PLUMS AND PRUNES WITHOUT STONES AND SEEDS
HOW ALL FRUITS MAY BECOME SEEDLESS
A number of years ago a distinguished pomologist who was not in the secret of my newest plant development, visited my place at Sebastopol in company with the eminent botanist Professor Hugo de Vries. Standing by one of the plum trees, de Vries asked his friend to cut through a plum and examine the stone. Then with obvious amusement he watched the pomologist work his knife carefully around the center of the plum-to avoid a stone that was not there. As he told of it afterward, he declared that even the boots of the pomologist indicated surprise when the knife cut at last through the center of the plum without meeting any obstruction. This was a case in which a man's surprise would be somewhat proportionate to his knowledge of botany and plant physiology. The more he had studied the subject, the better he would be able to appreciate what stonelessness in a plum really means. The more he had worked in plant development, the fuller would be his appreciation of the labor represented in the reproduction of this anomaly. And my visitor, being both a botanist and a practical plant experimenter, was certainly greatly surprised.
WHAT THE STONE MEANS TO THE FRUIT
The story of the development of the stoneless plum has been told in an earlier chapter. It will be recalled that I worked primarily with a small, partially stoneless plum that was found in France-a sour, acrid fruit of no interest except for its partial lack of seed-covering. I crossed this inedible fruit with a cultivated plum, and selected and re-crossed through successive generations, until I had segregated the characters of stonelessness and good quality of flesh and re-assembled them in a single individual. Further mention of the development of the stoneless prune, through crossing the stoneless plum with the French prune, with the ultimate production of the Conquest prune, was given in the preceding chapter. Here it is not necessary to repeat the details of the method through which the stoneless plums of various kinds, including the prune, were developed. It seems desirable, however, to examine at some length the relations that obtain between the stony seed-covering and the general and especial needs of the plant; and to correlate this type of seed-covering with other types of protective seed-covering that serve the same or a similar function in the case of other tribes of plants. When man takes a plant under his care, some of its many parts may become of little use, because of the changed conditions of the artificial environment. Thus the wild oat has a pointed, saw-like beard, which, turning and twisting under influence of moisture and heat, helps the seed to burrow into the earth. This is obviously useful to the plant in a state of nature. But it becomes a useless piece of baggage when the plant has been tamed and grown by man, for man will see that the seed is planted, in return for the crop it yields. The blackberry, domesticated, has no further use for the thorny armor that was originally developed to protect it from destruction by animals that would browse on its leaves and stems or trample it to death. In the same way the cactus, when taken under cultivation, can dispense with the spines that were so necessary a protection to it while it grew in the desert, where, in the old days, buffalo and antelope, and in more recent times cattle and horses would feed on its succulent slabs were they not carefully guarded. The apple, which armed itself with sharp thorns when in the wild state, has given up the thorns since it came into the orchard. Among other families of plants we find that protection has been secured by the development of acrid or astringent or poisonous properties, offensive odors, or imitative colors that serve no useful purpose except to safeguard the plant against its enemies. And such protective devices and mechanisms often become a burden when the plant is brought under the guardianship of man. Of a piece with these protective devices is the peculiar covering that the plums and their allies have developed about the seed that grows at the heart of their fleshy and succulent fruit. This stone is like an armor-plate covering that successfully protects the seed from the action of even the strongest jaws, or from almost any forces of nature to which it is likely to be subjected. Possibly one reason why the stone fruits have developed this unusual seed-covering is that each fruit of this family bears but a single seed. The many-seeded apple does not need to protect its seeds quite so jealously; but the plum, with its single seed, can afford to take no chances of the destruction of that seed. The case illustrates a familiar principle of nature. Everywhere it is observed that the more prodigal the supply of reproductive mechanisms, the less the seeming care with which they are guarded. Among forest trees that are fertilized by the action of the wind, pollen is produced and wasted by the ton. But in flowers pollenated by insects, relatively small quantities of pollen are produced, and its distribution is carefully prepared for by the auxiliaries of color and fragrance and nectar which guide the pollen-distributing insects. The mustard produces thousands of seeds for each plant, and it does not even take the trouble to imitate the grains of other plants, in size and form, as some of the seeds are obliged to do in order that they may be distributed with the grain when grown. The peach, on the other hand, produces but a single seed for each flower and fruit, and armors that seed with so strong a covering as to make it difficult for the germinating cells to make their exit when the time comes for their development. Thus these stone fruits conform to a great familiar principle of Nature. Their exceptional covering has been developed by natural selection to insure continuance of the species under natural conditions. But it is obvious that, now that man has taken the plant under his care, the species will be perpetuated with his aid, and hence the extraordinary armor about the seed might well be dispensed with. But as a matter of course the plant cannot drop all at once a structure that heredity and environment have worked thousands of years to build up. Man cannot take the Indian and say to him: "Be civilized," and expect him in a generation to drop the tendencies that have become a part of him through centuries of inheritance. The hunter cannot take the wolf and by treating him like a domesticated animal make a dog of him in a single generation-even though the ancestor of the dog was a wolf. And similarly when the fruit grower takes the plum under his protection, he cannot hope that this plant will give up at once the protective device that has served it so well in the long past. Heredity will have its say, and the seed armor will persist long after it has ceased to be of real utility.
