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The Royal Walnut

SPEEDING THE GROWTH OF A LEISURELY TREE

If on visiting my grounds you were to notice two trees, one ten times as large as the other, growing side by side, you would perhaps be surprised to be told that the two are of the same age and grew from seed of the same parent. And it perhaps would not greatly clarify the matter in your mind to be told that these are varying individuals of a remarkable hybrid known as the Paradox Walnut. But probably your interest would be aroused in a tree that could show such diversity of progeny. The tree in question was developed more than twenty-five years ago. One of its parents was the native California black walnut tree; the other parent was the European tree usually called the English walnut, but with somewhat greater propriety spoken of as the Persian walnut. The European tree had been introduced in California a number of years before the time of my experiments, and it thrives in our climate and produces abundant fruitage. I had heard of a supposed natural European hybrid walnut, and I determined to make the experiment of fertilizing the flowers of the California species with pollen from the Persian. The experiment itself presented no particular difficulties and the results were of a striking character. The nuts that grew from the hybridized flowers were to all appearance unchanged. This, of course, is quite what might have been expected, for the influence of foreign pollen on the ovum of a plant manifests itself in the innate qualities of the seed, and not in the exterior qualities of the fruit immediately produced. But when the hybrid nuts were planted the following season, a part of the seedlings that sprang from them showed at once the effects of the intermingling of racial strains. As compared with seedlings of either the California or the Persian walnut, they manifested an enormously enhanced capacity for growth. Indeed they sprang forward at such a rate as presently to dwarf their pure breed relatives. The phenomenal growth of these hybrid trees continued year after year. The tree so far out-stripped all competitors in the matter of growth that it might fairly be said to represent a new type of vegetation. On this account, and in recognition of sundry other anomalies, I named them Paradox. At sixteen years of age these trees were sixty feet in height and as much in breadth of branches, the trunk being two feet in diameter at about four feet from the ground. Meantime English walnuts on the opposite side of the street averaged only eight or nine inches in diameter at thirty-two years of age, and had a spread of branches only about one-fourth that of the youthful Paradox. In addition to its quality of rapid growth, the Paradox has wide-spreading branches with a tendency to droop. It makes a beautiful shade tree. The leaves are of extraordinary length, sometimes measuring three feet, although usually only about half that. Another curious characteristic is that the foliage has a delicious apple-like fragrance, of which the foliage of the parent tree gives no suggestion. These anomalies of growth and foliage show the mingling of racial strains. A further result of this mingling is shown in the fact that the hybrid tree produces very few nuts. It is obvious that the two strains brought together are so variant that their progeny is made relatively sterile. The sterility is not absolute, however, for the few nuts produced germinate readily if planted. But another anomaly manifests itself in the characteristics of the seedlings thus produced; for these are the ones that show such extraordinary variation in size. In the same row, as already intimated, there will be bush-like walnuts from six to eighteen inches in height side by side with trees that have shot up to eighteen or twenty feet; all of the same age and grown from seeds gathered from a single tree. This rate of growth continues throughout life, and the fraternity of dwarfs and giants has been a puzzle to layman and botanist alike. These second generation hybrids vary as much also in regard to foliage and general characteristics of form and development as in size. Some resemble the California walnut, others the Persian ancestor, and there are scores of variations, the manner of growth of some of which-notably those that trail their limbs along the ground like a gourd or squash-bears scant resemblance to that of any walnut. From this extensive variation, it has been possible to select trees of even more rapid growth than the second generation hybrids, and the field seems to be open for the production, through selection in successive generations, of trees of still wider diversity of form and growth. Curiously enough the wood of the Paradox walnut is exceedingly hard, even harder and more close-grained than that of the ordinary black walnut. This is surprising in view of the rapid growth of the tree. Ordinarily trees that grow rapidly have soft wood, as every cabinet-maker knows. The Paradox further justifies its name by producing a wood that has great firmness of texture and is well adapted to take on a cabinet finish. All in all the production of the Paradox hybrid, and the development of a race of hard-wood trees of exceedingly rapid growth, constitutes a genuine triumph in tree culture. A tree that grows to the proportions of a handsome shade tree and furnishes material for the cabinet-maker in six or eight years, has very obvious economic importance.

