124 PLANT STUDIES 
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 color 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- 
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 
