122 On Flowers and Insects. 



subject, but it would seem, from the few observa- 

 tions that have been made, that half a dozen pollen 

 grains are sufficient to fertilise a seed. But in 

 plants in which the pollen is carried by the wind, 

 the chances against any given grain 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 Paeony the pollen is carried by insects, and 

 far less therefore is required ; yet even here the 

 quantity produced is still large; it has been esti- 

 mated that each flower produces between 3,000,000 

 and 4,000,000 grains. The Dandelion is more spe- 

 cialised in this respect, and produces far less pollen, 

 about 240,000 grains to each flower ; while in the 

 common Avens only ten times more pollen is pro- 

 duced than is actually used in fertilisation. 



4. 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 this does occur in 

 some species ; 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. 



