INTRODUCTION. 11 



remarks made under the last class with respect to the 

 amount of difference between the male and female 

 flowers are here applicable. It is at present an in- 

 explicable fact that with some dioecious plants, of 

 which the Eestiacese of Australia and the Cape of 

 Good Hope offer the most striking instance, the dif- 

 ferentiation of the sexes has affected the whole plant 

 to such an extent (as I hear from Mr. Thiselton Dyer) 

 that Mr. Bentham and Professor Oliver have often 

 found it impossible to match the male and female spe- 

 cimens of the same species. In my seventh chapter 

 some observations will be given on the gradual con- 

 version of heterostyled and of ordinary hermaphrodite 

 plants into dioecious or sub-dioecious species. 



The fourth and last Class consists of the plants which 

 were called polygamous by Linnseus ; but it appears to 

 me that it would be convenient to confine this term to 

 the species which co-exist as hermaphrodites, males, and 

 females ; and to give new names to several other com- 

 binations of the sexes — a plan which I shall here 

 follow. Polygamous plants, in this confined sense of 

 the term, may be divided into two sub-groups, accord- 

 ing as the three sexual forms are found on the same 

 individual or on distinct individuals. Of this latter or 

 trioicous sub-group, the common Ash (Fraxinus ex- 

 celsior) offers a good instance : thus, I examined during 

 the spring and autumn fifteen trees growing in the 

 same field ; and of these, eight produced male flowers 

 alone, and in the autumn not a single seed ; four pro- 

 duced only female flowers, which set an abundance of 

 seeds ; three were hermaphrodites, which had a dif- 

 ferent aspect from the other trees whilst in flower, and 

 two of them produced nearly as many seeds as the 

 female trees, whilst the third produced none, so that it 



