356 



BOTANY 



phyte. The primary nucleus of the embryo-sac divides into sixteen, 

 instead of eight, nuclei, and these nuclei are uniformly distributed 

 through the peripheral cytoplasm, instead of forming a definite egg- 

 apparatus and antipodal cells. In this respect the gametophyte of 

 Peperomia resembles the early stages of that in Isoetes and Selaginella, 

 and still more the condition found in the mature embryo-sac of Gnetum. 

 As in Gnetum, apparently any nucleus may become differentiated to 

 form that of the egg. In Peperomia no polar nuclei are developed, 

 but after fertilization several (usually eight) of the nuclei fuse into 

 one very large nucleus, which by division gives rise to the endosperm, 

 or secondary prothallial tissue. 



The Antipodal Cells 



The antipodal cells generally remain unchanged, and apparently 

 take little part in the further development of the embryo-sac. There 



FIG. 321. A, Naiasflexilis. Pollen-tube entering the embryo-sac (X400) ; pt, pollen- 

 tube; s, synergids; o, egg. B, Sparganiuin simplex, embryo, em, surrounded by 

 the young endosperm-cells; en, free endosperm nuclei (X 200). 



are, however, many exceptions to this. Thus in most Grasses the 

 number of antipodal cells is much increased, and they become large 

 and conspicuous, and are evidently actively concerned in the nutri- 

 tion of the developing embryo-sac and embryo. A similar condition 

 has been observed in many Compositse, and the very large antipodal 

 cells of some Ranunculaceae show a division of the nucleus, although 

 no cell-division occurs. The most remarkable case yet observed is 

 that of Sparganium simplex (Fig. 322), where the three small antipo- 

 dal cells of the unfertilized* embryo-sac subsequently give rise tc ? 

 mass of one hundred and fifty or more active cells. 



Pollination 



The pollen-spores are sometimes so placed that they fall sponta- 

 neously upon the stigma of the same flower. More commonly cross 



