THERE is perhaps no device of nature that more perfectly accomplishes its purpose than the one with which all living things are endowed—the instinct for the renewal of life. In man the dawn of the mating instinct has ever been the theme of poets, and some of its manifestations are the despair of ascetics. Through it some of the noblest of man’s emotions have arisen, and because of its perversion our daily newspapers chronicle the basest and most sordid tragedies. But whether noble or ignoble, this instinct for mating is, in its simplest terms, only a provision of nature that all life contains within itself the means of renewing life. Without this, life, so far as we know it, would end with the present generation. Perhaps our understanding of this decree of an all-wise nature to increase and multiply will be heightened by looking at it not only from its familiar manifestations in man, but more broadly. Seen from this broader viewpoint, it is the inherent legacy of all living things from the dawn of life on the earth down to the present. Even the simplest one-celled organisms have the faculty of increasing. In all plants, both the flowerless ones and those producing flowers, the process is carried to a perfection almost unbelievable in its intricacy and in provisions against its failure. From the matings of flowers much may be gleaned; even man himself can The equipment which different flowers have developed for this purpose, their almost uncanny devices to make certain that only the distant and foreign male can ever impregnate the female, the enormous wastage of both unfertilized females and males that will never become fathers, and the overwhelming effectiveness of it all, in spite of this prodigality—these manifestations of the production of young in the plant world will take up the rest of this chapter. All the first part will tell of this process in flowering plants, while the second shows how flowerless plants accomplish the same end in more secret ways. Finally, in a brief third part, we shall see how, without mating of the sexes, nature has still one other way to see to it that there is a constant supply of young. We have already made clear that all plants are divided upon the basis of whether they bear flowers and their mating goes on before the world, or whether they bear none and the process is accomplished in more secret ways. Because flowers are so much better known, and it is simpler to see how the act is consummated in them than in the cryptogamous plants, we shall first consider the phanerogams or flowering plants, and in the second section of this chapter the cryptogams or flowerless plants. 1. Visible Marriage of Flowering PlantsIn the first chapter, under the section devoted to flowers, we found that the stamens are the male and the pistils the female organs of reproduction. As the Pollen is made up of individual pollen grains, which are very often stuck together so that we see only the mass, not the individual pollen grain. Sometimes the pollen is not sticky, as in the case of pine trees or in the ragweed—a fertile cause of hay fever. In these, and hundreds of other plants, the wind will blow great clouds of pollen through the air. When we stop to consider that a single, or at most a very few pollen grains are all that are necessary—in fact, are all that can be of real service—the enormous wastage of the male fertilizing substance, in order that mating be secured, gives us some idea of how prodigal is nature in this supreme function. The pistil, or female organ of reproduction, is more cautious in the expenditure of its resources. As we have seen, it is composed of a swollen base, the ovary, a slender shank, the style, and a swollen or branched tip, the stigma. In some plants the ovary is divided into several compartments or cells, each with one or more ovules, which are only immature or unfertilized seeds, often very tiny, but usually quite easily seen if the ovary is cut open. It is the entrance of the pollen grain into this ovule that consummates the act of fertilization. As the ovule is carefully secreted within the ovary of the flower, and as the male fertilizing stuff or pollen is found only on the anther, it is obvious that some method of bringing the two together must be provided for. In some plants this is accomplished by the anthers being just above the stigma, and when the pollen is ripe and the ovule ready, the stigma is found to be covered with a sticky substance. As the falling pollen grains touch the stigma, they are caught in this sticky substance just as surely as flies are caught once they touch a fly paper. But just here one of the most wonderful processes of nature begins. The pollen grain begins, slowly at first, to grow, and in the act it penetrates the outer coat of the stigma with a minute pollen tube. This slender threadlike tube, carrying with it the male germ, grows straight down through the stigma, into the narrowed style, and through this to the ovule. Once the pollen is caught on the stigma, nothing is so sure of fulfillment as that this male fertilizing stuff will ultimately reach the ovule. For the hitherto virgin ovule this impregnation starts a new phase in life. It means the beginning of the end, but in the process fruit and seed will be developed, and the young bride, already a mother, has triumphantly accomplished that for which she exists. If fertilization of all flowers were as simple as this, there would be no need of what follows, but actually in surprisingly few plants are the stamens and pistils so arranged, the ripening of the pollen and readiness of the ovule for impregnation so timed that the act can be accomplished in such direct fashion. For it is quite obvious that in flowers in which the whole drama of mating goes on within the petals, without the interference or help of any outside agency, the result will be a crop of young who know no other characters than those of the parents, and have nothing to look forward to but a closely inbreeding progeny, very little, if at all different Certain structural features of flowers have been so developed that fertilization of the ovary by the pollen of the same flower is impossible. The commonest case is in those flowers where the stamens are shorter than the pistils, as they always are in the common snowdrop, hyacinth, the sassafras tree, 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, 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 VISITORSOn any summer day, especially when the sun is shining brightly, we may see bees and butterflies 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 While color of flowers seems as though it were attraction enough, it is very likely that their fragrance or perfume is still more seductive in its power of luring insect visitors and repelling useless ones. Poets have called this perfume the soul of the flower, and in its almost intangible beauty it might well be so called were it not for the fact that