104 COBN CROPS 



corn. This is called xenia, or " the immediate effect of 

 pollen on the endosperm." The effort of xenia, however, 

 is limited to the kernel, there being no apparent effect on 

 the cob or on the stalk. 



In ordinary hybridization, only the germ is a hybrid and the 

 endosperm sm-rounding is not affected. Xenia is accounted for 

 by "double fecundation." In ordinary fertiUzation, only one 

 of the two generative nuclei which are formed in the pollen tube 

 is supposed to pass into the embryo sac and unite with the egg 

 cell. In double fecundation both nuclei enter the egg sac, one 

 fusing with the nucleus of the egg cell, and the other with the 

 polar nuclei to form the embryo sac nucleus, the di^dsion of which 

 gives rise to the endosperm. The endosperm would then be a 

 hybrid and partake of the dominant characters of the male 

 parent. 



The fact that only a part of the kernels show xenia, means 

 that double fertilization does not always take place. It is neces- 

 sary that the germ cell be fertilized, but it appears at present 

 that fertilization of the endosperm nucleus is incidental rather 

 than necessary. 



Mendel's laws 



76. If crossed or hybrid kernels of dent and sweet corn 

 be planted, they will produce ears having both dent and 

 sweet corn kernels, with a ratio of three dent corn grains 

 to one sweet corn grain. This is explained by assuming 

 that the germ cells (pollen grains or ovaries) are either 

 pure sweet corn or pure dent corn. 



When a plant is grown from a hybrid seed, then, one- 

 half the pollen grains will represent pure sweet corn, and 

 one-half pure dent corn, and the same with the ovaries. 

 While the plants may be hybrid, the sexual elements re- 

 main pure. In the process of fertilization a union produc- 

 ing a hybrid (sweet X dent or dent X sweet) will occur 

 twice as often as a pure dent (dent X dent) or a pure 

 sweet (sweet X sweet). 



