DOUBLING THE PRODUCTIVENESS OF THE CHERRY
MORE AND BETTER CHERRIES
When I chance to see mention in the newspaper headings of the doings of New York's celebrated Four Hundred I am sometimes reminded of the Four Hundred of Sebastopol. The particular Sebastopol that I have in mind is the place where my fruit farm is located, about seven miles from Santa Rosa. By the Four Hundred of Sebastopol I mean a very aristocratic colony, comprising four hundred families of pedigreed cherries, that are colonized on a single big tree in my cherry orchard. I could speak only from vaguest hearsay as to the lineage of New York's aristocratic coterie, but may claim to discuss the pedigrees of the Four Hundred of Sebastopol with final authority. And I can vouch for the blueness of blood, so to speak, of every one of them. That there are about four hundred families in my patrician cherry colony is a matter of accident, quite uninfluenced by any thought of imitation. It chances that year by year the process of elimination about balances the process of addition to the family, and the census of the colony is not greatly altered. Reference has been made in various earlier chapters to the origin and development of the patrician cherries. They are closely related as to their remote ancestry, as I suppose is the case with the members of every other aristocracy. Yet, as we have seen, the ancestral traits are variously blended in the different families, and there is notable diversity among them as to individual traits. Some of them bear fruit that is vividly red in color, others fruit that is pallid; and there are corresponding divergences as to flavor, freedom of stone, sugar content, and all the rest of the complex characteristics of a well-bred cherry. Of course these qualities are variously recombined in the progeny of each new generation. So I can never tell what surprise is in store for me when I raise seedlings from the fruit. And there are always new additions to the colony that will only come into bearing next season or the season after and reveal what they hold in store. Thus it chanced that in the season of 1908 I found among the cherries one that bore quite the largest fruit I have ever seen; fruit, moreover, of the most inviting color and having qualities of flesh to match. Cions from this new stock will be sent out and will in due course colonize many an orchard with a new variety of fruit that is sure to find great favor. But if I thus from time to time have pleasant surprises, I am also too often chagrined to find among my patrician cherries offspring that seem unworthy. But of course one hears of black sheep among the scions of even the noblest families, so it is not surprising that the blueblood cherries of Sebastopol offer no exception. And as the black member of any human family is always held up as a warning example, I have thought that I might in the same way make the black sheep of my cherry colony serve a useful purpose by explaining somewhat in detail the reason for their appearance. In so doing I shall be able, perhaps, to make a somewhat clearer exposition than has hitherto been attempted of certain aspects of heredity that are peculiarly important from the standpoint of the practical plant developer.
UPPER CASE QUALITIES
We have learned something in earlier chapters about unit characters and the way in which they are blended or mosaiced together to make up the personality of any individual plant. It will be recalled that where the two parents of a given individual have opposing qualities as regards a given characteristic-where one, let us say, is black and the other white-it is quite the rule for one quality to dominate the other in such a way that the offspring precisely resembles, as regards that quality, the dominant parent-in this case the black one-and resembles the other parent seemingly not at all. And we have learned also that the latent or recessive character that is thus subordinated-in this case whiteness-will reappear in a certain proportion of the offspring of the succeeding generation. Now, it has been found convenient by recent experimenters to adopt a graphic method that will make the printed accounts of their experiments more readily comprehensible. The expedient in question is the simple one of using a capital letter to designate the dominant factor of any pair of unit characters, and a corresponding lower case or small letter to designate the recessive factor. Letting "D," for example, stand for the dominant trait of blackness in the illustration just given, and "d" for the recessive trait of whiteness, we may concisely state the facts of inheritance as just noted in the following formula: Parent "D" being mated with parent "d," the offspring, whether few or many, bear in each individual case in their germ plasm the factors "D" and "d" in combination. But if two of these offspring are interbred, there will be a splitting up of the factors and re-combination in such wise that in any average group of four of their progeny the result will be this: One member that is pure dominant (DD), two members that are mixed dominants (Dd), and one member that is pure recessive (dd). The DD individual is "homozygous" for