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The Stoneless Plum


I was showing some specimens of the remnants of stones in various specimens of my new plums to a visitor one day. I indicated a stone that was like the crescent of the new moon in shape. "This," I said, "is my plum as it was when the stone was only partially taken out of it. "And this"-indicating another one with only a fragment of stone not as large as a grain of wheat-"is the same plum four or five generations later." The visitor laughed. "That," said he, "reminds me of the museum that showed a skull labeled 'The skull of William Shakespeare,' and another labeled 'The skull of William Shakespeare when he was a boy.' There is this difference, however, that Shakespeare's head, according to the museum record, got larger as he advanced in age, whereas your plum stone became smaller." And then, becoming quite serious, my visitor inspected a series of fragmentary plum stones that had been placed before him, and added: "To make a stone grow smaller was certainly a notable feat. How did you manage it?" This is a question that has been asked more often, in connection with the stoneless plum, than in the case of almost any other of my plant productions. For a plum which looks on the outside precisely like any other, but which is found to be stoneless, never fails to excite surprise. Even visitors who know what to expect, when asked to bite through one of these specimens, can seldom refrain from exclamations of wonder when the teeth go right through the fruit as readily as they would through a strawberry. Many persons are not greatly interested in the daisy that combines four specific strains, because they know nothing of the difficulty of making such a union, and are quite unmoved by the spectacle of a white blackberry or a fragrant calla, because they have seen white fruits before, and because fragrant flowers are rather the rule than the exception. But no one ever saw an edible stone-fruit without a stone until one was produced here on my farm. So "How did you do it?" is the universal question of laymen and scientific botanists alike on seeing this really remarkable fruit. And when an attempt to answer the question is made, the story seems absurdly short and simple; yet to my mind it recalls reminiscences of what was perhaps the most strenuous series of experimental efforts that I ever undertook-a quest that occupied a considerable share of my time for a period of fifteen years, and which even now is not altogether completed. As you follow the outline of this story, please recall that while it takes but a phrase to tell of the pollenizing of two plum flowers and the production of one anomaly in the first generation and of some other anomaly in the second, in reality a period of five or six years has elapsed between the pollenizing experiment and the observation of the second generation results. When this is borne in mind and it is further recalled that breeding through many generations is necessary to secure the results desired, it will be clear that the production of a stoneless plum was an achievement that required its full share of patient waiting.


At an early stage of my almost endless series of experiments in the hybridizing of plums, I chanced to hear of a so-called seedless plum that was said to grow in France, where it had been known for a long time as a curiosity. About 1890 I sent to the Transom Freres Nurseries in France and secured twigs of this plum, which was known merely as the Sans Noyau. These were grafted on one of my plum trees, and in due course produced a crop of fruit, which as expected, proved to be a blue-black, cranberry-sized fruit, extremely sour, soft, and unfit for eating either raw or cooked. The original shrub, as I have been informed, and as it grew here, is a rambling, thorny bush rather than a tree, utterly worthless for any purpose except the one for which I desired it. The fruit, besides being flavorless and unpalatable, was scanty in yield. Moreover the fruit was by no means seedless, notwithstanding its French name. It was only partially stoneless, as most specimens produced fair-sized kernels in the fruit, and every kernel had a thick rim of stone around one side partially covering the kernel. While it therefore lacked much of exhibiting the condition of stonelessness that I had hoped to see, it did, nevertheless, show a tendency to abandon the stony covering that has always characterized all the fruits of the plum family. From the outset I was convinced that by proper hybridizing and selective breeding it could be made valuable. Next season the blossoms of the freak plum were fertilized with the pollen of the French prune and with that of numerous other plums and prunes. The seedlings from these crosses were grafted to ensure their earlier bearing. In the first generation I obtained some plums fully twice as large as their seed parent. Most of these had stones, however, and were soft, sour fruits. A very few of them were partially stoneless, and from these the work was continued.


The next generation showed some general improvement in the growth of the tree and the size and quality of the fruit. All the seedlings of the cross from the Sans Noyau upon the French prune were grafted and fruited, even though many of them showed the thorny, dwarfed, ill-shaped type of tree of the uncultivated ancestor. After two or three generations there was a marked tendency to improvement. In a large lot of seedlings, in 1904, I obtained two that seemed to me of favorable appearance-for much can be known from the quality of leaf and stem long before the time of fruiting. And when, two years later, the grafts thus selected bore fruit, it was delightful to find my predictions verified; the fruit was almost absolutely stoneless, only the faintest splinter of stone occasionally appearing. And combined with this stoneless condition there were qualities of size and flavor that made the fruit practically equal to the French prune. Moreover, as is often the case with hybrids one strain of which is wild stock, the new plum proved to be a very good bearer. So my ideal of an eatable plum having no stone about its seed was almost achieved. I say almost achieved because there still remained, in the case of the plums of best quality, a fragment of shell which varied from a small crescent about one side of the kernel to an almost invisible granule. There were some individual plants among the numberless seedlings that bore fruit in which the stone was absolutely eliminated and, in some cases, the seed also. But it proved extremely difficult to combine this quality of entire stonelessness with the desirable qualities of size and flavor, lacking which the fruit could have no practical value. Further hybridizing experiments, aimed at the production of an absolutely stoneless plum of fine flavor, are still under way; but in the meantime there are several varieties actually in hand that are of most admirable quality and yet stoneless. In the ordinary French prune, from three to six percent of the entire fruit is stone; while in my stoneless prune called the "Conquest" the fragment of stone does not represent more than a thousandth part of the bulk or weight of the fruit. And among the eight or ten hundred varieties of stoneless plums now growing in my orchard, there are sure to be some that will show still further improvement.


