The Burbank Cherry
THE EXPLANATION OF A DOUBLE IMPROVEMENT
"How many assistants have you in your orchard?" a visitor asked me. And when I replied, "About a hundred thousand this morning, I fancy," my visitor looked quickly this way and that across my eighteen acre Sebastopol farm, and then seemed politely incredulous. "I don't see quite so many," he remarked. "In fact I can see but eight." "No," I said, "you don't see them; but you can hear them if you listen. They are mostly up there among the cherry blossoms. Notice how their wings hum as they go from flower to flower." "You mean the bees?" "Just so; the bees-they are my most important helpers at this season. I should get no cherry crop without them, and for that matter no plum crop, no apple crop, and very few flower seeds. In fact, most of us who grow fruit would soon go out of business, or reduce our farms from acres to square feet, if it were not for the bee helpers buzzing about from blossom to blossom." "But do you depend entirely upon the bees to pollenize your cherries?" my questioner continued. "Not altogether. I am obliged to do some pollenizing, particularly at the beginning of an experiment, to make sure of the exact cross that I desire. But after the experiment is under way, I for the most part leave the work to the bees. They operate, as you see, on a large scale, making a thousand pollenizing experiments where I could make one. And in the end the results of their work are highly satisfactory."
HOW POLLENIZATION IS EFFECTED
To illustrate the necessity for the aid of the insect helpers, I usually show the method by which cross pollenizing is effected when done by human hands. I select a blossom that is almost mature but has not opened, and cut it across with a very thin, sharp knife, taking the petals about half way down, thus amputating all the stamens, but leaving the pistil. Pollen which has previously been collected upon a watch crystal from some open flowers-is applied by lightly touching the finger to it, then to the stigma, taking care to cover the top of the stigma completely with the pollen. This is a simple enough procedure, but it must be done carefully, as the number of tests that one experimenter can manage is limited. Moreover, it is necessary, of course, in a case that calls for hand pollenizing, to mark the blossom with a tag of some sort, else there would be no record of the experiment, and no way of telling whether it finally proved successful. Again, it is usually desirable to remove other blossoms from the cluster in which the artificially pollenized one grows, to give a better opportunity for development of this individual. If, finally, we are to make absolutely certain that no other pollen comes in contact with the stigma, thus guarding against the possibility of fertilization of the flower by other pollen than that intended, it may be desirable to tie a paper bag over the flower. The latter procedure is not usually necessary, particularly if care has been taken to cover the stigma with pollen, as once this is done there is almost no danger that any foreign pollen will find lodgment. Moreover, the flower from which the petals have been cut, as just described, will not attract the bees, and would probably not be fertilized at all if our experimental pollenization should fail.
TIME THE LIMITING FACTOR
But even when restricted to the essentials, the process takes time; and although some thousands of hand-pollenations are done annually in my gardens and orchard, yet, as intimated, we try to leave the bulk of this work to the bees. Of course, these otherwise admirable helpers make no distinction between different varieties of blossoms, passing freely from one tree to another, regardless of the variety; but they usually confine their attentions on any given day to trees of a single species; that is to say, they do not ordinarily pass from cherry blossoms to the blossoms of the plum or almond, even if all are in season. They seem to prefer not to mix their sweets. So they do not distribute pollen to the wrong flowers as often as might be supposed. Where I wish to make pollenizing experiments on a larger scale, I sometimes place a branch of a cherry tree in full bloom among the branches of the tree of another variety, with which I wish to effect hybridization. The bees then transfer the pollen from the borrowed limb to the flowers on the surrounding branches, and a thoroughly satisfactory cross pollenation is often thus brought about. If a visitor who observes my cherry trees in the blossoming time chances to visit my orchards a little later, at the time of fruiting, he will probably be disposed to admit that my method of experiment has had very satisfactory results. For the cherries that grow on my trees are among the largest and most luscious, as well as the most abundant, that have ever been produced. The visitor will perhaps be surprised to find many scores of cherries quite different in appearance growing on the same tree. This, however, is the result of grafting. Seedlings grown from seed produced on a single tree may vary widely, but the immediate fruit of any individual tree is fairly uniform, unless the tree has been grafted. But trees on my farm always are grafted, so the phenomenon of divers varieties of fruit on the same tree is a familiar one.
