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In the early years of my experimenting, soon after I began importing plants, I attempted to cross the Japanese plum with the almond. The cross was made without very great difliculty, and the results were exceedingly interesting. Each species was fertilized with the pollen of the other, and here as elsewhere it appeared to make no particular difference in which way the cross was made. The hybrid seedlings partook somewhat of the character of the earliest of the hybrids produced by crossing the plum and the apricot. Most of the seedlings outgrew either parent, their enhanced vigor suggesting that of the hybrid walnuts. But on the other hand some of them almost refused to grow at all, being permanently dwarfed, and in this regard suggesting a certain number of the second generation of the walnut hybrids. This wide diversity of form and vigor in the first generation hybrids is a rather unusual phenomenon. As a rule, we have observed that first generation hybrids are somewhat uniform in character, and that the tendency to wide diversity appears in the second generation. Indeed, attention has more than once been called to the fact that the discovery that such is the tendency among hybrids was the one that put me on the track of most of my successful plant developments. At the time when my experiments in hybridizing the Japanese plum and the almond were commenced, there were few, if any, other plant experimenters anywhere in the world who seemed fully to grasp the principle that variation occurs in the second generation, and that it is by raising large numbers of second generation hybrids from which to make selection, that the development of new and useful varieties of plants may best and most rapidly be carried out. This principle is so familiar to-day that horticulturists and botanists who refer to it very commonly overlook the fact that the recognition of the principle is very recent. Twenty-five years ago I found it impossible to convince most well known horticulturists and botanists and biologists-with many of whom I had some spirited discussions on the subject-that the great individual variations occur in the second and a few succeeding generations. Today all these men, in common with horticulturists and biologists in general, acknowledge that these variations and recombinations do occur. Indeed, nothing more is necessary than the most casual inspection of the new varieties that have been developed at Santa Rosa in the intervening period to establish the validity of what was generally regarded as an heretical view only twenty-five years ago. And yet the case of the first generation hybrids between the Japanese plum and the European almond, showing the wide diversity just recorded, suggests that it is not always easy to lay down rules of thumb. Observation of the phenomenon of plant development in the field may present complexities that make the sifting out of principles difficult. No one whose first hybridizing experiments happened to be performed with chance hybrids of the plum and almond, and who saw among his first generation seedlings all the range of forms from dwarfs to giants, would have been likely to conclude that the first generation hybrids are generally uniform in character and that variation takes place in the second generation. Looking back now, and being able to check the observation with knowledge gained through noting the effect of hybridizing hundreds of other species, it is interesting to make inquiry as to why the first generation hybrids of the plum and almond showed such anomalous diversity. I am inclined to think that the answer may be found in the assumption that either one parent or the other was itself a hybrid. Perhaps both parents were hybrids. The fact that almonds are known to cross with the peach and the nectarine-to which reference will be made more at length presently-lends color to this assumption. And of course there is no question that the Japanese plums are largely hybridized. In a word, then, the hybrids produced by cross-pollenizing the Japanese plum and the almond were probably in reality second generation hybrids having the strains of other species than the almond and the Japanese plum in their veins. Be this as it may, the facts as to the curious diversity among the plum-almond hybrids have more than passing interest. It should further be recorded that the diversity in size was matched by the wide range of diversity in minor characteristics. The bark and leaves varied extensively among the different hybrids; on some trees the buds were round and plump, and on others long and sharp. Many of the trees produced somewhat abundant blossoms, and the individual blossoms varied widely in color and in size. But there were other trees that produced no blossoms whatever under any circumstances. These would form great clusters of buds, but instead of bursting into flowers the buds would drop off and ordinary branches would come out in their stead. In the case of buds already opened to form flowers, the blossoms not only varied as to size and color, but they showed the most astonishing diversity as to their essential fructifying organs. Some of the blossoms had numerous pistils and no stamens. Others had numerous stamens and no pistils. In yet other cases there were blossoms having stamens and pistils but absolutely without petals. In no case was fruit formed. The blossoms one and all were sterile. An attempt was made to fructify the blossoms by pollenizing them with pollen from each of the parents. But the effort was futile. The ovaries were seemingly incapable of maturing. It would appear, then, that the Japanese plum and the almond, as represented by the particular specimens that were used in these hybridizing experiments, were just at the limits of affinity that permitted cross-fertilization, but imposed sterility on the offspring. The parents were a shade more widely removed from each other genetically than were, for example, the plum and the apricot or the Persian and California walnuts. Conceivably this fact, and not the mixed ancestry of either parent, may have accounted for the diversity of form of the progeny. As the plum-almond hybrids were sterile, it is obvious that the experiments through which I had hoped to develop new varieties and perhaps new species of fruits could go no further in this direction. It is of course possible that individual plums and almonds or different varieties of the two races might be found that would combine to produce fertile offspring. This supposition finds support in the fact that my earliest crosses between the plum and the apricot were also sterile; whereas later ones produced the fertile plumcot, as the reader is aware. So it is obviously worth while to continue the experiments of hybridizing the plum and the almond, and there is every reason to hope that interesting and valuable results may be attained. My own experiments, however, although they have been repeated occasionally and have never been quite lost sight of during the twenty-five years that have intervened since the first tests were made, have produced only the anomalous results just related. Yet even these, in addition to their scientific interest, may be thought to point the way to more practical developments. At least they prove that there is no barrier between the tribe of plums and the tribe of almonds that may not be partially broken down.


