172 Plant Genetics 



in the first case each parent contains three factors, the 

 small letters representing merely the absence of factors. 

 The F! generation, therefore, contains six factors, and 

 for this reason is more vigorous than either parent. It 

 is stated in the diagram that in the first case there is 

 "more hybrid vigor' 7 and in the second case "less 

 hybrid vigor," simply because hybrid vigor is a relative 

 term. It represents merely how much more vigorous 

 the hybrid is than either parent. In the first case the 

 parents have three factors and the hybrid six, and the 

 increase is three, which measures the amount of hybrid 

 vigor. In the second case each parent has two and the 

 hybrid four; the increase therefore is only two, and for 

 this reason there is less hybrid vigor in the second 

 case than in the first. 



Assuming that the majority of factors are desirable, 

 and the desirable factors make for general vigor, it would 

 follow that the most vigorous plant will be the one 

 containing the greatest number of positive factors. 

 It has been shown that plants that contain the greatest 

 number of factors are hybrids, and for this reason 

 hybrids are more vigorous. 



The following question may be raised. If it is 

 granted that all desirable factors tend somewhat to 

 increase the general vigor, do they all do this to the 

 same degree? The obvious answer is in the negative, 

 and this has no bearing upon the validity of the explana- 

 tion. On the other hand, if heterozygosis be accepted 

 for an explanation the question would present a diffi- 

 culty. Heterozygosis would claim that Aa induces 

 vigor, not because of any particular factor that it repre- 

 sents, but because it is a heterozygous set. It seems to 



