INTRODUCTION. 11 



plicable fact that with some dioecious plants, of which 

 the Eestiaceae of Australia and the Cape of Good Hope 

 offer the most striking instance, the differentiation 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 specimens of the same spe- 

 cies. In my seventh chapter some observations will be 

 given on the gradual conversion of heterostyled and of 

 ordinary hermaphrodite plants into dioecious or sub- 

 dio2eious species. 



The fourth and last Class consists of the plants which 

 were called polygamous by Linmeus; but it appears to 

 me that it would be convenient to confine this term to 

 the species which co-exist as hermaprodites, 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 fe- 

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

 was in function a male. The separation of the sexes, 

 however, is not complete in the Ash; for the female 

 flowers include stamens, which drop off at an early 



