RESULTS WITH HYBRIDIZATION 



113 



greater increases than do others. Hartley has found that, 

 while certain crosses give an increase, others give a 

 decrease, and in some cases the cross is almost sterile. 

 Probably those varieties that have been longest selected 



Fig. 34. 



-A breeding plat where many tassels and ears are covered with 

 paper bags, for artificial pollination. 



as to type, and therefore are the nearest to a pure (homo- 

 zygous) state, will respond most readily to crossing. 



83. Shull has found that when a variety has been 

 resolved, by inbreeding, into pure strains, certain of these 

 pure strains when crossed give yields superior to the yield 

 of the original corn, while other combinations give poor 

 yields. He suggests that the maximum yields will be 

 secured by first reducing a variety to its elemental strains, 

 and then producing hybrid seed each year from only 