Since the almond can be crossed with the plum it may reasonably be expected that mating would be effected with its closer relatives, the nectarine and peach, with even greater facility. And in point of fact it has been observed that the almond crosses with the nectarine so readily that it is practically impossible to prevent cross-fertilization when the two trees grow in the same neighborhood. The bees appear to visit them indiscriminately, and to effect hybridization so commonly that it is impossible to raise fruit from the seed with any degree of certainty when there has been an opportunity for cross-fertilization. The same thing is true, as might be expected, of the peach; which, indeed, as we have elsewhere seen, is scarcely separable botanically from the nectarine. Most varieties of almond blossom very early in the season, before nectarines or peaches are in bloom. But where the trees are blossoming at the same time in the same neighborhood the bees are almost certain to mix them indicriminately. It is nothing unusual in California where almonds and peaches are growing in the same orchard, and where peach seeds are planted, to have one third of the seedlings turn out to show marked characteristics of the almond; or, contrariwise, to find that a number of the almond seedlings show the characteristics of the peach. This, of course, is sometimes annoying to the practical orchardist, but it suggests interesting possibilities for the plant developer. Wishing to see just where the experiments might lead, I have crossed the almond with the nectarine, using great care to make sure that the experiment was not vitiated by accidental pollenizing. In some cases I have used the old method of tying a sack over the flower, which I do not usually consider necessary in pollenizing if properly performed. Hybridizing experiments of this type have been carried on somewhat extensively for at least fifteen years. I have thus produced a hybrid almond-nectarine that has an absolutely smooth skin, with nothing of the roughness and comatose condition usually found in the almond. The hybrid reproduced the color and quality of the flesh of the white nectarine parent as well as its smooth skin. And as the almond quality of seed and stone was fairly reproduced, the combination was a very curious one-to all intents and purposes a smooth-skinned peach, with white flesh, bearing at its core an almond nut. Further experiments in selective breeding will be necessary to develop the hybrid to a stage at which its qualities of flesh and nut respectively will give it commercial importance; but the foundation for such development is supplied in the hybrid already secured. This hybrid, it may be noted incidentally, is a most remarkably vigorous grower. An allied series of experiments of equal interest was inaugurated by hybridizing the Languedoc almond and the Muir peach, using, as in the other case, the utmost precaution to prevent foreign pollenization. Many seedlings were grown from this cross and a large number of them have been under observation for years. The most notable thing about these hybrid seedlings from the outset was the tendency of many of them to take on rapid growth. Some of them grow five or ten times as fast as the average seedlings of either parent. This propensity of hybrids to rapid growth is something that we have seen manifested in many other cases. It is, indeed, a rather common result when species that vary by just the right amount are hybridized. The hybrid walnuts furnished the typical illustration of this on the most spectacular scale. The fruit of these almond-peach hybrids varied a good deal on different trees. Sometimes the fruit was leathery like that of the almond, but in other cases it was edible and quite peach-like. In a few cases the pulp was so fully developed that it might be considered a fairly good peach. The seed covering was usually in the shape of an almond and smoother, thinner, and generally more elongated than the peach stone. It was hard-shelled and corrugated, but had not the texture of the peach stone. The meat within was sweet or slightly bitter, suggesting a rather inferior almond. Thus the fruit of this hybrid might be said to be fairly intermediate between the fruits of the parents, yet on the whole the flesh of the peach and the stone of the almond, respectively, tended to be prepotent. This is what would perhaps be expected, when we recall that the flesh is the specialized modern development in the case of the peach, and that the seed is similarly specialized and developed in the case of the almond. We have found occasion to believe that prepotency or dominance is conditioned on newness of development; the case of the peach-almond hybrid gives a measure of support to this theory. But while the specific qualities of peach and almond, representing their specialized development in comparatively recent times, thus tend to be segregated along Mendelian lines, yet the traits in each case