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The Sugar Prune


"Probably you have heard the story of the General who declared it impossible to build a bridge across a certain stream that obstructed the march of his army until he had plans and specifications and blue prints for the work. While he waited for these-so the story goes-a subordinate built the bridge, and reported to his superior with the suggestion that it might be well to march the men across the bridge forthwith and then make the drawings at leisure afterwards. A visitor at my orchard told me this story, and applied it to the case of some of my newest varieties of plums. "It appears to me," he said, "that your custom resembles that of the young soldier who built the bridge without the plans and specifications. You appear to have developed a good many of your fruits on the same principle. You seem to have gone ahead and produced the fruit, while a more cautious experimenter would have been occupied in designing hybridizing methods and testing unit characters, and would not have been fully prepared to start on the actual constructive work until about the time you finished." Whatever the force of this comparison, it is true that I have often succeeded in producing a fruit of the finest quality by methods that to a less practiced experimenter might look haphazard; methods that did in point of fact lack something of the precision that an investigation conducted solely for purposes of scientific record rather than for practical results might have required. Such is the case with a large number of experiments in plum breeding. Here I have dealt with such vast numbers of individuals and brought into the hybridizing tests such varied and so many races, that accurate record of every step of a series of experiments extending over a term of years was quite out of the question. My "Combination" plum has a pedigree, could it be accurately traced, that includes strains of almost every race of plums under cultivation. From the seed of this strange hybrid you may produce trees that will bear fruit closely similar in all respects to at least a score of entirely different well known varieties or races of plums. The mixed pedigree of the product is recorded in this motley galaxy of offspring; but details as to all the parental crosses, tracing back along an experimental search of thirty years duration, are not to be had. The original parents used in the first cross are of course known; but successive generations deal with tens of thousands of seedlings. So it was impossible for anyone who was carrying out, as I have been, not less than three thousand different plant breeding experiments each year, involving in the aggregate not fewer than six thousand different species, to trace accurately, much less to record, each and every cross-fertilization among the myriad blossoms of my orchard. Yet a chance hybridization might by good fortune effect precisely the needed combination of qualities to give me a fruit that had eluded my most earnest efforts at systematic breeding. Very often, to be sure, I can judge from the result wham the racial strains most probably were that were blended to produce the new hybrid. But even this is not always possible, and not a few among the thousands of new varieties of plums that have originated in my orchard are of untraced and untraceable pedigree, at least as regards some of their strains. When I say that something like seven and a half million seedlings of the plum have passed under my hand and eye in the course of my many series of experiments in the perfection of this fruit, the reader will not wonder that there are gaps in the record.


On the other hand, it must be understood that there are almost numberless instances in which the hybridizing of different strains of plums has been effected by hand, in accordance with the most rigid scientific methods, and accurately recorded in my plan books. Indeed, this is true in almost all cases of the first cross through which a tendency to variation has been brought about. The first generation hybrids are usually very much alike, and inspection of them often gives no clues to the ultimate results to be expected. But in the next generation all the divergent characteristics of both racial strains fight for representation, and the diversity of forms produced may battle accurate description. Beyond this stage it is usually necessary for the practical breeder to turn over the task of cross-fertilization to the bees, contenting himself with keeping a sharp outlook for seedlings that show desired combinations of traits. How diversified these traits may be in case of a market fruit has been illustrated at some length in the preceding chapter. In this respect, most plums are at least as complex as the cherry, and the requirements in the case of the "perfect" prune are even more exacting. The word prune, it should perhaps be explained, is applied in California to any plum that can be dried with the stone in place without fermentation of the pulp. The quality that permits such drying is largely dependent on the amount of sugar that the fruit contains. There are prunes and prunes, as even the most unpracticed observer must know, and there are gradations of size, flavor, and sugar content that are vastly important from the standpoint of the orchardist and by no means without interest from the standpoint of the consumer. One of the tasks I early set myself was to produce a prune that should excel all others in the qualities, singly and combined, that make for perfection in this valuable fruit. I think I may fairly claim to have accomplished that end, although I shall not pretend that my ideal of a perfect prune has thus far been quite attained. I am not sure that I should be over-pleased if it had been; one does not really wish to reach the end of a trail, leaving nothing to strive for, no unknown territory to explore. It is a matter of more or less authentic record that the prune was originally introduced into California by a French sailor named Louis Pellier, who came to San Francisco in 1849 with the first horde of gold seekers.


