146 Plant Genetics 



evidently the vegetative body of the female gameto- 

 phyte, and this tissue is called the endosperm. 



In angiosperms, however, a new situation intro- 

 duces doubt. It can be said as before that after the 

 germination of the megaspore everything within the 

 megaspore wall is a female gametophyte tissue, but 

 it cannot be said that fertilization affects the egg only, 

 for one of the sperms fuses as regularly with the fusion 

 nucleus as does the other sperm with the egg. It will 

 be remembered that the fusion nucleus is formed by 

 two nuclei, which have migrated from each end of the 

 sac, so that when the sperm enters into the fusion there 

 is a triple fusion. After fertilization the fertihzed egg 

 of course forms the embryo sporophyte, but usually 

 every nucleus of the old gametophyte disappears except 

 the fertilized fusion nucleus, which then forms the 

 endosperm in which the young sporophyte is imbedded. 

 For this reason the fertilized fusion nucleus is usually 

 called the endosperm nucleus. 



A comparison of angiosperm and gymnosperm seeds 

 reveals the following contrast (see fig. 34) . In the appear- 

 ance of their essential structures they aje exactly aHke, 

 and for this reason some botanists claim that the endo- 

 sperm of angiosperms is the same as that of gymno- 

 sperms, that is, gametophyte tissue. The opposing claim 

 is that, although the gymnosperm endosperm is gameto- 

 phyte tissue, the situation in angiosperms is essentially 

 different. In angiosperms the endosperm does not arise 

 from morphologically unmodified gametophyte tissue, as 

 in gymnosperms, but entirely from the endosperm nu- 

 cleus, and this nucleus is clearly the product of the fusion 

 of male and female nuclei. With such an origin the endo- 