dominant factors and will breed true to blackness. The dd individual is homozygous for the recessive factors and will breed true to whiteness. The two Dd individuals are heterozygous for the color factors, and whereas they are individually black their offspring will repeat the formula 1 DD + 2 Dd + 1 dd; they will reproduce, in other words, the conditions of the second filial generation itself as just analyzed. Let me re-state all this, using only the letters, to show the convenience of the formula and at the same time to fix it in memory: D mated with d in the first generation gives us Dd + Dd + Dd, etc., in the second generation. Dd mated with Dd gives us in the third generation 1DD + 2Dd + ldd. If this is not absolutely clear, you will do well to re-read the above paragraphs, and it is quite worth your while to consider the matter somewhat attentively. If you have only theoretical interest in plant breeding you should be concerned in the matter no less personally, because the same laws of heredity that are about to be illustrated apply with full force to all life, including human offspring. If, on the other hand, you have thought of undertaking some experiments in plant developing, which I hope is the case, it is doubly important that you should get the full significance of these simple formulae. Like other formulae, they are devised solely for convenience in promulgating ideas. As used in the following illustration, they will make it possible to present vividly the case of our black-sheep cherry, and through this to clarify a large number of obscure cases that must prove very puzzling to the novitiate in plant development.
EXPLAINING THE BLACK SHEEP
Let us now stake our way, as it were, with the aid of the upper-case and lower-case letters, along the line of a series of plant experiments through which a certain Patrician cherry was developed. To avoid complications and to escape getting into a tangle of ideas and a maze of letters, let us consider only a single quality in detail, keeping in the background of our minds the idea that the actual experimenter is at all times considering almost innumerable other qualities as well. The one quality that we will consider at the moment is, let us say, the matter of size. We wish, for some special purpose, to develop a cherry that shall be a giant among cherries, yet which of course shall combine size with quality. Now we have at hand a cherry that bears very large fruit of poor quality. We have also at hand a tree that bears small fruit of delicious quality. Our first step will be to transfer pollen from the stamens of one of these to the pistils of the other. We carefully mark the limbs bearing the hybridized flowers; and subsequently we gather the fruit and save the seed and in due course plant it and nurture the seedlings by methods hitherto fully explained. So when a year and a half has passed from the inauguration of our experiment we have a row of hybrid seedlings ready for grafting. The one thought that is uppermost in our mind, for purposes of the present exposition, is that of securing a plant that will bear fruit of large size. Now we have learned that there are certain correlations of parts that will enable the plant experimenter to predict, from the quality of the seedling, a good many things about the quality of the fruit it will subsequently bear. Utilizing this knowledge, we pass along the row of seedlings and select from among the thousand or five thousand individuals the ten or twelve that seem to us to give greatest promise. Nor at this particular stage of the development is the selection very difficult, for the first generation hybrids usually show no very great tendency to variation. That tendency is revealed in subsequent generations, as we have seen. In point of fact, as a moment's reflection will tell us, the seedlings before us are really all of one quality as regards the particular characteristic of their innate tendency to bear large or small fruit. One of their parents bore large fruit; the other bore small fruit. If, then, we assume that here, as in many other cases of plant breeding, the quality of largeness is dominant to the quality of smallness, it may be expected that all the hybrids of the first generation will tend to bear large fruit. If, introducing our convenient system of symbols, we designate the dominant quality of bigness with the letter B, and the recessive quality of smallness with b, we may designate the members of the hybrid generation as all being mixed dominants, each bearing the factors Bb. This means that the factor B dominates the factor b, and that the individuals in question will all bear large fruit. So we may expect (on this assumption), having grafted our selected seedlings, that each of them will show, two or three years hence, fruit of large size. But of course the other qualities of this fruit will not be all that we could desire, so it will be necessary to continue the experiment. Suppose we do this by cross-pollenizing different members of the same group. We shall thus mate Bb with Bb. And the result of this mating, as we know, will be to produce, in each group of four, one BB individual, two Bb