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A good many years ago I had a talk with an official connected with the Department of Forestry, at Washington, in which I suggested that the problems of his department could best be met by the development of new types of forest trees. The official regarded the suggestion as grotesque. In common with nearly everyone else at that time he looked upon the tree as a fixed product of nature, quite beyond the possibilities of any change that man could direct. That was the time when Darwinism, although it had pretty fully established itself in the scientific world, was still on trial in the minds of the people in general. And even those who accepted the general truth of the Darwinian doctrine of evolution for the most part did not realize that evolution is a process that is going on about us today along the same lines that have characterized it in the past. To accept the doctrine of evolution at all required the overturning of the most fundamental ideas. After the conception had been grasped that in the past there had been eras of change and development, it was a long time before even the most imaginative scientist fully grasped the notion that our age also is a time of change and transition, and that the metamorphoses of plants and animals through which new forms have evolved in the past are being duplicated under our eyes in our own time. And in particular, as regards so massive and seemingly stable a structure as the tree, was it peculiarly difficult for botanists to conceive of flexibility and propensity to change, or to evolve, in the present time. It is true that no very keen eye was required to observe that trees differ among themselves within the same species, but it is also true that these divergencies always fall within certain limits and that on the whole they may be regarded as insignificant when weighed in the balance against numberless characteristics in regard to which the trees of a species seem practically identical. Take, for example, all the individuals that one could observe of, let us say, the common shagbark hickory, the variations of which were referred to in the preceding chapter. Attention was called to the fact that the hickories that I used to observe as a boy in the neighborhood of my New England home differed a good deal in size and form, and that the nuts that they bore were sometimes oval, sometimes rounded in form, sometimes rough, sometimes smooth, sometimes thick, and sometimes thin of shell, and equally diversified as to the quality of their meat. But of course I should be foremost to admit that all these diversities were in the aggregate of minor significance in comparison with the characteristics that even the most divergent of the hickories had in common each with all the rest. All of them were trees that attained a fair size as trees go. All have roots and trunks and branches of the same general form and aspect-as much alike, for example, as the bodies and arms and legs of human beings. All of them had leaves that could at once be distinguished as being leaves of the hickory and of no other tree. All had bark with the same characteristic whitish color and the same propensity to scale off in layers; and although the bark of some was much rougher than that of others, any fragment of bark of any hickory tree could readily enough be distinguished as characteristic of the species, and as not by any chance having grown on any other kind of tree. Then, too, if the hickory tree were felled and cut into fire wood, the texture and fiber of the wood itself enabled anyone who glanced at it to pronounce it hickory as definitely and with as much certitude as if he had seen the tree while living and in full leaf. No other wood had quite the same whiteness, quite the same strength and elasticity of fiber. The Indians had learned this in the old days, and had used the hickory of a preference always in making their bows. We boys, in our barbaric age, followed the Indians' example. We knew that a bow of hickory had shooting qualities that no other bow could hope to match. All in all, then, the hickory, despite the trivialities of variation which are mentioned in the preceding chapter, stands apart when we come to scrutinize it comprehensively, as a tree differing from all others and obviously entitled to stand as a unified and differentiated species. And what is true of the hickory is no less true of each and every species of tree in our forest. Each walnut and oak and beech and birch and pine and linden and locust has a thousand points of unison with every other member of its own species, could we analyze its characteristics in detail, for every conspicuous point of divergence. If we consider minutiae of detail as to size and exact form of leaf and all the rest, no two individuals are identical. But if, on the other hand, we take the broad view, it is clear that each recognized species stands out in a place apart, grouped with all the other members of its own kind, and somewhat isolated from all other species. Such being the obvious fact, it was perhaps not strange that the botanists and foresters of twenty-five years ago looked almost with suspicion on anyone who suggested that the different species of forest trees might be interbred and modified and used as material for building of new species that would better fulfill the conditions of re-forestration than any existing species. Even botanists who thought that they fully grasped the idea of Darwinian evolution looked askance at such a suggestion. It seemed to bid defiance to the laws of heredity, as they understood them. It appeared almost like an affront to Nature herself to suggest that her handiwork might thus be modified and improved.


