256 



BOTANY. 



481. While these changes are going on, the nuclei of the 

 embryo-sac increase rapidly (by mitotic division) and form 

 cells which fill up a considerable part of its cavity. These 

 cells constitute the endosperm, and serve somewhat later to 

 nourish the growing embryo. This nourishing tissue is the 

 homologue of the sexual plant (prothallium) of the Fern- 

 worts, here greatly belated. 



482. The embryo in its growth gradually absorbs the en- 

 dosperm. In many cases growth is checked in the ripen- 

 ing of the seed, before much of the endosperm is used up 

 (Fig. 154, ^ to D ) ; in such seeds the embryo is small and 



J H G F 



Fig. 154.— Magnified sections of seeds, showing emtryos and endo- 

 sperms. A, Oat ; B, Sedge ; C, Coffee ; D, Marsh-marigold ; E Bitter- 

 sweet ; F, Goosef oot ; G, Nettle ; H, Oak ; I, Sweet Pea ; J, a Mustard In 

 -A to D, small or minute embryo in large endosperm ; E to O larger 

 embryo and smaller endosperm ; H to J, large embryo and no endosperm. 



poorly developed. In other cases more (Fig. 164, E to G), 

 or in still others all (Fig. 154, ^to /), of the endosperm 

 is absorbed; in these the embryos are much larger and 

 better developed. Where endosperm remains in a seed, its 

 cells are generally filled with starch, or less frequently with 

 oily matters ; where no endosperm remains, there is always 



