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It not unfrequently happens that a visitor from the east expresses a particular desire to see a fresh prune. And when the fruit is shown the visitor usually expresses surprise at its appearance. "Why, that looks just like a big plum," said a young woman who was shown a specimen of my finest variety. "Taste it," I said. "It tastes exactly like a plum, too," she declared. "There is every reason why it should," I assured her; "for it is a plum. Not only so, but you have probably eaten any number of prunes in New York, even though you supposed that you had never seen a fresh one. The prune is an excellent table fruit and my best varieties are very good shippers. So a fair proportion of the best plums that are sold in the eastern market are really prunes. Yet, of course, they are called plums when sold to be eaten fresh. And this is proper enough, for every prune is a plum, even though every plum is not a prune by any manner of means." It is rather curious that this elementary bit of botanical information should not be more widely known. But my experience tells me that comparatively few persons living away from a prune growing district realize that the fruit with which they are so familiar in the dry state was neither more nor less than a plum before it was dried. In point of fact a prune might be spoken of as a plum educated-and educated in a particular way. In a sense all plums of the present day are educated. Each one has been brought, by selection, in the course of centuries to a point where it is a highly edible fruit. My famous quartet of developed plums, named in the preceding chapter, are assuredly educated in a high degree. Each of them is large in size, attractive in color, delicious in flavor, and of such firm quality of flesh as to bear shipping to distant markets. Yet no one of them has the particular kind of education that is absolutely essential for a prune. Neither Wickson nor Santa Rosa nor Formosa nor Beauty plums would have the slightest value as additions to the orchard of the prune grower. The smallest and the poorest prune in the orchard would be preferred. Yet the qualities that these educated plums lack are very few. Or, stated otherwise, the points of education that the prune has acquired, over and above other plums, are few. But they are absolutely essential. The qualities in question are simply these: A capacity to produce a large percentage of sugar and store it in the juices of the fruit; and, secondly, a capacity to produce a skin-covering having a peculiar quality of cracking in just the right way when the fruit is plunged into an alkali bath. Granted these qualities, any plum is a prune; lacking them, no plum is a prune of value. As to the varying degrees in which the qualities may be attained by different races of prunes, we shall have more to say in a moment.


In order to get a clear view of the matter, it will be well for us to make inquiry as to just how the prune came to take on the particular kind of education that now gives it distinction. By so doing we shall perhaps be enabled to understand better why it is that the prune finds it so easy to lapse back from the standards its forebears have established. If I have been engaged in a forty year long quest of a perfect prune, without quite attaining the ideal, it is chiefly because this fruit shows such a propensity to forget what it has learned and to revert to the standards of the ordinary plum. And the reason, stated in a word, is that the traits that now specifically characterize the prune have been acquired in comparatively recent generations; whereas the main characteristics that make the ordinary plum an edible fruit have been traditional in the family for untold centuries. When I find our almost perfect prune lapsing back in the next generation to a condition that robs it of all value as a prune, I am reminded of the story of a young Indian who was taken from his tribe and given every advantage that the Government could furnish him. Years were spent in teaching him the studies of the modern curriculum, mathematics, history, literature, language, and even a smattering of art. At twenty-one he had a better education than many of our presidents, and his future was considered very promising by those who had to do with his training. Ten years later this educated Indian was one of the most worthless of his tribe. He had simply "gone back to the blanket stage of existence." The pull of past heredities was too strong upon him. The transitory influence of a few years of education could not efface the racial instincts that had been implanted through thousands of generations of breeding of a more primitive sort. And so it is with the prunes. Through extreme specialization in recent times they have developed certain properties that were not of value to their ancestors, and, like the Indian, they are very ready to throw these off and revert to their blanket stage of existence. So when we hybridize a prune with some fine variety of plum, or even cross two varieties of prunes, in the hope of getting a larger and more productive prune, we very commonly secure a fine fruit-a fruit sometimes that is in many ways superior to either parent-but a fruit that is not a prune at all in the technical sense; a fruit, in short, lacking the refinements of large sugar content and peculiar quality of covering; being, therefore, a mere plum-in a word, a blanket Indian. And all this tends to show that we are right in assuming that the peculiar property of depositing a large quantity of sugar in the fruit is one that was not inherent with the ancestors of the prune until man undertook the education of the fruit and trained it for that particular purpose.


