KEPKODUCTION 453 



to the visiting insect, for when it touches the stamens of a 

 short-styled form, it covers with pollen the part of its 

 body which will come into contact with the stigma of the 

 next long-styled flower it alights upon. Another portion of 

 its body will be dusted with the pollen from the latter, which 

 will be suitably placed to be deposited upon the stigma of 

 the 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. Lyihrum Salicaria 

 is tnmorphic, 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 in a corresponding position. 



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 flowers show self -sterility that is, their 

 oospheres are incapable of being fertilised by generative 

 nuclei developed from their own pollen ; 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 