THE ROYAL WALNUT

At about the same time when the Paradox was produced, I undertook another series of hybridizing experiments with walnuts that resulted in a tree scarcely less anomalous. These experiments consisted of the mating of the California walnut with the black walnut of the Eastern United States. The latter tree produces perhaps the finest cabinet wood grown in America, but it has almost disappeared from our eastern forests owing to the rapacity and lack of foresight of the lumberman. The California and eastern walnuts are rather closely related, yet the divergence is sufficient to give the hybrid a character markedly different from either parent. In some respects this hybrid, which was christened the "Royal," showed characteristics analogous to the Paradox. It had the same tendency to extraordinarily rapid growth, and in subsequent generations it showed the same tendency to produce a varied company of dwarf and of giant progeny. There was also a considerable variation in foliage, although not the extraordinary diversity shown by the second generation seedlings of the Paradox. In one important respect, however, the Royal hybrid differed fundamentally from the other. Instead of being relatively sterile, it showed the most extraordinary fecundity. The first generation hybrids probably produce more nuts than any other tree hitherto known. At sixteen years of age one of these trees produced a harvest of nuts that filled twenty apple boxes, each about two feet long by one foot in width and depth. In one year I sold more than a thousand dollars worth of nuts from a single tree. The nuts themselves are closely similar in appearance to those of the parent trees, but are individually larger. Unfortunately seedlings grown from the nuts cannot be depended upon to reproduce all the good qualities of their hybrid parents. Like most second generation hybrids, they tend to "throw back" to the divergent grandparental strains. To propagate the race extensively, therefore, it is advantageous to adopt the well-known method of grafting. It has been found that root stocks of the Royal hybrid furnish very valuable stocks on which to graft the English walnut in California. On most soils a tree grafted on this hybrid will produce several times as many nuts as a tree of corresponding size growing on its own roots. The trees are also much less subject to blight when they are thus grafted.

GRAFTING THE WALNUT

The importance of the new walnut and the fact that it may best be propagated by grafting makes it desirable to add a few details as to the method by which grafting is effected; for in the case of this tree the process is far more difficult than with ordinary fruit trees. Grafting the walhut is not, indeed, as difficult as grafting the pecan or the hickory, with both of which species the process was until recently found impossible of accomplishment. In this regard the walnut is rather to be likened to the fig, both being difficult to graft, yet not presenting insuperable difficulties for the skilled operator. Persons who first attempted to graft the walnut in California often failed four times out of five; and budding was even less successful. But the importance of the subject led to a careful study of methods, and today grafters who thoroughly understand their work are so successful that they scarcely have more than two or three failures in a hundred successful grafts. To attain such success, however, it is necessary to attend carefully to the various stages of the process. The grafting should not be attempted until quite late in the season; just after the buds begin to start is the most opportune moment Hard wood should in all cases be selected for grafting; the pithy tips are utterly worthless for this purpose. Some grafters claim that only about two cions should be used from the base of the last year's growth where the wood is very firm. Of course the principle of fitting the inside bark or cambium layer of stock and cion accurately together applies here as in the case of every other tree. Further details of the method will be given in a subsequent chapter, where the special methods of grafting and budding will be more fully examined. It suffices for the moment to emphasize the fact that these methods of propagation are as advantageous in the case of the walnuts, whether hybrid or of pure strains, as in the more familiar case of fruit trees. Of course the stocks on which to graft must be grown from nuts, and I have already pointed out that the seedlings are likely to show diversity. But all that is necessary is to plant the seeds rather thickly, and then to save the seedlings that show the best qualities.

