Germination.—Under the influence of moisture and warmth the embryo in the seed begins to swell and unfold its parts. The radicle makes its appearance first (Fig. 5), breaking through the seed-coat at the micropyle; it is the young root. The radicle always grows downwards, that is, toward the centre of the earth. If the seed lies in such a position that the micropyle is directed upwards, the point of the radicle turns over and grows downwards as soon as it escapes from the seed-coat. As the young root becomes longer and thicker (Fig. 6) the seed-coat opens more and more, showing the cotyledons beneath, and these, too, are gradually forced apart. [Pg 13] Fig. 5.—An early stage in the germination of a Broad-Bean seed. R, radicle; s.c., seed-coat. (× ⅔.) Fig. 6.—A slightly later stage in the germination of a Broad-Bean seed. cot., cotyledon; pl, plumule; R, radicle; s.c., seed-coat. (× ⅔.) 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 [Pg 14] 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.