LUTHER BURBANK 



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, hybridizing between these species will 

 never occur, however close their affinity. Simi- 

 larly there are two closely related species of 

 evening primrose that are not hybridized under 

 natural conditions because the flower of one 

 opens only for a brief period at midday and that 

 of the other only during the night. 



Again it occasionally happens that the physical 

 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 probable that the fructifying 

 substance of the pollen is debarred from finding 

 its way to the ovule. 



Such cases are probably exceptional, however, 

 and the usual barrier erected by Nature between 

 species is not so much physical as chemical. That 

 is to say, the antagonism is inherent in the plants 

 themselves. 



Allied species are of such chemical constitu- 

 tion that the protoplasm from one mingles readily 

 with protoplasm from the other. 



[44] 