Nevertheless all that we know of heredity suggests that the effort on the part of man to develop such a trait as this would not have been successful had it not chanced that there were among the ancestors of the prune some races that possessed a tendency toward the peculiar property of producing very sweet fruit. There is nothing anomalous in that supposition, however, for it is well known that many tropical fruits tend to have a high sugar content. Such is the case, for example, with the date, the fig, and the pineapple. The orange, also, in some of its varieties, is a very sweet fruit, and there are numerous others among the fruits still confined to the tropics that show the same quality. Indeed, in general it may be said that fruits growing in the tropics tend to have a high sugar content, the reason being, perhaps, that in hot climates this is necessary to insure preservation of the fruit long enough to permit it to serve its purpose in protecting the seed during its growth and preparation for germination. But as fruits migrate to temperate zones, they tend to give up this habit of sugar production. All pulpy fruits, to be sure, develop a certain amount of sugar, but the percentage is relatively small with most fruits of temperate climates. The contrast in this regard between the average wild plum and such a fruit as the fig or the date is very striking. But we have seen illustrated over and over that a habit once ingrained in a race is with very great difficulty shaken off altogether, so it is not strange that, under exceptional circumstances or conditions of soil and climate, an individual plum tree might show reversion to the state of its tropical ancestor and produce a fruit much sweeter than other plums. Such an individual, if its fruit came to the attention of the orchardist, would be likely to be preserved and propagated; and in the course of time, through selection among the seedlings of this tree, a race of sweet plums would be developed. But it is only under conditions of artificial cultivation, in all probability, that such a race could be preserved. For, of course, the production of a large amount of sugar must draw on the energies of the tree, and if this increased sweetness of fruit did not prove beneficial to the tree itself, natural selection would presently weed it out. So, as I said, we may fairly assume that it is only within the comparatively recent period since the plum was under cultivation that the development of a race of sweet plums, which we now term prunes, has taken place.


As to the other characteristic prune trait, that of developing a skin of such texture that it will crack in precisely the right way when put into the alkali bath, this may fairly be assumed to be an even more recent acquisition. Yet here, again, we may assume that there were ancestors of the plum that developed characteristics of skin of which this is a reminiscence. And it is not very difficult to conceive how this may have come about. The wild plum quite commonly grows along the water courses and by lakesides. It may chance that plums growing along the shores of the Mediterranean, or perhaps by some inland body of salt water like the Dead Sea, were covered on occasion with salt spray from dashing waves or saturated with the brine when they fell to the earth. In such case, varieties that chanced to endure this treatment best would be the ones preserved, and in due course a race of plums having the right texture of skin to stand this treatment would be developed. This particular quality of skin would doubtless be subordinated when the plant migrated to regions away from the salt water and hybridized with other races. But here as before the latent trait would be preserved as a submersed factor in the germ plasm, ready when the occasion arose to make itself again manifest. But how, it may not unnaturally be inquired, would man himself discover the value of the alkali bath in preserving the prune? Granted that a prune had been evolved through artificial selection that had a sufficiently high sugar content to make it a drying prune, how chanced anyone to hit upon the particular method of drying that is now employed, an essential preliminary of which is the submersion of the fruit in the alkali bath? The question is doubly pertinent because even to this day in France the use of this method is by no means universal. In many cases the prune is still dried with the aid of artificial heat, the fumes and smoke of wood or charcoal taking the place of the alkali bath in giving the right quality to the skin and aiding in preservation. So we may assume that the simpler method of using an alkali bath is of very recent origin. Not unlikely the discovery was made altogether by accident. Many of us can recall that in our boyhood days it was customary in New England to make lye for use in the manufacture of soft soap by percolating water through barrels filled with wood ashes. The lye thus made is closely similar in composition to the fluid that is now used in preparing the prune. It seems a reasonable conjecture that the discovery of its value in this connection may have resulted from observation that plums which chanced to drop into a bucket of lye, when removed and thrown aside were more resistant to decay than other plums. Such a chance observation would have sufficed to give the clue to some ingenious person, and the value of lye as an aid in making the plum into a dried fruit would thus come to be understood. But whether or not this was the manner of discovery, the fact remains that the lye bath is an essential part of the process of curing the prune. Therefore the quality of skin that adapts the fruit to respond properly to this treatment is one of the absolute essentials that the fruit developer must have constantly in mind.


It may seem rather curious at first glance that a high sugar content should be essential to the preservation of the prune, when we reflect that sugar is a very fermentable substance. Everyone knows, for example, that starch is transformed into a form of sugar before it is fermented in the manufacture of alcohol. How, then, does the sugar in the prune prevent the fermentation of the fruit and insure its preservation? The answer is that sugar ferments only under influence of certain living micro-organisms, and that these micro-organisms cannot work in a too concentrated solution of sugar. There are myriads of the microbes spread broadcast everywhere on the wind, and of course they find lodgment in the skin of the prune as on every other exposed surface. But the alkali bath to which the prune is subjected, destroys these germs at the same time that it cracks the skin of the fruit. Other germs would find lodgment, however, and set up fermentation, were it not that the cracked skin permits a very rapid evaporation of the water content of the fruit. This quickly brings the sugar content to a degree of concentration that makes it a powerful antiseptic-that is to say a germicide that destroys any micro-organisms that enter it. But unless the prune has at least 15 per cent of sugar in its pulp, it will take too long to desiccate it sufficiently to give the sugar the right degree of concentration. And unless the conditions are very exceptional, even when the plum has a sugar content of more than 20 percent, it still will not dry rapidly enough to escape fermentation unless its skin cracks in just the right way. A difference of the hundredth of an inch in the average interval between the cracks may make all the difference between a satisfactory prune and a nearly useless one. Of course in the pure dry air of many regions of California, under a cloudless sky, a very sweet prune will often dry perfectly without the aid of the alkali bath; but it would not do for the prune raiser to depend upon these conditions as a general thing. He must control his prune, for he cannot control the weather.


