It must be taken for granted in this paper that the reader has such
knowledge of the parts of the flower as could be obtained from the
paper on "A Typical Flower," printed in the June number.
When flowers first appeared it became necessary to secure the transfer
of the pollen grains to the stigmas. This was necessary in order that
the ovule might be developed into a seed containing a young plant or
embryo. At first the currents of air were selected as the agents of
this pollen transfer, and the flowers were adapted to what is known
as wind-pollination. As the wind is an inanimate agent any transfer
by it is largely a matter of chance. In order to increase the chances
of successful pollination it was necessary for pollen to be developed
in enormous quantities, so that it might fall like rain. In this way
stigmas would be reached, but at the same time an enormous amount
of pollen would be wasted. The evergreens are good illustrations of
wind-pollinated plants, and their showers of pollen are very familiar
to those who live near pine forests. When these showers come down in
unaccustomed regions they are often spoken of as "showers of sulphur,"
and the local newspapers are full of accounts of the mysterious
substance.
In wind-pollinated plants not only must the pollen be excessively
abundant, but it must also be very light and dry. Sometimes the
buoyancy is increased by the development of wings on the pollen grains,
as in the case of pines. This habit of pollination is found not only
among the evergreens, but also among many important families of the
higher plants, as in the ordinary forest trees, the grasses, etc.
When the higher forms appeared, however, flowers of a different
character gave evidence that a new type of pollination was being
devised. Instead of the old wasteful method, insects were called in to
act as agents of the transfer. By securing an animate agent there is
a definiteness in the pollination and a saving in pollen production
which is quite in contrast with the wind method. It must not be
supposed that all flowers have learned to use insects with equal skill,
for many of them may be said to be clumsy in their arrangements. On the
other hand, certain families have reached a high degree of organization
in this regard, and arrange for insect visits with a skill and
completeness of organization which is astonishing.
In order to secure visits from insects, so that pollination may be
effected, flowers have been compelled to do several things. In the
first place, they must provide an attractive food. This has taken two
prominent forms, namely, nectar and pollen. There are insects, such
as butterflies, which are not only attracted by the nectar, but whose
mouth parts have only been adapted for sucking up a liquid. There
are other insects, however, like the bees, wasps, etc., which are
able to take the more substantial pollen as food. Accordingly insects
which visit flowers may be roughly divided into the two classes,
nectar-feeders and pollen-feeders.
In the second place, the flower must notify the insect in some way that
the food is present. This is done primarily by the odors which flowers
give off. It must not be supposed that odors which are sensible to
us are the only ones sensible to insects, for in general their sense
of smell is far keener than ours. It is also probably true that the
display of color, which is so conspicuously associated with flowers, is
an attraction to insects, although this has become somewhat doubtful
lately by the discovery that certain insects which were thought to be
attracted by color have proved to be color blind. At present, however,
we have no reason to suppose that color is not associated in some
prominent way with the visits of insects.
It should be noticed, also, that two kinds of pollination are possible.
The pollen may be transferred to the stigma of its own flower, or it
may be carried to the stigma in some other flower, and this other
flower may be some distance away. The former method may be called
self-pollination, the latter cross-pollination. It seems evident that
flowers in general have made every effort to secure cross-pollination.
This would seem to imply that it is a better method for some reason,
although we may not be able to explain why. Apparently, however, while
flowers in general have tried to secure cross-pollination, they have
not entirely abandoned the chances of self-pollination, so that if
one should fail the other may be used. In this way it will be found
that a great many plants have two kinds of flowers, the ordinary showy
kind, and in addition to them inconspicuous flowers which are never
seen except by those acquainted with their presence. For example, in
the common violet, in addition to those flowers with which everyone
is familiar, others are developed which are concealed by the cluster
of leaves, which never open, but which are able to produce very well
developed seeds.
With nectar and pollen provided as food, and with odor and color
notifying the insects of their presence, it remains to be noted that
the suitable insects are those which fly. A creeping insect is of no
avail in the work of pollination, since the pollen will be rubbed from
its body as it crawls from one flower to the next. How the flowers ward
off the visits of creeping insects, which are attracted as well as the
flying ones to the food provided, will be described in a subsequent
paper.
A good illustration of the workings of insect pollination may be found
in the sweet pea, or in any member of the pea family. The flower has a
rough resemblance to a butterfly, whose projecting body is represented
by a structure like the keel of a boat. In this keel is a cluster of
stamens, and also the pistil with its stigma at the top. While lying in
this keel the stamens shed their pollen upon the style, which usually
has hairs or some sticky surface to receive it. Accordingly the style
bears the stigma on top and masses of pollen stuck to its sides below.
