| 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.]
3. THE STRUCTURE OF GRAINS OF
MAIZE AND WHEAT.
1. Preparation of the seeds.—Soak grains of maize (Indian corn) and wheat in water until they are soft. The grains of maize will need soaking for several days. Plant about a dozen of each in damp sawdust, and in the meantime examine others.
2. The maize grain.—A grain of maize is really a fruit, as a pea-pod is. A pea-pod contains several seeds; a maize fruit contains only one seed, which fills it. Notice the shape of the grain—flattened, rounded along one edge, and bluntly pointed at the opposite edge ([Fig. 14]). Notice a whitish patch on one of the flattened sides; a ridge (E) down the middle of this marks the position of the embryo. Cut through the grain lengthwise, so as to divide the embryo into two equal parts, and examine the cut surface ([Fig. 15]). Identify:
(a) The embryo, lying somewhat obliquely, and to one side. The radicle (rad.) is directed towards the pointed end of the grain, and the plumule (pl.) towards the rounded end.
(b) The endosperm (end): a mass of material outside the embryo, and forming at least half of the grain.
(c) The scutellum (scm): a plate lying between the endosperm and the embryo.