r of the flower, when on withdrawing it, the two pollen masses
will be removed from the flower. If we now examine these carefully, we
shall see that they change position, being nearly upright at first,
but quickly bending downward and forward (Fig. 89, _D_, ii, iii), so
that on thrusting the stem into another flower the pollen masses
strike against the sticky stigmatic surfaces, and a part of the pollen
is left adhering to them.

The last arrangement that will be mentioned here is one discovered by
Darwin in a number of very widely separated plants, and to which he
gave the name "heterostylism." Examples of this are the primroses
(_Primula_), loosestrife (_Lythrum_), partridge-berry (_Mitchella_),
pickerel-weed (_Pontederia_), (Fig. 84, _I_), and others. In these
there are two, sometimes three, sets of flowers differing very much in
the relative lengths of stamens and pistil, those with long pistils
having short stamens and _vice versa_. When an insect visits a flower
with short stamens, that part is covered with pollen which in the
short-styled (but long-stamened) flower will strike the stigma, as the
pistil in one flower is almost exactly of the length of the stamens in
the other form. In such flowers as have three forms, _e.g._
_Pontederia_, each flower has two different lengths of stamens, both
differing from the style of the same flower. Microscopic examination
has shown that there is great variation in the size of the pollen
spores in these plants, the large pollen from the long stamens being
adapted to the long style of the proper flower.

It will be found that the character of the color of the flower is
related to the insects visiting it. Brilliantly colored flowers are
usually visited by butterflies, bees, and similar day-flying insects.
Flowers opening at night are usually white or pale yellow, colors best
seen at night, and in addition usually are very strongly scented so
as to attract the night-flying moths which usually fertilize them.
Sometimes dull-colored flowers, which frequently have a very offensive
odor, are visited by flies and other carrion-loving insects, which
serve to convey pollen to them.

Occasionally, flowers in themselves inconspicuous are surrounded by
showy leaves or bracts which take the place of the petals of the
showier flowers in attracting insect visitors. The large dogwood
(Fig. 110, _J_), the calla, and Jack-in-the-pulpit (Fig. 86, _A_) are
illustrations of this.

CHAPTE