LUTHER BURBANK 



In the case of more widely divergent species, it 

 may come to pass that the juices of one plant are 

 actually poisonous to another. In such a case it 

 is futile for the pollen grain and pistil to meet, 

 because no fertilizing influence will be trans- 

 mitted. 



Even if the degree of chemical antagonism 

 developed has not reached a stage that makes 

 fertilization wholly impossible, it may be suffi- 

 cient to prevent the development of a thrifty 

 offspring. 



Or, as is quite usual, it may result in the 

 sterility of the hybrid progeny, and thus put a 

 barrier upon farther advance along that line. 



If proof were needed that such a chemical 

 antagonism prevents the cross-fertilization of 

 species separated too widely, further evidence 

 may be found in the negative results that attend 

 the attempt to graft a branch of one of these 

 species upon the stock of the other. 



Generally speaking, it will be found that 

 species that cannot be cross-fertilized, also cannot 

 be cross-grafted. 



In exceptional cases, it is possible to effect the 

 graft where efforts at hybridization have proved 

 futile. Such was the case, for example, with my 

 grafted tomato and potato vine. But in general, 

 the plant that refuses to mate with another plant 



[46] 