It is obvious, then, that the plant developer must always bear in mind the two particular features of the fruit's education he has to contend with. But it is also understood that there are many other features that cannot be ignored. A prune tree, like any other plum tree, must be a good grower and a full annual yielder. The fruit must ripen early in the season while the days are long and warm. It must drop from the tree in exactly the right stage of ripeness that the orchardist may not be put to the trouble and expense of picking it. The fruit should have a small stone and if possible a free stone-overlooking for the moment the question of entire stonelessness which will doubtless be required of the prune of the future. Again, the trade demands a glossy black prune, for-owing, perhaps, to the fact that the French prunes, especially those cured in the smoke, are black-the average purchaser is prejudiced against the prune of lighter color even though it be of better quality. When we consider how many of these traits are different from those required in the ordinary plum, and hence have been developed in recent times under conditions of artificial selection, it will be obvious how largely the task of the prune developer must be carried out in opposition to the main stream of heredity; and it will not seem strange that forty years has proved none too long a time in which to develop the perfect prune. If I were to attempt to make a guess-it, of course, would be only that-as to the number of generations that have elapsed in the history of the prune since the qualities that chiefly characterize it were developed, my estimate would be something like this: The tendency of the fruit to drop promptly at the right time has been in vogue for perhaps only five or ten generations out of the thousands of generations since plums were brought under cultivation. The quality of producing sufficient sugar in the right form for drying may have been developed during perhaps the last twenty-five generations; but it has been brought to its present high percentage during the most recent half dozen generations. The condition of the skin which allows it to crack in just the right way has without doubt been cultivated for only a few generations. But on the other hand the fairly edible flesh, not having a high sugar content, has been the heritage of the plum for thousands of generations. So we can readily understand that the plant developer may secure among many thousands of seedlings, nearly all of them producing plums of fair quality, perhaps only one or two that show the qualities that specifically characterize the prune even in a minimum degree. The progenitors of the seedlings may have been prunes of fair quality; but the seedlings themselves have gone back to the blanket stage of plum development. The chances against securing even a single fruit that combines all the desired qualities among any given lot of seedlings are so small as to be almost disheartening. Indeed when the plant developer brings together two strains, each carrying its galaxies of more or less antagonistic characters, it is not altogether unlike scattering the letters of the alphabet in a whirlwind and expecting them to fall together in some chance eddy in such a way as to spell out some specified word.


I was not unmindful of the difficulties of the project, but nevertheless the obvious need of a better prune than California growers had been able to secure by importation appealed to me from the time of my first coming to the state; and when I undertook plant experimentation on a large scale, the development of the prune was one of the things that first engaged my attention. This work began about 1885, when I was growing seedlings of the European plum, Prunus domestica, from which practically all the prunes have been developed. I have told in an earlier chapter of the success that ultimately attained the effort, through the development of the sugar prune. Here I wish to tell a little more at length of some of the tentative efforts and partial successes that paved the way for the final realization of an ideal. As already told, I began experiments by hybridizing the French prune with the larger and handsomer but less sugary variety known as Pond's seedling, and in California often called the Hungarian prune. The little French prune was selected as the parent tree and many thousands of blossoms were pollenated from the Hungarian. This was in 1885. Four years later, at the meeting of the California State Horticultural Society, I had the pleasure of exhibiting fruit of seventy different varieties of these crossbreed seedlings. During the next winter a purchaser of the commercial part of my nurseries, being ignorant of the value of these crossbreed prunes, destroyed sixty or more of them. Fortunately, however, cions from several of the most promising had been grafted on older trees. Among these selected grafts were two that gave much promise. These were advertised in New Creations of 1893.


