8 ON FLOWERS AND INSECTS. [LECT. 



reaching the pistil of another flower are immense. 

 Consequently by far the greater part of the pollen is 

 lost. Every one for instance must have observed the 

 clouds of pollen produced by the Scotch fir. In such 

 flowers as the Pseony the pollen is carried by insects, 

 and far less therefore is required ; yet even here the 

 quantity produced is still large ; it has been estimated 

 that each flower produces between 3,000,000 and 

 4,000,000 grains. 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 Gsertner, 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 coexist 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 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 anguslifolium 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. 



