FERTILIZATION OF FLOWERS.
If we compare the flowers of different plants, we shall find almost
infinite variety in structure, and this variation at first appears to
follow no fixed laws; but as we study the matter more thoroughly, we
find that these variations have a deep significance, and almost
without exception have to do with the fertilization of the flower.
In the simpler flowers, such as those of a grass, sedge, or rush among
the monocotyledons, or an oak, hazel, or plantain, among dicotyledons,
the flowers are extremely inconspicuous and often reduced to the
simplest form. In such plants, the pollen is conveyed from the male
flowers to the female by the wind, and to this end the former are
usually placed above the latter so that these are dusted with the
pollen whenever the plant is shaken by the wind. In these plants, the
male flowers often outnumber the female enormously, and the pollen is
produced in great quantities, and the stigmas are long and often
feathery, so as to catch the pollen readily. This is very beautifully
shown in many grasses.
If, however, we examine the higher groups of flowering plants, 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
stamens ripen first, discharging the pollen and withering before the
stigma is ready to receive it, _e.g._ willow-herb (Fig. 113, _D_),
campanula (Fig. 123, _A_, _D_), and pea; in the two latter, the pollen
is often shed before the flower opens. Not so frequently the stigmas
mature first, as in the plantain (Fig. 121, _G_).
In many flowers, the stamens, as they ripen, move so as to place
themselves directly before the entrance to the nectary, where they are
necessarily struck by any insect searching for honey; after the pollen
is shed, they move aside or bend downward, and their place is taken by
the pistil, so that an insect which has come from a younger flower
will strike the part of the body previously dusted with pollen against
the stigma, and deposit the pollen upon it. This arrangement is very
beautifully seen in the nasturtium and larkspur (Fig. 99, _J_).
The tubular flowers of the _Sympetalæ_ are especially adapted for
pollination by insects with long tongues, like the bees and
butterflies, and in most of these flowers the relative position of the
stamens and pistil is such as to ensure cross-fertilization, which in
the majority of them appears to be absolutely dependent upon insect
aid.
The great orchid family is well known on account of the singular form
and brilliant colors of the flowers which have no equals in these
respects in the whole vegetable kingdom. As might be expected, there
are numerous contrivances for cross-fertilization among them, some of
which are so extraordinary as to be scarcely credible. With few
exceptions the pollen is so placed as to render its removal by insects
necessary. One of the simpler contrivances is readily studied in the
little spring-orchis (Fig. 89) or one of the _Habenarias_ (Fig. 90,
_G_). In the first, the two pollen masses taper below where each is
attached to a viscid disc which is covered by a delicate membrane.
These discs are so placed that when an insect enters the flower and
thrusts its tongue into the spur of the flower, its head is brought
against the membrane covering the discs, rupturing it so as to expose
the disc which adheres firmly to the head or tongue of the insect,
the substance composing the disc hardening like cement on exposure to
the air. As the insect withdraws its tongue, one or both of the pollen
masses are dragged out and carried away. The action of the insect may
be imitated by thrusting a small grass-stalk or some similar body into
the spur 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.