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.