In the eelgrass it is the specific gravity of the male flower, or, the secreted air bubble, which makes the flower lighter than the water, and actually causes the flight from the depths to the surface. Because of this, fertilization can only take place on the surface, although the flowers and fruits otherwise mature under water. But in sea wrack, in Naias, and in ditch grass, all submerged aquatics, the flowers are even fertilized under the water. Pollen in such plants is much modified, and instead of being in the ordinary form of pollen grains, it is, at the time of ripening, lengthened out into tubular, hairlike structures. These delicate prolongations of the male fertilizing stuff are carried by the currents of the water, just as a thread would be, but with the difference that the pollen threads are so beautifully weighted to fit their watery environment that they float, suspended, in the depths of the water at or near the level of the female flowers. The pollen is set free at maturity, just as it is in the eelgrass, but to meet the female, which never rises, it must float with the current of the stream. There must, as in the wind-carried pollen, be a tremendous wastage, yet sufficient quantities of it do fertilize the females, particularly in the ditch grass, which fruits very freely.
Whether it be any of the various contrivances for insect fertilization, or by the winds, or, as in the eelgrass, by the water, the climax of the flower’s life is always reached in this act. For all annuals the plants, also, begin to die down then, a process that is completed with the production of seed, which is, of course, the object of all those varied modes of fertilization. Perhaps no answer to the question of why plants do not always self-fertilize themselves is so eloquent as the hundreds of ways they have adopted to avoid doing so, a few of which we already know. Many volumes have been written on this subject, but all of them, intricate as the methods they describe nearly always are, merely confirm what we have already seen—that rather than submit to self-fertilization, plants will adopt almost undreamed-of expedients. Sometimes, as in the eelgrass and in the visits of nocturnal insects to those night-blooming flowers that carry on their matings in the glamour of moonlight or in the dusk of eventide, the drama, in the eyes of imaginative writers, is one of singular beauty and charm. And, on the other hand, we have seen the well-nigh heartless cruelty of the Dutchman’s-pipe in keeping as prisoners its absolutely necessary insect deliverers. Even this is outranked for matchless ruthlessness by a wild arum, a relative of our jack-in-the-pulpit, from the East Indies. It produces a club-shaped inflorescence composed of tiny flowers that need cross-fertilization, but so offensive is the odor of the flower that no insects will tolerate it. A snail, a voracious eater of foliage, is attracted to the flower partly by the fine fleshy leaves, but mostly by a juice secreted at the apex of the flower column. To this the snail crawls, and fertilizes the tiny flowers over which it drags its body. When this is accomplished it speeds on hungrily to the juice just above it and eagerly devours the poison. Death follows almost immediately. The secretion of this murderous liquid to lure the only creature that will visit such an offensively malodorous plant, which, without it, would very likely be itself destroyed by the foliage-eating snails, is a gruesome contrast to that happy flitting of butterflies which completes the fertilization of most flowers in equally effective but more pleasing fashion.
Once impregnation of the ovule has been consummated, it begins a slow process of change, involving sometimes the modification of the ovary, or of the calyx, and very often of the swollen apex of the flower stalk upon which these organs are borne, known technically as the receptacle. We have seen, in the first chapter, what greatly different types of fruits are developed from different ovaries, and they of course produce seeds in varying size and amount. In the coco de mer, a palm from the Seychelles, the seed often weighs forty or fifty pounds, while in some orchids a single capsule will contain over a million almost microscopic seeds. Some of the devices of fruits and of seeds to secure the utmost spreading of the species over the earth will be considered in another chapter. All the devious methods of plants in producing their young become significant, so far as the earth’s vegetation is concerned, only when we find out what this enormous progeny has done with their opportunity. The chapter on the Distribution of Plants will tell us how well that opportunity has been used.
2. Hidden Marriage of Flowerless Plants
As we stated in the first chapter cryptogams, while they produce no flowers, must bear organs that perform the functions of flowers in the reproduction of new individuals. Because, generally speaking, the process is more hidden in its manifestations, and nearly always requires the aid of the microscope to detect it, it is not so well known as the reproductive processes of flowering plants by those who have not the opportunity to manipulate such instruments. The act, however, is just as interesting, and, as we shall presently see, it may well be considered the ancestor of those more showy methods of producing young, which have been all too inadequately treated in the preceding pages. While the parts having to do with reproduction in flowerless plants are microscopic in size, it is possible to understand the broad outlines of what goes on and perhaps the life history of such plants is as well illustrated in ferns as in anything else.
THE LIFE HISTORY OF A FERN
In the discussion of ferns in the first chapter we found that on the back of some of their leaves, or occasionally on special leaves devoted to the purpose, were many small brownish or dark spots, arranged in rather definite fashion, and known as sori. ([Figure 63].) Each sorus contains many minute bodies known as spores, not unlike very miniature seeds in general appearance, but quite unlike them in behavior and mode of life. No better idea of their size can be gleaned than to record the fact that in each sorus there may be about one hundred small, often short-stalked spore cases, known as sporangia, and that in each sporangium well over forty, and sometimes over sixty, spores will be crowded. A healthy specimen of many of our common ferns will bear about ten or a dozen leaves, each of which is divided into many divisions, and among these divisions of the leaf there may be at least fifty that bear from fifteen to twenty sori. It can be easily figured from this that a healthy plant of this fern may and usually does produce over forty-five million spores, each of which contains within it the opportunity of developing into a new plant. There is thus a prodigality in producing the means of renewal of life among ferns that far outstrips the production of seeds in even the most prolific of flowering plants.
When the spores in the sporangium are mature and therefore ready for the next stage in their life history several things must happen. With somewhere about six thousand of them crowded together under each sorus, more room to develop is obviously the first consideration. This is provided for by the fact that when the spores are ripe the sporangia have the ability to throw them considerable distances; then of course the wind can carry them much farther. To be of any use they must fall upon damp ground, for some degree of moisture is absolutely necessary for what is about to happen to them. In nature countless millions never do fall in a favorable location, or, if they did, such an enormous production of fern spores would soon make the world exclusively a fern garden. The comparatively minute fraction of them that ever do find congenial surroundings, once they are expelled from the spore case, then begin a process that is not unlike the germination of a seed. For the spore must take in water from the soil, which by osmotic pressure finally bursts it open. From the burst spore a minute tube, known as the protonema, or literally first thread, begins to develop. It is, of course, of microscopic size, and yet near its base there is a branch tube formed, differing from it in structure and ultimately forming rhizoids, which are rootlike hairs. Both the protonema and the rhizoids begin growing, the first forming, usually flat on the ground, an often heart-shaped body having the characteristic green coloring matter of all plants. The rhizoids multiply and look not unlike roots. This young, still microscopic plant, grows apace, and may soon be distinguished with the naked eye. It looks not unlike a heart-shaped mass of greenish tissue quite flat on the ground, and is called a thallus.
Up to this point, then, we may trace the story of any fern which has thrown off its cloud of spores and from which develops this tiny thallus, looking not in the least like a fern nor as though it could ever be modified into one. Because this thallus is,