432 



BOTANY 



as in Gymnosperms. Fertilisation is effected, as throughout the 

 whole vegetable kingdom, by the fusion of the protoplast of a male 

 generative cell with an egg-cell (cf. p. 67). 



The fertilised egg-cell gives rise to the embryo, which, while still 

 enclosed within the embryo-sac, acquires a considerable size and 

 differentiation. After the embryo has attained a definite stage of 

 development, varying in different species, its further growth ceases, and 

 parting with its constituent water, it passes into a dormant condition, 

 from which, after the lapse of a longer or shorter period, it emerges 

 only when abundantly supplied with water. The other parts of the 

 ovule also increase in size, after fertilisation has been effected, and 

 undergo extensive internal modification. As a result of the changes 

 incident upon fertilisation, the ovule becomes converted into a SEED. 



The development of seeds, or enclosed macrosporangia 

 containing embryos, is a distinctive characteristic of the 

 Phanerogams. In conformity with this distinction, they are also 

 termed Seed- plants or Spermaphytes. 



The essential parts of the seed, which are always present, are the 

 seed - coats developed from the integuments of the ovule and the 

 embryo (Fig. 362, B). In many cases there also arises, from the 



Fig. 362. — Capsella bursa pastoHs. A, Longi- 

 tudinal section of a ripe seed ; h, hypocotyl ; 

 c, cotyledons ; v, vascular bundle of the 

 funicle (x 26): B, longitudinal section of 

 the seed -coat, after treatment with water ; 

 e, the swollen epidermis ; c, brown, strongly 

 thickened layer ; *, compressed layer of 

 cells ; a, aleurone grains ( x 240). 



Fid. 363. — A, Seed of Hyoscymrms niger, showing 

 the Dicotyledonous embryo embedded in the 

 endosperm ; B, seed of EleUaria Cardamomwm, 

 enveloped by a thin aril; the white, mealy 

 perisperm next to the seed -coat encloses an 

 oleaginous endosperm (shaded), in which the 

 Monocotyledonous embryo lies embedded. 

 (After Beho and Schmidt, magnified.) 



chalaza of the fertilised ovule, a fleshy envelope, the seed-mantle or 

 ARILLUS. Frequently a parenchyma rich in nutritive material is 

 formed between the embryo and the seed-coats. When this nutritive 

 tissue, or so-called albumen, is derived from the nucellus, it is termed 

 the perisperm (Fig. 363, B) ; when developed within the embryo-sac, 

 the endosperm (A). If the seed is provided with neither endosperm 

 nor perisperm (Fig. 362, A), the cells of the embryo itself are filled 

 with accumulated reserve material. 



