146 Outlme of Genetics 



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, 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 the 

 ''triple fusion" involves the transmission of hereditary 

 characters. Fertilization of the fusion nucleus is just 

 as essential as fertilization of the Qgg, and so far as inherit- 

 ance is concerned the endosperm and embryo are sister- 

 sporophytes. 



The exact function of double fertilization is not 

 clearly understood. Nemec (7) has sought to account 

 for endosperm hybridization as an adaptation which 

 results in a better adjustment of the composition of the 

 reserve food supply to the needs of a hybrid embryo. 



Xenia throws considerable light on the nature of 

 endosperm. Because of its behavior in inheritance, 

 geneticists would naturally regard the endosperm as a 

 sporophyte, an abnormal sister to the embryo. 



The phenomenon of xenia is not limited to the case 

 of yellow endosperm, but appears in connection with 

 quite a number of endosperm characters. The red- 

 grain and purple-grain characters in corn, which were 

 employed to illustrate types of factor interaction, are 

 also governed by this mechanism. In these cases, how- 

 ever, an additional detail appears. A section of a grain 

 of corn appears in hg. 24. 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 alcurone layer, which is the outer 



