124 PLANT STUDIES 



and pollen-feeding insects, represented by the nnmerons 

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

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

 The presence of these snpplies of food is made known to 

 the insect by the display of color in connection with the 

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

 attraction of insects l)y color has been doubted recently, as 

 certain experiments have suggested that some of the com- 

 mon flower-visiting insects are color-lilind, Imt reniarkultly 

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

 seems to he sufflciontly established that many insects rec- 

 ognize their feeding ground 1)y the dis])lay of color. 



Sd. Suitable and unsuitable insects. — It is evident that 

 all insects desiring nectar or pollen for food are not snit- 

 alilo for the work of pollination. I'or instaaice, 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 Ijrushed otf a,nd lost. The most favorable insect is 

 the flying one, tlia.t ca,n pass from flowei- to flower througli 

 the air. It will 1)e seen, therefore, tliat the flower must not 

 only secure the visits of suitalile insects, but must guard 

 against the deprcdati(uis of unsuitable ones. 



S7. Danger of self-pollination. — There is still another 

 problem which insect-pollinating flowers must solve. If 

 cross-2iollim-ition is more advantageous to the plant tlian 

 self-pollination, the latter slnndd 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 ilowers. In those phmts which 

 have stameir-prodncing flowers upon one plant a.nd ca.rj)el- 

 producing flowers u[ion another, there is no su(di da.nger. 



88. Problems of pollination. — In most insect-pollinating 

 flowers, thei'efore, there are tliree problems ; (1) to prevent 

 self-pollination, (2) to secuire tlie visits of suitalile insects, 

 and (.']) to ward off tlie visits of unsuitalile insects. It 

 must not be supposed that flowers are uniformly successful 



