The Endosperm in Inheritance 149 



gametes fertilizes the egg and produces a hybrid embryo, 

 which, in the next generation, behaves as a heterozygote 

 for yellow endosperm. The other male gamete fertilizes 

 the fusion nucleus and produces the endosperm nucleus, 

 which therefore contains the factor for yellow endosperm, 

 the result being that the endosperm is yellow, although 

 the ovule belongs to the white race. Xenia means, 

 therefore, that the endosperm is a hybrid as well as 

 the embryo, and "triple fusion" involves the trans- 

 mission of hereditary characters. Fertilization of the 

 fusion nucleus is just as essential as fertilisation of the 

 egg, and so far as inheritance is concerned the endosperm 

 and embryo are sister sporophytes. 



Xenia has caused much discussion in genetics. It 

 throws light upon the nature of endosperm and suggests 

 that it belongs to the sporophyte generation because it is 

 a product of an act of fertilization. Because of its 

 behavior in inheritance geneticists would naturally 

 regard the endosperm as a sporophyte, an abnormal 

 sister to the embryo. 



Cases of xenia are not limited to yellow endosperm. 

 Xenia appears also in the crosses between sweet and 

 starchy corn. Crosses of red corn and purple corn also 

 show xenia, but in this case additional details appear. 

 A section of a grain of corn is shown in fig. 35. There is 

 first the pericarp or "seed coat," which is the ovary 

 wall, belonging to the old sporophyte, and therefore 

 does not concern us. Within this is a thin aleurone 

 layer, which is the outer layer of endosperm, while the 

 bulk of the seed consists of the starchy endosperm. 

 Since aleurone is endosperm, colors pecuUar to it would 

 show xenia in inheritance. This was shown in the case 



