48 GEORGE HARRISON SHULL 



plants remained rosettes or formed only weak depauperate stems. But this new 

 hybrid became a vigorous upstanding form in this impoverished area as well 

 as on better soil elsewhere. I recorded this as a notable example of making 

 heterosis take the place of manure or commercial fertilizers. 



Figure 2.10 is a notable hybrid, which represents my first direct personal 

 contact with a recognized case of hybrid vigor. This hybrid resulted from a 

 cross I made in 1905 between the so-called "Russian" sunflower and the wild 

 Helianthus annuus of our western prairies. Both of these forms have been re- 

 ferred, botanically, to the same species. Both are of approximately equal 

 height, scarcely as tall as the six-foot step-ladder shown in the figure. The 

 tallest of these Fi hybrids was 14.25 feet in height. 



Returning now to the question which I sidestepped in the beginning — 

 what we mean by the expression the heterosis concept — I suggest that it is the 

 interpretation of increased vigor, size, fruitfulness, speed of development, 

 resistance to disease and to insect pests, or to climatic rigors of any kind, 

 manifested by crossbred organisms as compared with corresponding inbreds, 

 as the specific results of unlikeness in the constitutions of the uniting parental 

 gametes. 



I think the first clear approach to this concept was involved in a statement 

 which I have already quoted, that "a different explanation was forced upon 

 me" (in my comparisons of cross-fertilized and self-fertilized strains of 

 maize). That is, "that self-fertilization simply serves to purify the strains, 

 and that my comparisons are not properly between cross- and self-fertiliza- 

 tion, but between pure strains and their hybrids." Since heterosis is recog- 

 nized as the result of the interaction of unlike gametes, it is closely related to 

 the well known cases of complementary genes. It differs from such comple- 

 mentary genes, however, mainly in being a more "diffuse" phenomenon in- 

 capable of analysis into the interactions of specific individual genes, even 

 though it may conceivably consist in whole or in part of such individual 

 gene interactions. 



