f»16 



BOTANY 



PAllT II 



pro thallium ( = endosjoerm). This fills the embryo-sac and nourishes the 

 embryos, which grow down into it. The surrounding tissue of the 

 nucellus becomes crushed so that the embryo-sac extends to the seed- 

 coat. The cells of the endosperm are packed with reserve materials 

 (starch, fat, proteid) and these are utilised in the further development 

 of the embryo which takes place on germination, usually after a 

 period of rest. 



Tlie nutritive tissue in the Angiosperms arises on the other hand 

 after the egg-cell has been fertilised. It is derived from the secondary 

 nucleus of the embryo-sac derived by the fusion of the two polar 

 nuclei. This is stimulated to division after fusion with the second 

 generative nucleus. The nuclei derived by this process of division are 

 distributed in the protoplasm which lines the wall of the embryo-sac, and 

 Avhen a large number has been formed the protoplasm divides to form 





Fig. 480. — Part of section througli one of the coty- 

 ledons of the Pea, showing cells with reserve 

 material, am, Starch grains; al, aleurone grains; 

 p, protoplasm; n, nucleus, (x 100.) 



Fig. 481. — Cell ftom the endosperm of 

 I'hyteh'.phas maciocarpa, with reserve 

 cellulose. (X 225.) 



numerous cells. These by further division fill the whole embryo-sac 

 with the tissue of the endosperm. 



Gnetwn is the only Gymnosperm in which the process resembles that in 

 Angiosperms. In it after one or more free egg -nuclei has been fertilised the 

 remaining nuclei increase in number and by a process of free-cell-forniation give 

 rise to the tissue of the endosperm which fills the embryo-sac. 



In Angiosperms also the endosperm as a rule compresses the 

 remains of the nucellus. Reserve materials such as starch, fatty oil, 

 and aleurone grains are accumulated in the endosperm cells (Figs. 69, 

 70, p. 67) ; in other cases the greatly thickened walls form a store of 

 reserve-cellulose (Fig. 481). In a few cases, as in Piperaceae, Scita- 

 mineae, etc., the nucellus persists and also serves as a nutritive tissue ; 

 it is then termed PEUisrERM (Fig. 482). In very many cases, how- 

 ever, not only is the nucellus absorbed by the endosperm, but the 

 latter is completely displaced by the embryo. The reserve materials 

 are then stored up in the cotyledons or in the whole body of the 

 embryo. 