it appears to be of not the slightest use, except as a lure. In all the equipment of seduction there is none like this fragrance of flowers for attracting insects. Flowers, then, have things to offer to insects which the latter need. Nectar and pollen are the chief, and where these merely bread and butter objects are not enough, or sometimes in addition to them, the flower is dressed out in gorgeous colors or perfumed with a fragrance beyond the dreams of the fairest bride. What insects do to complete the fertilization of such a legion of beauties makes up the romance of the flowers. Perhaps not even in man himself is this creation of new life so surrounded with beautiful ideas. Also, as in man, it sometimes is bound up with an almost fiendish cruelty and cunning. The common blue columbine, much grown in gardens for its beautiful blossoms, always has the flowers hanging upside down, a habit that admirably serves to keep its pollen from rain. The opening and closing of many flowers in cloudy weather, or at night, may be for the same reason. Everyone knows the five blue spurs into which the petals of columbine are produced. At the very end of each spur, which is always curved, the flower secretes a considerable quantity of honey. This, one of the greatest attractions to bees, leads inevitably to a visit from one. The bee, in order to reach the honey, hangs on to the inverted flowers, clutching the base of the spur with its foreleg, and further securing itself by the mid or hind legs, which grasp the slender column into which, in the columbine, the stamens and pistils are crowded. In its anxiety to reach the honey the bee pokes its head as far into the spur as possible, but it gets in only a fraction of the full length of the tube. To reach the honey it extends its sucking apparatus, which is a complicated mechanism for this purpose on the head of nearly all insects, and which will hereafter be called by its true name of proboscis. It happens that bees can easily bend the proboscis downward or toward their own body, but only with considerable difficulty can they bend it in the opposite way. And yet the honey in the curved tip of the columbine can only be reached by curving the proboscis to fit the tube, and in this process the bee’s body for nearly half its length is forced to touch the anthers. While these While the columbine by its spurs and other interior structure succeeds nearly always in holding a bee long enough to insure its being dusted with pollen, the common barberry bush of Europe (Figure 70), also much planted in American gardens for ornament, actually drives bees away by sharp blows of its stamens, so that self-fertilization shall not result from the visit. In this shrub the petals partly cover the stamens unless the latter are disturbed, and, in fact, the curved tip of the petals forms a kind of socket into which each of the six stamens are fitted. The position of these is such that any insect can go straight to the honey glands which are at the base of the flower, without touching the anthers. But their filaments are broadened out at the base, so much so that their edges touch. The honey glands are so placed that the insect must touch the broad bases of at least two filaments, between which, in fact, it must force its proboscis in order to reach the honey. The moment any particular pair of filaments are irritated by the bee, two pollen-dusted stamens fly out from their position among the petals and the anthers strike the bee with a sharp blow. Many observations prove that almost never does the bee go on with his honey sucking after this rude interruption, which has resulted in at least its head being dusted with pollen. The low, sticky stigma is so placed that it is one of the first things the bee’s head strikes as it reaches the center of the flower. Because of the position of the stamens, while they are undisturbed, it is impossible that pollen from them could have been brushed off at the bee’s entrance of the flower. And by an almost miraculous adjustment of the power of the blow by the irritated stamens, this drives off the intruder only after he has brushed his pollen-laden head over the stigma. His head at this stage is, of course, covered only with foreign pollen gathered elsewhere, but just as soon as the bee tries to get what he came for, sometimes even before he gets his reward, out fly the pair of stamens, thoroughly dusting the bee, and seeing to it that the blow is just sufficient to drive off the pollen-laden insect. No device to secure cross-fertilization could be more effective. If the blow of the stamen were only ever such a slight fraction less than it is, the bee would only stop a moment and then go on honey sucking; which, because of the To attract insects and then repel them seems a little like using them as some flirts notoriously use men, only to throw them over when they are no longer interesting. In the large-flowered magnolia tree from the southeastern United States, insects, however, fare somewhat better than this. In this magnolia, which has flowers several inches long, self-fertilization is impossible as the stigmas are ready to receive a mate several days before the laggard stamens are provided with the wherewithal. Without some insect or other outside help there would be only a childless old age for this particular tree. The flower opens rather early in the season, while the nights are still cool, and as a protection from the cold, rose beetles habitually fly into them. They find a pleasant shelter under the three inner petals which arch over the honey-coated stigma, and form a snug little chamber so much warmer than the outer air that its heat is appreciable to the touch. The rose beetles, once they are inside this warm shelter, cannot get out and are often held for a few days. Then, as the stigma passes its period and the stamens are furnished with pollen, the chamber opens by the gradual withering of the petals. But the insects, in their efforts to get out, have raised a perfect dust storm of pollen with which they There are some other flowers that hold visiting insects in a trap until cross-fertilization has been completed, and all of them by no means furnish their visitors such a snug little heated chamber as the magnolia. One vine from the eastern United States, known as the Dutchman’s-pipe, or sometimes as the pipevine (Figure 71), is singularly ruthless in this respect. Its flowers are of such evil odor that only carrion-loving insects, such as certain kinds of flies and gnats, ever visit them. The flower is of very peculiar structure, being formed of a hollow tube bent from its stalk first downward, and then upward. The upper part ends at the opening which is provided with a three-lobed lip or doorway. Through this the insects crawl, and they finally reach the bottom of the curved part of the flower. Behind and above them is the entrance through which they have just come. And above them, in the other