The task of producing a stoneless plum had proved very difficult chiefly because it had all along been necessary to bear in mind a number of quite different objective points. It was not sufficient to produce a stoneless plum. From the practical standpoint there would be no object in that unless the fruit about the stoneless kernel was of good size and of palatable quality. And, unfortunately, there appeared to be no tendency to correlate stonelessness with good quality of fruit. In point of fact the tendency was quite the other way; and, indeed, this was to be expected in view of the fact that the original partially stoneless plum was a small, acid fruit growing on a wild bush. The problem was to combine two lines of ancestry that were in many respects directly in conflict. It would have been impossible to do this had it not proved that stonelessness and good quality of fruit, although not originally combined, have the attributes of what may be called unit characters, and hence can be assembled in a single fruit in the later generations of a hybrid progeny.


A very natural question arises as to what had originally caused the little French "bullace"-as the Sans Noyau is sometimes called-to develop the extraordinary tendency to give up the stony seed-covering which no other member of the family had ever been known to renounce. The question is doubly significant when we recall that some sort of shell or stony covering is almost absolutely essential to the preservation of the seeds of plants in general. The shell is often very thin, as with the seeds of most garden plants. It may be reduced to a mere filament of cellulose, as in the case of a grain of wheat. With pulpy fruits it is usually a very significant covering, of which the seeds of the apple and orange afford typical examples. And with the great tribe of fruits represented by the plums, cherries, peaches, apricots, and almonds, this shell has been developed until it is veritably stone-like in texture. Just why this extraordinary development of the protective seed covering was necessary or advantageous in the case of this particular tribe of plants, it would perhaps be difficult to say. It is altogether probable that the original progenitor of the family of stone-fruits grew in central Asia. I have received from that region a shrub that may perhaps be regarded as the prototype of the entire race of the stone-fruits-not perhaps the direct progenitor, but an early offshoot from the ancestral stock which has remained in the original environment and has not, perhaps, very markedly changed from the original state during the hundreds of generations in which the other branches of the family were spreading southward and westward across Asia and Europe. If we could know just what the enemies of the primitive Asiatic stock of the stone-fruits were like, we could perhaps surmise the reason for the development of the unusual seed-cover. Perhaps the stone was necessary to protect the kernel from the teeth of monkeys or primitive men; perhaps it was more particularly needed as a protection against climatic conditions, to ensure preservation during semi-arctic winters; or to keep vitality in the kernel during protracted periods of drought, since, unlike most other fruits, the seeds will rarely germinate if fully dried. As to all this we can only surmise. But we may have full assurance that the thick, stone-like seed-cover served a useful purpose, else it would never have been developed and so persistently preserved in all the divergent races of stone-fruits that were evolved under the new conditions of southwestern Asia and southern Europe to which these fruits found their way. The roving tribes of Arabia developed a modified form of the fruit adapted for preservation by drying, and now termed the apricot. Other people consciously or unconsciously selected and developed the almond; and yet others the juicy and luscious peach; while the plum ran wild and put forth a galaxy of hardy offspring that made their way to the north of Europe and also, along some now obliterated channels, to the Western Hemisphere. But each and all of these de0scendants maintained, and some of them like the peach intensified and elaborated, the unique characteristic of a horn-or stone-like protective covering for the seed. And so, it becomes matter for wonderment that with all these uncounted generations of heredity clamoring for fruit with a stony covering there should have developed in France a member of the tribe, even though it be an inconspicuous outcast, that rebelled against the family tradition and dared to produce a seed that lacked a part of the habitual covering.


