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PRACTICAL POLLENATION

A SURVEY OF WORKING METHODS

Once upon a time-it may have been about the year five million B.C.-a plant imbued with nascent wisdom made a tacit compact with a fellow creature of the world at large that was fraught with strange and fateful meanings for races of beings yet unborn. The fellow creature in question was at that time probably the most highly developed citizen of the world, although in modern terminology he would be termed "merely an insect." The compact the plant made with him was to the effect that one should manufacture sweet nectar and freely supply it as food; and that the other in return should carry the fructifying pollen grains from flower to flower. Doubtless no more important compact was ever entered into in the history of animate creation before or since. For out of this compact grew the rivalry that stimulated development and made possible the evolution of the whole race of plants that bear beautiful flowers and exhale sweet perfumes. But for this eventful alliance, there would never have developed in the world a conspicuously colored or a scented flower of any kind. And it requires no argument to show that a world without beautiful and sweet-scented flowers would be a world robbed of a large share of its attractions as an abiding place. But that is not all. The alliance between insect and flower did not merely suffice to give us things of beauty. It bespoke utility as well. It made possible the bringing together of germ-plasms from plants growing far apart, thus insuring virile and variant strains; and this determined in large measure the amount and direction of the evolution of the highest orders of plants. For it must be observed that, with rare exceptions, the higher plants are precisely those that long ago entered into this cooperative scheme whereby they trusted their fate absolutely to the insects. They hazarded much, for if anything should lead to the destruction of a few insect races, entire orders of plants would suffer race suicide. But if they risked much, they also profited much; for the cross-pollenizing effected by the insects afforded the constant stimulus to variation that underlies all evolution, and enabled the plants that entered into the coalition presently to outstrip their fellows. Wherever you find a tribe of plants that shows great diversity of form, large numbers of species, and ready adaptability to improvement, you will as a rule find a tribe of so-called "entomophilous," or insect-loving flowers, dependent upon the winged messengers for the consummation of their matings. Vast responsibilities then were implied in this coalition of the plants and the insects; but the results have justified the hazard.

PLANTS THAT DID NOT JOIN THE UNION

We shall presently see illustrated in detail the curious adaptations of form and color and structure to which the plants of various species were led in their rivalry to secure the good graces of the insects and thus to make sure of perpetuating their species. Every blossom of the entire orchard, every flower of the garden, and with a few exceptions all of the vegetables under cultivation furnish illustrations in point. But it should be recalled that there are large numbers of plants of a lower order that from the outset refused to enter into the coalition, and that even to this day have declared themselves independent of the plant-insect union. Plants of this non-union clan are the entire races of lowly mosses and lichens; a goodly number of aquatic forms that maintain the appearance and manner of their remote ancestors; and the familiar tribe of ferns; and some of the trees which depend mainly upon the wind. All of these, and a large company of forms less familiar to the amateur, have obstinately retained throughout the ages the primeval habit of propagating their kind not with immobile pollen grains but with the aid of self-moving germ-cells. These motile germ-cells, of microscopic size, find their way through the water-supplied in case of land plants by a film of rain or of dew-from one plant to another, and effect cross-fertilization without calling in the aid of any allies. They do not need to attract insects, and so they have not adopted the system of advertising through the development of large and showy blossoms and nectar cups to which the members of the plant-insect alliance are obliged to resort. But if the lowly plants thus maintained their independence, they have done so at a very great sacrifice. They are not more independent than they are unprogressive; and indeed they are unprogressive precisely because of their independence. The method of cross-fertilization that they have adopted does indeed enable some of them to blend the strains of different individual plants; but in every instance the parents must be growing in the immediate vicinity of each other. Except by the accidental and most unusual transfer of a plant through the agency of a passing animal, there is hardly the remotest chance of effecting cross-fertilization between individual mosses or lichens or ferns growing in widely separated regions. But we have already seen that it is precisely this blending of traits brought from parents growing under different environing conditions that is chiefly responsible for making plants vary and furnishing the materials for evolutionary progress. So it goes without saying that the plants that are restricted, in the choice of possible mates, to individuals growing under the same conditions to which they themselves are subjected, cannot expect to change rapidly and therefore do not evolve in any such ratio as plants having the other habit. And in point of fact we find that the plants that retain this primitive custom of fertilization with the aid of motile germ cells, acting quite independently of insect or wind, are plants of a low order of development, showing relatively little diversity of form and small capacity for adaptation. The most conspicuous of them with which the ordinary observer is familiar, namely the ferns, bear a striking resemblance in contour to plants of the remote Carboniferous Era, traces of which have been preserved in the coal beds. And there can be no doubt that this persistence of the primitive form has been largely due to the special method of fertilization which the ferns have retained. If it be permitted to carry personification one stage farther, we might say that the ancestors of the ferns belonged to a conservative family, jealous of its independence, and unwilling to enter into outside alliances. And the penalty of conservatism here, as so often in the range of human experience, has been racial stasis.

