SEEDS AND FRUITS. 143 
which are gradually converted into the soluble form, and diffuse 
into the young plant, forming its first nutriment. Active respira- 
tion here takes place, carbon dioxide being given off in consider- 
able quantities. Hence the necessity for air, or rather for the 
free oxygen which it contains. Acorn, pew, and most forms with 
large cotyledons, germinate like the bean, and in nature the coty- 
ledons remain below ground, enclosed in the seed-coat. Such 
germination is therefore called hypogean. It is to be noted that 
only a small part of the so-cailed radicle is really root. The 
region between this and the cotyledons is the base of the stem 
or hypocotyl (figs. 2 and 3). Where the cotyledons are small and 
comparatively poor in reserve materials, as in mustard and 
cress, they escape from the seed-coat by elongation of the hypo- 
cotyl, and becoming green, function as the first leaves. Such 
germination is epigean (fig. 3). 
The chief point in which a germinating albuminous seed differs 
from the preceding is that the cotyledons (or cotyledon) act as 
organs of absorption. They remain within the seed-coat, and so 
influence the reserve materials in the endosperm that they become 
transformed into the soluble state, when they can readily be 
absorbed. Castor-oil seeds furnish a good dicotyledonous example. 
Among monocotyledons grasses (maize, wheat, oat, barley, &c.) and 
date may be mentioned. The scwtellum (cf. p. 133) of the former 
effects absorption, and the radicle and plumule elongate in oppo- 
site directions, the former having to break through a layer of 
tissue known as the root-sheath (fig. 54). In date the tip of the 
sheath-like cotyledon remains within the seed, while its base elon- 
gates considerably, thus pushing radicle and plumule (which latter 
it surrounds) completely out of the seed. 
