyledons of these plants shrivel up as the seedling grows, and this seems to indicate that during its early stages the plant lives upon this food. In Chapter II. we shall make experiments to see if this explanation is the true one. If so, the lupine and vegetable-marrow seeds evidently rely partly upon their stored food and partly upon setting the cotyledons to work as leaves, whilst the plant is still very young.

Fig. 10.—Germinating Vegetable Marrow seed. p, the peg by which the seed-coat (s.c.) is held down to allow the cotyledons (cot.) to be withdrawn. (×1.) (After Bailey.)

Fig. 11.—A germinating Pea; cot, cotyledon; pl, plumule; R, radicle; r.h., root-hairs; S.c., seed-coat. The radicle has been marked with Indian ink at intervals of 1/10”.

The true leaves.—The cotyledons are really makeshift leaves, which are already formed in the seeds. Even when they expand and become green they do not live long, but as soon as the next few leaves are well established, shrivel up and wither. The true or “foliage” leaves first make their appearance as a bud which surrounds the growing point of the stem. As this part of the stem increases in length, the foliage leaves become separated from each other and spread out to the light and air.

The lengthening of the stem and root.—Unless an experiment to test the truth of the matter is really made, it might be supposed that the different parts of the stem and root of the seedling grow in length at the same rate. This can be tested by marking the stem and root with lines of Indian ink at equal distances. In one experiment with a pea seedling five lines were marked upon the young root at regular intervals of one-tenth of an inch, beginning at the tip (Fig. 11). The seedling was carefully replanted and examined again a few days later. Between the tip and the first mark there was then (Fig. 12) a distance of seven-tenths of an inch; that is, this part had grown to seven times its former length. The second interval was four times as long as before, the third was one and a half times as long, while the fourth and fifth intervals had not increased in length at all. Such experiments prove that the root grows in length either at or just behind the tip. When a young stem is treated in the same way the lengthening is found to take place more evenly.

Fig. 12.—The Pea seedling of Fig. 11, a few days later. cot, cotyledons; pl, plumule; R, radicle; S.c., seed-coat. (×1.)

Fig. 13.—The tip of a root, showing the root-cap. (Magnified.)

Rootlets.—After a time the radicle begins to put out branches called rootlets. These come off the main root in rows. In some cases rootlets make their appearance whilst the radicle is still very short, as in the vegetable marrow of Fig. 10, but in others the radicle may be a few inches long before it produces rootlets.

The root cap.—The tip of the root, and of each of its branches, is covered by a little cap, shaped somewhat like a thimble (Fig. 13). This protects the tender growing point from the friction of particles of soil, and is continually renewed by growth from within as its outer layers are worn away.

Root hairs.—When a young root is held against a dark background it appears fluffy. This appearance is caused by a large number of very fine hairs upon its surface. The hairs are not found all over the root and its branches, but only for a short distance a little way behind the tips (Figs. 7 and 11). These root hairs are of very great importance to the plant, as will be seen in Chapter II.