148 Plant Genetics 



The geneticist is not much concerned about the exact 

 morphological or cytological nature of endosperm, but he 

 is much concerned about its behavior in inheritance, and 

 perhaps the phenomena of endosperm inheritance may 

 help to decide whether endosperm is gametophyte or 

 sporophyte or neither. 



Certain races of corn have yellow endosperm, while 

 in other races it is white (colorless) . If a cross is made 

 with pollen from the yellow endosperm race on the 

 silks of the white endosperm race, what would the infer- 

 ence be as to the result ? We could assume that yellow is 

 dominant over white, since yellow is probably due to the 

 presence of a factor which is lacking in the white. In 

 making such a cross, therefore, we would expect a hybrid 

 embryo to be formed which would show the dominant 

 character of yellow endosperm when this embryo 

 becomes a plant bearing ears the next season. On the 

 contrary, we find that the dominant yellow character 

 appears, but it appears that same year. The cross 

 of course puts the yellow endosperm factor in the young 

 hybrid embryo, but we cannot imagine that this embryo 

 passed the character out into the endosperm that sur- 

 rounds it. The real mechanism is as follows. 



Some time after this phenomenon was discovered in 

 1872, it was named xenia (in 1881), the definition of the 

 term being the direct effect of foreign pollen upon the 

 endosperm. At the time of its discovery the mechanism 

 involved in xenia was not understood. Later, double 

 fertilization was discovered, and this furnished a complete 

 mechanism. A pollen grain from the yellow endosperm 

 race contains two male gametes, and each gamete con- 

 tains the factor for yellow endosperm. One of the 



