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 to be withdrawn.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.
