i66 Plant Genetics 



fleshy fruits, such as tomato and eggplant, there is a 

 marked increase. On the individual plant there are 

 distinctly more flowers and fruits, and in some cases 

 separate inflorescences are longer, such as the ears of 

 corn. 



EAST next studied tobacco as representing those 

 species which are generally self-fertilized in nature. 

 It is the 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 frequently 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. As yet there is no very satisfactory explana- 

 tion of the tobacco situation, and we must be satisfied 

 at present with the tentative conclusion that in tobacco 

 there appear certain unknown limitations to hybrid 

 vigor. In general it may be said that hybrid vigor 

 appears prominently in species normally cross-pollinated 

 in nature and less prominently in species generally self- 

 fertilized in nature. 



In this connection it should be stated that hybrid 

 vigor is also manifested in some regions of the animal 

 kingdom. It seems, however, that it is not so general 

 a phenomenon among animals as among plants. In 

 fact, many zoologists refuse to recognize in hybrid 

 vigor any general law, pointing out the many cases 

 among animals in which hybridizing apparently results 

 in loss of vigor. It should be noted, therefore, that the 

 present discussion of hybrid vigor applies primarily to 

 plants and should not be extended in any general way 

 to the animal kingdom. 