PLANTS THAT HAVE LEFT THE UNION

It would appear, however, that there are certain races of plants that were once members of the plant-insect alliance but which are now no longer in the union. These apostates include two quite different tribes of plants. On one hand there are numerous gigantic trees that no longer depend upon insects for the fertilization of their flowers. On the other hand there are little cowering plant-waifs that nestle close to the earth and which, in quite a different manner, also assert their independence. The trees that have thus revoked the treaty of alliance include such familiar forms as the pine, the oak and the walnut. These trees, and a goodly number of their fellows, long ago declared against further cooperation with the insect, and adopted the method of producing large quantities of pollen and scattering it in the air to be carried by the wind to the pistillate flowers, which in some cases grow on neighboring branches and in other cases on quite different trees. The method is in one sense wasteful, inasmuch as it involves the production of vast quantities of pollen, only an infinitesimal portion of which will ever come in contact with a receptive pistil. And of course the production of this pollen must draw heavily on the energies of the living substance of the tree. But on the other hand the tree that thus depends upon the wind rather than upon the insects is under no necessity to develop large and conspicuously painted flowers. Nor need it produce nectar to feed the insect allies, since these have been renounced. And it may very well chance that the saving of energy thus effected more than counterbalances the waste through excessive pollen production. At all events the plants that have adopted this system of pollenizing give evidence that their plan is not a bad one in the very fact of their extreme abundance. Moreover the "wind-loving" or "anemophilous" plants, as the botanist terms them, have not only produced a great variety of species and vast numbers of individuals, making up the bulk of our forests, but the individuals themselves are of such virility of constitution as to attain gigantic size. Indeed a moment's consideration makes it clear that the plants that had depended on the wind rather than on insects for fertilization are quite in a class by themselves in the matter of size, inasmuch as they constitute the bulk of our forest trees. This relation between size and habit of spreading the pollen broadcast on the winds cannot be altogether accidental. But whether the trees grew large because they had given up the alliance with the insects, or whether they gave up the alliance because they were growing large, it would be hard to say. We know that, in the main, insects tend to keep near the surface of the earth, and it may be that the plants that tended to grow very tall were relatively neglected by the insect messengers. But on the other hand, there are insects that haunt the highest trees, and we can hardly doubt that had even the tallest of plants desired to retain the services of insect messengers, races of these would have been developed that would have proved equal to the most exacting demands. What seems on the whole most probable, then, is that the trees have changed their allegiance from insect messengers to wind because of the very nature of the conditions under which they grew. By raising their heads high and higher into the air they obviously put themselves more in contact with the wind and thus make it increasingly possible to spread their pollen broadcast across wide stretches of territory. As a matter of fact we know that the pollen of pine trees in particular may be carried almost in clouds for scores and even hundreds and hundreds of miles. So there is every opportunity for the cross-fertilization of individual trees growing in widely separated territories; and there is therefore no restriction put upon the possibilities of progress and evolution for these large-growing plants in penalty for their renunciation of the services of insect messengers. The case of the trees, then, simply illustrates the fact that there may be more than one way to effect a given purpose, and that a change of method may be no barrier to progress, even though the abandoned method still remains an admirable one for a vast coterie of organisms of slightly different habit.

