580 



BOTANY 



PART II 



tropous ovules develop into seeds resembling those derived from 

 anatropous ovules, but the embryo is curved. 



In some cases the function of the seed-coat is modified owing to the protection of 

 the seed or seeds being undertaken by the pericarp ; this or its innermost layers 

 are developed as sclerotic cells and form the stone of the drupe or shell of the 

 nut. In such cases (e.g. Almond, Cherry-Laurel, Cherry, Pepper, etc.), since any 

 special development of the seed-coat is unnecessary, it tends to become reduced ; 

 its cells do not thicken or modify their walls and the various layers become 

 simply compressed. 



The nutritive tissue in the seeds is developed, in the case of 

 Gymnosperms (except in Gnetum\ by the time of fertilisation and 



FIG. 563. Part of section throligh one of the 

 cotyledons of the Pea, showing cells with 

 reserve material, am, Starch grains ; al, 

 aleurone grains ; p, protoplasm ; n, nucleus, 

 (x 160. After STRASBURGER.) 



FIG. 564. Transverse section of the seed of CoZ- 

 chicum, showing the reserve-cellulose of the 

 endosperm within the seed-coat. 



constitutes the prothallium (cf. p. 565). This fills the embryo-sac and 

 nourishes the embryo, which grows 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 ; this takes place on germination, 

 usually after a period of rest. 



The nutritive tissue in the Angiosperms (and of Gnetum) arises, on 

 the other hand, after the egg-cell has been fertilised. It originates 

 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 produced by this 

 process of division are distributed in the protoplasm which lines the wall 

 of the embryo-sac, and when a large number has been formed the 



