294 



A TEXTBOOK OP BOTANY 



[Ch. VI, 5 



bjr the insect (Fig. 205). All of these matters are described 

 in detail in several works devoted to the subject. The 

 student will find that to the original investigators, these re- 

 markable adjustments between flowers and insects seem most 

 reasonably explained as result of a gradual process of adapta- 

 tion of one to the other during the course of their evolution. 

 Insect-pollination prevails far more widely than any other 

 method, and moreover is characteristic of the highest kinds 

 of plants. A reason for its superiority over wind-pollination 

 consists obviousl}^ in its greater economy and efficiencj^ for 



in the one case the 

 pollen is simply cast 

 forth and its access to 

 a stigma left to chance, 

 while in the other the 

 pollen is carried di- 

 rectly from anthers to 

 stigmas. A phase of 

 this economy — mak- 

 ing the most, so to 

 speak, of insect visits 

 — explains the pres- 

 ence of stamens and 

 pistils in the same 

 flowers. 



While insects are overwhelmingly the most important, 

 they are not the only animal cross-pollinators of flowers. 

 Some kinds of large bright flowers are regularly pollinated 

 by small nectar-eating birds, especially humming-birds 

 (Fig. 206) ; and others, growing close to the ground, are 

 pollinated by snails, which are attracted by a succulent tis- 

 sue formed in the spike among the flowers. But insects, 

 from their combination of small size, active habits, and 

 nectar diet, make the most effective cross-pollinators. 



As with other plant organs, flowers have not only their 

 primary function, which they perform as their contribution 



Fig. 206. — Marcgravia nepenthoides, polli- 

 nated by humming birds; much reduced. 



In the pouched nectaries l^elow the flower, 

 there is secreted abundant nectar, in probing 

 for which the hovering birds bring their heads 

 successively against the flowers. (After H. 

 MUUer.) 



