SM JLUTHER BURBANK 



The cfaAracterittict that make it impossible to 

 croft two ipedet that have varied beyoml certain 

 limits are tometmies phytica]. 



Thus it may chance that the two spectes have 

 developed the habit of blooming at different 

 timet. 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 aflinity. Similarly 

 there are two closely related species of evening 

 prinux>se tliat 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 tlie 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 exceptiunully 

 slender style, it is possible that the fnictifying 

 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 



