Fig. 7.
—Mustard
Seedling, showing
root-hairs and
cotyledons. (× ½.)
The cotyledons.—During the germination of various seeds, a very marked difference in the behaviour of the cotyledons is to be seen. In the case of the broad bean and pea the cotyledons remain in their original positions, partially enclosed by the split seed-coat. Presently a hump (Figs.[ 6] and [ 11]) forms at the upper end of the radicle, as if the plant were making an effort to pull its plumule out of the seed. It soon succeeds ([Fig. 12]), and the plumule turns up to the light. It is the young stem. At its end is a little bud, formed by a number of small, overlapping, green leaves which surround the growing point. Henceforth the stem grows upwards, that is in a direction precisely opposite to that of the root’s growth. Both stem and root are attached to the cotyledons, which gradually shrivel up as the stem and root become larger and larger.
When, however, the seed of the mustard, or sycamore, germinates the cotyledons behave very differently (Figs.[ 7] and [ 8]). Soon after the root has become well established the cotyledons come quite out of the seed-coat and unfold themselves. Instead of remaining on or under the surface of the ground they are carried upwards at the end of a stalk toward the light, and for some time the little plant appears to consist of root, stalk, and cotyledons only. If, however, the cotyledons are gently pressed apart, a tiny bud is seen between them. This evidently corresponds to the bud at the end of the stem of the bean or pea.
In the case of the lupine ([Fig. 9]) or vegetable marrow ([Fig. 10]) the cotyledons appear to combine these two conditions. They are swollen and contain stored food; yet they come out of the seed-coat early, become green, and open out to the light. They are evidently leaves, though their shape differs from that of the later leaves.
Fig. 8.—Three stages in the growth of a Sycamore Seedling. cot., cotyledons; fol., first pair of foliage leaves. (Slightly reduced.)
The germinating vegetable-marrow seed possesses a curious contrivance for pulling its cotyledons out of the seed-coat. This is a peg (p, [Fig. 10]) which develops at the top of the radicle, and holds down the lower half of the seed-coat whilst the other half is forced upwards to allow the cotyledons to be withdrawn.