In 1895 one of the new prunes was introduced as the Giant. It was so well received that four years later it was placed on the lists of fruits recognized by the American Pomological Society. The Giant is a well balanced cross between its two parents the French prune (d'Agen) and the Hungarian. Fruits average 1 1/2 to 2 ounces each and are of a sweeter and finer texture than the Hungarian but not so firm and sugary as the prune d'Agen. The large size, handsome appearance and rare keeping qualities place this among the best canning, shipping, and market fruits; but, unfortunately, the Giant follows its pollen parent the Hungarian in having a low percentage of sugar; so it does not cure well as a prune. Here, then is a specific illustration of the tendency to revert to the characteristics of the plum and to give up the special qualities of the prune. The Giant is a valuable fruit, excellent for shipping and especially good for canning. When placed in boiling water the skin immediately rolls away from the fruit, leaving the rich honey-colored flesh ready for the can. The plum has made its way to distant territories, and is now grown extensively in Australia and New Zealand, being especially prized for canning purposes. In California it has proved a favorite and it is greatly superior to its staminate parent the Hungarian prune, especially for shipment. But it is sold as a plum and not as a prune.


Obviously, then, this was not the fruit I was seeking. But my experiments continued and after a few more generations of crossing and selection, I found among the seedlings one that produced a fruit in many respects more promising. This fruit was introduced in 1898 under the name of the Pearl prune. The Pearl prune originated as a seedling from the French prune. It is usually a little larger than its parent, but somewhat more flattened in form. The skin and flesh are pale amber and so translucent when ripe that the stone can be seen through them. It is really a delightful prune, of exceeding high flavor, delicious aroma, and melting flesh, surpassing even the true Green Gage plum. No prune excels it for attractive fragrance. When cured it produces one of the most delicious of prunes; but it requires care in handling, since it does not cure well in the open air. Its chief fault is that it is not very productive, although healthy and vigorous. It was sold to a New Zealand firm for introduction in the Southern Hemisphere in 1898. I myself introduced it in the Northern Hemisphere. The New Zealand nursery company recommends it for that country in a recent catalog as follows: "Pearl:-Raised by Luther Burbank. A seedling of the well-known French prune, which it surpasses in size of fruit. It is very handsome, flattened ovoid in form, white, semi-transparent, with a heavy bloom. In honeyed sweetness, combined with a peculiarly attractive fragrance and flavor it excels all other prunes or plums. It requires care in handling, and will not cure well in the open air. It is especially recommended for market and home use when fresh." The following quotation from "The Plums of New York," written in 1910, shows how this variety was regarded in New York at that time: "The variety now under notice is one to be pleased with if it came as a chance out of thousands; its rich, golden color, large size, fine form, melting flesh, and sweet, luscious flavor place it among the best dessert plums. In the mind of the writer and of those who have assisted in describing the varieties for 'The Plums of New York,' it is unsurpassed in quality by any other plum. The tree-characters, however, do not correspond in desirability with those of the fruits. The trees, while of medium size, and seemingly as vigorous and healthy as any, are unproductive here. In none of the several years they have been fruiting at this Station have they borne a large crop. If elsewhere this defect does not show, the variety becomes at once one of great value. "The fruits of Pearl are said to cure into delicious prunes-to be readily believed by one who has eaten the fresh fruits. This variety ought to be very generally tried by commercial plum growers and is recommended to all who grow fruit for pleasure."


Another prune that I developed somewhat earlier was named the Honey prune. This was one of my earlier seedlings and not a hybrid. It was of better quality and handsomer than the Green Gage, the standard of excellence at that time. The tree was not remarkably productive, but the variety has been welcomed as a home fruit in several localities of California. It was not considered worthy of general introduction but a few trees were sold to local growers who were interested in this variety and felt that it met the demands of their locality. A seedling of the prune d'Agen which I called Miller, was sold to Leonard Coates of Morgan Hill California, in November, 1898. This he introduced in 1908 as the "Improved French Prune." Later the name was changed to "Morganhill." The introduction of this prune as described by Mr. Coates himself furnishes an illustration of the length of time it usually takes for the public to become accustomed to a new fruit. In a letter Mr. Coates says: "We did not attempt a system of advertising in the start but rather tested it thoroughly for some ten years or so. It is very hard to introduce any new fruit as so many have been put on the market without real merits. Fruit growers, however, appreciate to a considerable extent, the value of selecting good varieties of fruit to propagate from. It seems that the chief introduction of pedigreed stock has taken place since our present nurseries were located and advertised on letter heads, etc., as specializers in pedigreed stock. The 'Miller' prune which we now call 'Morganhill' has been coming under the head of pedigreed prunes. We called it in the first description 'Improved French.' Very few people had enterprise to buy these trees at any increased figure and now we are propagating them at the same price as any kind of prune tree. About half the people seem to ask for pedigreed prunes and the others simply say 'French prunes.'" This, then, suggests a measure of success. It constituted at least a good beginning. Successes more unqualified were to follow; but the work just described was instrumental in laying the foundation for the later improvements-improvements that culminated in four prunes, one of which is already revolutionizing an entire industry, while the others have intrinsic values at least as great. An account of these perfected prunes will be given in the succeeding chapter.

This text is from: Luther Burbank: his methods and discoveries and their practical application. Volume 5 Chapter 3