Chapter III: How Plants Produce Their Young (2)

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.”

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,

(_A_^{1}) archegonia, (_A_^{2}) antheridia, and (_A_^{3}) the rhizoids. _B_: Prothallus, showing the young plant with its first leaf (_B_^{1}), its own roots (_B_^{3}) and the rhizoids of the prothallus (_B_^{2}). Drawing and legend for it slightly altered from Kraemer.]

in the truest sense, merely a preparation for the process that _will_ produce another fern, it is always known as a _prothallus_. The prothallus is thus the first stage in the reproduction of ferns, a very simple stage, with only the faintest indication that the thallus might be considered the vegetative and its rhizoids perhaps the rootlike counterparts of foliage and roots of mature ferns. As we shall see presently, even this differentiation has not the significance that such a structure in flowering plants would indicate. There is not, as yet, the faintest indication of sexes that need to mate in order to produce their young. The spore has so far only produced a tiny flat body of green tissues with a few rootlike threads, so unlike the fern from which it started that its true significance, or even the fact that it had ought to do with ferns was not known until about the middle of the last century.

This green cushiony prothallus keeps on growing, its heart-shaped mass becoming divided into an obviously left and right hand side and the rhizoids multiplying in number. They are always borne on the lower side next the ground, or next whatever the prothallus may be growing on. Near the notch of the heart-shaped prothallus are developed a few flask-shaped bodies which contain within them an egg cell or single ovum, the female reproductive body. By a series of changes this egg cell becomes embedded in a mucilaginous material. This flask-shaped body with the female egg cell inside is known as the _archegonium_. From among the rhizoids there may, at about the same time, be found developing small globular organs that have in them a number of tiny cells, each of which has attached many minute threadlike tails. The globular organs, with their minute, tailed cells are known as _antheridia_, and comprise the male reproductive equipment. Just as in flowering plants, neither the _archegonia_ (female) nor the _antheridia_ (male) can produce offspring without mating and the method by which this marriage is accomplished differs tremendously both in practice and in its significations from that in phanerogams. In the first place, the male and female reproductive cells are separated by a considerable distance, they are both inclosed in structurally different casings, and the whole operation is so microscopic that insects can be of no service. Nor can the wind do for them what we have seen that it does for the pollen of pines and grasses.

Of the aids to fertilization there remains then only the water, which plays such an important part in the mating of the eelgrass and ditch grass among flowering plants. But in these ferns a very different drama is about to be enacted. The male cells, as we have seen, are provided with slender tails, which are movable. They move, in fact, to such good purpose that the male cell can actually swim in the water. Of course its minute size demands only the merest drop of water, in which it will take the only excursion of its brief life. For just as soon as it is mature, a heavy dew or the tiniest particle of water will set free the little male messengers. The water too has not been without effect on the female cell. More remarkable still, this mucilaginous matter contains in it a substance that acts as a lure to the swimming male cells. In any event they do swim directly to the entrance of the female cell’s abode, through it and to her, when the union is effected. At once there is thrown across the entrance a membrane that excludes all other males, and the fertilization is complete. From this union of the male and female cells a true young fern begins to develop. First a young leaf and roots, finally a stem and in the end, of course, a full-grown fern producing spores, ready to renew the whole process.

Some ferns do not follow all the steps exactly as we have outlined, for all of them have not the structure of the typical one whose life history has been sketched above. In the adder’s-tongue fern, for instance there is a stalklike prolongation from the base of the only leaf the plant bears, on which all the spores are borne. In certain others, as in the ostrich fern, the spores are borne on leaflike growths that serve only this function. Most ferns, however, bear spores on otherwise unmodified foliage leaves and the great bulk of them on the under side of such leaves.

There are several things about the life history of a fern that differ fundamentally from any flowering plant and perhaps the chief is what is known as the alternation of generations. A spore, for instance, can never produce a fern as a seed will always produce a flowering plant. In this respect they are like many insects that always have two or sometimes three different stages in their life history. Only by the complicated method of first a spore then the prothallus, from which archegonia and antheridia are produced, followed by the free swimming male cells fertilizing the female, can a fern reproduce itself. As we shall see in the chapter on the History of the Plant Kingdom, this alternation of generations, the absolute necessity of water in which to carry on the fertilization, and above all the ability of the male cells for free swimming in the water, are all landmarks in the development of plant life. In its simplest form fertilization in flowerless plants is characterized by one or all these processes, as it is in the ferns, while in the flowering plants, the act is accomplished by processes, discussed previously, which, in the development of the plant kingdom, mark a period only comparable, in the history of man, to such tremendous achievements as the acquirement of speech or the ability to make a fire.