SELF-FERTILIZED PLANTS

But the case of the other company of plants that have back-slidden from the insect alliance is altogether different. The plants in question do not make up any great conspicuous tribe, comparable to the forest trees, but are a miscellaneous company of lowly vegetables of unrelated families. Familiar examples are the wheat of the fields, peas and beans in our garden, and a certain number of the more obscure species of violets. The jewel weed, the fennel, the rue, and the nettle, are other somewhat less familiar yet not uncommon tribes of plants whose flowers are habitually self-fertilized. There can be no question that these plants are the descendants of tribes that were at one time members of the plant-insect union. The fact that most of them retain more or less conspicuous flowers proves this beyond question. In the case of the wheat, which might be thought a possible exception, there is the evidence of certain species of wild wheat, growing to this day in Palestine, which have only partially renounced allegiance to the insects, still putting forth flowers that on occasion may be cross-fertilized with their aid or with that of the wind. Just why these various plants of different families have departed from the custom that has served their fellows so well, would be interesting matter for conjecture. Perhaps the most plausible suggestion is that the ancestors of the plants that now have closed flowers and thus depend exclusively upon cross-fertilization had fallen on evil days in which there was a dearth of insect messengers in the regions they inhabited. The story of the starved martins, told in an earlier chapter, furnishes a striking illustration of the fact that insects that ordinarily are abundant may in any given season fail to put in their appearance. And even if the insects themselves are abundant, the weather conditions, in a given season, may be such as to make it almost impossible for them to carry out their bargain by transferring pollen from flower to flower. Every orchardist knows that a protracted rainfall just at the time when his apple, pear or plum trees are in bloom, may prevent the bees from visiting the flowers; and in such case, as is only too well known, there will be a partial, or no crop that season. With trees and other perennial plants it is not matter of absolutely vital importance that there should be a crop of seeds produced each season. Failing progeny this year, next year or the year after will answer, in the case of a plant which grows on a permanent stalk or from roots outlast the winter. But the case of the annual plant is altogether different. Should such a plant fail for a single season to produce seed, its entire race would vanish instantly from the earth. That thought is rather startling when presented thus tangibly. Yet its truth is almost axiomatic. As a rule, the entire seed crop of an annual plant in a state of nature, either germinates or decays the ensuing season after its production. And it is absolutely incumbent on the plants that grow from this seed to produce in turn a store of seed that will carry on the racial stock. So it is not strange that a plant that is thus perennially threatened with destruction should adopt exceptional measures to ensure the fertilization of its flowers. Very often it may have happened that certain individual flowers that chanced to be self-fertilized were instrumental in saving the life of a species that otherwise would have been exterminated. And as, through the operation of heredity, the offspring of these flowers would tend to reproduce the self-fertilizing habit of their parent, the surviving representatives of the species might thus come to constitute a tribe in which the habit of bearing self-fertilized flowers was the prevailing custom. And thus it is, perhaps, that the method of reproduction followed by the wheat in our fields and the peas and beans in our gardens may be accounted for. Yet the fact that certain of these self-fertilized flowers, as for example the violet, retain the custom of putting forth showy flowers even though these for the most part are seedless, shows how powerful is the hold of remoter heredity, and how persistently the plant clings to a custom to which its ancestors owed their racial development. Moreover, it has been observed that the violet, when transplanted to a sunny spot and made accessible to insects, may resume the custom of growing seeds in its conspicuous flowers, whereas hitherto it had produced them only in the small inconspicuous bud-like flowers at its base which never open.

SCHEMES TO ENSURE POLLENATION

It is curious to observe how insistent is the inherent demand for fertilization of the flower, and how even flowers that openly advertise for the insects may strive to provide for self-fertilization in the event that their call remains unanswered and in vain. The common barberry (Berberis vulgaris) for example, opens and exposes its pollen-bearers only during the bright hours of a cloudless day. But in case an insect fails to visit it, provision is made that will ensure self-fertilization; for in due course the stamens dart forward and sprinkle their pollen over the pistil. In the case of the fennel flower of France, described elsewhere, which does not open at all, the pistils bend forward when they are ripened, and after taking the pollen from the stamens, straighten up again. With the rue, the arrangement is curiously complex and machine-like. Of the several stamens, each in turn bestows its pollen on the pistil at their common center. It has been observed that the stamens advance alternately, numbers one, three, and five in turn; numbers two, four, and six following in succession, as if the entire mechanism were actuated by clock work. But these and sundry other ingenious mechanisms for self-fertilization after all only evidence the resourcefulness of a plant in its struggle for self-preservation. It is better that a flower should be self-polllenized than that it should not be pollenized at all. But the process is in no wise comparable, in its value for the race, to the more usual process of cross-fertilization. The self-fertilized plant develops fixity of race. It lacks the needed stimulus of the blending of different racial strains. It will produce few varieties, thus giving little opportunity for the operation of natural selection. In a word, such a plant is really marked for ultimate extinction, unless, as in the case of the wheat, man steps in to give it the refuge of artificial selection. It may well be doubted whether the existing races of cultivated wheat could perpetuate their species, if put upon their own resources in competition with wild plants, for a dozen or two dozen years. The habit of self-fertilization may preserve for a certain number of generations a plant that would otherwise have been completely exterminated; but at best it marks a stage of degeneration and decline. The plant that follows it is in a sense retracing its steps down the ladder of evolution by which its ancestors have made ascent. And so it is not surprising to find that the vast majority of the useful plants of orchard and garden have kept up the traditional alliance with the insects to which they owe the multiplicity of their specific forms and the virility of the individual members of their organization.

