MORPHOLOGY OF THE SEEDLING - 29 
such a case as this the appropriateness of the name seed- 
leaf is evident enough, — one recognizes at sight the fact 
that the cotyledons are actually the plant’s first leaves. 
In the bean the leaf-like nature of the cotyledons is not 
so clear. They rise out of the ground like the squash 
cotyledons, but then gradually shrivel away, though they 
may first turn green and somewhat leaf-like for a time. 
In the pea (as in the acorn, the horse-chestnut, and 
many other seeds) we have quite another plan, the under- 
ground type of germination. Here the thick cotyledons 
no longer rise above ground at all, because they are so 
gorged with food that they could never become leaves ; 
but the young stem pushes rapidly up from the surface 
of the soil. 
The development of the plumule seems to depend some- 
what on that of the cotyledons. The squash seed has 
cotyledons which are not too thick to become useful leaves, 
and so the plant is in no special haste to get ready any 
other leaves. The plumule, therefore, cannot be found 
with the magnifying glass in the unsprouted seed, and is 
almost microscopic in size at the time when the hypocotyl 
begins to show outside of the seed-coats. 
In the bean and pea, on the other hand, since the cotyle- 
dons cannot serve as foliage leaves, the later leaves must 
be pushed forward rapidly. In the bean the first pair are 
already well formed in the seed. In the pea they cannot 
be clearly made out, since the young plant forms several 
scales on its stem before it produces any full-sized leaves, 
and the embryo contains only hypocotyl, cotyledons, and a 
sort of knobbed plumule, well developed in point of size, 
representing the lower scaly part of the stem. 