THE STONE BECOMES AN INCUMBRANCE
And yet it is easy to see, that under conditions of artificial cultivation, the stone is not merely useless to the fruit; it is a positive incumbrance, in the first place, it puts a tax upon the vitality of the plant-makes a strong draft on its energies. A plant is a manufactory for transforming elements of the soil and of the air, under the influence of sunlight, into grains, fruits, gums, essential oils, and the like. Its capacity to produce any one of these is more or less complementary to its capacity to produce the others. When the cultivated plum produces a useless stone, it has worked to no purpose; and the energy that goes to build the stone might far better have been utilized, even from the standpoint of the plant itself, in the production of fruit. For the perpetuation of any given race of cultivated fruit plants now depend not upon the character of its seed-covering but upon the appeal made by the pulp of the fruit to the palate of man. So the stone not only destroys a part of the usefulness of the plum for man directly, by its presence in the fruit, but it is also indirectly harmful in that it hampers the vigor of the tree in the production of foliage and larger quantities of fruit. Yet when the plant improver attempts to remove the stone that has thus come to be an incumbrance to the plant, he is obliged, as it were, to swim upstream against the hereditary current of the ages. Ten, fifteen, twenty years-these are but moments of time when working against tendencies that are fixed by thousands of repetitions under conditions that remained unchanged for numberless generations, and until the immediate present. Bearing this in mind, we gain a more vivid impression of the difficulties that confront the plant developer who would endeavor to relieve the plum of its burdensome stone.
AID FROM NATURE
But here as elsewhere Nature will sometimes seem to forget for a moment the very fundamentals of her plan; and through such a lapse the hereditary mechanism of a given organism may be changed more radically, perhaps, in a single generation, than it could be changed by almost any number of generations of selective effort on the part of man. Such a lapse was made, we do not know just when, in the case of a minor variety of plum that chanced to grow in Central Europe. Through this momentary lapse in Nature's memory, this plant found itself with a seed for which the customary stony covering had been nearly half forgotten. Only about half remained of the shell that to plum seeds in general is as a veritable armor plate. The plant that suffered this strange mishap was, as the reader already knows, a little French bullace of small significance, known as the sans noyau. Of course we must not be supposed to imply that the relative importance of this particular member of the plum tribe had anything to do with its mishap. The laws of heredity apply quite as rigidly to the most insignificant as to the most important of plants. Indeed, it is scarcely within man's province to decide as to which plants are really insignificant and which important in the scheme of things. But at least it may be affirmed that, according to ordinary human standards, the little bullace was of most inferior type. Yet, paradoxically enough, it became, in virtue of its misfortune, the most important race of plums in the world. For without the aid of this seemingly malformed race, the plant developer would have had no leverage with which to attack the problem of relieving the great family of stone fruits of their now useless and even obnoxious seed-covering. The malformation of the little bullace, through which it lost its seed protector, would doubtless have resulted under conditions of natural selection in exterminating the species. But the same transformation which would thus have worked destruction in a state of nature, sufficed to make sure that, under the changed conditions of artificial selection, this particular plum should become the progenitor of all the plums of the future. For we can little doubt, now that the stone has been taken from a few varieties of cultivated plums and prunes, that all other varieties will ultimately be brought into the stoneless coalition. And the only feasible way to bring this about will be to interbreed one variety after another with the descendants of the little stoneless bullace. The plums of the future will be diversified in form and size and quality. They will draw their chief ancestral traits from the plums of Japan or China or Europe or America, or from a blending of these strains. But each and every one of them will have the little sans noyau for one of its ancestors, and will owe to that plebeian ancestor the quality of stonelessness which will be regarded as one of its best prized characteristics.
