THE FLOWER. 85 



species is applied to the stigma; but it has been proved 

 that, as a general rule, there are great advaii- 



, , J Pollination, 



tages in having the pollen brought from another 

 flower. 1 Accordingly, while self-fertilization is possible in 

 most plants, various arrangements exist by which cross- 

 fertilization is favored. 



A number of external agents serve as efficient means of 

 pollination. The wind carries the light pollen of 'pine and 

 other trees to great distances, sometimes even External 

 hundreds of miles, insects of many different agents, 

 kinds are actively engaged in carrying pollen from one 

 flower to another, and humming birds visit a considerable 

 number of species. In comparatively few cases pollen is 

 conveyed to the stigma by the agency of water. 



Flowers themselves show many remarkable adaptations 

 that favor cross-fertilization. The most important of these, 

 as discussed at length by Darwin and other Adaptations 

 writers, are the following: of flowers, 



1. Diclinism, or the separation of stamens and pistils. 

 These are borne in different flowers, either on the same 

 plant, as in the hazel, oak, etc., or on different individuals, 

 as in the willows and poplars. In some families, as the 

 maples, both conditions prevail. Plants with staminate 

 and pistillate flowers on the same individual are said to 

 be monoecious, those in which the separated flowers are 

 on different individuals are dioecious, and those in which 

 either condition exists together with the production of 

 some perfect flowers are called polygamous. Of those in 

 which the separation is most complete, namely, perfectly 

 dioecious species, Darwin says, " About the origin of such 



1 Cf . Darwin, Cross- and Self-fertilization in the Vegetable Kingdom ; 

 Miiller, Fertilization of Flowers. 



