Hybrid Vit^or 167 



the individual woidd have been immediately eliminated. Oidy 

 strains that stood by themselves, that survived on their own merits, 

 would have been retained. On the other hand. weak, j^enotyfxjs in 

 cross-fertilized species were retained through the vigor that they 

 exhibited when crossed with other genotypes. The result is, 

 therefore, that self-fertilized strains that have survived competi- 

 tion are inherently stronger than cross-fertilized strains. On this 

 account weak genotypes may often be isolated from a cross- 

 fertilized species that as a whole is strong and hardy." 



4. Rise of the sporophyte generation. — The commonly ac- 

 cepted interpretation of hybrid vigor is based upon a Mendelian 

 mechanism that would be effective only in the diploid generation. 

 In the evolution of the plant kingdom, the haploid gametophyte 

 generation has been superseded in dominance by the diploid sporo- 

 phyte generation. Hybrid vigor may help to account for this. 



Some recent investigations have extended the scope 

 of hybrid vigor in an interesting and significant way. 

 The work was done originally by Collins and Kemptox 

 (3), and later confirmed and extended by Jones (8). In 

 brief, it is as follows. 



If corn sporophytes exhibit hybrid vigor, will the 

 endosperm also show the same phenomenon ? Endo- 

 sperms, as has been stated, are genetically equivalent 

 to sporophytes in several ways. If crossing increases 

 vigor and size of sporophytes, therefore, it might be 

 expected to increase the size of the endosperms also. 



Furthermore, the endosperms have considerable advantage 

 over sporophytes as material for such investigation. We say that 

 hybrid sporophytes are more vigorous than pure bred sporophytes, 

 but just how much more vigorous cannot be stated with exactness. 

 In order to demonstrate this clearly, it would be necessary to have 

 the hybrid and the pure bred stock growing side by side in e.xactly 

 the same conditions, but the conditions cannot be controlled 

 with exactness. The environmental factors affecting the size and 

 vigor of a corn plant are numerous, complex, and to a large extent 



