213. How the embryo backs out of the seed.—The embryo lies within the mass of the endosperm; the root end, near the smaller end of the seed. The club-shaped cotyledon lies near the middle of the seed, surrounded firmly on all sides by the endosperm. The stalk, or petiole, of the cotyledon, like the lower part of the petiole of the leaves, is a hollow cylinder, and contains the younger leaves, and the growing end of the stem or bud. When germination begins, the stalk, or petiole, of the cotyledon elongates. This pushes the root end of the embryo out at the small end of the seed. The free end of the embryo now enlarges somewhat, as seen in the figures, and becomes the bulb, or corm, of the young plant. At first no roots are visible, but in a short time one, two, or more roots appear on the enlarged end.
214. Section of an embryo.—If we make a longisection of the embryo and seed at this time we can see how the club-shaped cotyledon is closely surrounded by the endosperm. Through the cotyledon, then, the nourishment from the endosperm is readily passed over to the growing embryo. In the hollow part of the petiole near the bulb can be seen the first leaf.
Fig. 95.
Seedlings of castor-oil bean casting the seed coats,
and showing papery remnant of the endosperm.
Fig. 96.
Seedlings of jack-in-the-pulpit;
embryo backing out of the seed.
Fig. 97.
Section of germinating embryos of
jack-in-the-pulpit, showing young
leaves inside the petiole of the
cotyledon. At the left cotyledon
shown surrounded by the endosperm
in the seed; at right endosperm
removed to show the club-shaped
cotyledon.
215. How the first leaf appears.—As the embryo backs out of the seed, it turns downward into the soil, unless the seed is so lying that it pushes straight downward. On the upper side of the arch thus formed, in the petiole of the cotyledon, a slit appears, and through this opening the first leaf arches its way out. The loop of the petiole comes out first, and the leaf later, as shown in [fig. 98]. The petiole now gradually straightens up, and as it elongates the leaf expands.
