256 CONCLUDING REMARKS CHAP. VL 



spread from a single centre to such widely remote and 

 separated areas. The family of the Kubiaceae contains 

 not far short of as many heterostyled genera as all 

 the other thirteen families together ; and hereafter 

 no doubt other Rubiaceous genera will be found to 

 be heterostyled, although a large majority are homo- 

 styled. Several closely allied genera in this family 

 probably owe their heterostyled structure to descent 

 in common ; but as the genera thus characterised are 

 distributed in no less than eight of the tribes into 

 which this family has been divided by Bentham and 

 Hooker, it is almost certain that several of them 

 must have become heterostyled independently of 

 one another. What there is in the constitution or 

 structure of the members of this family which favours 

 their becoming heterostyled, I cannot conjecture. 

 Some families of considerable size, such as the Bo- 

 ragineae and Verbenacese, include, as far as is at 

 present known, only a single heterostyled genus. 

 Polygonum also is the sole heterostyled genus in its 

 family ; and though it is a very large genus, no other 

 species except P. fagopyrum is thus characterised. We 

 may suspect that it has become heterostyled within 

 a comparatively recent period, as it seems to be less 

 strongly so in function than the species in any other 

 genus, for both forms are capable of yielding a con- 

 siderable number of spontaneously self-fertilised seeds. 

 Polygonum in possessing only a single heterostyled 

 species is an extreme case ; "but every other genus of 

 considerable size which includes some such species 

 likewise contains homostyled species. Lythrum in- 

 cludes trimorphic, dimorphic, and homostyled species. 

 Trees, bushes, and herbaceous plants, both large 

 and small, bearing single flowers or flowers in dense 

 spikes or heads, have been rendered heterostyled. 