And it may well be questioned whether this point of view would have been altered even to this day had it not been for a conspicuous and notable demonstration of the possibility of modifying existing species of trees. The demonstration was made when I took pollen from the flower of a Persian walnut and transferred it to the pistils of the California black walnut. Here were two species of trees so notably different in form and shape of leaf and fruit and color of wood that not even the most casual observer could confound them. They were not even natives of the same continent, and no botanist would claim that they were as closely related as are many species of forest trees that grow side by side in our woodlands and maintain unchallenged their specific identity. Yet when these two trees were cross-pollenized they produced fertile nuts, and trees of a new order grew from these fertile seeds. The barriers between these not very closely related species were broken down, and a new type of forest tree was produced that differed so markedly from either parent that no one could confound it with either, and that excelled both in the capacity for rapid growth so conspicuously as to seem to belong not merely to a different species but to an entirely different tribe of trees. The reader has already learned details of the history of this Paradox walnut, and we shall have something more to say of it in connection with a further interpretation of the laws of heredity, in a subsequent chapter. Here I refer to it only in connection with the demonstration it gave of the possibility that new types of forest trees might be developed by hybridization and selection, quite as had been claimed in the comment that aroused such skeptical and even sarcastic response from the professional forester. But, as I said, after this demonstration had been made, it was no longer possible even for the hidebound conservatist to deny the possibility that forest trees, like other plants, are somewhat plastic materials in the hands of the plant developer. And in course of time it came to be recognized-though even now the knowledge has scarcely been acted on-that the new idea given by observation of the Paradox walnut could be utilized for the practical purpose of supplying us timber trees that might be expected to re-stock our woodland in a fraction of the time that would be required for the growing of trees of unmodified wild species. The row of Paradox walnut trees which at fifteen years of age were two feet in diameter and towered as beautiful and symmetrical trees to the height of sixty feet, standing just across the street from their Persian parent, which, at thirty-two years of age was nine inches in diameter and perhaps forty feet high, afforded an object lesson that even the most skeptical could not ignore. "If new trees are needed to make forests to supply the place of those that your thoughtless forbears have destroyed," the trees seem to say, "why not call upon me and my fellows?" And to such a question there seems but one rational response. The Paradox hybrid and its fellows must be called upon to re-stock the ravaged timber lands of America. New hybrids must be produced by the union of varied species of pines, oaks, and elms, and other timber and ornamental trees, to give diversity to the landscape and to supply different types of wood for the uses of carpenter and cabinet-maker. The Paradox walnut stood there-and still stands-as the working model for a new order of mechanism-a timber tree that shall be able to re-forestrate a treeless region in half a human generation with a growth ready for the axe and saw of the lumberman.


In preparing this new material for the making of forest trees, it will be possible, no doubt, to bring trees from foreign lands, either for direct transplantation or as hybridizing agents. Thus, as we have seen, one of the parents of the Paradox walnut was a tree not indigenous to America. But we may recall also that another hybrid walnut, the Royai, which sprang from the union of two indigenous species, the black walnut of the Eastern United States and the black walnut of California, rivals the Paradox in its capacity for rapid and gigantic growth. So it is obvious that we are by no means reduced to the necessity of making requisition on foreign lands for material with which to develop our new races of quick-growing forest trees. But, on the other hand, the plant developer is always willing, like Moliere, to take his own where he finds it. So if foreign species can be found that will hybridize advantageously with our native species, they will of course be welcomed. The reader will recall that I have invoked the aid of numberless exotic fruit trees and vegetables and flower bearers in the course of my experiments in plant development. There is every reason to expect that equal advantage will result from the utilization of forest trees from, let us say, Siberia in one hemisphere and Australia in the other to blend with the strains of American species. In some cases it will be possible to bring the foreign species and acclimate them without hybridization. This has been done with several species of eucalyptus which have been brought to California from Australia and have proved a wonderful addition to the ranks of our ornamental and timber trees. Everyone who visits California marvels at the eucalyptus, and those of us who watch it year after year marvel equally, because this tree has capacity for growth that seems little less than magical. No other trees, perhaps, ever seen in America, with the exception of the hybrid walnuts, have such capacity to add to their stature and girth year by year as has the eucalyptus. Moreover the eucalyptus may be cut down for timber, its trunk severed only a few inches above the ground; and it will send forth shoots that dart into the air and transform themselves into new trunks, each seeming to strive to rival the old one. From the roots of the fallen giant spring a galaxy of new giants, and each new shoot assumes the proportions of a tree with almost unbelievable celerity. Add that the wood of the eucalyptus, notwithstanding its rapid growth, is of the very hardest, and the remarkable character of this importation from the Southern Hemisphere will be more clearly realized. Unfortunately the eucalyptus is sensitive to cold; otherwise it would at once offer a solution of the problem of re-forestration throughout the whole of the United States. Perhaps the eucalyptus may be made more hardy by hybridizing and selection. If not, we must take to heart the lessons it gives-in common with the hybrid walnuts-as to the possibility that a tree may show almost abnormal capacity for rapid growth and at the same time may produce lumber of the hardest texture. Hitherto it has generally been supposed that a tree of rapid growth would as a matter of course produce soft timber. The hybrid walnuts and the various eucalyptus trees serve to dispel that fallacy.