are of such long standing that they do not Mendelize in the clear and satisfactory way that we have seen manifested in some other cases-for example, the color of the blackberry, and its thorns. There is, to be sure, a very marked segregation in the second generation, illustrated by the most astonishing variation among different second generation hybrids in the matter of size, rapidity of growth, and almost every quality of flower and fruit. This variation was so marked, indeed, as to rob the seedlings of the value they might otherwise have had as stock for grafting. The large-growing specimens have value for this purpose, but the diversity among the seedlings is so great that they cannot at present advantageously be grown with any hope of producing dependable stocks. In the matter of the fruit, the second generation hybrids are equally variable. There are some specimens that tend to reproduce the almond quality and others that tend to reproduce the peach quality. And as might be expected there are yet others that combine the quality of the two fruits. The best of these bear fruits that are obviously peaches, even peaches of fair qualities, yet that have at their center what would be at once recognized as an almond nut, with characteristic shell and seed. In a word, these are almonds grown inside the peach-a combination of obvious interest. But this anomalous fruit, notwithstanding its interest, did not present commercial possibilities that could at the moment be realized. The peaches that thus bear almonds are not of the best quality as compared with recognized varieties of commercial peaches. Neither, on the other hand, were the almonds borne by these peaches of a quality to enable them to compete in the market with the best varieties of commercial almonds. What had been produced, in a word, was a rather inferior peach bearing at its core a rather inferior almond. The combination has obvious scientific interest, but it has no immediate commercial value. There is no reason to doubt that a continuance of the experiment in which selection was made among the best specimens of this hybrid fruit, together with further hybridization in which the strains of the best peaches and the best almonds were successively introduced, might result in producing a peach-almond that would have flesh equal to the best varieties of peaches and a nut equal to the best almonds. Even now there are apricots that bear delicious nuts. Inasmuch as the apricot is already in this condition, there is no reason why the peach should not do the same. The apricot seeds of California are now nearly all shipped to France to make almond oil. At the time when the experiments above referred to were carried out, however, it was not clear that a fruit combining the qualities of the peach and the almond would have great commercial value. The peach industry and the almond industry are so entirely different that the inauguration of altogether new methods would be necessary to make them operable in combination. Hence the hybridizing experiments were not carried beyond the second generation, and the hybrid trees were thereafter used as stocks for the engrafting of cions that gave greater commercial promise, even though less interesting from a scientific standpoint.


A subsequent series of experiments was undertaken, however, to which reference has been made in another connection, in which the almond was combined with the purple-leafed peach. It has already been recorded that the first generation hybrids of this cross bore green leaves exclusively, but that purple leaves appeared in a certain proportion of the hybrids of the second and subsequent generation. In this cross, the purple-leafed peach was used invariably as the pistillate parent. There is every reason to suppose, however, that the results would have been the same had the cross been made the other way. Among the second generation seedlings were not only some with red leaves, but others that showed a combination of colors varying from the pure green almond leaves through different shades to the crimson leaf of the peach. There was thus exhibited a pronounced tendency to segregation of colors in certain cases, and a combination of the colors in others. Selection being made among the trees with the purple leaves, this characteristic, as might have been expected, reproduced itself, and a race of purple-leafed peach-almonds was developed. The fruit of this hybrid is purple fleshed, and as to its general characteristics it is a fair compromise between the peach and the almond, not unlike the hybrid form already described. This form of peach-almond has considerable merit as an ornamental tree, and it will probably prove of value as an acquisition for the garden and dooryard. Even though a peach that bears an edible seed has no greatly added commercial value, owing to the small size of the seed, such a fruit with large seed of thinner shell, and with peach flavor, should certainly be appreciated.


