i.] MODES OF TRANSFERENCE OF POLLEN. 9 



from the one could easily fall on and fertilize the other. 

 And in fact in some species this does occur ; but as we 

 have seen, it is a great advantage to a species that the 

 flower should be fertilized by pollen from a different 

 stock. How then is self-fertilization prevented ? 



There are three principal modes. 



Firstly, in many plants the stamens and pistil are in 

 separate flowers, sometimes situated on different plants. 



Secondly, even when the. stamens and pistil are in 

 the same flower, they are in many species not mature at 

 the same time ; this was first observed by Sprengel in 

 Epilobium angustifolium (Fig. 13) as long ago as 1790; 

 in some cases the stigma has matured before the anthers 

 are ripe, while in other and more numerous cases the 

 anthers have ripened and shed all their pollen before 

 the stigma has come to maturity. 



Thirdly, there are many species in which, though the 

 anthers and stigma are contained in the same flower 

 and are mature at the same time, they are so situated 

 that the pollen can hardly reach the stigma of the same 

 flower. 



The transference of the pollen from one flower to 

 another is, as already mentioned, effected principally 

 either by the wind or by insects, though in some cases it 

 is due to other agencies, as for instance, by birds, or by 

 water. For instance, in the curious Vallisneria spiralis 

 the female flowers are situated on long stalks which are 

 spirally twisted, and grow very rapidly, so that even if 

 the level of the water alters, provided this be within 

 certain limits, the flowers float on the surface. The 

 male flowers on the contrary are minute and sessile, 

 but when mature they detach themselves from the 



