124 PLANT RELATIONS. 



and pollen-feeding insects,, represented by the numerous 

 bees and wasps. When pollen is provided as food, the 

 amount of it is far in excess of the needs of pollination. 

 The presence of these supplies of food is made known to 

 the insect by the display of^aokn: in connection with the 

 flowers, by odor, or by form. It should be said that the 

 attraction of insects by color has been doubted recently, as 

 certain experiments have suggested that some of the com- 

 mon flower-visiting insects are color-blind, but remarkably 

 keen-scented. However this may be for some insects, it 

 seems to be sufficiently established that many insects rec- 

 ognize their feeding ground by the display of color. 



86. Suitable and unsuitable insects. It is evident that 

 all insects desiring nectar or pollen for food are not suit-r 

 able for the work of pollination. For instance, the ordi- 

 nary ants are fond of such food, but as they walk from plant 

 to plant the pollen dusted upon them is in great danger of 

 being brushed off and lost. The most favorable insect is 

 the flying one, that can pass from flower to flower through 

 the air. It will be seen, therefore, that the flower must not 

 only secure the visits of suitable insects, but must guard 

 against the depredations of unsuitable ones. 



87. Danger of self-pollination. There is still another 

 problem which insect-pollinating flowers must solve. If 

 cross-pollination is more advantageous to the plant than 

 self-pollination, the latter should be prevented so far as 

 possible. As the stamens and carpels are usually close to- 

 gether in the same flower, the danger of self-pollination is 

 constantly present in many flowers. In those plants which 

 have stamen-producing flowers upon one plant and carpel- 

 producing flowers upon another, there is no such danger. 



88. Problems of pollination. In most insect-pollinating 

 flowers, therefore, there are three problems : (1) to prevent 

 self-pollination, (2) to secure the visits of suitable insects, 

 and (3) to ward off the visits of unsuitable insects. It 

 must not be supposed that flowers are uniformly successful 



