ants, we see
that the outer leaves of the flower become more conspicuous, and that
this is often correlated with the development of a sweet fluid
(nectar) in certain parts of the flower, while the wind-fertilized
flowers are destitute of this as well as of odor.

If we watch any bright-colored or sweet-scented flower for any length
of time, we shall hardly fail to observe the visits of insects to it,
in search of pollen or honey, and attracted to the flower by its
bright color or sweet perfume. In its visits from flower to flower,
the insect is almost certain to transfer part of the pollen carried
off from one flower to the stigma of another of the same kind, thus
effecting pollination.

That the fertilization of a flower by pollen from another is
beneficial has been shown by many careful experiments which show that
nearly always--at least in flowers where there are special
contrivances for cross-fertilization--the number of seeds is greater
and the quality better where cross-fertilization has taken place, than
where the flower is fertilized by its own pollen. From these
experiments, as well as from very numerous studies on the structure of
the flower with reference to insect aid in fertilization, we are
justified in the conclusion that all bright-colored flowers are, to a
great extent, dependent upon insect aid for transferring the pollen
from one flower to another, and that many, especially those with
tubular or zygomorphic (bilateral) flowers are perfectly incapable of
self-fertilization. In a few cases snails have been known to be the
conveyers of pollen, and the humming-birds are known in some cases, as
for instance the trumpet-creeper (Fig. 121, _A_), to take the place of
insects.[14]

[14] In a number of plants with showy flowers, _e.g._ violets,
jewel-weed, small, inconspicuous flowers are also formed, which are
self-fertilizing. These inconspicuous flowers are called
"cleistogamous."

At first sight it would appear that most flowers are especially
adapted for self-fertilization; but in fact, although stamens and
pistils are in the same flower, there are usually effective
preventives for avoiding self-fertilization. In a few cases
investigated, it has been found that the pollen from the flower will
not germinate upon its own stigma, and in others it seems to act
injuriously. One of the commonest means of avoiding self-fertilization
is the maturing of stamens and pistils at different times. Usually the
sta