individuals, and one bb individual. Being interpreted in terms of our actual row of seedlings, as they stand in our orchard in this, the fourth or fifth year of our experiment, this means that in every lot of four thousand seedlings one thousand are pure dominants as regards large fruit, two thousand are mixed dominants, and one thousand are pure recessives. But now comes a very tangible and very practical complication. As regards their external traits, and as regards the fruit that they will individually bear, the one thousand pure dominants (BB) and the two thousand mixed dominants (Bb) are identical. There is nothing in their exterior appearance, and there will be nothing in the appearance of their fruit, to indicate which of them contain only the factors of dominance (BB), and which contain the recessive factor combined with the other (Bb). Yet for the purpose of future experimentation, in which we shall be obliged to call on succeeding generations, it makes a vast difference which individuals are selected. We are well aware of this as we walk along the row of our seedlings, but we are also aware that there is no method by which we can fathom the secrets of the germ plasm of our seedlings, to determine which are BB and which are Bb stock-save only the method of future breeding. In spite of our best endeavors it may very well happen that the ten or twelve seedlings that we now select, to be grafted for the continuance of our experiment, include not a single pure dominant (BB), but are made up exclusively of mixed dominants (Bb). We have seen that the latter are twice as numerous as the others, and that the two look just alike; therefore the chances are two to one that they will be chosen in the majority, and it will not be strange if they are inadvertently chosen to the exclusion of the others. Yet this choice will insure that the factor of smallness which we are striving to eliminate was carefully preserved in the germ plasm of the cions of this second generation that we now graft into membership in the aristocratic cherry colony. And when, after another interval of two years, these cions come into flower and are mutually cross-pollenized, the seeds they bear, being the offspring of mixed dominants (Bb X Bb), will produce a generation of seedlings precisely repeating, as regards the quality under consideration, the formula of their parent generation. In a given lot of four thousand, let us say, one thousand will be BB, two thousand will be Bb, and one thousand will be bb. And precisely the same difficulty in selection confronts the experimenter that confronted him before. If he could only know which are the pure dominants and which the mixed one, all would be well. But not only is it impossible for him to know this, but he may not be able even to determine with certainty, from examination of the foliage of the seedlings, which ones belong to the group of three thousand that bear the dominant factor (either BB or Bb), and which to the group of one thousand that bear only recessive factors (bb). It must be borne in mind that the experimenter is really considering a large number of qualities, and it must be understood also that there may not be any clearly established point of correlation between the foliage or stem or buds of the seedling and the qualities of its future fruit as regards the matter of size. So it may quite conceivably happen that the experimenter, using his best endeavors to make right selection, picks out for preservation, among the ten or twelve chosen out of the thousands, individuals that (though they have only large-fruited ancestors in the two generations back of them), yet themselves are pure recessives (bb) as regards that quality, bearing no factor of large fruit whatever. And in that event the experimenter will be confronted, after another two-year or three-year interval of waiting, with an array of fruit, borne on the branches of his long-nurtured and carefully selected cions, not a single specimen of which is other than insignificant in size. Other good qualities the fruit may have. But in the essential quality that we are keeping under consideration it is utterly lacking. In the matter of size it reverts to the recessive member of its great-grandparental ancestry. And so its telltale progeny, hanging there among the luscious fruits of surrounding branches (of other lineage), are like the black sheep in a patrician family. Not an enheartening experiment, thus far, for the would-be developer of a colossal cherry. Yet the case is not really quite so bad as it seems. There is an old familiar saying that "blood will tell," and our new formula, if properly applied, gives full support to the saying. Making application of it, we may say that the dwarf cherry which we have developed as the result of about nine or ten years' efforts at the production of a giant, is after all a thing of quality, even though it lacks one of the qualities that we are seeking. It is a scrub as to size, but it is none the less a thoroughbred as regards a number of other qualities. In the matter of color, let us say, it is a vivid red; it is sweet and appetizing; it is resistant to disease; it will bear shipping, and so on.
