certain kind of geranium to another similar one for instance, but perhaps to a rose? The answer to this is simple enough, but its implications are limitless. Only pollen of a certain species or variety is useful to the stigmas of that variety. To practically all others the stigma is simply unreceptive, except in those closely related plants that may all have a common parentage. When crosses between such closely related plants do occur the result is known as a hybrid, which will be considered elsewhere. To this extent, then, flowers are peculiarly exclusive in their matings and promiscuity occurs in the vast number of cases only in plants of the same species. We do not yet understand the impotence of pollen of one species upon another; all that we do know, which has many times been proved by experiments, is that it fails to act. If it did act, no one could picture the chaos into which the vegetative world would be thrown.
WIND AND WHAT IT DOES FOR FLOWERS
While, as we have seen, thousands of plants rely upon insects for producing their young, still other thousands put everything to the hazard of the wind. Pollen is so light that it can easily be blown very great distances, and while the wastage is enormous, the process works so well that the greater part of the vegetation of the earth is thus fertilized. This is true not as to the number of different kinds of plants, for in that respect insect fertilization is more important than that accomplished by the wind. But in the number of individual plants concerned the wind is incomparably the greatest fertilizing agency that is known to us. This for the reason that all grasses and sedges, most catkin-bearing trees such as oak, hickory, birch, practically all pine trees and their relatives rely wholly on the wind for fertilization.
For reasons that will be enlarged upon in another chapter, all of these great groups of plants must be considered as of simple structure, some, like the pines, relics of a remote past when no flowering plants, as we know them to-day, existed on the earth. In any event the reliance upon the wind is certainly hazardous, and while it of course insures nearly universal cross-fertilization, it may well result in scanty fertilization or, in exceptional cases, complete failure of it. Quite obvious also is the amount and direction of the wind in the process, for in very open and windy places grasslike vegetation, or at least a predominance of species fertilized by wind, is likely to be found, rather than those plants that rely upon insects, that, unable to stand the full force of the wind, seek more sheltered places. While such a thing is not the cause of prairies, or the predominantly grasslike vegetation along sand dunes, or the exclusive spruce forests of the bleak and windy north country, it unquestionably aids in maintaining the often exclusive nature of such pure associations of plants. Over thousands of square miles on our own great plains or on the steppes of Russia, both subject to violent winds, the great bulk of the vegetation is wind fertilized. It could hardly be expected that pollen, once in the grip of such a wayward and shifting thing as the wind, should not be wasted in great quantities. This is particularly true of pine trees, which at pollen time may often be seen giving off golden clouds of dust, of which perhaps 95 per cent is wasted.
WATER AS AN AID TO FERTILIZATION
Those submerged aquatic plants upon which neither the winds nor honey-seeking insects can work the magic of cross-fertilization, seem to be about the poorest equipped for perpetuating their kind through impregnation of their tiny flowers. And yet, for at least two of them, which will be described presently, the process is accomplished by an adaptation of their mode of life to their watery environment that seems incredible. These two have been selected as illustrating two peculiar adaptations in the weight of pollen or pollen-holding flowers that is common to some other submerged aquatic plants. In one the male flower, or pollen from it, with the very nicest adjustment of function to environment in all the realm of the plant world, is just of the right specific gravity to float to the surface with dramatic suddenness and perfectly timed effectiveness. In the other the pollen is just enough heavier than the water to float betwixt the surface and the bottom, so that at the proper moment it is where it can fulfill its destiny.
The common eelgrass or tapegrass is a submerged aquatic which roots in the mud and has long grasslike leaves which may often be seen waving gently in the current of many quiet streams in this country and in Europe. Down near the base and in among its swaying verdure, it bears tiny flowers which have no petals, and in which, as if recognizing the futility of display in such a secluded watery home, even its calyx is reduced to small scales. Some of these minute flowers are females, others again all males, and as they appear in their early stages it looks as though never the twain could meet. And the hopelessness of their ever meeting is increased as the maturing female begins slowly to uncoil the fine stalk upon which it grows. Steadily but surely the loose spirals of the stalk of this ever more mature female flower uncoils, until, when quite ready for the pollen, it is at last upon the surface. The male flowers, in the meanwhile, are down near the bottom with their small freight of pollen ready to perform their function, but firmly anchored to a stalk absurdly inadequate to reach the surface where alone they can be of service. A great Belgian, Maurice Maeterlinck, who studied this plant with more sympathetic vision than any botanist has yet been able to equal, wrote in one of his essays on “The Intelligence of Flowers” the solution of this little drama of apparent hopelessness. No other words can ever convey the meaning of what happens to the eelgrass quite so well as his. “Is there any more cruel inadvertence or ordeal in nature? Picture the tragedy of that longing, the inaccessible so nearly attained, the transparent fatality, the impossible with not a visible obstacle! It would be insoluble, like our own tragedy upon this earth, were it not that an unexpected element is mingled with it. Did the males foresee the disillusion to which they would be subjected? One thing is certain: that they have locked up in their hearts a bubble of air, even as we lock up in our souls a thought of desperate deliverance. It is as though they hesitated for a moment; then, with a magnificent effort, the finest, the most supernatural, that I know of in all the pageantry of the insects and the flowers, in order to rise to happiness, they deliberately break the bond that attaches them to life. They tear themselves from their peduncle, and, with an incomparable flight * * * dart up and break the surface of the water. Wounded to death, but radiant and free, they float for a moment beside their heedless brides and the union is accomplished, whereupon the victims drift away to perish, while the wife, already a mother, closes her calyx, in which lives their last breath, rolls up her spiral, and descends to the depths, there to ripen the fruit of the heroic kiss.”