Failing to make his fortune in the mines, this young man, in association with his brother who had presently joined him, established a nursery and conducted it with a certain measure of success until 1856 when one of the brothers returned to France to bring back a bride. He brought also some prune cuttings. And these, notwithstanding the long journey by way of the Isthmus, were still alive when California was reached. They were immediately grafted upon plum stock, with entire success. The most important of the varieties of prune thus introduced was the common French prune, sometimes known as the prune d'Agen. The descendants of this stock made up the large prune orchards of California for the ensuing half century. The French prune, while not without its good points, is by no means a perfect fruit. It is a cling-stone, which is a serious defect in a prune. Moreover, the stone itself is rather large in proportion to the flesh. The fruit ripens too late to be profitable in some parts of the country, and the risk of having the crop destroyed by the early rains is a serious defect everywhere. Neither is the tree a strong grower, or a very reliable producer, or of the most symmetrical growth. It occurred to me, therefore, when I first took the matter in hand, that among the essential qualifications of the ideal prune at which I must aim would be early ripening and the production of a larger, still sweeter free-stone fruit that would be borne in profusion.


We have had occasion to point out that the common orchard fruits do not breed true when grown from the seed. Explanations of this fact have been given, and fuller explanations will appear in subsequent chapters. Here it suffices to note that the prune is no exception to the rule. Very seldom does the seed of a prune tree produce a fruit that much resembles the prune. Usually the fruits are of all sizes, shapes and colors. They are sweet, sour, bitter, as the case may be. Some of them crack and others remain smooth. The trees on which they grow are many of them ill-shaped, weakly, or subject to disease. Although the parent form may have been an early ripener, the seedling may produce fruit that ripens so late as to be useless. All of which serves to give an inkling of the difficulties that beset the plant experimenter who sets out in pursuit of an ideal prune. Moreover, the variety of characteristics required to make up the ideal prune is far greater than the novice might suppose. It is a matter of course that the fruit should be large and well flavored-though not too large, lest it become too difficult to dry; and that it should be produced in abundance. But there are various equally essential points that the novice might overlook. There is, for example, the matter of quality of skin, determining the fitness of the fruit to undergo the lye bath which is an essential part of prune curing. It is necessary to dip the prunes in this bath, consisting of a solution of potash or lye, in order that the skin may crack in such a way as to permit the rapid evaporation essential to quick drying. But in a very large number of cases, prunes that have every other essential quality fail when subjected to this final test. It is not too much to say that I have developed hundreds of new varieties of prunes that were well nigh perfect as to quality, but which had no commercial value whatever because they failed to stand the acid-or to be literal the alkali-test. So the experimenter is always confronted with the possibility of failureat the very last, even when his efforts seem to have met with complete success at the earlier stages. With the utmost solicitude, therefore, he must watch the fruit as it passes through the potash bath. If the skin peels from the fruit instead of cracking, that particular variety is worthless, no matter what its other good qualities. Moreover, the cracks in the skin must be very small and numerous. If they are too far apart by the hundredth of an inch the prune will have a rough appearance that mars it from the commercial standpoint. If the skin is too thin, so that in gathering and handling the fruit is bruised, it can never make a commercial prune. But on the other hand, the skin must not be too thick as then it would not be properly cut by the lye. In a word, there must be the most nicely balanced qualities of the skin of the fruit, and without this final touch, the prune is a failure, even though it grows to seeming perfection on the tree. The intrinsic qualities, in addition to perfection of skin, that I aimed at from the outset, were large size, increased production of sugar, and early ripening. The matter of size is doubly important because this largely determines the price that a prune brings in the market. The sugar content is obviously important because upon this chiefly depends the drying quality of the fruit. And the matter of early ripening is at least as essential as any other quality, because the prune is dried in the sun, and the fruit that ripens late in the season not only often lacks sunshine to complete the process, but may be absolutely ruined by the rains which begin to fall in the early autumn.


