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 [Pg 17] 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 [Pg 18] 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.