CHAP. VI. ON HETEROSTYLED PLANTS. 261 



only half of the latter, that is the females, yield 

 seeds. On the other hand, heterostyled plants seem to 

 have no advantage, as far as cross-fertilisation is con- 

 cerned, over those which are sterile with their own pol- 

 len. They lie indeed under a slight disadvantage, for 

 if two self-sterile plants grow near together and far 

 removed from all other plants of the same species, they 

 will mutually and perfectly 'fertilise one another, 

 whilst this will not he the case with heterostyled di- 

 morphic plants, unless they chance to belong to opposite 

 forms. 



It may be added that species which are trimorphic 

 have one slight advantage over the dimorphic; for if 

 only two individuals of a dimorphic species happen to 

 grow near together in an isolated spot, the chances are 

 even that both will belong to the same form, and in 

 this case they will not produce the full number of vigor- 

 ous and fertile seedlings; all these, moreover, will tend 

 strongly to belong to the same form as their parents. 

 On the other hand, if two plants of the same trimorphic 

 species happen to grow in an isolated spot, the chances 

 are two to one in favour of their not belonging to the 

 same form; and in this case they will legitimately fer- 

 tilise one another, and yield the full complement of vig- 

 orous offspring. 



The Means by which Plants may have been rendered 



Heterostyled. 



This is a very obscure subject, on which I can throw 

 little light, but which is worthy of discussion. It has 

 been shown that heterostyled plants occur in fourteen 

 natural families, dispersed throughout the whole vege- 

 table kingdom, and that even within the family of the 

 Rubiaceae they are dispersed in eight of the tribes. "We 

 may therefore conclude that this structure has been ac- 