When I began my quest of a perfect prune, in the year 1879, it at once occurred to me that something might be accomplished by hybridizing the French prune with another variety known as the English Pond's seedling but usually called in California the Hungarian prune. This was a large and handsome fruit, while the French prune brought to the combination the qualities of rich flavor and relatively high sugar production. If these diverse qualities could be combined in a single fruit, I saw that a great advance would be made. The little French prune was selected as the mother tree and many thousand blossoms were hand pollenated from the Hungarian. The offspring of this cross were as variable as had been expected, and among the seedlings were some that produced fruit of superior quality. Four years later, at the meeting of the California State Horticultural Society, I had the pleasure of exhibiting no fewer than seventy varieties of these crossbred seedlings. And in 1893 two new plums were introduced as representing the best selection among the almost myriad forms of the hybrid progeny. One of these new plums was named the Giant, the other the Splendor. The former is a handsome plum practically intermediate in qualities between the original parents. It has peculiar value as a shipping plum, and in particular it gained popularity with the canners because its skin has the property of rolling away from the fruit when placed in boiling water, leaving the rich, honey-colored flesh. But these, of course, are not the qualities desired in the prune. The other variety, named the Splendor, is about one-third larger than the common French prune and contains something like five percent more sugar; its quality and flavor are also superior. It has, moreover, the drying qualities of the prune, and it was freely predicted by many who knew it that it would soon completely displace its French progenitor. But unfortunately it had one single peculiarity that placed it at a disadvantage; namely, the propensity of the fruit to cling to the tree when ripe. It dries into a first class sweet prune, but it dries on the tree, and that is an insuperable defect, because the prune grower demands that the fruit shall fall naturally to the ground. He does not wish to be obliged to take the trouble even to shake the tree. So the unfortunate propensity of the new prune to hold to its moorings, so to speak, greatly marred its value.


In the year 1899, however, after almost twenty years of continuous and laborious effort, I was finally able to present a prune which met the expectations of the most sanguine; a prune which combined all the good qualities of its progenitors and combined them in superlative degree, and which, in addition, had the peculiarly desirable quality of ripening about the first of August, three or four weeks in advance of the usual period of the prune harvest. This almost perfect prune was placed on the market in 1899 under the name of the Sugar Prune. A description of the new fruit was given by Mr. B. M. LeLong, secretary of the California State Board of Horticulture, as follows: "The sugar prune is an extremely early prune, ripening August 1st; it grows superbly with yellow flesh, tender, and rich in sugar. The skin is very delicate, at first of a light purple tinted with green, changing at maturity to dark purple, covered with a thick white bloom. The form is ovoid, slightly flattened, measuring five by six and a half inches in circumference, average size fifteen to a pound, which is two or three times larger than the French prune; the fruit stock is short, and severs very easily from the stem as the fruit reaches maturity; the pit is of medium size, flattened, slightly wrinkled and most often separated from the flesh; the skin is so thin or porous that the fruit begins to shrink on the tree as soon as ripe." To add to the value of the sugar prune, the tree on which it grows is unusually vigorous and very productive. Analysis of the fresh fruit at the State University discloses the fact that it is nearly one-fourth sugar-the exact amount being 23.92 percent, contrasted with the 18.53 percent sugar content of the French prune, and the 15.33 percent of prunes in general. Not only does the sugar prune contain far more sugar than any of the varieties from which it sprang, but it fully equals the French prune in flavor, and it is two to three times as large. It is far more productive, and can be grown for one-third to one-half the cost of producing the French prune. In flavor it is fully equal to the celebrated Imperial, and, in most striking contrast to that fruit, it is exceedingly productive. Add that the new prune excels all other varieties in the extreme earliness of its time of fruiting, and it will be obvious that the sugar prune marks at least a long step towards the ideal at which I aimed. It ripens at a time when the weather is hot and dry, so that it can be rapidly cured. A month or so later when the other varieties are maturing, the weather is often foggy and cloudy and sometimes even rainy, so that fruit curing is carried on under difficulties and often with serious loss. It is not strange, then, that the sugar prune met with an immediate and enthusiastic welcome from many fruit growers, although of course there were regions in which a prejudice was shown against it, such as always meets any new product. In the markets of the East, the demand for the sugar prune was soon far in excess of the supply.