All this has to do with the production of a compound fruit in which the almond seed is only an accessory. It remains to say a few words about the almond itself as a commercial nut. The importance of the subject will be obvious when we record that in a recent year more than three thousand tons of almonds were produced in California alone. When it is further recalled that numberless unsuccessful attempts have been made to establish almond orchards in various warmer regions of the United States, and that the failure of these orchards has been due almost exclusively to a single remedial defect, the importance of the almond from the standpoint of the plant developer will be more clearly understood. The one great defect of most varieties of almond is that they bloom so early that their blossoms are likely to be destroyed by frost. A second minor defect is that many of the varieties of almond do not bear well unless they are cross-fertilized with pollen from other varieties. The later defect is obviously one that requires only reasonable intelligence in the planting of different varieties in contiguous rows, so that cross-fertilization may readily take place, or the production of varieties with perfect blossoms. To overcome the defects due to too early blooming is a somewhat more difficult matter. Fortunately, however, there is a rather wide range of variation among different kinds of almond as to the matter of time of blooming. It follows that there should be no great difficulty in producing, by selective breeding, a variety that combines desirable qualities of nut production with the habit of late blooming. The difficulty has been that until recently orchardists have not recognized the possibility of thus segregating and recombining characters, and they have "trusted to luck" in setting out their almond orchards, so in a large number of cases the profitless trees were cut down or regrafted to Burbank prunes. Latterly, the California orchardists have learned that there are two or three varieties that may be depended on, notably the Nonpareil and the Ne Plus Ultra, both of which originated in California from seedlings grown by A. T. Hatch of Salinas County. These may best be polenized, in the opinion of experienced orchardists, by the variety known as Texas Prolific. Unfortunately neither of the varieties mentioned produces nuts of the largest size, but their certainty of bearing gives them advantage over varieties that would otherwise be superior but which cannot be depended upon. It should not be difficult, except that such an experiment necessarily takes time, to crossbreed the different varieties that have individual traits of exceptional value, and thus to produce in the second generation, or through successive selections, varieties that will combine the best qualities. Indeed, something has already been accomplished in this direction, notably in the case of such a variety as that known as Drake's Seedling, a late blooming variety that is prolific and a regular and abundant bearer, notwithstanding its parent form was the Languedoc which has been pretty generally condemned for irregular bearing. There is no good reason why the almond should not bear as regularly and as abundantly as the apple or peach or cherry. As to the shell of the almond, this has been so specialized through selective breeding that in the best varieties it is perhaps as soft and thin as desirable. If it becomes too soft, it is liable to injury in shipping, and thus the appearance of the nuts is marred and their market value impaired; also being subject to destruction by birds before it is harvested. Perhaps, however, selective breeding may advantageously be carried out with an eye to the whitening of the shell of the nut. At present it is necessary to bleach the shells after the nuts are thoroughly dried, first with low pressure steam and then with the fumes of sulphur. Such bleaching is necessary to meet the demands of the consumers. It would obviously cheapen production and save a good deal of trouble if a variety could be produced that would have the desired color of shell in the natural state. Another defect is that the almond tends to cling to the tree too tenaciously, requiring unnecessary labor. Any almond grower would appreciate these two experiments. My own experiments of late have been in all the directions mentioned, and I have reason to suppose that I now have better almonds than any heretofore grown. It is clear, then, that there are various directions in which the almond may profit by the attentions of the plant developer. The steady and increasing demand for this nut warrants the expectation that systematic efforts for its improvement may meet with an adequate financial reward. Already the cultivation of the almond is an industry that exceeds in importance that of any other nut except the walnut and pecan. And it is an industry that will increase in proportion as the efforts of the plant developer make the almond a more certain bearer. What has just been said will sufficiently indicate the lines along which the plant developer must work in order to produce these results.

-It is obviously worth while to continue the experiments of hybridizing the plum and almond, and there is every reason to believe that interesting and valuable results may be attained.

This text is from: Luther Burbank: his methods and discoveries and their practical application. Volume 11 Chapter 3