AN UNSTABLE RACE
The cherry is at best a variable fruit. Like most orchard fruits, it cannot be grown dependably from seed. But, of course it is necessary in producing new varieties to work from seedlings, and from the standpoint of the experimenter who wishes to produce new varieties, it is fortunate that the tendency to vary exists. For, as our other experiments have taught, in the case of plants already described, it is only when a tendency to vary from a fixed racial type has been brought about by hybridization, or otherwise, that the material is furnished upon which the experimenter can build. In the case of the cherry, all the familiar varieties are the result of hybridizing experiments performed either consciously or unconsciously in the past. By working with the seed of any existing variety, one secures plants of numerous types that suggest different possibilities of development.
THE IDEAL CHERRY
In the course of my experiments, however, I have had occasion to bring together, through artificial pollenization, various standard varieties of the cherry, and, although I have not found it necessary to send to foreign countries, yet the stock with which I have worked represents races which have been developed in regions as widely separated as Russia, the eastern United States, California, and Japan. It has been my aim to combine the desirable qualities of different races of cherries from these widely separated regions, and the task here, as in so many other instances, has chiefly consisted in persistent selection among multitudes of seedlings of widely diverse types. The foundation stock with which I chiefly worked was the variety known as Early Purple Guigne, crossed with the Black Tartarian; but in subsequent crosses the qualities of Russian, French and American cherries and of numerous others were introduced, in an attempt to achieve the ideal cherry. A familiar but notable characteristic of the cherry, in which it differs markedly from most other fruits, is its habit of ripening at the very beginning of summer, while many of the small fruits are not yet in blossom. This characteristic gives the cherry peculiar commercial value, as it comes on the market at a time when there is a scarcity of fruits. It occurred to me many years ago that there would be a still greater advantage if a cherry could be produced that ripened several weeks earlier than any variety then on the market. So early ripening was one of the first ideals at which I aimed. With that object in view I naturally selected for my early hybridizing experiments specimens growing on trees that were observed to bear earlier, even if by only a few days, than surrounding trees. To come at once to the sequel of the story, I may say that I was able after many years of experimentation to produce a cherry that ripens about three weeks earlier than any variety-hitherto grown in California. This result was achieved by persistent selection, generation after generation, of specimens that manifested the early-fruiting propensity. But the full bearing of the story cannot be understood unless attention is given to the almost numberless complications that were involved.
SEEKING MANY ENDS AT ONCE
Had the only object sought been the production of a cherry that ripened very early, it would not have been very difficult to attain success. In that case all other qualities could have been disregarded, and attention given solely and exclusively to the question of time of fruitage. The cherries that ripen earliest each season being selected, I should presently have produced a race of early bearers, beyond peradventure. Selection carried through a comparatively small number of generations would have sufficed to give me what I sought. But a moment's reflection makes it clear that there would be no commercial value in a cherry that ripened earlier than its fellows, unless this cherry combined with the quality of early ripening other qualities of size and abundance and fitness for shipping, that give the cherry its value as a market fruit. It is obvious that in selecting my cherries it was constantly necessary to bear in mind not merely one quality but several qualities, and it requires no great knowledge of plant experimentation to see that this greatly complicated my problem.
DIVERSIFIED QUALITIES REQUIRED
In point of fact, the qualities that are required in a really satisfactory commercial fruit are much more diversified than the ordinary observer would ever suspect. In the case of the cherry there are at least a dozen quite distinct qualities, which might be spoken of as unit characters, that must constantly be borne in mind. A cherry that will bring a good price in the market must be large in size; it must be attractive in color; it must be sweet and savory to the taste; and it is of prime importance, particularly from the California standpoint, that the fruit shall be of such texture and quality of skin as to bear shipment across the continent, and so reach the Eastern market in good condition. As much as this will be obvious to every eater of cherries. But from the standpoint of the fruit grower, there are many other qualities that are no less important. It is necessary that the tree that bears the cherries shall be hardy and able to withstand the frosts; that it shall have the quality of vitality that makes it immune to the attacks of insects; that it shall have abundant foliage to protect the fruit from the sun; and that it shall be a prolific bearer no less than a bearer of fruit of marketable quality. All this, in addition to the quality of earliness of bearing to which reference has already been made. If we add that there are certain minor qualities, to be borne in mind, such as the question of length of stem, number of cherries to the cluster, and tendency of the fruit to cling to the stone in one case or leave it readily in another, an inkling will be gained of the complications of the problem in heredity that confronts the developer of an improved race of cherries. But the full significance of these complications can scarcely be appreciated wholly by any one who has not been confronted by them in actual practice. If I have been able to overcome them in a relatively brief number of years, it is because I have worked persistently, selected with discrimination, and invoked the aid of the bees in making experiments on a large scale. The modern student of heredity, in dealing with cases such as this, is able to give a somewhat tangible illustration of the difficulties involved with the aid of simple mathematics. He does this on the basis of the Mendelian interpretation of the method of transmission of unit characters of which we have learned something in an earlier chapter.
