SPERMATOPHYTES 



269 



the embryo sac by way of the micropyle, but pierces directly through 

 the region of the ovule beneath the embryo sac (chalaza) and enters 

 the embryo sac from below. Among the more familiar plants in which 

 chalazogamy has been found are 

 the walnuts and elms. Entrance 

 by the micropyle is called porog- 

 amy; and there are other routes 

 used by the pollen tubes of cer- 

 tain plants, intermediate between 

 true chalazogamy and porogamy. 



Fertilization. — After the tip 

 of the pollen tube has entered 

 the sac, it enlarges very much, 

 usually destroys one of the 

 synergids, and finally discharges 

 the two male cells or nuclei 

 (fig. S99). One of the male 

 cells passes to the egg and fer- 

 tilization is accomplished. The 

 other male cell passes deeper 

 into the sac, comes into contact 

 with the fusion nucleus, and fuses 

 with it. Into the structure of the 

 primary endosperm cell (or nu- 

 cleus), therefore, three nuclei 

 have entered : an antipodal 

 polar, a micropylar polar, and a 

 male nucleus. This participa- 

 tion of both male cells in nuclear 

 fusions in the same embryo sac 

 has been called double fertiliza- 

 tion, and it is perhaps the greatest puzzle connected with the embryo 

 sac of angiosperms. 



Double fertilization. — This phenomenon was first described in 1898, 

 but subsequent investigation has indicated that it is probably of universal 

 occurrence among angiosperms. It means that one male cell enters 

 into the formation of the embryo, and the other into the formation of the 

 endosperm. This raises a question as to the nature of the endosperm of 

 angiosperms. The old view was that it is belated tissue of. the female 



Silphium : 



Fig. 599. — Fertilization 

 sy, undestroyed synergid; pt, swollen tip of 

 pollen tube, still with some contents (»); 

 spi, coiled male cell in contact with egg 

 nucleus (o); sp2, curved male cell in contact 

 with fusion nucleus (c). — After Land. 