curved part of the flower, is the stigma. As in the magnolia, this matures several days before the pollen from its surrounding stamens is ripe, so that self-fertilization is never possible. What is now the plight of the insect at the bottom of the upward-pointing tubes, one leading to the organs of reproduction, the other to the exit? By an almost diabolical cunning the inside of this flower is so smooth that no insect can crawl up its slippery sides. It takes some time for the prisoner to find this out, and, in the meantime, it has explored every nook and corner of the flower by flying. In the course of this exploration it reaches and covers the stigma with pollen, for as we shall see presently, it always comes into the flower pollen-laden. Evidently becoming panicky about getting out, the insect then flies with very considerable force in every direction. Toward the true exit it naturally flies the most, and by a refinement of cruelty this is the lighter end of the flower, and therefore the obvious mode of escape There appears nothing very romantic about the cross-fertilization of the Dutchman’s pipe, in fact, the whole affair seems but a sordid and, it must be confessed, a very efficient trick to get what the flower needs from the insect, rewarding it by many hours of apparently hopeless captivity. But most flowers do have something that insects want, and none so well fulfill the expectations of butterflies as the meadow pink. This is a graceful little perennial native in the fields in central Europe, but often grown in American flower gardens. It has beautiful pink flowers, with a long tubular calyx, at the bottom of which are rich honey glands, accessible only to the long proboscis of different kinds of butterflies. No other insects, and many try, are able While such plants as the meadow pink and thousands of others have lost, if they ever possessed, the power of self-fertilization, and rely absolutely on insect visitors for their perpetuation, there are many hundreds of kinds that apparently hope for cross-fertilization, but, in default of it, due to their inability to absolutely compel it, they finally accept self-fertilization as a last resort. Darwin once said that “Nature abhors perpetual self-fertilization,” and the frequent visits of insects and their rÔle in preventing it, together with the flowers’ adaptations to A more remarkable case of leaving one final chance for self-fertilization is the gas plant. It exhales such a strong and peculiar odor that only certain kinds of insects will visit it. In fact, the odor is so strong and is so heavily charged that a lighted match held near it has been known to slightly ignite—hence the plant’s name. The flower bears a low, squat stigma, profusely covered with honey, which is perfectly accessible to any insect visitor. It has ten stamens, which at first are quite out of the way of insects, two being folded back in each of the five yellow petals. First one stamen begins moving gently from the shelter of its petal, and the anther, pollen-coated, hovers over the stigma, which would inevitably lead to self-fertilization if the stigma were only ready. It never is, and, as though realizing this, the stamen gently moves back out of the One of the largest families of flowering plants is the pea family (Figure 72), with over five thousand members, practically all of which rely on bees for cross-fertilization. In some kinds, where bees occasionally fail them, the flowers wither without self-fertilization and, of course, no seed are then produced. In such a large family of plants there are naturally many different adaptations for securing cross-fertilization—some of them of such extreme complexity that they could hardly be included here. All the family have the characteristic pealike flowers familiar enough in the sweet pea, which have already been described and figured on page 44. In all of these stamens and pistils are hidden inside the keel, at least in the early stages of the flower. In some, such as clovers, for instance, the organs emerge from the keel, and after fertilization by insects re-enter their retreat. There are scores of different plans for securing the desired object, but the common alfalfa, with a few other related plants, has the most startling. The flower begins life with its 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 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 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 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 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 complicated than in any so far noted. Darwin’s book, “On the Various Contrivances by Which British and Foreign Orchids Are Fertilized by Insects,” reads like a fairy tale. Yet it is the result of years of patient observation by incomparably the greatest naturalist of recent times. To it the reader must go for the details of a drama of absorbing interest, but too long to sketch even briefly here. Perhaps one illustration may be mentioned of how far the principle of cross-fertilization has been carried, and to the deadly effects of its failure in at least one case. In a certain orchid from Brazil, known as butterfly orchid, the pollen is nearly always carried out of the flower by an insect visitor, but, if by mischance it is not, and falls on the stigma, not only does it fail to fertilize the ovules, but it kills the pistil forthwith. It must have struck many thoughtful readers to ask a rather obvious question at this point. Why, if untold millions of insects are constantly flitting from flower to flower, does not the pollen get mixed, as it is quite certain that they will not fly from a 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 WIND AND WHAT IT DOES FOR FLOWERSWhile, 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 WATER AS AN AID TO FERTILIZATIONThose 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 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 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 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 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 2. Hidden Marriage of Flowerless PlantsAs 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 FERNIn 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 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, 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 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 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 MOSSEver 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 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 LIFE HISTORY OF A MUSHROOMThe 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 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, THE PRODUCTION OF YOUNG PLANTS WITHOUT MATINGIt 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 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 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.” |