70 PLANT-LIFE ON LAND [OH. 



of flowers is great. This should impress weightily 

 upon the mind the importance which attaches to 

 intercrossing. 



Due recognition is readily given in popular books 

 to the wonderful mechanisms by which intercrossing 

 is carried out. But it often happens that the actual 

 process of fertilisation, as it is effected in the Higher 

 Flowering plants, is left entirely out of account. Its 

 main features are, however, of the highest importance 

 as giving material for comparison leading to broad 

 views of descent It will therefore be necessary to 

 recount how fertilisation is carried out in an ordinary 

 Flower. A basis of comparison will thus be provided 

 with plants lower in the scale. 



The stamen is an organ which usually bears four 

 pollen-sacs. Each contains many pollen-grains, which 

 are set free by rupture of the protecting wall. The 

 grains are commonly dry and dusty, but incapable of 

 spontaneous movement : so that each is independently 

 open to transfer by external agencies. The numbers 

 of the grains are roughly in relation to the precision 

 of the removing agent. Where the method is hap- 

 hazard, as in the case of transfer by the wind, the 

 production of pollen is usually profuse. Where the 

 method is exact, as in the case of insect-agency 

 working in relation to a highly specialised flower, 

 the production of pollen is limited. Extreme cases 

 of economy are seen in Orchis where a single stamen 