LIFE HISTORY OF A MOSS

Ever since the war, the peat-forming mosses, known as sphagnum, have become more widely known to the general public than any of the ten or twelve thousand mosses known to grow on the earth. Its power of absorption, greater than linen bandages, made it extensively used to pad surgical dressings. Hundreds of thousands of these sphagnum

dressings were made, and the collection of sphagnum from the bogs in which it nearly always grows was the task of many who could render no other service.

The reproduction of sphagnum is not unlike that of ferns already described. There is the same necessity of a film of water in which the free swimming male can fertilize the female. But some other things about their reproduction of young differ from ferns.

In the first place sphagnum is a nonvascular cryptogam, in that its leaves have no veins or ducts in them and its minute stem is also without those conducting passages that characterize all ferns, and the flowering plants, which are considered the most highly developed of all plant life. (See Chapter I for a discussion of this point, in the section devoted to “Flowerless Plants.”)

In this moss, also, there are small branches, some of which bear only the tiny leaves, but some bear leaves and the reproductive organs. The female or _archegonia_ are much like those in the ferns, and the _antheridia_ or male are also, as in the ferns, minute globular organs in which are the male cells. The branches bearing males are greenish, yellow, or even reddish, quite unlike the ashy gray foliage leaves which give to sphagnum its characteristic ashy gray color. Unlike the ferns, the male cells of sphagnum have only two tails, but they nevertheless swim, tail first, to the female, when the time for fertilization comes. The female branches are found mostly toward the upper end of the plant and bear the archegonia at their extremities.

From what we know of the reproductive stages in the ferns it is now obvious enough that in sphagnum moss, as we ordinarily see it, we have, because it bears antheridia and archegonia, a quite different condition from the ordinary spore-bearing leaves of ferns. For as yet spores have not been developed on the moss. The mating of male and female cells, directly on the plant, proves that in this “plant,” at least, our ordinary notion of this moss is mostly confined to a stage in its life history comparable in ferns to the production of archegonia and antheridia on the fern prothallus. From this mating of the male and female cells there results, as in the ferns, the production of a spore-bearing structure. This consists of a spore case, matured for the most part in the chamber occupied by the fertilized female cell, but ultimately its cap is carried upward. Later on the spore case ruptures, releasing the spores. As in the ferns, these germinate, forming a short green protonema followed by a prothallus. From this a short leafy branch develops, which completes the life cycle, as this is the young moss plant.

In other words, sphagnum, as we ordinarily see it, produces, on the plant, male and female cells which unite to form a spore case with spores in it. These are shed, develop into a protonema which is followed by the prothallus and from this the young moss plant develops. In ferns the conspicuous well-known stage is the spore-bearing one, in sphagnum it is the production of male and female cells directly on what appears to be the mature plant.

There are many other kinds of mosses than sphagnum, and their life histories differ in slight degrees from it. But they all agree in this, that the greenish, feathery little moss plant is a stage in its life history bearing male and female cells, the mating of which produces a spore-bearing contrivance. In most of the familiar green mosses this is a capsulelike body on a short stalk, usually well elevated above the green mass of plants. From this the spores are shed and develop into a protonema or “first thread” just as in ferns. Unlike them, and unlike sphagnum, the green mosses produce no thallus, and the young leaves of the moss are developed directly from this protonema.

LIFE HISTORY OF A MUSHROOM

The common mushroom that we eat is easily enough divided into a thick stalk, known as a _stipe_, and a broad hood called a _pileus_. The under side of the pileus is seen to be composed of thin plaits set closely together and radiating from the center toward the edge. These are known as _gills_. From among the gills the spores are shed when they are mature, usually foretold by the changing of the color of the gills from whitish to purplish and even to brown or blackish. The spores are then shed and ready for the next stage. From what we already know about ferns and mosses, it is clear that from these spores a mushroom cannot develop without the production of male and female cells and all the rest of that process of hidden marriage that characterizes all flowerless plants. But in most mushrooms no one has ever seen, nor have the most carefully conducted experiments ever demonstrated the germination of the spore. So far as we know at the present, many mushrooms may or may not produce their young through the germination of their spores in their native fields and meadows and the subsequent production of male and female reproductive organs. But if their spores do produce such organs, which all our knowledge of spores makes probable, it is, in a truer sense than in most cryptogams, a case of hidden marriage. The process of producing their young is thus a secret one that scientists have not yet been able to disclose. Of course it is a common practice of mushroom growers to purchase _spawn_ from seedsmen which under favorable conditions will produce many young mushroom plants. This, however, is the production of young without mating of the sexes, a fairly common characteristic of many other plants which will be considered presently.