We have seen that the essential quality of the prune, and that which differentiates it from plums in general, is its inherent tendency to produce a large percentage of sugar. A great number of fruits share with the prune the capacity to manufacture sugar, but few other fruits have the power in such supreme degree. The manufacture of sugar by fruits is so familiar a phenomenon that we usually take it for granted and give it no thought. Yet a moment's consideration makes it clear that this capacity is one of the most extraordinary functions in the whole list of vital phenomena. Holding a ripe prune in my hand I am sometimes led to reflect that this is in many ways the most remarkable of chemical laboratories. Within the cellular structure of this fruit, a combination and metamorphosis of chemical products is brought about that the most skillful of human chemists is unable to duplicate. Every chlorophyll bearing plant, to be sure, possesses in greater or less measure the capacity to manufacture starch and to transform this substance into a soluble sugar. But the fact that this attribute is characteristic of plants in general, does not make it the less mysterious for the thoughtful observer. The chemist is able to analyze starch, and he tells us that it is a compound each molecule of which contains six atoms of carbon, ten atoms of hydrogen, and five of oxygen. But while he makes his analysis and determines the proportions of the component elements, he is careful to assure us that these elements are doubtless associated in very complex combinations of which his analysis gives him only a vague inkling. If we glance at the formula by which the chemist represents a molecule of starch-C6 H10 05 -the thought at once suggests itself that this seems to be a union of six atoms of carbon with five molecules of water; for of course we are all familiar with the formula H20 as representing water, however little we may know of the other niceties of chemistry. And in point of fact, this is about the way in which the chemist regards the matter. Starch is a compound of water and carbon. The plant secures the water from the soil and the carbon from the atmosphere, where it exists in the form of carbonic acid gas, which is given out constantly from the lungs of every living animal. With these simple and universally present materials, then, the wonderful chemist of the plant laboratory builds up the intricate substances that we term starch. This substance is stored away in the plant cells, not for the moment available for the purpose of nutrition, but constituting a reserve store of food material upon which the tissues of the plant can draw at need. Starch itself is insoluble in the juice of the plant, but to make it available whenever needed, it is only necessary for the plant chemist to add to the compound the constituents of a molecule of water, namely two atoms of hydrogen and one of oxygen, and the starch is transformed into a soluble sugar called glucose or levulose. This substance, dissolved in the juice of the plant, may then be transferred to the place where it is needed; which, in the case under consideration is the flesh of the fruit. The process of starch manufacture and of transformation of starch into sugar, with the final storing of the sweet product in the flesh of the prune, constitutes, as I have just suggested, one of the most marvellous manifestations of the power of vegetable cells. Indeed, it is precisely this capacity that differentiates vegetable tissues from all animal tissues whatever; for the biologists tell us that no living organism, high or low, save only the vegetable, is capable of manufacturing a single molecule of starch, much less a molecule of sugar out of inorganic materials. So a thoughtful person can scarcely fail to regard even so plebeian a thing as a prune with a certain measure of wonderment, almost of awe, if he allows himself to reflect on the mysterious processes that have taken place within its structure.