As to just how this break with tradition came about, we can perhaps make a better guess than we can as to the precise origin of the tradition. It seems likely that the little bullace lost the power to produce a protective stony covering for its seed through the impoverished condition due to some defect in the condition of the soil in which it chanced to grow. Unquestionably the production of the stone makes a strong draft upon the resources of the tree. Obviously the material to supply this dense horny structure must come from the soil, and in case the exact chemicals needed are supplied in scant quantity, the shrub might be forced to economize in producing a shell for its fruit kernel, just as a hen is forced to economize in the shell covering of her egg in case lime is lacking in her food. The same sort of economy is practiced when the human child finds inadequate nourishment. In such case the bones may be not only small but defective in mineral substance, a well-recognized type of abnormality resulting with which medical men are familiar. So it seems plausible that a paucity of proper food materials was the explanation of the origin of the original Sans Noyau. It is in keeping with this explanation that the Sans Noyau, is, as we have seen, a small scraggly shrub, a mere dwarf as compared with the average stature of trees of its family; and that its fruit is reduced to the proportions of a small berry, and is utterly lacking in those qualities of sweetness and flavor that are the almost universal characteristic of other stone-fruits. In a word, then, it is highly probable that the plum that supplied the character of stonelessness, upon which my experimental endeavors in the production of a marketable stoneless plum was founded, was a pathological product. I may add that many other "sports" or mutations in the vegetable world that have furnished a basis for the evolution of new races or species may very probably have had the same origin.


This explanation of the origin of the Sans Noyau makes it easier to understand the difficulties that attended the progress of this experiment. Had the little plum been absolutely stoneless-so that no factor whatever bespeaking a stony fruit remained as part of its heritage-there would probably have been no very great difficulty in producing through hybridization a stoneless fruit of good quality in the second or third generation. All experiments seem to show that the stone condition is, as might be expected, prepotent, or, in the Mendelian phrase, dominant. So in crossing an ordinary plum with a stoneless one, it was to be expected that the offspring of the first generation would bear stone-fruit. But the latent or recessive trait of stonelessness may be expected to reappear in a certain proportion of the offspring of the second generation; and the stoneless fruit thus produced may be expected to breed true. Such is what might be expected provided one were dealing with an absolutely stoneless plum as one of the progenitors. But unfortunately we are not dealing with an absolutely stoneless plum, but only with one in which the tendency to produce a stone has been minimized or partially suppressed. And so our relatively stoneless plum of the second generation still retains traces of the hereditary propensity to produce the stony covering; and, as we have seen, this propensity manifests itself in the fragmentary stone, sometimes reduced to a mere speck in size, that many of my stoneless plums exhibit. Nevertheless there remains not a doubt that from subsequent generations, from the stock in hand, an absolutely stoneless plum that retains all the valued qualities of the fruit and in all sizes, colors and flavors desired will be produced. That it has been possible to eliminate the stone altogether, advancing thus markedly in this regard upon the original partially stoneless form with which the experiment began, suggests the truth of a view now held by some prominent biologists, notably by Professor William E. Castle of Harvard, that a unit character may be modified in successive generations-not merely blended or made into a mosaic with other characters, but actually modified as to its potentialities. Professor Castle instances in support of this view the case of guinea pigs bred by him that developed a full-sized fourth toe on the hind foot from a rudimentary stump of a toe. The experiments just cited illustrate the opposite condition of causing a rudimentary organ in this case a plum stone-to be altogether eliminated. It should not be overlooked that both experiments are perhaps capable of interpretation in other terms. In each case what actually happens may perhaps be better explained as reversion to a very remote ancestor. Doubtless there were among the ancestors of the guinea pig races with four toes; and doubtless if we go far enough back we should find ancestors of the plum that produced a seed having no stony covering. And we are perhaps not far wrong in assuming that it was the long-subordinated influence of this vastly remote ancestor that, in the case of my plums, sided with me, so to speak, against the forces of the more recent heredity, and made possible the ultimate success of my hybridizing experiments.


