164 Outline of Genetics 



pure lines may bring into the hybrid a maximum number 

 of heterozygous sets, quite as many as are present in 

 cross-fertilized families. 



5. Reciprocal crosses are equivalent.^ — This would 

 obviously follow from any Mendelian hypothesis such 

 as heterozygosis. 



East next studied tobacco as representing those 

 species which are generally self-fertilized in nature. It is 

 a common impression that tobacco is a striking exception 

 in the matter of hybrid vigor. In tobacco crosses the 

 hybrid progeny, instead of being more vigorous, are fre- 

 quently less vigorous than either parent. East admits 

 that there are certain cases of this kind, but points out a 

 number of other cases which are quite " normal" in show- 

 ing hybrid vigor. In any event, the tobacco situation 

 strongly suggests the idea that hybrid vigor appears less 

 prominently in species that are generally self-fertilized 

 in nature than in species normally cross-fertilized. 



It may be that the "subnormal" tobacco hybrids are products 

 of such wide crosses that hybrid vigor can no longer operate (see 

 p. 169). 



The phenomenon of hybrid vigor appears also in a 

 great many other plants. It has of course been noted 

 most frequently in cultivated forms, but there is also 

 some evidence as to its occurrence among wild plants. 

 Not only has it been observed among many angiosperms, 

 woody as well as herbaceous, but also among gymno- 

 sperms and pteridophytes; and there is even some slight 

 evidence that hybrid vigor occurs in the sporophyte of 

 the bryophytes (see Britton i). 



As for the exact nature of the phenomenon, quite 

 a number of features arc involved. Primarily, hybrid 



