ANGIOSPERMyE 



355 



(Tulip), but usually it divides by transverse walls into a row of 2-4 

 cells. Sometimes (Rosa, livida, Ariscema triphyllum) there may be 

 several of these sporogenous cells. 



The primary archesporial cell usually has cut off from it an outer 

 cell, the tapetum, which, by further divisions, gives rise to the tissue 

 at the apex of the nucellus. The inner cell may at once form the 

 embryo-sac, but more commonly divides into two or more cells, one 

 of which grows faster than the others, and destroys them. It may 

 ultimately destroy the whole of the nucellar tissue, except the apex, 

 and forms the single large macrospore, or embryo-sac. 



The primary nucleus of the embryo-sac divides, and in the typical 

 Angiosperms (Fig. 320) one nucleus moves to each end of the embryo- 



D 



FIG. 320. A, B, Naias flexilis. A, young embryo-sac with two nuclei. B, older 

 embryo-sac with four nuclei. C, diagram of typical angiospermous embryo-sac; 

 at the upper (micropylar) end, the egg-apparatus consisting of the synergids, sy, 

 and the egg, o; at the lower (chalazal) end, the three antipodal cells, ant; pn, the 

 two polar nuclei. D, Peperomia pellucida, section of young embryo-sac, which 

 contains sixteen free nuclei, not all shown in the section (X 400). 



sac. The upper end is the micropylar end, the lower the chalazal, 

 or antipodal end. Each nucleus then divides twice, and of the four 

 nuclei at each end one moves toward the centre of the embryo-sac, 

 where these " Polar-nuclei " unite to form the " Endosperm nucleus." 

 This fusion of the polar nuclei usually occurs before the fertilization 

 of the egg-nucleus, but it may not occur until afterward. The three 

 micropylar nuclei become invested with thin cytoplasmic membranes, 

 and one of them is the egg-cell (o), the other two being known as 

 Synergids. The three antipodal nuclei form a similar group of cells, 

 the antipodal cells, which, unlike the cells of the egg-apparatus, 

 very often develop a cellulose wall. 



Peperomia. The genus Peperomia (Fig. 320, D) shows a marked de- 

 parture from the other Angiosperms in the development of the gameto- 



