28 THE PARTS OF A FLOWER. [CHAP. 



in many familiar instances forms a small head at the 

 top of the style. In some cases the style is absent, 

 and the stigma is consequently sessile. 



Thus, the pistil is normally surrounded by a row of 

 stamens, and it would seem at first sight a very 

 simple matter that the pollen of the latter should 

 fall on the former. This in fact does happen in 

 many instances, and flowers which thus fertilize 

 themselves have evidently one great advantage 

 few remain sterile for want of pollen. 



Such cases, however, are much less frequent than 

 might at first be supposed, and there are three prin- 

 cipal modes by which self-fertilization is prevented. 

 Firstly, in many species, the stamens and pistil are 

 situated in different flowers ; such species are called 

 diclinous ; when the male and female flowers are on 

 the same plant, they are termed monoecious ; when on 

 different plants, dioecious. 



Secondly, in other instances, as was first discovered 

 by Sprengel, though the stamens and pistil are both 

 situated in one flower, they are not mature at the 

 same time, and the pollen, therefore, cannot fertilize 

 the stigma. These plants are called dicJiogamous. 

 Sometimes, as in the Arum, the pistil matures before 

 the anther, and these plants are called proterogynous ; 

 but much more frequently the anther matures before 

 the pistil ; and such plants are called proterandrous. 



Thirdly, there are some plants in which, as was first 

 discovered by Mr. Darwin, the same object is secured 

 by the existence, within the limits of the same species, 

 of two or more kinds of flowers, differing in the rela- 

 tive position of the stamens and pistil, which are so 

 placed as to favour the transference by insects of the 



