Читать книгу An Introduction to Nature-study - E. Stenhouse - Страница 8

Fig. 17.—Young wheat seedling.

C, cotyledon; r, r, r, r, roots. (×3.)

The endosperm.—The endosperm, which at first made up more than half the seed, gradually shrivels up as the little plant continues its growth. The food material which it contains is absorbed by the scutellum and is passed on to afford the plant the necessary nourishment for those early stages when it is too young to feed itself. By the time the first few foliage leaves are well developed, all that remains of the grain is an empty husk.

Comparisons and contrasts.—The examination of these seeds and seedlings will enable the student to see that differences, which at the first glance appear great, are often of only minor importance; while apparently small variations may prove, on closer inspection, to be caused by deeply-seated differences of structure and habits of life. He should always set himself the questions, “In what ways do such and such objects resemble each other; and in what ways do they differ from each other? Which of the differences and resemblances are of most importance?” He should also notice that a mere difference of size is often of very small consequence.

Above all, the student should get into the habit of asking the reasons for the differences and resemblances which he notices in his nature-study. To learn what these reasons are he must observe closely, think carefully, and then make experiments to test the accuracy of his conclusions. “Be sure you are right; then look again”[4] should be his motto.

It is at once plain that the seedlings fall into two classes, according to the number of cotyledons or seed-leaves which they possess. The wheat and maize have only one such seed-leaf, while the mustard, bean, pea, sycamore, and vegetable marrow have two each. We shall see later on that one-seed-leaved plants differ from those with two seed-leaves not only in the number of their cotyledons, but also in the characters of their leaves and flowers and in their method of growth. These differences are so constant and so important that botanists have agreed to call all plants of the first class (such as maize and wheat) Monocotyledons, and plants of the second class Dicotyledons.

One of these differences is that the main roots of dicotyledons are formed directly by the growth of their radicles; while in monocotyledons there is, after a short time, no such main root to be found, but several roots of almost equal size spring from the base of the stem and spread outwards in all directions (Fig. 17).

Both maize and wheat seeds contain—outside the embryo—a large store of food called endosperm (Figs. 15 and 16), which is not seen in any of the dicotyledonous seeds described in this chapter. This is not a very important difference, for, if we examined a very large number of dicotyledon seeds, we should find that most of them possessed endosperm. On the other hand, many monocotyledonous seeds are destitute of endosperm. Only after observing a very large number of facts is it safe to make general statements.

Confining our attention to dicotyledons, we are impressed by the great variation in size of the cotyledons. Those of the bean and pea are swollen with food material and form a large proportion of the bulk of the seed. As a consequence, the seedling has enough food to enable it to grow into quite a sturdy little plant before it needs any foliage leaves. The cotyledons of the mustard and sycamore, however, are thin, and they unfold almost immediately into green leaves, and set to work to help to maintain the plant until the first foliage leaves can be formed. The cotyledons of the lupine (Fig. 9) and vegetable marrow (Fig. 10) serve a double purpose. They not only contain a store of food ready to hand, but they also set to work early to make new food, until the new leaves are sufficiently advanced to take up their duties. It should be remembered that cotyledons are makeshift leaves.

EXERCISES ON CHAPTER I.

1. Make a collection of the seeds of various trees; try to find, in each seed, the cotyledons, radicle, and plumule. Which of the seeds contain stored starch?

2. Soak pine and larch seeds in water for several days and then sow them, with a covering of half an inch of soil. Make notes of the number, shape, size, and behaviour of the cotyledons. How large are the seedlings at the end of the first season?

3. Make similar observations on the growth of sycamore, ash, and beech. Cover the seeds with an inch of soil.

4. Plant seeds of oak and chestnut two inches deep, and make drawings and notes of the stages of growth.

5. Investigate the structure and method of germination of a barley seed, and find out whether barley is a dicotyledon or a monocotyledon.

6. Make experiments to discover the effects, upon the germination of various seeds, of differences of temperature, moisture, and light, and write full accounts of the results obtained.

7. Draw from memory a young seedling of maize, and notice its chief peculiarities. (1898)

8. Draw the seedling of the sycamore in two or more stages, and add short notes. (1898)

9. Draw the root of any seedling that you have studied, giving its name. Mark the exact position of the root-hairs. (1898)

10. Open the nut provided. Draw what is to be found in it in one or two positions. Name the parts and give short explanations. (1901)

11. Explain, with drawings, how certain seedlings withdraw their seed-leaves from the seed-coat. (1901)

12. Describe and explain as far as you can the principal changes to be observed during the germination of a bean or pea. (1901)

13. Describe the germination of a bean, and compare it with that of a grain of wheat. (1898)

14. Describe the structure of a grain of wheat, and contrast it with that of an acorn. (1896)

15. Plant seeds in wet, sticky soil (so that the air cannot easily get to them), and compare their growth with that of similar seeds in a light, open soil.

16. Two acorns are allowed to germinate, one in the neck of a bottle full of water, and the other in an ordinary flower pot. What differences will be noted in the two plants as they grow? (Certificate, 1904)