ii.] DICLINOUS PLANTS. 31 



they touch the anther with one side of the proboscis 

 and the stigma with the other. There are, however, 

 in flowers a great many admirable and beautiful 

 contrivances, tending to prevent the fertilization of a 

 flower by its own pollen ; in consequence of which 

 insects habitually carry the pollen from the anthers 

 of one flower to the stigma of another. 



As already mentioned, there are three principal 

 modes in which self-fertilisation is prevented. Firstly, 

 by the stamens and pistil being situated in different 

 flowers, either on the same plant, or, more commonly, 

 in different plants. These differences form the char- 

 acteristics of the classes, Monoecia, Dicecia, and Poly- 

 gamia, of Linnaeus ; but it is obvious that such classes 

 are not natural, since we have in very nearly allied 

 species, even within the limits of what is generally 

 considered a single genus, cases in which the one is 

 diclinous, that is to say, has the stamens and pistil 

 in separate flowers, while in the other, the flowers 

 contain both. 



Secondly, in other cases, the self-fertilization of 

 plants, as was first observed by Sprengel in Epilobium 

 afagustifglium in the year 1790, is guarded against 

 by the fact that the stamens and pistils do not ripen 

 at the same time. 



In some few cases the pistil ripens before the sta- 

 mens ; these species are called " proterogynousr Thus 

 the Aristolochia has a flower which consists of a long 

 tube with a narrow opening closed by stiff hairs which 

 point backwards, so that it much resembles an ordi- 

 nary eel-trap. Small flies enter the tube in search of 

 honey, which from the direction of the hairs they can 