The one fault of the eucalyptus, its inability to stand extreme cold, is likely to be shared by other trees that are imported from the southern hemisphere or from sub-tropical regions of our own hemisphere. Although, as just suggested, it may be possible to overcome this fault through selective breeding, a long series of experiments will doubtless be necessary before this can be accomplished. In the meantime we shall be obliged to place chief dependence, in all probability, upon our native stock of trees, hybridized perhaps with allied species of Europe and northern Asia. But, even so, there is no dearth of material. America is richly stocked with forest trees. Moreover these represent, so the geological botanists assure us, a flora of very ancient origin which has shown its capacity to maintain itself through successive eras during which there have been tremendous climatic changes. It follows that our native forest trees have in their heredity the reminiscence of many and widely varying environments. And by the same token they have capacity for variation, and therefore afford exceptional opportunity for diversified development. It is not necessary here to analyze in great detail the qualities of the different groups of forest trees. A brief summary of the characteristics of a few of the more important groups will serve to suggest the abundance of native material, and to give at least an inkling as to what may be expected, in the light of what was revealed by the experiments with the walnuts, as to possibilities of development of the different tribes. Of course the great family of cone-bearers stands in the foreground, represented by many species, and known as the timber trees that give us the pine lumber which has everywhere been the chief material for the carpenter, and an important foundation material for the cabinet-maker. We have but to recall the giant sequoia and redwood of California, the largest trees existing anywhere in the world, to be made aware of the possibilities of growth that are present in the racial strains of the family of cone-bearers. And even if these giants shall be regarded as representatives of an antique order that has outlived its era, there remain numerous pines and firs and hemlocks of magnificent proportions to test the skill of the plant developer for their betterment. Moreover there is every probability that redwood and big tree may be crossed, and a variety produced that will be adapted to the new conditions, and which will outgrow all other trees. Nothing could be easier than to cross-pollenize members of this tribe, inasmuch as the pollen is produced in the utmost profusion, and the pistillate flowers are exposed when mature in the nascent cones awaiting fructification. That cross-fertilization occurs among the wild trees through the agency of the wind is a matter of course. Doubtless there are hybrid species of pines and their allies, everywhere often unrecognized or classified as good species. Quite large forests mostly composed of hybrid cypresses are found in California, and the oaks are known to hybridize frequently; also the eucalyptus trees of various species. If study were made of individual conifers in any forest region where different species are found, it would doubtless be possible to secure by mere selection new races that would admirably serve the purposes of the forester. But of course still better results may be expected when hand-pollenizing is carried out intelligently, and the racial strains of different species of conifers are blended and tested to find just what are the best combinations. It would be nothing strange if among the hybrids there should be found one or more varieties that will attempt to rival the Sequoia itself in giantism, and that will quite outrival it in rapidity of growth. What the pines are as producers of white and relatively soft wood of straight grain and uniform texture, the members of the great family of oaks are as producers of wood of hard texture, irregularly grained and knotted, but capable to taking on a polish and serving almost every essential purpose of the cabinet maker. The most famous of oaks, doubtless is the typical British species, but the American white oak is a close second. Perhaps these two might be hybridized. If the hybrid thus produced were by any chance to show the capacity for rapid growth that the hybrid walnuts have shown, while retaining the hardness of texture of its parents, as the hybrid walnuts do, the tree thus produced would by itself go far toward solving the problem of re-forestration. The oaks quite frequently hybridize in a state of nature. Granted a producer of soft white wood such as probably can be made by combining the white pine with some of its allies; a producer of hard cabinet wood such as a hybrid between the British oak and the American white oak would probably constitute; and the hybrid walnuts already in existence as producers of woods of the hardest and finest texture for cabinet purposes-granted further that the other new trees have the capacity for growth which the hybrid walnuts show-and a triumvirate of trees would be attained that could be depended on to go forth and gladden the devastated hillsides and valleys with trees that would jointly meet every need of carpenter and cabinet maker, adding incalculable billions to the wealth of our nation. And of course we need not by any means confine attention to these few most typical trees. There are beeches and chestnuts that are near relatives of the oak, each of which serves its own particular purpose as the provider of wood having unique quality. The beech, for example, is prized by the chair maker for his furniture, and by the turner for the making of carpenter tools and such like instruments. The chestnut makes railroad ties that are thought to have no equal and telegraph poles of requisite strength and straightness. Then there are other families that have their valued representatives. The hickories have already been referred to. The maples must not be overlooked, as they furnish highly prized white woods to the cabinet maker. The tulip tree supplies a light-colored wood used by cabinet maker and coach builder. The basswood or linden gives a wood of peculiar fiber that meets the needs of carvers and instrument makers. The willows and their allies; members of the birch family; the buttonwood tree or sycamore; and the locusts and their allies are other native trees that are of value as they stand and are well worth developing. The plant experimenter who works with these different trees, being guided by their botanical affinities, but making careful tests even where he doubts the possibility of hybridization, will be almost certain to have his efforts rewarded by the production of some trees of new varieties that will not only duplicate the unexpected qualities of the hybrid walnuts, but will doubtless also reveal unpredicted traits that will give them added value. Patience will be required in carrying out the work, for the tree is long-lived and experiments in its development are quite different from those in the development of annual plants. Yet something of the probable results of an experiment can be judged even from observation of seedlings in their first year. And by hurrying the hybrid plants by the method of grafting, it will be possible greatly to shorten the generation. Still it is not to be denied that the work of developing new races of trees is one that should preferably command the attention of the younger generation. In particular, it should be carried on under government supervision, as part of the great work of re-forestration, the necessity for which has only in recent years been clearly realized by those in authority or by the community in general.


