Fig. 98.
Seedlings of
jack-in-the-pulpit,
first leaf arching
out of the
petiole of
the cotyledon.
Fig. 99.
Embryos of
jack-in-the-pulpit
still attached to
the endosperm
in seed coats,
and showing
the simple
first leaf.
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.
216a. Germination of the pea.—Compare with the bean. Note especially that the cotyledons are not lifted above the soil as in the beans. Compare germination of acorns.
[Digestion.]
[216]b. To test for stored food substance in the seedlings studied.—The pumpkin, squash, and castor-oil bean are examples of what are called oily seeds, since considerable oil is stored up in the protoplasm in the cotyledons. To test for this, remove a small portion of the substance from the cotyledon of the squash and crush it on a glass slip in a drop or two of osmic acid.[16] Put on a cover glass and examine with a microscope. The black amorphous matter shows the presence of oil in the protoplasm. The small bodies which are stained yellow are aleurone grains, a form of protein or albuminous substance. Both the oil and the protein substance are used by the seedling during germination. The oil is converted into an available food form by the action of an enzyme called lipase, which splits up the fatty oil into glucose and other substances. Lipase has been found in the endosperm of the castor-oil, cocoanut, and in the cotyledons of the pumpkin, as well as in other seeds containing oil as a stored product. The aleurone is made available by an enzyme of the nature of trypsin. Test the endosperm of the castor-oil bean in the same way. Make another test of both the squash and castor-oil seeds with iodine to show that starch is not present.
Test the cotyledon of the bean with iodine for the presence of starch. If the endosperm of corn seed has not been tested do so now with iodine. The endosperm consists largely of starch. The starch is converted to glucose by a diastatic “ferment” formed by the seedling as it germinates. Make a thin cross-section of a grain of wheat, including the seed coat and a portion of the interior, treat with iodine and mount for microscopic examination. Note the abundance of starch in the internal portion of endosperm. Note a layer of cells on the outside of the starch portions filled with small bodies which stain yellow. These are aleurone grains. The cellulose in the cell walls of the endosperm is dissolved by another enzyme called cytase, and some plants store up cellulose for food. For example, in the endosperm of the date the cell walls are very much thickened and pitted. The cell walls consist of reserve cellulose and the seedling makes use of it for food during growth.