SEEDS AND FRUITS. 1 43 



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, peu, 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 liypogean. It is to be noted that 

 only a small part of the so-called 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, kc.) and 

 date may be mentioned. The scutellum (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. 



