But, perhaps, the most hopeless of all is the well-known partridge berry, whose red berries are common in the woods during August and September. This seems as though it fought off any chance of securing a mate by a flower structure and behavior that would certainly so result if some way out of the difficulty were not at hand. The partridge berry bears two kinds of flowers that outwardly look much alike, but whose sexual organs differ in this way: in some flowers the stamens are all shorter than the pistil, and in others the pistil is much outtopped by the stamens. The extraordinary feature of it is not so much this structural difference, however, but the fact that pollen from the short-stamened flower is useful only to its neighboring short-styled relative, while the pollen from this long-stamened but short-styled neighbor is nearly useless where it is found and really useful only on the long-styled plant. By this device, but again only with outside aid, this plant does not prevent maternity, but increases its chances of being fruitful, for, as we have already seen, cross-fertilization appears to be the rule rather than the exception, and the partridge berry not only needs it, but can exist only when its offspring are the result of such crosses.
In all those plants that bear the different sexes in different flowers on the same plant, as in the hickory, or even on different plants, as in the willow, there must, of course, be some method arranged for cross-fertilization or they would promptly die out. So general is this cross-fertilization, so much a part of the economy of nature does it appear to be, that we can only think that there must be in the production of this vast horde of the cross-fertilized some advantage. Besides securing greater virility, which almost certainly results from this promiscuity, greater variability is promoted. If virility is the result, the price paid for it is tremendous, for the hindrances to self-fertilization are so many and so effective that most flowers would inevitably die as perfectly pure but ineffectual virgins if that fatality were not prevented. How they are saved from such a sterile fate, how they finally secure a mate by devices that outshine the most bewitching tricks of the daughters of Eve, is one of the most fascinating stories in all the history of the plant world.
For, of course, flowers do secure a mate, and they are aided in this enterprise by the most formidable array of helpers, one might almost call them conspirators. The chief of these are insects, thousands of different kinds of which are constant flower visitors. Some of the smaller birds, and even snails, also help flowers to meet their mates. The wind, too, bears pollen through the air to some expectant bride-to-be. And, finally, in the water, by a series of acts the like of which no one could improve for cunning, the cross-fertilization of certain aquatic plants is consummated. It would take a book larger than the present one to give even the briefest account of how these different aids to maternity do their work and how the flower responds to this help. As that is quite out of the question, only some of the best-known examples of cross-fertilization will be given, and these will be grouped according to what agency the flower is indebted for its chance of doing that for which it is created.
INSECTS AS FLOWER VISITORS
On any summer day, especially when the sun is shining brightly, we may see bees and butterflies flitting from flower to flower, busy as the proverbial bee. We already know enough about nature’s ways of doing things to be certain that these, and hundreds of other kinds of insects, do not come for nothing, and that the flower must have something to offer. Bees, especially, are thrifty creatures whose business demands exacting and prolonged toil. They would not waste five seconds upon idle flower calling if the blossoms did not yield a rich store. And thousands of flowers do yield the sweetest and richest kinds of stores of nectar or honey, which is, in fact, by the help of insects who alone can extract it, our sole source of honey. Many flowers which produce no nectar do have such plentiful stocks of pollen that the bees come for that alone. In the peony, for instance, over three million pollen grains are produced in each flower, only a minute fraction of which can ever fertilize an ovule. All the rest would be wasted were not pollen in itself a particularly nutritious food for young bees, and consequently much sought after by the careful bee mothers. They are the only insects that feed their young on pollen, or beebread, as it is called by the beekeepers, so that the enormous overproduction of this male fertilizing agent, from the point of view of the flower, is a decided attraction, one might almost call it a trap, to insure constant visitations from bees. For they are perhaps the most useful of all insects in the great game of securing cross-fertilization, as we shall presently see. Many other kinds of adult insects eat pollen directly and so add to the number of insect visitors.
No one has ever been able to explain the beautiful coloring of flowers, except that it serves as an attraction for insects and small birds. Like the honey or nectar, it seems to play no real part in the home economy of the flower, to be of not the least use otherwise. While honey and the gorgeous colors of flowers are a delight to man, that would be no sufficient reason for the ability to produce them. Both of these attributes of flowers, as attractions for insect visitors, are, however, so absolutely essential to cross-fertilization that we must think of them as having grown up out of that demand. As we shall see a little later, even the structure of some insects has been modified so that they can reach the nectar or pollen only by automatically doing for the flower what it cannot do itself.