442 VEGETABLE PHYSIOLOGY 



next short-styled form it may visit. The best seeds are 

 produced when each stigma is supplied with pollen from 

 stamens occupying a corresponding position to itself. This 

 method of cross-pollination is not thoroughly effective, as 

 the insect after a short time will be carrying pollen from 

 stamens of both lengths, having visited several flowers of 

 both kinds. The size of the pollen grains in each case is, 

 however, correlated with the features of the corresponding 

 stigmatic surface, which helps to secure the most advan- 

 tageous result. 



This arrangement is termed heterostylism or dimorphism, 

 of which however it is only one form. Ly thrum Salicaria 

 is trimorphic, bearing two sets of stamens of different 

 lengths, and a style which differs from both. There are 

 three modes of arrangement of these organs, and as in the 

 Primrose, the most serviceable pollination is that which 

 takes place when pollen from a stamen of a particular 

 length is applied to a stigma of the same length. 



Other arrangements are physiological rather than struc- 

 tural. Of these the strangest is what is called, prepotency. 

 When a stigma of a flower exhibiting this property is 

 pollinated by pollen from its own stamens, and at the same 

 time by pollen taken from another flower, the latter is 

 always the originator of the gamete by which fertilisation 

 is effected. Some flow r ers show self -sterility, that is 

 fertilisation never takes place if they are only pollinated 

 by pollen from their own stamens ; in some few cases their 

 own pollen acts as a poison to them. 



Though cross-pollination is generally most advantageous 

 it is not universal. Self-pollination occurs in many plants ; 

 in some indeed, special means have been developed to 

 secure it, either exclusively, or in cases in which cross- 

 pollination fails to be effected. Only one of these need 

 here be alluded to ; this is cleistogamy, or the production 

 of special flowers which do not open, in addition to the 

 normal ones. The most conspicuous instances of this are 

 afforded by several species of the genus Viola. In one of 