An insect being attracted to such a flower naturally lands upon the
keel as upon a shelf, with its head toward the center of the flower,
where the nectar is deposited. If the insect is heavy enough the
weight of its body pushes down the keel, but the contained style is
anchored, so that it seems to dart out, and strikes the insect's body,
first with the stigma at the tip, and then glancing along rubs its
side against the body of the insect. The insect flies away with pollen
rubbed upon its body, and when it goes through the same performance at
another flower, the new stigma strikes it first and gets some of the
pollen, and then some more pollen is smeared on, and so the pollen is
carried from one flower to the stigma of another flower. It is easy to
see the effect of the weight of a heavy insect by pressing down the
keel with a pencil, when the style will be seen to dart forth at the
tip.
Perhaps one of the most common ways of securing pollination is that in
which the pollen and stigma are not ready at the same time in the same
flower. The pollen may be ready to shed, but the stigma is not ready to
receive, or the reverse may be true. This would seem very effective in
preventing self-pollination. Illustrations of this kind are exceedingly
numerous, but perhaps as common a one as any is furnished by the great
fireweed, Epilobium. It has a conspicuous purple flower, and if a patch
of the plants be examined the flowers will be found in two conditions.
In one set the cluster of stamens will be found projecting straight
out from the flower, while the style with its stigma is turned back
out of the way under the flower. In the other set the stamens, having
shed their pollen, are turned back behind the flower, while the style
has straightened up, and the mature stigma holds the same position
that the anthers did the day before. An insect, in visiting such a
group, therefore, may fly straight towards a flower whose stamens are
projecting and shedding, and its body will be dusted with the pollen.
If it now flies to a flower which is a little older, whose stamens
are out of the way, but whose style is projecting, its body carrying
the pollen will strike the stigma. In this way the pollen is very
effectively transferred from one flower to another.
It would be impossible to give any adequate account of the subject of
insect-pollination in general, as it is an immense subject with an
ever-increasing literature. Every kind of flower has its own particular
way of solving the problem, so that the subject will never be completed
until all flowers have been questioned and their answers obtained.
Any account, however brief, should not omit mention of the orchids,
which in the matter of insect-pollination have reached the highest
degree of organization. So detailed are their adaptations that
each kind of flower is adapted to a particular kind of insect. The
accounts given of the various ways in which orchids attract insects
and secure pollination really surpass belief, until one has actually
observed some of the plants and their insects at work. Any greenhouse
furnishes abundant examples of orchids, and our illustration represents
one of the most common of our native orchids, the ordinary yellow
Lady-slipper. In most orchid flowers there is a long tubular spur, at
the bottom of which the nectar is found, which is to be reached by
long probosces, such as can be found only in moths and butterflies. In
Lady-slippers, however, there is a different arrangement. The flowers
have a conspicuous pouch in which the nectar is secreted, and a flap
overhangs the opening of the pouch. Behind the flap are the two pollen
masses, between which is the stigmatic surface. A bee crowds itself
away into the pouch and becomes imprisoned, and may frequently be found
buzzing about uneasily. The nectar is in the bottom of the pouch, and
after feeding the bee moves toward the opening overhung by the flap,
and rubs itself against the stigma and then against the anthers,
receiving the pollen on its back. A visit to another flower will result
in rubbing some of the pollen upon the stigma, and in receiving more
pollen for another flower.
One of the most remarkable cases of insect-pollination is that shown
by the ordinary Yucca, which is pollinated by a small moth, the plant
and the moth being very dependent upon one another. The flowers of
Yucca occur in very large prominent clusters, and hang like bells. In
each bell-shaped flower there are six hanging stamens, and a central
ovary ribbed lengthwise like a melon. At the tip of the ovary is a
funnel-shaped opening, which is the stigma. During the day the moth
hides quietly in the recesses of the flower, but at dusk she becomes
very active. She travels down the stamens, and, resting on the open
anthers, scrapes out the somewhat sticky pollen with her front legs.
Holding the little mass of pollen she runs up on the ovary, stands
astride of one of the furrows, pierces through the wall with her
ovipositor, and deposits an egg in an ovule. After depositing several
eggs, she runs to the apex of the ovary and begins to crowd the mass
of pollen she has collected into the funnel-like stigma. These actions
are repeated several times, until many eggs are deposited and repeated
pollination has been effected. As a result of all this, the flower is
pollinated and seeds are formed, which develop abundant nourishment for
the moth larvae, whose eggs had been laid in the ovule. Just how the
insect learned that this behavior on her part would secure food for her
young is hard to imagine.
In studying any flower there are three questions that should be asked:
(1) How does it hinder self-pollination?; (2) How does it secure
cross-pollination?; (3) How does it discourage the visits of unsuitable
insects?
John Merle Coulter.