284 DIOECIOUS AND Chap. VII. 



even to the whole plant ; and in this latter case the 

 species becomes dioecious. In those rare instances men- 

 tioned in the Introduction, in which some of the indi- 

 viduals of both monoecious and hermaphrodite plants 

 are proterandrous, others being proterogynous, their 

 conversion into a dioecious condition would probably be 

 much facilitated, as they already consist of two bodies 

 of individuals, differing to a certain extent in their 

 reproductive functions. 



Dimorphic heterostyled plants offer still more 

 strongly marked facilities for becoming dioecious ; for 

 they likewise consist of two bodies of individuals in 

 approximately equal numbers, and what probably is 

 more important, both the male and female organs 

 differ in the two forms, not only iu structure but in 

 function, in nearly the same manner as do the repro- 

 ductive organs of two distinct species belonging to 

 the same genus. Now if two species are subjected to 

 changed conditions, though of the same nature, it is 

 notorious that they are often affected very differently ; 

 therefore the male organs, for instance, in one form of 

 a heterostyled plant might be affected by those un- 

 known causes which induce abortion, differently from 

 the homologous but functionally different organs in 

 the other form ; and so conversely with the female 

 organs. Thus the great difficulty before alluded to is 

 much lessened in understanding how any cause what- 

 ever could lead to the simultaneous reduction and 

 ultimate suppression of the male organs in half the 

 individuals of a species, and of the female organs in 

 the other half, whilst all were subjected to exactly the 

 same conditions of life. 



That such reduction or suppression has occurred 

 in some heterostyled plants is almost certain. The 

 Rubiacese contain more heterostyled genera than any 