We are so accustomed to putting up with the annoyance of the stone in the fruit that we for the most part never give it a thought. But a moment's reflection makes it clear that the plum stone serves man no useful purpose, while the inconvenience it gives us is obvious. It requires no argument to show that a solid fruit without a stone would be far more acceptable. But this is not the only reason, although perhaps a sufficient one, for the development of the stoneless fruit. The other reason looks to economy of production and saving of material from the standpoint of the tree itself. It has been estimated that a tree requires several times as much solid material and the expenditure of far more energy, to produce the stony covering of the fruit seed than to grow the flesh of the fruit itself. So it might well be expected that other things being equal, a tree bearing stoneless fruit would prove at least twice as productive as one bearing stone-fruit. Under the conditions of nature, this increased fruitage would by no means compensate for the loss of the protective stony covering, for the seed unprotected by its coat of mail would be at the mercy of any bird or animal or insect that attacked it. There would probably be no representative of the stone-fruit family in existence today were it not for the protection afforded the seed by its hard and indigestible covering. Regardless of animate foes, the seed would perish from the effect of sun, wind, rain, and frost, if denied protection. And this is by no means a mere matter of inference. One of the great difficulties that attended the experiments which I have just narrated, was the preservation of the stoneless seeds from one generation to another. It was found to be exceedingly difficult. Various insects, especially aphides, millipedes and eel-worms, would get among them and quickly destroy them. Fungous diseases also attacked them. And for several years more than three-fourths of the seeds kept for planting were thus lost. At a fairly early stage of the experiment I had large quantities of seeds in hand, for I was operating on an expansive scale in order to have wide opportunity for selection. Several hundred thousand plum seeds, all stoneless, were once placed in cold storage, at freezing temperature, as soon as they were gathered and cleaned. Some were placed in sterilized sawdust, and some in charcoal dust, and some in sand. Another assortment, similarly packed, was kept in boxes in a cool shady place until the first of January, when all were planted. In both lots, the seeds that had been kept in sand were in better condition than those preserved in the sterilized redwood sawdust. Those kept in charcoal differed little from the other lots. The ones in cold storage had suffered from blue mold more than the others, but both lots were in fair condition. All were planted on the same day in rows side by side. The seeds that had been kept in cold storage germinated at once, and in a week were all practically above ground. The seeds of the other lot, which had come from the same trees, did not commence to germinate for about six weeks. Yet later in the season very little difference could be seen between the two lots; on the whole the cold storage seeds showed rather the poorer growth.


An even better method of preserving the seed was presently developed, and I was finally able to preserve the stoneless seeds almost as securely as if they had their original protective covering. My new method consists in washing the stoneless seeds in clear fresh water when first removed from the fruit; immersing them for a few minutes in a week solution of "Bordeaux mixture" (sulphate of copper and lime-water), then rinsing for a brief period in fresh water, and placing them in damp sawdust that has been sterilized by boiling, care being taken that the sawdust is barely moist, not wet. The box containing the seeds is placed on the north side of a building, in a cool, shady place, and examination is made from time to time to see that the seeds do not become too dry or infested with insects or mold. If treated in this way, the seeds are practically all saved; they may be planted out of doors like other plum seeds, and they will germinate promptly. It is obvious that a seed requiring such careful treatment to preserve it all the winter would stand small chance of being able to perpetuate its kind in a state of nature. But on the other hand, it must be admitted that it is well worth while to give the amount of attention required to the preservation of these seeds, in view of the enhanced value of their product. It will be understood, however, that the average fruit-grower will not be required to concern himself about the seeds, as his orchards will be propagated by grafting in case of this fruit as is customary with all orchard fruits. There can be little doubt, then, that the time is almost at hand when all our plums will be grown without stones, since the experiment of removing the stones from a large number of varieties can now be followed up without great difficulty. The pioneer work has been done, and the cross breeding of my best present varieties of stoneless plums, to secure all the desirable qualities of any existing plum, may readily be effected. Even though the fruit should not be of better quality than that which it supplants, the fact that the elimination of the stone permits an increased abundance of fruit, to say nothing of the value of the stoneless fruit itself, will offer an inducement that the progressive fruit raiser will find conclusive. It should be added that the plum which has been induced to vary in the matter of seed production, is not always content merely to have cast out the stone but sometimes tends to eliminate the seed itself.


One of my stoneless plums has nothing but a jelly-like substance to take the place of the seed. It is probable that plums actually seedless as well as stoneless will prove favorites with some fruit growers. Of course plums that present this anomaly cannot be propagated from the seed. But in this regard they do not differ from a number of cultivated plants, including the potato, the horseradish, and the sugar-cane. And for that matter it must be recalled that very few orchard fruits are reproduced from the seed. The favorite varieties of apples and pears are so blended that they do not breed true from the seed. If you were to plant the seed of a Baldwin apple, a Bartlett pear, or a sugar prune, there is only the remotest chance that you would produce a seedling that would resemble the parent. Yet apples and pears and prunes are propagated year after year by means of buds and grafts. The same method of propagation would of course suffice for the seedless plum. It would still be possible, however, to produce new varieties of seedless plums by using the pollen of these varieties to fertilize the flowers of other plums that were stoneless but not seedless. The seedlings from such a cross would tend to vary in successive generations, as all hybrids do. A certain number of the offspring of the second and later generations would doubtless be seedless, and it would thus be possible to develop new varieties of seedless fruit from a parent stock that is itself incapable of producing viable seed. The stoneless hybrids already produced represent almost every color of the plum-white, pale yellow, orange, scarlet, crimson, violet, deep blue, almost black, striped, spotted, and variously mottled. They vary indefinitely in quality. Some of them are of abnormal size. They ripen from the middle of June until Thanksgiving. So the stoneless plum already constitutes a new race having numberless varieties, and the possibilities of further improvement are limitless.

-In producing stoneless fruits, we are simply helping plants to catch up with evolution.

This text is from: Luther Burbank: his methods and discoveries and their practical application. Volume 2 Chapter 4