THE COMPLICATIONS ILLUSTRATED
It will be recalled that we had occasion to consider such opposing traits as blackness and whiteness in our white blackberry, large size and dwarf size in the case of our walnut trees, stone fruit versus stoneless fruit in cases of our plums, and perfume versus lack of perfume in cases of the calla, as pairs of unit characters that are mutually exclusive in case of any individual, but which both tend to recur in the second generation of hybrid offspring. It will be recalled, too, that a specific illustration of the formula according to which such recurrence takes place, was found in Professor Castle's experiments in crossing a black guinea pig with a white one; in which case, although all the offspring were black, the quality of whiteness reappeared in one-fourth of the descendants of the second filial generation. Now it should be observed that this ratio of one in four is a ratio that has been found to hold good in a very great number of experiments applied to various races of animals and plants, when a cross has been made and a record kept of the results with reference to a single pair of unit characters, such as blackness versus whiteness in the case of the guinea pigs. In such a case, where the offspring of the second filial generation are interbred, it has been clearly demonstrated, that on the average, one-fourth of the offspring of the second filial generation will resemble the paternal grandparent, and one-fourth the maternal grandparent; the remaining half being of mixed heredity. Stated otherwise, there is an even chance that in any group of four offspring of the second filial generation, one individual will resemble each grandparent as regards a given unit character. Applying this rule to the case of our cherres, and considering for the moment only the matter of early-bearing versus late-bearing, it should result, if these qualities constitute a pair of unit characters, that by crossing an early-fruiting cherry with a late-fruiting one, the descendants of the second generation would show one specimen in four growing early fruit, one in four growing late fruit, and two of intermediate tendencies. -All that would then be required would be to breed exclusively from the one-fourth that were early-bearers, destroying the three-fourths that lacked this quality or had it mixed with the undesirable quality.
NOT SO SIMPLE IN ACTUAL WORK
But, unfortunately, the simplicity of the formula vanishes as soon as we come to consider a second, and third, and fourth pair of unit characters. Here also the formula has been worked out in mathematical terms; and it appears that when several characters are involved, we at once come to deal with numbers that are no longer easy to keep track of. Moreover, the various pairs of unit characters may be juggled in an almost infinite variety of ways. We are seeking, for example, (1) an early-bearing cherry of (2) good size, (3) fine color, (4) sweet taste, and (5) good keeping quality. Suppose, for the sake of argument, we consider each of these to constitute, as contrasted with the opposite condition, one member of a pair of unit characters. Then it appears that, according to the theory of chances which underlies the interpretation of the Mendelian formula, the probability that any given combination of these five qualities will appear in an individual specimen of the progeny of the hybrid generation is only one in about five hundred. We shall have early-bearers that are of good size and taste, but lack shipping quality; other early-bearers that are good shippers but lack size or taste; yet other specimens that have size and taste and shipping quality, but lack the quality of early bearing; and so on throughout all the possible combinations of five pairs of qualities. But the combination of all the desired characters in a single individual will take place very rarely indeed. And when we advance from five pairs of unit characters to ten or twelve, as we have already seen that we must do in the case of our cherry, the matter becomes almost infinitely complex. As we increase the number of qualities under consideration, the number of possible combinations among them increases at an alarming geometrical ratio. It appears that whereas there is an even chance, when only a single pair of qualities was in question, of producing one offspring like each parent in each group of four; and whereas there is the same even chance of producing one offspring like each parent in every group of 256 individuals when four pairs of unit characters are in question-when we have to deal with ten pairs of unit characters the possible arrangements have become so bewildering and complex that there is even chance of producing a single offspring like each grandparent only in each group of more than a million progeny!