The oft-cited hybrid walnuts supply us with tangible evidence of the possibility of developing new races of trees having much-to-be-desired qualities of rapid growth, through hybridization of the existing species. Such evidence, as I have suggested, doubtless is more forceful and convincing than any amount of theoretical argument. But it may be of interest to support this evidence, and in so doing to reveal additional reasons for belief that the same principles will apply to other forest trees, by recalling briefly the story of the vicissitudes through which the existing trees have passed and through which the diversified hereditary factors were implanted in their racial germ plasms. A knowledge of this story we owe to the geological botanists. They have sought diligently in the rocks for fossil remains, and by joint effort, searching all around the world, have been able to reproduce a picture of the main story of the evolution of existing forms of vegetable life. It is by recalling the story which they tell us, and thus alone, that we are enabled somewhat clearly to apprehend the possibilities of variation, and through variation of so-called new development-consisting essentially of the re-combination and intensification of old ancestral traits-that we have witnessed in the case of many tribes of plants in the course of our experiments. A brief resume of this story of plant life in the past, with particular reference to our own flora, will serve i.n the present connection to explain why there is every warrant for believing that each and every one of our forest trees contains submerged in its heredity the potentialities of a development of which its exterior appearance gives but faint suggestion. It appears that there is full warrant for the belief that the modern flora originated in the northern hemisphere, and probably in the region of the north polc During the so-called Mesozoic age, the conditions of the northern hemisphere were those that would nowadays be described as tropical or sub-tropical. There were palms growing in Europe, and such species as the sequoia, the plane trees, maples, and magnolias grew even at a relatively late period as far north as the seventieth degree of latitude. Remains of conifers have been found within nine degrees of the pole itself; remains of palms in Alaska coal measures, and of the sassafras along the western coast. At this early period the flora of the entire northern hemisphere was, as regards its trees, essentially comparable to the existing flora of America today. There were oaks and beeches scarcely distinguishable from existing species. There were birches and planes and willows closely related to the living species known as Salix camnbida. There were laurels not unlike their modern representatives, the sassafras and cinnamon tree, and myrtles and ivies that are represented by existing descendants of allied forms. And there were magnolias and tulip trees of which the existing tulip tree of the United States is an obviously direct and not very greatly modified descendant. All these trees grew far to the north, and luxuriated, as has been said, in a temperature that we of today would call subtropical, but which the inhabitants of that time, had there been a human population, would have described as arctic; for in that day it is probable that the north pole was tilted far toward the sun, and that the conditions that we now think of as tropical existed only in the region of the pole itself. Then there came the slow progressive period of refrigeration. The tropical climate of the pole was succeeded by an age of ice, and the successive ice sheets slowly pressed southward, driving the plants no less than the animals before them along all parallels of longitude, until the flowers and faunas that intermingled in the arctic region were scattered along diverging paths to people the continents separated by the wide stretches of the Atlantic and the Pacific oceans. It may seem strange to speak of plants fleeing before the ice sheet. But it must be understood that the plant is a migratory being, when considered as a race, notwithstanding the stationary habit of the individual. Plants put forth mobile seeds, and devise many strange ways of insuring their wide dissemination. They are always seeking new territories, and, granted proper conditions, always finding them. And it was only such plants as could migrate with relative celerity that were able to maintain existence and escape extermination by fleeing southward when the era of cold succeeded to the warm era in the arctic regions and when the arctic chill gradually spread southward and encompassed all the higher and middle latitudes of the northern hemisphere. The plants that chanced to flee southward along the land surface that we now term Europe found their further flight checked when they reached the stretches of mountains extending east and west that we now term the Alps. Here thousands of species made a last stand and ultimately perished. But the plants that were fortunate enough to choose the other avenues of escape, passing down across the land surface that we now term America and Asia, were not obstructed in their flight. Indeed, the long ranges of the Appalachians and Rockies and Sierras