LUTHER BURBANK 



It shows that in the heredity of the plant there 

 are strains of spinelessness that might presumably 

 be utilized by the plant developer in the produc- 

 tion of a spineless race. 



In particular it was learned that there is in the 

 Hawaiian Islands cactus that develops specimens 

 that are partially thornless when grown on moun- 

 tain sides in positions absolutely inaccessible to 

 browsing animals. Also in California, Mexico, 

 Colorado, New Mexico, and Texas, as I learned 

 from various reports, small patches of half thorn- 

 less cactus are sometimes found, always in inac- 

 cessible crevices among the rocks. These all appear 

 to be species of Nopalia and not Opuntia. 



In some of the South Sea Islands where vegeta- 

 tion is abundant, and where browsing animals 

 are few, the Opuntias have either reverted to a 

 spineless condition, or have retained spines that 

 have become merely hairlike appendages. 



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. 



[198] 