As we saw in the section devoted to Flowerless Plants in Chapter I, there are many other kinds of fungi than the familiar edible mushroom and their close relatives, the often deadly poisonous toadstools. The reproductive processes in these other fungi are fairly well understood, but they can hardly be included here. In the mold on bread, the yeast used in baking, the rust of wheat and the diseases of other plants and of animals, the individual organism is so minute that it can only be detected under the microscope. Their reproductive processes are, of course, on such a minute scale that they could be followed with profit only by those equipped to study them. They have been described in many botanical textbooks, and those interested in them should consult such books.

In recapitulating the reproductive processes in cryptogamous plants the thing that distinguishes them from all flowering plants is that they bear, in some stage of their life history, a spore. From this, in the great bulk of them, a mature plant never develops. Only by the production from the spore of some contrivance for bearing male and female cells, which may, as in some seaweeds, even be on different plants, can a mating of these be accomplished, and from this union will develop the mature plant. There are many modifications of this plan, but in nearly all of them the presence of water, for the free swimming of the male cell to its mate, is essential. Just as in flowering plants and in all the larger animals, however, the reproduction of young in cryptogams is a sexual process depending on the union of male and female. While in phanerogams that process may well be spoken of as visible marriage, with all the pageantry of insects and beautifully colored flowers, in cryptogams the process is not only a hidden marriage, its ways are sometimes so secret that, even in the common mushroom, the actual mating is conjectured rather than demonstrated.

THE PRODUCTION OF YOUNG PLANTS WITHOUT MATING

It is so generally true in all plants that a union of male and female is necessary for the production of young, and, as we have seen in most of them, the process is so uniformly successful that still another mode of producing them seems almost unnecessary. Yet in a surprisingly large number of plants new individuals, both of flowering and flowerless plants, are regularly produced without such a union and where sexuality has nothing to do with the increase.

In the life plant--a thick-leaved shrub from Mexico commonly grown in greenhouses--the leaves are wavy margined. From their edges, especially when injured, many tiny new plants will often start to grow. Even if the leaf is cut up into fairly small pieces many of these will develop young plants, and in various forms of the common rex begonia the leaves are usually cut into small pieces by gardeners for the production of young plants which always sprout from such pieces. It is useless to multiply such cases, as everyone knows of the production of young plants from the ends of strawberry runners, the cutting up of potatoes, the universal garden practice of making cuttings, and the sprouting of willows, all of which are effective by virtue of this faculty of plants to produce young quite without the intervention of different sexes. Not so well known are the cases of a liverwort, a small relative of the mosses, which, if chopped into fine pieces, each will develop into a new plant. We have already spoken of the spawn of mushrooms; and even on sphagnum moss, in addition to its sexual reproduction, it produces sterile branches that will root and, after separation from the old plant, form a new one.

Wherever this tendency is found, whether it be in a microscopic seaweed, some of which know no other means of reproduction, or in the showy begonia, it depends for its success upon a property of the ultimate unit of its structure, the cell. Sometimes, as in bacteria or the most minute seaweeds and in some other kinds, the whole plant consists of a single microscopic cell, when it is said to be a _unicellular_ plant. All others, in which the grouping or modifications of the cell makes more complex structures, such as trees or shrubs and all the plants that grow, both flowering and flowerless, are called _multicellular_ plants. Whether they be of one or many cells, these have the faculty of dividing, and by this division making two where one existed before the division. This division of cells is what happens in the normal growth of plants and it is this division, in more unusual ways, that results in the production of new plants without mating of the sexes. As cells are themselves microscopic, of course their division is equally so, and cannot be described in detail here. It has been many times described and pictured both in books on plants and animals, as it is the ultimate unit of the structure of both.

Plant life, then, seems to be better provided with means to renew itself than most animals, for, as we have seen, it has several methods to rely on. These may be divided into sexual, which includes both that in flowering plants with their visible mating and in flowerless plants with invisible mating, and _asexual_, literally without sex. In the latter are all those unicellular plants that reproduce themselves by simple division of the cell, and also those flowering plants that either naturally, as in life plant, or by the gardener’s art of making cuttings, produce new plants quite without the intervention of the sexes. Whether it be sexual or asexual, nature has more than fulfilled its obligation to the plant world in providing it opportunities for self-renewal. No matter what apparently unfavorable condition arises and often in spite of an almost unbelievable wastage of potential life stuff, the renewal goes on, or else there is the total disappearance of the species. So strong is this tendency to provide for renewal of their kind that many plants, if injured or cut by a mower, will almost in their last gasp hurriedly flower and set seeds, and we have already seen that the little liverwort, even if cut to pieces, also obeys that nearly universal law of nature: “Be fruitful and multiply.”