QUANTITY PRODUCTION NECESSARY
Such a computation, as made in accordance with the Mendelian formula, in itself serves to supply a ready answer to those Mendelians who have questioned the necessity of making experiments on the elaborate scale that I have all along followed out. According to strict Mendelian reasoning, it is clear that we must deal with thousands of seedlings in order to stand a chance of securing a single one that shows a desired combination of qualities, when six or eight qualities are in question-and I seldom work with less than twice this number in view. And the case is even more complex than this computation would show, because I am always concerned not merely to combine a half dozen or a dozen desirable qualities, but to have a wide range of choice among numerous individuals showing this combination, that one may be found which exhibits the desired qualities in the superlative degree. It is fair to assume, then, that I should never have secured the Burbank Cherry, and following it my newer varieties of cherries that: (1) fruit weeks before the usual cherry season, and (2) produce a superabundant crop of fruit of (3) the largest size, (4) best color, (5) firmest texture, and (6) finest quality; growing in (7) easily gathered clusters on (8) trees of fine shape that are (9) hardy and (10) immune to the attacks of insects, had I not extended my experiments far beyond the narrow limits of hand pollenation, with the aid of my hosts of indispensable helpers, the bees. So the biometric computations give fullest support to the practical methods that I have employed for the past forty years. Meantime, the results of my experiments-proving the possibility of segregation and reassembly of these diverse qualities-give vivid illustrations of the fundamental truth of the theory of unit characters, if these be properly interpreted.
GOOD FRUIT FROM BAD ANCESTORS
As a further illustration in point, note this curious circumstance: I have in various instances used as a parental stock, for purposes of hybridization, a cherry that produced a totally worthless fruit. The object of this selection was to introduce into a developing strain of cherries some good quality-say prolific bearing-that the otherwise worthless cherry showed in high degree. The immediate progeny of this cross would be of no value as the bad qualities of the worthless cherry were dominant. But among the remoter descendants I have been able to discover individuals that combine the quality of prolific bearing with the good qualities of the other parent stock, and in which the undesirable qualities of the original worthless ancestor were quite eliminated. It must be clear that this result could not have been brought about if the various pairs of qualities-large size versus small size, sweetness versus sourness, prolific versus shy bearing, and the like-had not been separated in the germ plasm of the hybrid in such a way that the unit characters could be sorted out and any good quality transmitted to the later generations, unimpaired by its contact with the opposing bad quality. In other words, had there been a blending of traits in the sense in which the older experimenters imagined the traits of hybrids to be blended, we should have had at best a cross in which the qualities of the worthless cherry were mingled with those of the valuable one; a race which, if somewhat better than its worthless ancestor, was somewhat worse than its valued one. And it would never have been possible to breed out altogether the undesirable qualities that the original cross had introduced.
SEPARATING THE TRAITS
But we have seen in the case of the cherries, as we had previously seen in the case of some other plants, and as we shall have occasion to see in numberless others in future, that it is possible to breed traits into a hybrid strain, and then breed them out again. In point of fact, no progress in the production of new varieties could have been made along the lines of my experiments, were it not for this possibility. My Shasta daisy, for example, is not intermediate in size between the species from which it sprang, but larger than any of them. My white blackberry is not intermediate in color between the parental strains, but is of a far purer white than its light colored ancestor. My stoneless plum is more stoneless than the race from which it sprang, although that race has been crossed again and again with strains of plums that invariably produce a stony seed covering. Some of my hybrid walnuts are far larger than either parent stock, and some are far smaller than either. And so on throughout the list of the hybridizing experiments through which the new races of plants have been developed at Santa Rosa. Everywhere we find evidence of the segregation of unit characters and their re-commingling and re-assortment in later generations. Nowhere else, probably, can there be found such an aggregate mass of testimony to the operation of this principle as will be supplied in the pages that tell of my various experiments in plant breeding. We shall have occasion to see that there are cases in which there is a blending of traits, and we shall find an explanation of such blending. But, as the cases already presented sufficiently illustrate, the carrying forward of characteristics unblended, and the possibility of their restoration after long submergence in new combinations, constitutes the underlying principle that makes possible the rapid development of new forms of plant life. And, reverting to the cases in hand, there is no better illustration of the truth of this proposition than that furnished by the new cherries which present in superlative measure, in a single individual, ten or a dozen clearly definable qualities that have been sorted out and brought together from the commingling of widely divergent ancestral strains. The traits that were developed through response to the environment in widely scattered geographical territories and through hundreds of generations, have been brought together, in combinations never hitherto presented; with the result that my early-bearing, large sized, bright colored, and highly flavored cherries constitute essentially a new variety of fruit, while at the same time they evidence with full force the all-compassing influence of the laws of heredity.
-According to strict Mendelian reasoning, it is clear that we must deal with thousands of seedlings in order to stand a chance of securing a single one that shows a desired combination of qualities, when six or eight qualities are in question - and I seldom work with less in view.
This text is from: Luther Burbank: his methods and discoveries and their practical application. Volume 2 Chapter 7