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. 
