PLAXT AFFINITIES 225 



is to say, the antagonism is inherent in the plants 

 themselves. 



Allied species are of such chemical constitu- 

 tion that the protoplasm from one combines 

 readily with protoplasm from the other. 



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 

 transmitted. 



Even if the degree of chemical antagonism 

 developed has not reached a stage that makes 

 fertilization wholly impossible, it may be suf- 

 ficient 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 further 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 can- 

 not be cross-grafted. 



Vol. 2 Bur. H 