A RETROSPECTIVE GLANCE
In this view, then, the stoneless plum may be considered perhaps the most interesting of fruits. Possibly a future even more important than that just suggested may be in wait for it. It is at least within the possibilities, as hinted in our discussion of the peach, that the quality of stonelessness may be extended from the plums to the allied tribes of stone fruits by hybridization. Conceivably the descendants of the little bullace may include not only the races of cultivated plums but even all races of apricots and plumcots and cherries as well. But even though the view be confined to much narrower limits, it still remains true that the stoneless plum is among the most important of plant developments. So it may be worth while even at the risk of a certain amount of repetition to review the history of this development, and in particular to add a few details that have not hitherto been presented. It will be recalled that the little sans noyau, despite its name, was not altogether stoneless, inasmuch as each fruit had a rim of stone more than half way around the kernel; also that the fruit itself was only about the size of the ordinary cranberry, and was harsh, acrid, and unpalatable. Yet when this unpromising fruit was crossed with the French prune, and with numerous other plums and prunes, some of the crossbred seedlings produced fruit larger than the French prune, and nearly all of the hybrids were superior to the wild parent. All the seeds of these hybrids were carefully saved and planted. The seedlings were grafted on older trees, and a few seasons later still better ones were obtained; plants bearing larger fruits and many of them showing the tendency to abandon the stone. The first generation hybrid seedlings of this type, which were quite numerous, had mostly the French prune for the pistillate parent. A good many, however, were from the reciprocal cross. Of the latter, the crooked thorny seedlings which indicated that they were not crossed, or had reverted to the wild type, were generally destroyed even if they bore stoneless fruit. Those which showed the French prune or ordinary plum type were grafted into older trees to bear. All the seedlings from the cross of the sans noyau pollen upon the French prune were grafted and fruited even though many of them exhibited the thorny, dwarf, ill-shape of the wild parent. After the first generation the seeds of all were mixed, as there seemed no object in keeping them separate. For two or three generations there were all sorts of trees, the greater tendency being towards the bullace, which, being a wild type, would naturally be expected to have its characters more thoroughly fixed. In the first generation some plums were obtained fully twice as large as the fruit even of the cultivated parent. But most of these had stones, and were, moreover, soft, sour, undesirable fruits. All but a few of the more promising grafts were removed from the trees, and the experiment was continued with the selected ones. In the next generation there was some general improvement in the growth of the seedlings and the size and quality of the fruit. And in later generations the quality of the fruit rapidly improved-combined with stonelessness-until I obtained two or three fine plums and prunes. These were grafted extensively and seedlings raised and selected for still further improvement. Some of the earlier results of these experiments were exhibited at the Pan-American Exposition at Buffalo, New York, in 1901, and aroused much interest among fruit growers. None of these, however, was worthy of introduction as a commercial fruit. The plum called Miracle was the first of the stoneless plums to be introduced. This is borne on a rather slow-growing tree and has the size, flavor and appearance of a small Damson, being about an even balance between the French prune and the original sans noyau in most of its characters. Some years it is quite productive, but it is not an altogether dependable bearer. A representative of the Oregon Nursery Company, on a visit to my Sebastopol grounds in 1903, was greatly pleased with this variety, and at once purchased it. It has been advertised and grown quite extensively. Its flesh is of such quality as to be chiefly valuable for the making of jam. At that time it was the best stoneless plum in existence. But its chief merit was that it was the forerunner of a race of stoneless plums and prunes which will in time be grown wherever these fruits are raised.
