1 68 Outline of Genetics 



uncontrollable. Thus two different plants, growing side by side, 

 might be in a distinctly different environment without the fact 

 being recognized. It cannot, therefore, be said with much cer- 

 tainty just how much hybrid vigor a given plant shows when there 

 are so many unknown factors that might affect size and vigor. 

 On the other hand, if it is claimed that the endosperm of one grain 

 shows a given amount of hybrid vigor as compared with the grain 

 that grows next to it upon the same ear, the statement would be 

 more exact, for the two endosperms have developed under con- 

 ditions which are unquestionably much more constant than the 

 conditions surrounding the different sporophytes in a corn field. 



Jones selected a plant with white endosperm and 

 pollinated it with a mixture of its own pollen and pollen 

 from a yellow endosperm race. In the resulting ear, 

 therefore, he had a mixture of yellow endosperm grains 

 and white endosperm grains. The former grains of 

 course wxre hybrid, since the yellow factor was introduced 

 by the foreign pollen, while the white endosperm grains 

 must have resulted from own pollen and were homozygous. 

 In this way, Jones obtained side by side in the same ear 

 endosperms obviously hybrid and endosperms obviously 

 homozygous. When he weighed these two types he 

 found that the hybrids exceeded the homozygotes in 

 weight by from 5 to 35 per cent. 



He made the reciprocal cross, using the same mixture 

 of yellow^ and white pollen on silks of the yellow race. 

 Of course all the resulting endosperms were yellow, but 

 the hybrids, which had the yellow factor only from the 

 female side, were distinctly lighter yellow than the 

 homozygotes, which had the yellow factor from both 

 male and female sides. Weighing these two types, 

 Jones obtained the same results as before, the hybrids 

 exceeding the others in weight by an average of 20 per 



