230 PHYSICAL BASIS OF HEREDITY 



in the embryo sac. Hence half the embryo sacs are F and 

 half /. The former, F (+F), fertilized by F pollen gives 

 FFF endosperm, by / pollen give FFf; the latter, / (+/), 

 fertilized by F pollen gives ffF endosperm, by / pollen 

 /// endosperm. The four kinds of endosperm fall into two 

 classes, soft and hard, in the ratio of 1 : 1 in the F2, seeds. 



There are races of maize with yellow dominant endo- 

 sperm and others with recessive white. If the mother 

 belongs to a yellow race and the father to a white one, 

 the Fi endosperm is yellow like the mother. In the recip- 

 rocal cross it is also yellow. If, however, races with floury 

 seeds are used, the F-^ yellow endosperm in the former 

 cross is somewhat paler than the pure yellow of the yellow 

 race. Races with purple or with red endosperm crossed to 

 white give the same results, except that in these crosses the 

 quantitative effects seen in the floury flint crosses are not 

 observed, for, one dose of the dominant (purple) to two 

 doses of white gives the same color as two doses of purple 

 to one dose of white. 



There are two kinds of maize with white endosperm. 

 These when crossed together give F^ colored endosperm. 

 In these cases one race has one of the factors for color, and 

 the other, another complementary factor — ^like the two 

 white sweet peas. There is also a race with a dominant 

 white endosperm factor. The occurrence of these kinds of 

 whites led to some confusion in the earlier experiments of 

 Correns on endosperm inheritance. The word Xenia, that 

 had earlier a different meaning, is used to-day for these 

 cases of double fertilization in which the pollen has an 

 influence on the seed (the endosperm) that is not a part of 

 the Fi plant itself. East and Hayes sum up the results 

 given above (exclusive of the floury-flint cross) as follows : 



When two races differ in a single visible endospenn character in 

 which dominance is complete, Xenia occurs only when the dominant 

 parent is the male; when they differ in a single visible endosperm 

 character in which dominance is incomplete, or in two characters both 

 of which are necessary for the development of the visible difference, 

 Xenia occurs when either is the male. 