STARTING A WALNUT ORCHARD

A practical method of producing a permanent and profitable orchard with a foundation to last for a century, is to plant some seeds of the Royal hybrid in groups of three or four at intervals of fifty feet each way. By the end of the first season the strong growers will have asserted themselves, and the others can be weeded out. There will almost surely be at least one good tree in the group. Failing that, there will be other groups in which there are extra seedlings of good quality that may be transplanted. The seedlings should be allowed to grow for four or five years, the ground about them being cultivated and may be used for crops of corn, potatoes, beans, or pumpkins, but preferably not sown with grain, lest the growth of the trees be checked. At the end of five or six years there should be a fine walnut orchard with trees having trunks three to six inches in diameter. Now the stock is ready for grafting. The stock branches selected for this purpose should not be over two or three inches in diameter. The cions grow rapidly and an orchard produced in this way surpasses all others. Its trees have a natural black walnut vigorous system of roots, with undisturbed tap root. A year's growth has been saved by not transplanting, and a start equivalent to the growth of several years has been gained by using the faster-growing hybrid. So the English walnut grafted on this stock becomes a producing tree at a very early age, and an orchard of English walnuts thus grafted is worth at least twice as much as one on its own roots. The tree thus grafted has not only the advantages mentioned, but it is more wide-spreading and therefore more productive than the original tree; and the spread of limb is duplicated by the root system, which thus ensures a good supply of nourishment and the capacity to produce large crops even in dry seasons. We have seen that the hybrid walnuts of both the Paradox and the Royal types have the peculiarity of producing trees of quick growth and gigantic stature in the first filial generation, and a mixture of dwarfs and giants in the second generation.

THE STRANGE TRAITS OF HYBRIDS

The tendency to surpass their parents in size is a characteristic that is very commonly manifested when plants of different species are hybridized. It is a familiar and now well-recognized fact that the crossing of diverse strains of living creatures, plant or animal, tends to result in what for lack of a better term is usually described as increased vitality. It would appear as if the conflict of new tendencies so stimulates the cellular activities as to give them an unwonted capacity for reproduction. In this case we are not concerned, as we were in some of the other hybridizing experiments already examined, with the prepotency or dominance of the qualities of one parent. Instead of this there is a distinct blending of characteristics so that the new product is in many respects intermediate between its parents in matters of foliage and fruit. But in growing capacity it far surpasses them both. Thus we have produced, as the offspring of the slow-growing English walnut and the not very rapidly growing California species, a tree that grows so rapidly as presently to tower far above either of its parents. As to form of leaf and fruit the hybrid may resemble one parent in one direction and the other parent in another. The leaf of the Paradox walnut, for instance, more closely resembles the leaf of the English parent. The outside appearance of the Paradox nut is also similar to that of the English walnut. But on breaking the shell we find that it is thick and strong like the shell of the American species, and the kernel is relatively small, quite different in form as well as in flavor from that of the English walnut. It cannot be said that any one has a very clear notion as to precisely what the changes are that give to a hybrid race this enhanced vitality. But this mystery is after all only part of the great all-pervading mystery of heredity, which in turn is merged with the mysteries of life processes in general.

WHY SOME ARE DWARFS

What I shall consider a little more at length here, however, is the conduct of the seedlings of the second generation grown from either the Royal or the Paradox hybrids. How does heredity explain the observed fact that some of these are dwarfs that can by no process of urging be made to attain anything like the average stature of walnuts in general, whereas others, sprung from nuts grown on the same stalks, are giants that surpass even their hybrid parent, not to mention their moderate-sized grandparents. The fact of this diversity is unquestionable. It affords a surprise to all who inspect the trees of this strangely diversified fraternity. But how explain it? A clue to the explanation is gained when we learn that the California walnut, which, it will be recalled, was a parent form in each of the hybrid strains, is a tree showing great variability in the matter of size when growing in a state of nature. In the northern and central parts of California it is usually a large spreading tree, often with gracefully drooping limbs. But farther to the south it becomes a mere shrub, and on the mountains and hills about Los Angeles it is only a bush. The nut diminishes in size correspondingly until, in Texas and Mexico, it is scarcely larger than a pea. When growing still further to the south, in New Mexico and Texas, the black walnut is sometimes classified as a different species. It appears to me, however, that these dwarfed southern forms are only varieties that have acquired different characteristics through the influence of what for them has proved an unfavorable environment. In any event there is no reason to doubt that the dwarf form and the relatively large one are descended from the same original stock, though doubtless divergence has gone on through numberless generations. Meantime the English or Persian walnuts the other parent of the Paradox, is also a variable tree. In its native home it is very small, and even the cultivated variety cannot be depended upon to reproduce a given racial strain when grown from the seed. It is obvious, then, that the tendency to dwarfness, which appears in such conspicuous fashion in some of our second generation hybrids, may be accounted for as reversion to dwarfed ancestral strains in both parents in the case of the Paradox and of one parent in the case of the Royal. The tendency to grow relatively large prevailed in the strains of walnuts that were used in my hybridizing experiments, and the prepotency or dominance of this tendency is clearly shown in the hybrids of the first filial generation. But the latent tendency to dwarfness, which in the Mendelian phraseology would be termed a recessive trait, is able to reassert itself in a certain number of the offspring of the second filial generation, causing these to "throw back" to their dwarfed ancestors in the fullest measure. The capacity for large growth has been absolutely left out of their individual make-up. In the Mendelian phrase they are pure recessives; or, using the more technical terminology, they are "homozygous" as to the hereditary factors or determiners of the unit character of dwarfness. The reader may or may not feel that the new terminology adds to our comprehension of the phenomena. But in either case the fact of the appearance of the dwarf specimens of the second generation among the hybrids is at least in a sense explained by our knowledge that there were dwarfs in their ancestry.