THE WISEST OF PLANTS

It is flowers of the great brotherhood of insect-lovers, then, that chiefly claim the attention of the plant experimenter, because these are the ones that make up the chief census of orchard and garden. As a matter of course it is plants of this fraternity that are of interest to the amateur, because, generally speaking, it is these alone that put forth blossoms that please the eye. Whoever is interested to undertake experiments in plant breeding must then, familiarize himself with the mechanisms by which the plant makes known its appeal to the insect and those through which the perpetuation of its kind is effected; the mechanisms, that is to say, of the typical flower. As we come to study flowers in detail, it will appear that among those dependent upon insect-fertilizers, no less than among the wind-fertilized, there are individuals that bear the essential organs of the flower in separate blossoms. Reference was made to this in the case of our hybridizing experiment with a certain species of dewberry, and we shall see other illustrations of it from time to time. But the major part of the most familiar cultivated plants, including all the conspicuous fruit trees of our orchards, bear flowers each of which contains within the same blossom both the staminate and the pistillate organs. Ordinarily it is the function of the bee to carry pollen from one blossom to the pistil of another. But on occasion even these flowers may be self-fertilized. Thus it may be said that the most important, from a human standpoint, among the existing plants have adopted a compromise, in which cross-fertilization is the rule, yet which makes possible self-fertilization in case, under the stress of circumstances, cross-fertilization should fail to take place. Doubtless on the whole this was the best course of all. The plants that adopted it might be said to be the wisest of their race.

THE TYPICAL FLOWER

What may be regarded as the typical or perfect flower, then, is one that contains both pollen bearing and pollen-receiving parts, surrounded by the conspicuous insect signal that we term the corolla; and having also a less conspicuous outer shield termed a calyx. The calyx is the original shield about the flower bud, and its function is over when the flower opens. The botanist ordinarily speaks of the calyx as a modified leaf. He refers to the petals of the corolla as being also modified leaves or enlarged and beautified modifications of the calyx. He thinks of the stamens and the pistil as modified petals; and he justifies this estimate by showing that under cultivation it is often possible to transform these essential organs into petals. Thus, for example, are produced such double flowers as the cultivated rose and the chrysanthemum. To the human eye, these are things of beauty but from the standpoint of plant economy they must be regarded as travesties of flowers, since they are far less able and often Wholly incapable of producing seed. But it is perhaps a somewhat more philosophical view of the flower to consider it as a mechanism developed about the all-essential central organ, the pistil. This, the female organ of the plant, consists, in the developed form, of a basal structure, the ovary, containing the ovules or embryo seeds, and a more or less protuberant style at the end of which is the stigma that receives the fertilizing pollen. Considered as to its origin, the pistil is in effect a modified bud. Everyone is aware that individual buds of a plant may have the property of being able to reproduce the entire plant. The pistil is a modified bud each embryo seed of which, when fertilized, has the same potentiality. By the most wonderful miracle of the organic world, this infinitesimal structure is enabled to epitomize all the possibilities of a future plant, of predetermined size and form and habit. It differs from the bud from which it is developed chiefly in that it requires to be fertilized by union with a pollen cell, before it is capable of taking on development; and in the further very essential fact that when mature it may be cast off from its original moorings and carried to any distance, thus in a way making amends for the limitations put upon vegetables by their incapacity for locomotion. The stamens that normally grow in a circle about the central pistil develop at their ends anthers that produce, usually in relatively large quantities, pollen grains of exceedingly minute size. And each pollen grain contains, somewhat as does each ovule, all the hereditary potentialities of the entire plant. The pollen grain cannot, indeed, be made to develop into a plant; but its union with the ovule is essential to the development of that organism, and it is certain that the pollen grain, despite its infinitesimal size, brings to the union factors that represent its parent plant effectively and in full measure. It would be unbelievable, if we did not know it to be true, that a fleck of matter of scarcely more than microscopic size should contain the potentialities of a mammoth tree, and should predetermine the details of structure of a future tree even to its remotest leaf and to the finest details of its flowers and fruit. But that the pollen grain actually has these potentialities has been demonstrated thousands of times over by the plant experimenter. Any amateur who wishes to test the matter may do so, to his complete satisfaction, by making the simplest experiment in cross-pollenizing and watching the growth of the hybrid seedlings his work brings forth. The pollen grain effects union with the ovule by sending out a thread-like filament of protoplasm, like a tiny root, which penetrates the stigmatic surface, passes down along through the style, and carries the nucleus of the pollen grain to the nucleus of an ovule. When the two nuclei come in contact, fertilization has been accomplished. When pistil and the stamens have been considered, we are through with the really essential mechanisms of the flower. From the human standpoint, of course, chief interest centers in the corolla with its wide-spreading petals of varied colors. To the plant itself this structure is in a sense essential, inasmuch as it supplies the visible signal that attracts the attention of the insect. But beyond this it has no share in the process of fecundation. We shall have occasion to consider the form and structure of this showy portion of the flower in a multitude of individual cases, and to observe how it may be modified by process of selection, but from the present standpoint it does not call for further consideration. From the standpoint of the pollenizer, the stamens with their pollen-bearing anthers and the receptive pistil-with or without a stigma at its tip but always having one or more ovules in the egg-case at its base-are the organs that claim exclusive attention.