in particular served, as it were, to guide the line of march and aid the flight. So the American species made their way to the region of the gulf, and some of them even to the southern continent. And when the ice sheet finally receded, they were able to make their way northward again, though never to their former habitat; whereas Europe was treeless until the plant life of Asia spread westward to re-people it. Such is the explanation that the paleo-botanist gives us of the fact that the indigeneous vegetation of America today is closely similar to that which stocked the sub-arctic regions of the entire northern hemisphere in the geological period known as the Mesozoic-a period that seems infinitely remote when measured in terms of human history, yet which in the scale of time as measured by the geologist is relatively recent. Such trees as the sequoia, we are told, are survivors of that ancient regime that chanced to find hospitable shelter on the western slopes of the Sierras. Similarly the tulip tree of the east, with the blossoms that seem anomalous for a tree, should be regarded as the souvenir of a past age-a lone representative of vast tribes that once flourished in tropical luxuriance in regions that now give scant support to moss and lichen and stunted conifers. All in all, we are told, the remaining vegetation of today, varied though it seems, is but a scant reminiscence of that of the period preceding the ice ages. Only a few species, relatively speaking, were able to make their migration rapidly enough to escape destruction. These included a certain number, like the sequoia and the tulip tree, that were able to reach coigns of vantage that permitted them to exist without changing essentially from their sun-loving habit. But in the main the tribes that escaped destruction were those that developed a hardiness that enabled them to withstand extremes of temperature not far beyond the limits of the ice sheet. Others made their way northward again so soon as the ice sheet receded. And as the climate of ensuing ages, after the successive periods of intense refrigeration, everywhere retained, throughout the central and eastern portions of America, curious reminiscences of both the tropical and the arctic, the plants that finally repopulated the devastated territories were those that had learned, through the strange vicissitudes of their ancestors, to thrive where the thermometer in summer might rise to the one hundred degree mark, and where in winter the mercury might freeze. Such are the conditions under which pines and oaks and willows and beeches and black walnuts and allied trees exist to-day in the regions of northern America where they flourish. They can withstand the glare of a tropical sun in summer because their ancestors reveled in a tropical climate. And they can withstand equally the arctic cold of winter because their ancestors of other ages were forced to subsist under arctic conditions. The versatile tree that, thanks to the racial recollection of these vicissitudes, can adapt itself to the inhospitable conditions of our modern climate are but dwarfed representatives of ancient races of giants. To preserve life at all it was necessary for them to conserve their energies; and gigantic growth is feasible only for plants that can send their roots into rich, well watered soils and can likewise draw sustenance perennially from the atmosphere, unhampered by long periods of dormancy when life itself is threatened. But these dwarfed races carry in their germ plasm, submerged but not eliminated, factors for giant growth; factors for such development as would adapt them to life in the tropics; factors also for such development as would adapt them for life in the arctics. Their hereditary factors, in a word, are as varied as have been their past environments. So, as I said, what each tree is exteriorly gives us but faint suggestion of what it might be were its unrealized hereditary possibilities to be made actualities. So far as we know at present, the only way in which these unrealized possibilities may in any conspicuous measure be brought out is by hybridizing species that have so far diverged that they lie almost at the limits of afflinity. By such union of hereditary factors that have long been disunited, racial traits that are reminiscent of the old days when the northern hemisphere enjoyed a tropical climate may be revived, and a tendency to repeat a gigantic growth that characterizes ancestors vastly remote will be revealed. Such, I take it, is the explanation of the strange and otherwise inexplicable phenomena of gigantism manifested by my hybrid walnuts. And such is our warrant for believing that all other species of native trees have possibilities of development that are unrevealed in the exterior appearance of their present-day representative, and that can be revealed, so far as we know, only by hybridization.

-New hybrids must be produced by the union of varied species of pines, oaks and elms, and other shade and ornamental trees, to give diversity to the landscape and to supply different types of wood for use of carpenters and cabinet-makers.

This text is from: Luther Burbank: his methods and discoveries and their practical application. Volume 11 Chapter 6