NOT SO BAD AS IT SEEMS
Indeed, it is not unlikely that, as regards all desirable characteristics but one, our cherries are of such quality that, even in the patrician ranks in which they find themselves, they must be admitted to be "upper crust," to use a phrase that is said sometimes to pass current in human patrician circles. Or upon reverting to our formulae, and therefore to the terminology of the printer, we may say that they are "upper case" as regards all qualities other than size. As to bigness, to be sure, they are pure recessives and must be labeled bb; but as to juiciness they are JJ; for shipping qualities they are SS; for resistance to disease RR; for hardiness HH; and for productivity PP. That is to say, they are pure dominants for each of these qualities. Their germ plasm requires only an infusion of the dominant factor for bigness and their progeny will prove that breeding does tell. There is a tradition that passes current among dog breeders which I do not vouch for but which suggests a condition so comparable to that of our cherry that I cite it by way of illustration. It is said that the greyhound had been bred so exclusively for speed that it developed all the desired speed qualities of a hunting dog, able to overtake any quarry, but lacked the courage to seize the quarry once it had been overhauled. To overcome this defect, so the story goes, some one crossed the greyhound with the bulldog, thus breeding in a strain of courage; and in subsequent generations eliminated all the bulldog traits except courage by selective breeding; and so gave us a race of greyhounds in which the one missing quality had been supplied. This greyhound legend seems much more plausible today, now that attention has been so generally called to the segregation of unit characters, than it formerly seemed. But whatever its truth, the case of the hypothetical greyhound strongly suggests the case of our black-sheep cherry. This also lacks but a single quality. Can we not then breed this quality into our cherry and by remedying the one defect attain our ideal?
SOLVING THE DILEMMA
Fortunately, yes. This is precisely what we can do, and what the wise plant experimenter will do. We have but to look about in our cherry colony and we shall find another family, habiting perhaps a neighboring branch, the fruit of which exhibits in imposing measure the quality of size that our protege of the moment so notably lacks. This big cherry may even be the original dominant parent with which our experiment started. But it is a fruit which, although being everything that could be desired in size, is unfortunately quite lacking in color. In spite of its inviting bigness, it cannot make its way in the market be-ause, even at full maturity, it has the appearance of unripeness. But it is big, and bigness is the thing we are seeking. So we cross-fertilize the flowers of our little cherry with those of this big one. The result is readily foretold. Bigness, as we have seen all along, is dominant, and so the offspring of this union are individually big. They are mixed dominants (Bb), to be sure, but that, as we have seen, is something that concerns their descendants rather than themselves. Individually, they will bear big cherries, and that is all that we demand. But what as to the color of our new fruit? Here fortune again favors us. For it is very commonly observed that color of flower or fruit is likely to be dominant over lack of color. So our little red cherry, pure dominant as to color (CC) will stamp its influence in this regard on the progeny; the recessive color factor of the other parent (cc) being subordinated or made latent. In regard to color, as in regard to size, the progeny will be mixed dominants only (Cc). But here again the fact that they have the recessive factor (c) is of no consequence, since as we have seen the mixed dominant tangibly presents the quality as markedly as if it were a pure dominant. So when we have raised seedlings of this union of our little red cherry with the big white one, and when we have waited yet another pair of years, we shall finally be rewarded with the appearance on the cions, of fruit that meets our original ideal as to size, is as red as could be desired, and exhibits the other good qualities that entitle it to a permanent place in our patrician colony. It has taken us about twelve years to accomplish this result. And even now our new fruit must be propagated by grafting and budding, for it cannot be depended upon to breed absolutely true from the seed. The recessive factors for size and for color, as we have seen, are in its germ plasm; and these will make themselves manifest in the progeny. But so long as we confine ourselves to the method of grafting, we may hold the type of the new variety and spread broadcast our big red cherry with its combination of desirable qualities, with full assurance that, given reasonable conditions as to soil and climate, it will reproduce forever the qualities of the patrician fruit, the ancestral history of which we have just traced.
