In many of the plants already noted cross-fertilization is accomplished by virtue of the fact that the stamens mature before the stigma. But in the common strawberry the reverse is true. As the insect stands on the white petals, it must, in order to reach the honey at the base of the flower, put its head down so that if pollen were available there could only result self-fertilization. But while the stigma is ready, the pollen at this stage never is, and the insect which comes usually laden with pollen from other flowers cannot avoid impregnating the stigma with this foreign pollen. Later the stamens mature and, as insect visits continue so long as honey is to be found, they become dusted with pollen which is used for the fertilization of other plants. Scores of other plants also produce ripened stigmas before the pollen matures, and they must rely on insects to cross-fertilize them.
The cross-fertilization of the strawberry is such a comparatively simple process—seems in fact almost inevitable—that we are lost in wonder at the almost mathematical complexity of the act in the common purple loosestrife, which has been introduced into American gardens from Europe, and sometimes runs wild. In this plant there is a long terminal spike of showy, purplish-pink flowers, the color of which is sufficient to attract many insects from even a fairly swift flight. The petals are streaked with “pathfinders” toward the center of the flower. This consists of a tubular calyx; at the bottom of this is the honey, which secures the insect’s further interest once the color has attracted it. But it finds a condition of the reproductive organs almost without parallel. In some plants the style is hidden down in the calyx tube, while one set of stamens just peep out of the end of the tube, and a second set are still further and quite obviously protruded. In a neighboring plant the style will be found outside the tube, one set of stamens hidden in it, and the other set outtopping everything, except the petals. In still a third type of the loosestrife, the style exceeds everything but the petals, one set of stamens just emerge from the tube and the lowest set are hidden. It would appear as if the loosestrife could scarcely escape self-fertilization, except possibly in that form where the stigma outtops all the stamens, and this would result always if the pollen were indiscriminately useful from all three lengths of stamens. But it never is, only that from the short-stamened plants will fertilize the short-styled one, the mid-stamened ones the mid-styled counterpart, and the long-stamened ones the long-styled flower. The pollen grains of the different-lengthed stamens are even of different size and color. If this were not so, it is a simple mathematical problem that, with three different sets of style lengths and six sets of stamens, two in each flower, eighteen different crosses might be possible. As it is, only six crosses are ever possible and these only by the aid of insects, for it must be remembered that stamens and pistils of the same length are never found in the same flower. By an adjustment of the size of the body of the different insect visitors it works out so that, while all three body sizes frequently visit each flower, only that particular size of insect suited to the carrying of pollen from stamens of one definite length to a style of similar length actually accomplishes the cross-fertilization of that particular flower. And this without interfering with the visit of another different-sized insect which will accomplish the work for its particular set of stamens and pistils. So marvelous is the adjustment of style length and stamen length to each particular body size of the visiting insects, so perfect is the arrangement of the organs in each flower, that each contributes to the fertilization of its neighbors, never to its own, nor does it interfere with the process in other lengthened styles. Cross-fertilization is thus almost always accomplished in the six different combinations possible in this truly remarkable case of adaptation between insects and flowers.
From the almost mathematical complications of the cross-fertilization of the loosestrife it seems a far cry indeed to that of the Italian honeysuckle. This often runs wild over fences, but is unlike the more widely known Japanese honeysuckle, in that its stem passes through the different pairs of opposite, bluish-green leaves, which are joined together at the base. The Italian honeysuckle falls back on the more simple seductions of odor and honey for securing its really important insect visitors. It has such a long tube that only certain night-or evening-flying moths or butterflies can reach the honey. There is, even during the day, such a plentiful supply of this that it frequently fills half the tube, but even then it is quite out of reach of bees which never succeed in getting any. Toward evening, particularly on quiet, still evenings, the flower begins to send off in much increased quantity a heavy rich-scented odor almost overpowering in its sweetness. The butterflies and moths of the dusk having a long proboscis, succumb to this really enchanting lure, which, with the large store of honey, insures quantities of eager suitors. The stigma, while ripening simultaneously with the anthers, protrudes beyond them, so that the butterflies touch pollen only after touching the stigma, which of course is impregnated with pollen from an earlier visit of the insect to a different flower. So assiduous are the butterflies, that on a still night there will be not one grain of pollen left on the much-brushed anthers. If the night is cold or windy much pollen remains and cross-fertilization is left to pollen-eating insects such as mother bees or flies. While these can never get the honey they often do accomplish cross-fertilization and, sometimes by misadventure, self-pollination. It is only those moths and butterflies which, forcing their long proboscis down the honey-laden tube, must accomplish cross-fertilization. But failing this, the plant is more or less at the mercy of hosts of insect triflers, mere pollen eaters, who may or may not insure cross-fertilization, but in any case provide for self-fertilization.
Another use which certain plants make of honey, besides acting as a lure of insects, is found in the common lilac. The flower in this has considerable honey at the bottom of the tube, which can only be reached by insects with a proboscis sufficiently long to reach it. The lilac has only two stamens inserted near the top of the tube, the passage to which they very nearly obstruct. The stigma is hidden in the tube, and it matures simultaneously with the ripening of the pollen. As the insect inserts its proboscis between the stamens no pollen clings to it due to the character of the pollen grains. But as the proboscis is withdrawn from the tube its lower end is covered with honey to which pollen sticks. If a needle is inserted between the stamens and pushed only far enough to be still clear of the honey, no pollen will be found on it when withdrawn, but if pushed all the way down, its honey-coated point will catch considerable pollen. In the lilac, if insect visitors do not accomplish the work of cross-fertilization, the flower is self-fertilized ultimately by the protruding of the stigma far enough out of the tube to catch some of the remaining pollen grains.
It is perhaps useless to multiply instances of flowers which by various devices secure the cooperation of insects in getting pollen from a foreign source. To recapitulate some of those devices it is necessary only to recall what some flowers have done to force cross-fertilization. The heated chamber of the magnolia, the cruelty of the prison cell in Dutchman’s-pipe, the blow from the stamen of the barberry, the faithful rotation of the ten stamens of the gas plant, the explosive flower in some members of the pea family, the lure of honey and seductive odor of the Italian honeysuckle, the mathematical complexity of the loosestrife—these and hundreds of others all point to the necessity of cross-fertilization and a means to produce it almost beyond belief. There is the best of evidence that not only flowers but insects themselves have been modified in this great work, and that for every flower needing cross-fertilization some agency has been developed to secure it. Insects, beyond all other animal life, do this work, but it is accomplished by humming birds often, and in one plant even by a snail.
Two of the very largest plant families, not so far mentioned in this account, depend almost absolutely upon insects. In the daisy family, with over eleven thousand members, the large heads of flowers, often containing scores of individual flowers, are constantly brushed, over and over again, by the pollen-coated bodies of insect visitors. And in all or nearly all orchids ([Figures 73-75]), comprising over five thousand kinds, the same process is accomplished. In these plants, in fact, the act is, if possible, more
