There appears nothing very romantic about the cross-fertilization of the Dutchman’s pipe, in fact, the whole affair seems but a sordid and, it must be confessed, a very efficient trick to get what the flower needs from the insect, rewarding it by many hours of apparently hopeless captivity. But most flowers do have something that insects want, and none so well fulfill the expectations of butterflies as the meadow pink. This is a graceful little perennial native in the fields in central Europe, but often grown in American flower gardens. It has beautiful pink flowers, with a long tubular calyx, at the bottom of which are rich honey glands, accessible only to the long proboscis of different kinds of butterflies. No other insects, and many try, are able to get the honey from this plant. It begins flowering by opening its five beautifully fringed petals, all of which are marked with lines from their edge toward the center. These obvious “pathfinders” are common on many other flowers and all seem to be there to act as a guide for alighting insects, and, as it were, steer them to the center of the flower. As a butterfly alights he finds five protruding anthers covered with pollen. These are of no use to him, but nevertheless his head is covered by their pollen. In fact, many other visitors, who can never reach the honey, find the bright color and good stocks of pollen sufficient attraction for them, and after a pollen feast fly off to other flowers. If the butterfly comes at this state of the flower’s life, it pokes its long proboscis down into the tube of the calyx, but finds the passage almost blocked by another set of five stamens not yet ready to discharge pollen, and, as though ashamed of the fact, quite hidden from view. Also, the proboscis is very nearly stopped by the style, which, if its stigmas were ready for mating, would then and there become impregnated. But they are rolled together into a tight spiral, and their pollen-receptive faces tightly pressed together in addition to the whole structure being twisted, so that the very probably pollen-laden proboscis of the butterfly finally gets to the honey without leaving one grain of pollen where it could possibly self-fertilize the flower. Honey-laden, it now flies away, and so far as this particular flower is concerned, simply nothing has happened but to coat several insects with pollen. A little later the anthers of the first set of stamens cease work and drop off. Then the five stamens hitherto hidden in the calyx burst out and furnish a second crop of pollen, but the stigmas are still safely coiled away from the possible danger of self-impregnation. During this second stage the flower may repeatedly be visited, but until this second crop of anthers become useless there is not the slightest risk of the stigmas becoming self-fertilized. For the hole through which the second set of stamens has come is so small that still only the proboscis of the butterfly can penetrate it. When finally the second crop of anthers also fall off, then the style slowly uncoils and thrusts its now receptive stigmas above the calyx and in plain view of passing visitors. These come, pollen-laden of course, from other flowers, and cross-fertilization is assured. Here the flower furnishes two crops of pollen, plentiful supplies of honey, and asks only that in gathering these the butterflies see to it that its stigma be covered only with foreign pollen. Keeping carefully out of the way while they are about their business and there is danger of self-fertilization, they come out boldly once that danger is past. In this meadow pink self-fertilization is simply impossible; everything in the production of its young it owes to the butterflies, to which it surely makes adequate returns.
While such plants as the meadow pink and thousands of others have lost, if they ever possessed, the power of self-fertilization, and rely absolutely on insect visitors for their perpetuation, there are many hundreds of kinds that apparently hope for cross-fertilization, but, in default of it, due to their inability to absolutely compel it, they finally accept self-fertilization as a last resort. Darwin once said that “Nature abhors perpetual self-fertilization,” and the frequent visits of insects and their rôle in preventing it, together with the flowers’ adaptations to such visits, support the contention, which has never been seriously questioned. But some flowers appear to have left the back door open, as it were, so that failing cross-fertilization, they may still rely upon self-fertilization. A geranium from the Pyrenees, a relative of our common woods geranium, is of this type. A day or two before its stigmas are ripe it produces first one set of five anthers, all pollen-coated, and then another. If these have not been brushed clean, as often or usually happens, then the stigmas ripen and, of course, are impregnated by their pollen. If they are brushed clean of pollen, then the stigmas must rely on foreign pollen, of which it is assured a supply from the visits of pollen-laden insects from other flowers, which are still attracted by the flowers’ color and honey. Perpetuation is assured, in any case, but the preference is still for cross-fertilization.