I have thought that by thus tracing in detail the history of a single experiment, paying heed chiefly to a single quality, but reminding the reader from time to time that other qualities cannot be ignored, we could perhaps gain a clearer notion than would otherwise be possible of the practical steps through which a new form of fruit is developed. It is through such series of experiments, leading sometimes forward and sometimes backward in successive generations, that the four hundred families of cherries of my patrician colony have been developed. No two among the four hundred show precisely the same combination of qualities, but all of them show one combination or another of good qualities. Those that reverted to undesirable ancestral traits have been weeded out. And this is equivalent to saying that the selected varieties of cherries represent a fixed stock as regards many of their good qualities. We cannot expect that any given one will reproduce its kind precisely from the seed, for reasons that have been fully explained. But we can expect that there will be a goodly proportion among any company of seedlings from this stock that would produce fruit of excellent quality. In a word, then, these perfected varieties of cherries represent stock that is immediately available for the purposes of further experimentation. What they have accomplished is an augury of still better things that may be expected of their descendants. And so the practical question arises as to what, specifically, are the qualities that the improved cherry still lacks; and as to what particular experiments in hybridizing should be undertaken to remedy the defects. The first and perhaps the most important defect that suggests itself is that the newly developed cherries, particularly the sweet ones, lack something of hardiness. They grow to perfection in California, but as yet they are little grown in the eastern United States, and not at all in regions north of Ohio and Missouri. Yet the race of cherries, taken as a whole, constitutes a very hardy stock. The wild cherries of the eastern United States grow far to the north and are able to withstand the winters even in regions where the mercury sometimes freezes. It should be possible, and doubtless it will prove possible, to combine the best existing varieties of cherry with some of the wild cherries, and thus to develop a race of cherries that will retain the present qualities and introduce additional qualities of hardiness fitting them for growth anywhere in the United States; in fact this is a work in which 1 am now engaged. The common choke cherry (Prunus Virginiana) is a very hardy tree, unusually productive, and almost indifferent as to soil and climatic conditions. I have made experiments in the cultivation of this tree, raising thousands of seedlings from fruit of a large, handsome specimen that grew by the roadside near Westfield, Massachusetts. The experiments as far as conducted have been satisfactory. Of course the fruit of this tree is astringent and almost as bitter as a green persimmon. But the little beach plum from which one of my finest plums was developed, was scarcely of better quality. Perhaps it is not unreasonable to hope that it may be possible to make some such improvement in the cherry, through combination with the choke cherry, as I produced by hybridizing the beach plum with the Japanese plum. In that event, we shall in all probability have a cherry surpassing any existing one in size (because of the virility that the cross with the wild species has given it), retaining the good qualities of the present Burbank cherries, and in addition being so hardy that it would thrive in any soil and in almost any climate. If the choke cherry should fail to prove a satisfactory parent, there are numerous other wild species from which to choose. The black cherry of the eastern United States (Prunus serotina), is a tree that grows from Nova Scotia to Florida and westward to Dakota and Texas. It is of large size, and bears a fruit resembling that of a choke cherry in color and appearance, but of less astringent flavor. Then there is a small red cherry, commonly called the bird cherry (Prunus Pennsylvanica), the fruit of which is sour and astringent, but which is not without qualities of virility and hardiness that might make it a valuable hybridizing agent. This is perhaps the hardiest of all cherries. I have seen it growing wild nearly as far north as Hudson Bay, in regions where it is not uncommon for the mercury to fall sixty degrees below zero. The California holly-leaf cherry and the Catalina cherry are species that may be available for the development of other desirable qualities-for it is not in hardiness alone that the best varieties sometimes are found wanting; though the species just named are so far separated biologically and physiologically that it may be impossible to combine them. Many cultivated cherries, for example, are unable to withstand the warm spring rains without serious loss from cracking of the fruit. Sometimes almost an entire