224 LUTHER BURBANK 



PLANT ANTAGONISMS 



The characteristics that make it impossible to 

 cross two species that have varied beyond certain 

 limits are sometimes physical. 



Thus it may chance that the two species have 

 developed the habit of blooming at different 

 times. If the flowers of a given species are alto- 

 gether out of bloom before the flowers of another 

 species open, it is obvious that, in a state of 

 nature, a cross between these species will not 

 occur, however close their affinity. Similarly 

 there are two closely related species of evening 

 primrose that do not cross under natural condi- 

 tions because the flower of one opens only for a 

 brief period at midday and that of the other only 

 during the night and early morning. 



Again it occasionally happens that the physi- 

 cal structure of the style which carries the pollen 

 tube to the ovules is such as to prevent the 

 carrying out of this essential process. In the 

 case of a large pollen grain and an exceptionally 

 slender style, it is possible that the fructifying 

 substance of the pollen is debarred from finding 

 its way to the ovule. 



Such cases are probably exceptional, how- 

 ever, and the usual barrier between species is 

 not perhaps so often physical as chemical. That 