THE STONELESS PRUNE
The next stoneless variety to be introduced was the prune named the Conquest, with which we have already made acquaintance. It will be recalled that this is one of the quartette of best prunes described in the preceding chapter. From three percent to six percent of the bulk of the French prune is stone. The specks of stone that remain in the Conquest do not constitute more than one-thousandth part of the fruit, which is thus edible practically without waste. The Conquest was offered in my catalog of 1911-12 with the following description: "There has been known for several hundred years a wild plum, an unproductive, thorny bush, which bore insignificant, acid, bitter, wild berrylike fruits with only half or two-thirds of a stone. Years ago it was hunted up in Europe with the plan in view of producing really valuable stoneless plums and prunes. The labor and expense incurred in these experiments have been enormous, but among the many thousand varieties, one really good stoneless prune was produced and is here offered for the first time in the history of this earth. "The tree is a vigorous, healthy, rapid grower and unusually productive. The fruit is very similar to its civilized parent, the common French prune, in form, size, color and golden, sweet, rich flesh. The stone has been eliminated wholly with the exception of a tiny speck. The fruit is so very valuable and the tree so very productive that I have consented to introduce it this season. It ripens with the common French prune and is in all respects very much like it in size, quality and appearance." The French prune is nearly oval but Conquest is slightly more flattened in form, like some of the German prunes.
FURTHER IMPROVEMENT IN PROSPECT
Among the later seedlings I found some very good fruits which have reverted to the stony type, one of them in particular being extremely large and of sweet, rich, superior quality. Thus, after several generations of plums without stones, those having ordinary stones again appear. There are others, however, that retain the stoneless condition, and are of exceptional size. Every color of the plum now appears in these stoneless hybrids-white, pale yellow, orange, scarlet, crimson, violet, deep blue, almost black, striped, spotted, variegated, and mottled in every way imaginable. They ripen from the middle of June until Thanksgiving, and while some varieties are no larger than a cranberry, others are larger than any other plum now generally cultivated, except perhaps the Climax, the Wickson, and Kelsey. After a time, no doubt, varieties may be produced with solid flesh throughout, as many seedlings now have indications of such a condition. The best stoneless plum thus far produced has a strong tendency towards this condition. I am often asked how the present plum with stones and seed will be replaced by the stoneless variety. Will the ordinary varieties be supplanted within a few years? There is no probability of that. It will be a long time before our present orchards are replaced by trees bearing stoneless fruit. Long years of selective breeding have been required to give the plum its good qualities. To hold to present standards of quality and make the fruit stoneless as well, will require a great amount of time, patience and effort. Of course, with modern methods it can be done in a much shorter time than in the past, but it must take a long time gradually to replace one and then another and another. The replacement of the ordinary plum by the stoneless plum will come about gradually, somewhat as the red potato was replaced by the white potato in California. Twenty-five years ago nothing but the red potato could be obtained in any of the markets of this state. Even my own brothers questioned whether the Burbank could make headway against it. Today the Early Rose and a few other varieties may be secured when the Burbank is out of season.
THE OUTLOOK FOR SEEDLESS FRUITS
It will be remembered that there have been seedless raisins grown for a century or more, yet everyone knows that seedless grapes are by no means universal. The well known Washington navel seedless orange has made a new world market for this fruit. Yet the bulk of the oranges in the markets of the world have seeds. There are good seedless lemons and limes; but they are very gradually finding their way into the markets. The change from stone to stoneless fruit will come about by imperceptible steps. The change will be so slow as hardly to be noticeable. Poorer varieties of all fruits are gradually replaced by the better; so gradually that the change is scarcely noticed. Odd forms are constantly coming up in nature-like the little, deformed bullace that was the parent of the new stoneless plums. Sometimes their inherent prospective value is recognized-oftener not. A hornless animal appeared as a sport or sudden variation in Argentina half a century or so ago. Possibly this freak may have appeared a hundred times before. But in this instance someone having imagination noticed the mutant and fostered it, and we now have hornless stock from that Argentine variation, not only of the original but of nearly all breeds. Among fruits, changes no less marked are constantly arising, and as time goes on these will be more and more recognized, and appreciated and used. As a greater knowledge of plant improvement is becoming disseminated, more pronounced changes for the better will be made-the elimination of stones and seeds being one of the most important of the many improvements required. The appearance of the stoneless plum, not as a chance sport but as the product of an arduous series of hybridizing experiments, may be taken as a sure augury that the conception of an age of stoneless fruits is not illusory-however long its coming may be delayed.
-Man cannot take the Indian and say to him: "Be civilized" and expect him in a generation to drop the tendencies that have become a part of him through centuries of inheritance.
This text is from: Luther Burbank: his methods and discoveries and their practical application. Volume 5 Chapter 5