crop will thus be ruined. Again many cherries are susceptible to blight. A bulletin issued by the State Commission of Horticulture of California lists more than twenty insects-leaf hoppers, scales, mites, caterpillars, and borers-that prey more or less upon root or bark or leaf of the cherry tree, or that attack its fruit. Then there are inherent maladies, such as the tendency to overflow and condensation of sap, forming an injurious gum that may induce decay of bark and wood (called gummosis), to which the cherry is peculiarly liable. Hybridizing with wild species, intelligently and systematically carried out, might produce varieties of cherry that would show exceptional resistance to insect pests as well as inherent vitality that makes for healthiness in the tree. It has long been my belief that a solution of the problem of protecting our fruit trees from both insect and fungus pests must eventually be found in the development of the qualities that make for immunity of the trees themselves, rather than in the resort to such expedients as spraying and "gasing." In this regard the plant experimenter may well take a leaf from the notebook of the physician, who has learned that immunity to disease often depends more upon the condition of the patient than upon the presence or absence of disease germs. It is possible, furthermore, that the cherry may be hybridized even more widely, and that a fruit differing markedly from any cherry hitherto produced may thus be developed. An inkling of the possibilities in this direction is given by some experiments made recently by Professor N. E. Hansen, of the South Dakota Experiment Station, who has cultivated a variety of wild fruit, called the Sand Cherry, Prunus Besseyi, which is a dwarfed, compact grower, of heavy forns. and good foliage, and which had previously been put upon the market as the Improved Dwarf Rocky Mountain Cherry. This native tree has a fruit nearly as large as the Richmond cherry and sometimes of fairly good flavor. The Prunus Besseyi has always been considered a cherry by horticultural and botanical writers. My experiments, however, seem quite clearly to demonstrate that it is more truly a plum. I have had the tree under cultivation for more than sixteen years. The fruits of the original plant were black and bitter, almost as astringent as a persimmon. By combining this plant with various other American and Japanese plums, I produced abundant seedlings, and in 1904 had developed one especially promising variety. The fruit of this hybrid seedling ripens in California about August 10th, and is extremely large for this type. It is globular, and about one inch and a quarter in diameter. The color is pure, deep crimson, with a semi-transparent amber flesh, firm, juicy, and of a rich, sweet flavor, resembling that of the American plum. The tree is intensely productive, even breaking with its own weight of fruit. It has been suggested that this tree gives great promise as an aid in the production of a hardy type of fruit that will withstand the rigorous climate and conditions of the cold northern plains of Nebraska, Minnesota, and the Dakotas. What has just been said suggests that the fruit is not truly a cherry, yet the botanists seem to feel that it occupies an intermediate station, and is more closely related to the cherry than any other fruit. Such being the case, it should be possible to hybridize this dwarf hardy species with the cherry. The tree has the further valuable property of being able to grow on dry, barren sands. A hybrid cherry having this characteristic from one of its ancestors might be expected to constitute a fruit that would grow in regions too arid for the existing cherry as well as in regions that are too cold. And this is but one of several lines of possible development that invite the plant experimenter who will give attention to this type of cherry. To suggest one other line of improvement, it is sufficient to call attention to the familiar fact that the cherry has a very brief season. The Burbank cherry fruits two or three weeks earlier than others, as we have learned in another chapter. But even so the total period during which cherries of different varieties are in fruit is very limited. One hears reports of an exceptional cherry tree that fruits a second time in the autumn. By the usual process of raising numerous seedlings, or by crossing and selection, a variety having this fall-bearing habit might be produced. The value of such a variety is obvious, though the early ripening of the cherry is at present what gives it greatest value, and it is well worth the while of the amateur to attempt experiments in this direction. The fact that cherry trees of one kind or another grow throughout the United States makes it possible for almost anyone to experiment with this fruit. And the opportunities for improvement are especially inviting.
-In cherry trees, as in the human plant, "blood will tell."
This text is from: Luther Burbank: his methods and discoveries and their practical application. Volume 4 Chapter 3