Si INTRODUCTORY REMARKS. CHAP.!. 



in which the pollen is mature before the stigma, and 

 proterogynous species, in which the reverse occurs; 

 this latter form of dichogamy not being nearly so 

 common as the other. Cross-fertilisation is also en- 

 sured, in many cases, by mechanical contrivances of 

 wonderful beauty, preventing the impregnation of the 

 flowers by their own pollen. There is a small class of 

 plants, which I have called dimorphic and trimorphic, 

 but to which Hildebrand has given the more appro- 

 priate name of heterostyled ; this class consists of 

 plants presenting two or three distinct forms, adapted 

 for reciprocal fertilisation, so that, like plants with 

 separate sexes, they can hardly fail to be intercrossed 

 in each generation. The male and female organs of 

 some flowers are irritable, and the insects which touch 

 them get dusted with pollen, which is thus transported 

 to other flowers. Again, there is a class, in which the 

 ovules absolutely refuse to be fertilised by pollen from 

 the same plant, but can be fertilised by pollen from 

 any other individual of the same species. There are 

 also very many species which are partially sterile with 

 their own pollen. Lastly, there is a large class in 

 which the flowers present no apparent obstacle of any 

 kind to self-fertilisation, nevertheless these plants are 

 frequently intercrossed, owing to the prepotency of 

 pollen from another individual or variety over the 

 plant's own pollen. 



As plants are adapted by such diversified and effec- 

 tive means for cross-fertilisation, it might have been 

 inferred from this fact alone that they derived some 

 great advantage from the process ; and it is the object 

 of the present work to show the nature and importance 

 of the benefits thus derived. There are, however, some 

 exceptions to the rule of plants being constructed so 

 as to allow of or to favour cross-fertilisation, for some 



