120 LUTHER BURBANK 



Mexico, Colorado, New Mexico, and Texas, as 

 I learned from various reports, small patches of 

 half thornless cactus are sometimes found, 

 always in inaccessible crevices among the rocks. 

 These all appear to be species of Nopalia and 

 not Opuntia. 



In some of the South Sea Islands where vege- 

 tation is abundant, and where browsing animals 

 are few, the Opuntias have either reverted to 

 a partially spineless condition, or have re- 

 tained spines that have become merely hairlike 

 appendages, but these are extremely tender 

 and of no use whatever, even in subtropical 

 climates. 



This tendency to produce partially spineless 

 races when the plant is grown under conditions 

 that make it inaccessible to browsing animals, 

 seems clearly to demonstrate that there are 

 obscure factors of thornlessness in its prehistoric 

 heredity. Our general studies in the effects of 

 hybridizing give adequate clues as to the way in 

 which these submerged factors may be brought 

 to the surface. 



The open secret, of course, is to blend the dif- 

 ferent strains of heredity by hybridizing the vari- 

 ous Opuntias, and to select for propagation the 

 seedlings that reveal the spineless condition in 

 combination with other desired qualities. 