From the present standpoint, however, we are not so much concerned with the mysteries of plant chemistry as with the extremely practical fact that the new sugar prune developed in my orchard has the fixed habit of setting its sugar-making laboratory in operation several weeks earlier than had been the custom with the ancestral races of prunes. This interesting and important change of habit had been brought about, as the reader who has perused the earlier chapters will surmise, by a process of selecting, generation after generation, the individual prunes that manifested a tendency to early fruiting. But here as elsewhere we are confronted with the question as to how it was possible thus to change so markedly the habits of a plant within a few generations. The answer carries us back in imagination, along lines we have followed in studying other plant histories, to the remote ancestors of the sugar prune. We are led to reflect that the time of fruiting of a given plant is largely dependent upon the climate in which the plant habitually grows. Now there must have been ancestors of the prune that grew far to the north, for the plum is a hardy plant. Among some of the remote and now untraceable ancestral strains, there were doubtless some that produced their fruit at least as early as the first of August, perhaps even earlier. And although (when interbreeding occurred) the hereditary tendency to early fruiting had been made subordinate to the late-fruiting tendencies of other races of plums that had grown in milder climates, yet the potentialities of early fruiting were never altogether lost. Hence among the multitude of seedlings that were produced by my hybridizing experiments, this trait, along with a multitude of other submerged ancestral traits, was now able to make itself manifest. And it was my task, by a comparatively simple process of selection, to make sure that the character was preserved. The matter is perhaps made a little clearer if we reflect that in any race of domestic plants, there is a considerable range of variation as to size of fruit, abundance of bearing, and time of fruitage. Such variations represent, as we have pointed out, the varying traits of diverse strains of ancestors. But it must be observed that there are always some clearly defined limits beyond which variation does not readily go. Among all the thousands of types of prunes grown on the seedlings of my hybrid colony or on grafts on some receptive tree, there will be individual fruits varying, let us say, from one-half inch in length to perhaps two and a half inches-but by no chance will there be a fruit four inches in length. Similarly among my seedlings there will be some that ripen their fruit as early as the first of August, but none that ripen so early as the first of July. Fruits of other species may ripen far earlier; the cherry does so habitually. But the ancestors of the plum have lived under conditions that made it unnecessary for them to mature their fruit much before midsummer. So their range of habit in this regard, as recorded in the stored hereditary tendencies, was strictly limited. And the possibilities of variation among my hybrid seedlings are correspondingly limited, because, as I have hitherto pointed out, heredity is but the symbol of the sum of past environments, and the hereditary limitations of any common race of plants today are determined by the aggregate limitations of all their ancestors.


Such an analysis, in which the varying conditions that environ the different strains of a hybrid's ancestry are kept constantly in mind, serves to give us a clue to the observed tendency of families or strains of animals or plants to revert in successive generations toward a given mean or average. It has long been observed that, as a general rule, the offspring of human parents that are exceptionally tall tend to be shorter than their parents; whereas, contrariwise, the offspring of dwarfs tend to be taller than their parents. In studying races of animals and plants, biologists have discovered that this tendency, spoken of as tendency to revert to a mean, is universal. The matter has been especially studied in recent years by the Danish biologist, Professor W. L. Johannsen, of Copenhagen. His studies of barley and of kidney-beans show that any given race of these plants is really made up of a number of subordinate races, representing different strains of the ancestral pedigree, and that when the plants; are self-fertilized, the progeny tend to group themselves into a few more or less permanent types. There are limits of variation as to size, color and qualities but the progeny as a whole do not tend to have offspring that approach the half-way mark between these two extremes. Rather they break up into groups, each group tending to reproduce itself in such a way as to form a new subordinate race or "pure type." Thus from the same mixed stock sundry races of relative giants and of relative dwarfs, as well as numerous intermediate races, are formed. Now it would appear that such a case as that of the prune, in which we are able to work out by artificial selection a race characterized by tendency to early fruitage, is in keeping with these studies of the so-called "pure lines" of descent to which Professor Johannsen has given attention. But it must be understood that it is exceedingly difficult to carry the experiment in the case of the prune to the stage at which the type becomes absolutely fixed, for the reason that there are so many other qualities to be considered. This matter of varying qualities represented in the same seed we have discussed before, and we shall have occasion to refer to it again and again. Here it suffices to note that the case of the prune is akin to others that we have examined, for example the hybrid walnuts and the early cherries, in that the qualities for which we have bred are so numerous and so varied that they can be aggregated only in one seedling among many thousands, and could not be fixed without a long series of generations of additional breeding. Fortunately this is of no practical consequence, because the prune, like other orchard fruits, may best be propagated by grafting. From a single seedling we may thus develop, in due course, an entire orchard or a series of orchards. Such is in practice the method of propagating the sugar prune. It is obvious that plants thus grown partake of the very substance of the original seedling; they are part and parcel of it, and fruit grown from such grafts will be uniform in quality, within the limits of variation that characterize the individual specimens of any fixed race.

-When I say that something like seven and a half million seedlings of the plum have passed under my hand and eye in the course of my many series of experiments in the perfection of this fruit, the reader will not wonder that each individual cross has not been recorded.

This text is from: Luther Burbank: his methods and discoveries and their practical application. Volume 2 Chapter 8