FLOWERS AND INSECTS 243 



flowers by color has been doubted, since it is claimed that 

 some of the common flower-visiting insects are color-blind, 

 but remarkably keen of scent. However this may be 

 for some insects, it seems to be sufficiently established that 

 many insects recognize their feeding ground by the display 

 of color. 



It is evident that all insects attracted by nectar or pollen 

 are not suitable for the work of pollination. For instance, 

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

 plant to plant any pollen dusted upon them is almost sure 

 to be brushed off on the way and lost. The most favorable 

 insect is the flying one, which can pass from flower to 

 flower through the air. It will be seen, therefore, that the 

 flower not only must secure the visits of suitable insects, 

 but also must guard against the depredations of unsuitable 

 ones. 



145. Self-pollination. It is evident that in many cases 

 self-pollination is likely to occur. In some flowers the sta- 

 mens and carpels are so related to one another in position 

 that when pollen is being shed some of it may fall upon the 

 stigma. Even the visit of an insect, which usually results 

 in cross-pollination, may result in self-pollination. 



It must not be understood that only cross-pollination is 

 really provided for, and that when self-pollination occurs 

 it is more or less of an accident. In addition to the numer- 

 ous cases of what may be called accidental self-polli- 

 nation in flowers usually cross-pollinated, self-pollination is 

 definitely provided for more extensively than once was 

 supposed. It is found that many plants, as violets, for 

 example, in addition to the usual showy insect-pollinated 

 flowers, produce flowers that are not at all showy, that in 

 fact do not open, and are often not prominently placed. 

 These inconspicuous closed flowers are called cleistogamous 

 flowers; and in these flowers self-pollination is necessary, 

 and very effective in producing good seed. 



