I. ] PRE VENTION OF SELF-FER TILISA TION. g 



between 3,000,000 and 4,000,000 grains of pollen. 

 The Dandelion is more specialised in this respect, 

 and produces far less pollen ; according to Mr. 

 Hassall about 240,000 grains to each flower ; while 

 in Geum urbanum, according to Gaertner, only ten 

 times more pollen is produced than is actually used 

 in fertilisation. 



It might, however, be at first supposed that where 

 stamens and pistil co-exist in the same flower, the 

 pollen from the one could easily fall on and fertilise 

 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 fertilised by pollen 

 from a different stock. How then is self-fertilisation 

 prevented ? 



There are three principal modes. 



Firstly, in many species 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 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. 



