=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.
[Illustration: Fig. 95. Seedlings of castor-oil bean casting the seed coats, and showing papery remnant of the endosperm.]
[Illustration: Fig. 96. Seedlings of jack-in-the-pulpit; embryo backing out of the seed.]
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.
[Illustration: Fig. 98. Seedlings of jack-in-the-pulpit, first leaf arching out of the petiole of the cotyledon.]
[Illustration: Fig. 99. Embryos of jack-in-the-pulpit still attached to the endosperm in seed coats, and showing the simple first leaf.]
[Illustration: Fig. 100. Seedling of jack-in-the-pulpit; section of the endosperm and cotyledon.]
=216. The first leaf of the jack-in-the-pulpit is a simple one.=—The first leaf of the embryo jack-in-the-pulpit is very different in form from the leaves which we are accustomed to see on mature plants. If we did not know that it came from the seed of this plant we would not recognize it. It is simple, that is it consists of one lamina or blade, and not of three leaflets as in the compound leaf of the mature plant. The simple leaf is ovate and with a broad heart-shaped base. The jack-in-the-pulpit, then, as trillium, and some other monocotyledonous plants which have compound leaves on the mature plants, have simple leaves during embryonic development. The ancestral monocotyledons are supposed to have had simple leaves. Thus there is in the embryonic development of the jack-in-the-pulpit, and others with compound leaves, a sort of recapitulation of the evolutionary history of the leaf in these forms.