THE FLOWER AND INSECT-POLLINATION 159 



" other flower " may be upon the same plant or upon a dif- 

 ferent plant, but the cross-pollination that is significant is 

 that which involves two distinct plants. Since flowers are 

 very commonly arranged to secure cross-pollination, it must 

 be of more advantage to plants in general than self-pollina- 

 tion. 



This relation between flowers and insects is mutually 

 helpful, for the flower secures pollination and the insect 

 secures food. The food supplied by the flower is either 

 nectar, a sweet secretion often wrongly called " honey," or 

 pollen. The insects that visit flowers may be grouped as 

 nectar-feeders, represented by moths and butterflies, and 

 pollen-feeders, represented by bees and wasps. The pres- 

 ence of these supplies of food is made known to the insect by 

 the display of color, by odor, or by form. Just what attracts 

 different insects is not always clear, but color, odor, and 

 characteristic forms belong to flowers visited by insects, so 

 that it seems safe to conclude that by these features the in- 

 sects recognize their feeding ground. 



The relation between flowers and insects is most striking 

 in those flowers arranged for cross-pollination. This ar- 

 rangement involves both the hindrance of self-pollination 

 and the securing of cross-pollination, and each kind of cross- 

 pollinating flower has solved these problems in its own way. 

 A few examples will be given, to suggest the kinds of arrange- 

 ments to be looked for, but the student should examine as 

 many cross-pollinating flowers as possible, and try to deter- 

 mine how self-pollination is hindered and how cross-pollination 

 is secured. 



In those plants that have staminate and pistillate flowers 

 on different individuals, the case is clear, for self-pollination 

 is effectually prevented by the absence of stamens from 

 flowers with pistils. However, in such cases, the wind is as 

 apt to be the agent of pollination as insects. 



The most difficult situation is where stamens and pistils 



