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 [Pg 23] 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 [Pg 24] 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.