HOW DO WE ACCOUNT FOR THE GIANTS?

But while we are thus supplied with a more or less satisfactory explanation of the appearance of the dwarf hybrids, the colossal companions of the same generation are as yet unaccounted for. It is a familiar fact, as just pointed out, that hybrids of different species do tend to take on new capacities for growth. But what hereditary warrant have the upstarts for thus out-doing their parents? So far as we are aware, there is no record of a pure bred walnut of any of the three species involved that ever showed such capacity for rapid growth or such propensity to continue growing until it attains colossal proportions as the hybrids manifest. There is no recorded or observed ancestor to whom we can appeal in explanation of the development of these new races of giants. As yet we are not denied at least a hypothetical explanation that may perhaps account for the observed colossal growth of these new races of trees. The explanation demands that we go back in imagination through very long periods of time, and consider the ancestors of our walnuts not merely for hundreds of generations but for thousands or perhaps for millions of generations. It is necessary, in short, to trace backward the ancestral history of the walnut to those remote epochs when the primordial strain from which the present trees have developed grew in tropical regions, and, in common with tropical vegetation in general, doubtless acquired the habit of luxuriant development. It is permissible even that we should place in evidence the exuberant vegetation of that remote geological era known as the Carboniferous Age. In that time, as the records in the rocks abundantly prove, the conditions of climate now restricted to the tropics prevailed even in the temperate zones, and the vegetable life was characterized by the abundant production of colossal forms. In successive ages the climate changed, and it became necessary for the plants that were unable to maintain existence under the changed conditions to adapt themselves in size and in structure to a less bountiful supply of food-stuffs drawn from both soil and air; for the soil of the temperate zone is relatively arid, and the air probably became progressively less rich in carbon, owing to the permanent storage of vast quantities of this substance in what ultimately became the coal beds. So it came about that all the descendants of the colossal plants of the Carboniferous Era formed races that were dwarfs by comparison. Here and there a straggling species, like the California redwoods, preserved a reminiscence of its imposing heritage. But in general the trees that make up our forests in the temperate zone are but insignificant representatives of a lost race of giants. These, then, are the remote ancestors that may be invoked in explanation of the rapid growth and relatively gigantic stature of our hybrid walnuts. In this view the exceptional growth of these hybrids betokens reversion to remote ancestral strains that for countless generations have not been able to make their traits manifest, but which have always transmitted these potentialities as submerged and subordinated tendencies. The admixture of the divergent racial strains-one from Europe, the other from California, or in the case of the Royal, from origins separated by the breadth of a continent-sufficed to bring together factors of growth that for all these generations had been separated, and the atavistic phenomenon of a giant walnut came into being. Thus interpreted, the case of the big walnut is not dissimilar to the case of our white blackberries or to that of the fragrant calla. In each of these instances, as in that of numberless others that we shall have occasion to examine, a mixture of racial strains brings about a reversion to the structure or quality of a remote ancestor. In the case of the walnuts we have had occasion to go back a few thousand generations farther than in the other cases, but there is ample warrant for believing that nature sets no limit on the length of time throughout which a submerged character may be transmitted, with full possibilities of ultimate restoration. We shall have occasion to examine further evidence of the truth of this proposition, drawn from a quite different field, in a later chapter. Here, for the moment, we may be contented merely to place our colossal walnuts in evidence. Towering above their dwarf blood-sisters, they present a vivid object lesson in heredity that appeals directly to the senses and strangely stimulates the imagination.

-Nature sets no limit on the length of time throughout which a submerged character may be transmitted.


This text is from: Luther Burbank: his methods and discoveries and their practical application. Volume 2 Chapter 5