PHANEROGAMIA. 409 



sperms, and the macrospore becomes more or less completely filled 

 with cellular tissue, commonly termed endosperm. 



The degree of development attained by the endosperm in Angiosperms 

 is various. Whilst, as a rule, it completely fills the embryo-sac, leaving 

 room, however, for the embryo, in some cases it occupies but a portion of 

 the embryo-sac, as in the Coco-nut, where it forms a thick parietal layer; 

 or, as in Nymphaea, Viscum, Lathrsea, Thesium, Rhinanthus, etc., where 

 the development of endosperm is confined to the upper half of the embryo- 

 sac. In some cases the endosperm is rudimentary, being represented 

 merely by a number of nuclei, as in Tropseolum, Alismaceae, Orchidaceae ; 

 and in Canna even this rudimentary development is wanting. 



The antipodal cells do not, as a rule, undergo any further development, 

 but in some cases (e.g. some Graminacese) they have been observed to 

 divide and give rise to a considerable mass of cells. 



The female organ is essentially an archegonium. In most 

 Grymnosperms it is actually an archegonium, like that of the 

 Pteridophyta ; it is developed from a single superficial cell of the 

 prothallium at the micropylar end, and has a neck containing a 

 canal-cell, leading to the ventral cavity in which lies the female 

 cell or oosphere. In the Angiosperms the female organ is reduced 

 to a single naked cell : the three cells constituting the egg- 

 apparatus represent each an archegonium reduced to a single cell ; 

 but one cell only is a true fertile oosphere, the other two (the 

 synergidse) being sterile as a rule. 



Pollination. In view of the fact that the female cell (oosphere), 

 and the prothallium bearing it, remain (as a rule) permanently 

 enclosed in the macrospore, and that the macrospore remains en- 

 closed in the indehiscent macrosporangium, it is clear that the 

 process of fertilisation can only be effected when the microspore 

 germinates in immediate proximity to the macrosporangium. The 

 bringing of the microspore into such close relation with the macro- 

 sporangium is what is termed pollination (see p. 232). "W hen the 

 pollen of any one flower is brought into relation with the macro- 

 sporangium of the same flower, the case is one of self-pollination : 

 when the pollen of an}' one flower is brought into relation with the 

 macrosporangium of another flower (whether on the same plant, or 

 on another plant of the same species), the case is one of cross- 

 pollination. 



The microspores when so brought are placed under conditions of 

 moisture and nutrition favourable to their germination. In Gymno- 

 sperms, where there is no ovary and no stigma, the microspore is 



