We need hardly insist, after what we have said, upon the necessity for some air and light, or remind the reader that he must not keep the seeds in a close, dark place, though light is not so necessary at first as air. The embryo connects the “cotyledons” or halves of the seed, and this develops into a tiny rootlet or “radicle,” and upwards into the stem, the commencement of which is known in botany as a “plumule.” The rootlet seeks nourishment from the ground. The albumen secreted in the cotyledons feeds the embryo, until (in some cases) it is exhausted and they die away. In other cases they grow up and obtain food for the young plant in the air. Some plants have (like wheat) only one seed-leaf, or cotyledon; and these kinds are called monocotyledons, or endogens, in which the growth is upright. The others are called dicotyledons, or exogens.
Fig. 748.—Polyadelphous stamens.
>Fig. 749.—Pistil of primrose.
So far now, perhaps, you may understand that the outer covering of the seed is called the testa; the opening which may be perceived in the ordinary bean near the dark spot is the micropyle, or little gate; that the halves of the covering are termed cotyledons, or cups, and that the embryo sprouts upwards and downwards, the upper part of the stem being the plumule, and the lower portion the radicle. Even if the seed be put micropyle upwards into the ground, or between layers of flannel, to germinate, you will find that the radicle will always curve downwards.
Fig. 750.—Diadelphous stamens.
Fig. 751.—Fibrous root.