HAND POLLENIZING

The essence of pollenizing is merely the transfer of pollen from the stamen of one flower to the stigmatic surface at the end of the pistil of another. This is the work that is ordinarily accomplished by the insect. It is all that the plant experimenter accomplishes when he wishes to effect the crossing of different plants of the same species or the wider crossing, commonly called hybridizing, of different species. There is nothing occult in the practice of the bee or in the imitation of his work as practiced by the hand of the pollenizer. What is accomplished in each case is the purely mechanical transfer of a certain number of minute pollen grains from one place to another. Beyond that, everything depends on the vital activities of the plant tissues themselves. We shall have occasion in another chapter to deal somewhat at length with specific methods that are necessary to effect cross-pollenizing in the case of sundry types of flowers that have developed blossoms curiously modified as to form or details of structure. But the general processes of hand pollenizing, as they apply to the chief flowers of the orchard and garden, may be stated in a few words. The essential thing is to secure a certain quantity of pollen, usually by shaking it from the flower on a watch-crystal or other small receptacle, and to transfer this pollen to the receptive pistil of another flower either with the finger tip-which furnishes in general the most useful piece of apparatus-or with a camel's hair brush. It is desirable to cover the receptive portion (stigma) of the pistil fully with pollen, partly to ensure complete fertilization, and partly to prevent the vitiation of the experiment through possible subsequent deposits of pollen from another source. If the flower to be fertilized has stamens of its own, these should be removed before they are fully ripe-which is often a few hours, or a day before the foreign pollen should be applied. This removal of the stamens may usually be done with a pair of small pincers. In case of flowers that have short pistils the cherry, apple, and other orchard fruits being good examples-the unopened flower bud may be cut around at about the middle, with a thin-bladed knife, the anthers being thus excised at a single stroke. With other flowers the mechanical details vary, of course; but the process is seldom complicated. It calls for common sense rather than for great ingenuity. So-called composite flowers, however, require special treatment. The daisy and the sunflower are familiar examples. Here the true flowers are very small and grouped in masses. Individual treatment is usually out of the question. The best method is to wash away the pollen with a carefully directed stream of water from a garden hose, or by spurting water from the mouth; after which the head of the pollenizing flower is rubbed rather vigorously against the one just depollenated, thus effecting fertilization en masse. In exceptional cases it may be desirable also to cover the fertilized flower with a paper bag to prevent the visits of insects; but in practicing pollenation on a large scale this may usually be omitted. If the stigma has been satisfactorily covered with pollen, it will present no exposed surface for the reception of other pollen grains. Pending the more detailed discussion of the specific methods of pollenizing adapted to particular flowers, I would give an all-compassing rule which, in itself, will serve as a sufficient guide to the experimenter who has clearly in mind the principles involved in the process of cross-pollenation. The rule is simply this: Seek Nature's plan and follow it. In other words, take a lesson from the bees, and pollenize the flowers somewhat as they do. Bear in mind the essentials of the process, which are the same for every flower. Study the mechanism of each new flower and adapt your precise method to the needs of the individual case. It does not matter just how the pollen reaches the reception stigma, provided it does reach it. A very short course of practice will give you the knack of cross-pollenizing, and enable you to enter on a course of experiments that will lead to surprising, fascinating and perhaps far-reaching results-results which may prove to be of time wide and world wide significance.

-The ferns belong to a conservative family; and the penalty of conservatism, whether in plants or in human beings, has always been racial stasis.

This text is from: Luther Burbank: his methods and discoveries and their practical application. Volume 3 Chapter 3