A more remarkable case of leaving one final chance for self-fertilization is the gas plant. It exhales such a strong and peculiar odor that only certain kinds of insects will visit it. In fact, the odor is so strong and is so heavily charged that a lighted match held near it has been known to slightly ignite—hence the plant’s name. The flower bears a low, squat stigma, profusely covered with honey, which is perfectly accessible to any insect visitor. It has ten stamens, which at first are quite out of the way of insects, two being folded back in each of the five yellow petals. First one stamen begins moving gently from the shelter of its petal, and the anther, pollen-coated, hovers over the stigma, which would inevitably lead to self-fertilization if the stigma were only ready. It never is, and, as though realizing this, the stamen gently moves back out of the way, still, in most cases, retaining some of its pollen. Then another tries and, again, as though realizing the futility of impregnating the unreceptive stigma, it also moves back. So it goes with all the other eight stamens, each of which moves gently out over the stigma, and gently back again, all of them failing to fertilize the laggard stigma. But there has been during all this time a constant procession of insects coming for the honey, and never for a moment have the stamens been off guard, so that each visitor goes off with at least some pollen clinging to it. Finally the stigma, after all the faithful ten are folded back among the petals, comes into its period, and is cross-fertilized by the insects which come from other flowers laden with pollen, as we now know. But if, by one of those accidents of nature, such as bad weather or what not, no insect ever does come, what then is to be the fate of the stigma? Has it staked all only to lose out in the end? It would almost seem that to ignore the steady attentions of the willing ten might make barren sterility a fitting punishment. But, in spite of what has happened, the stamens come to the rescue, if all insects fail, and this time, in a body, they rise up and shed upon the stigma from the half-withered anthers their few remaining pollen grains. There could be no finer example of having an anchor to windward.
One of the largest families of flowering plants is the pea family ([Figure 72]), with over five thousand members, practically all of which rely on bees for cross-fertilization. In some kinds, where bees occasionally fail them, the flowers wither without self-fertilization and, of course, no seed are then produced. In such a large family of plants there are
naturally many different adaptations for securing cross-fertilization—some of them of such extreme complexity that they could hardly be included here. All the family have the characteristic pealike flowers familiar enough in the sweet pea, which have already been described and figured on page 44. In all of these stamens and pistils are hidden inside the keel, at least in the early stages of the flower. In some, such as clovers, for instance, the organs emerge from the keel, and after fertilization by insects re-enter their retreat. There are scores of different plans for securing the desired object, but the common alfalfa, with a few other related plants, has the most startling. The flower begins life with its stamens and pistils concealed within the keel, which is apparently impregnable. When a bee alights on the flower and begins work he is welcomed by a small but violent explosion. When the dust of this clears away, and it is actually dusty with pollen, the dazed bee is seen to have fertilized with almost instantaneous rapidity the stigma of the exploding flower, which springs violently out of the burst keel. This is so arranged that, as it flies out of its trap, due to the explosion touched off by the bee, it strikes the under side of the bee’s body a distinct blow, brushing off in the process the pollen nearly always found there. This pollen has come from other plants. But, as in most plants of the pea family, the stamens closely surround the stigma, but are shorter than it. When the explosion occurs, the stigma, because it is slightly longer than the stamens, is the first thing to strike the bee’s body. Already impregnated, it is then indifferent to the cloud of pollen from the stamens of its own flower, which only a fraction of a second later also strikes the bee’s body. The first flower that a bee visits cannot, of course, be cross-fertilized notwithstanding the explosion which results, no matter from what angle the bee attempts to insert its proboscis. But in this case, as the stigma is unfertilized by foreign pollen, its own performs that service. In the vast majority of cases cross-fertilization is assured by certainly the most novel of processes, and, in the rare event of the bee not being covered with foreign pollen, self-fertilization is still possible.