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ACCOMPLISHING THE IMPOSSIBLE-THE PLUMCOT

A CROSS WHICH MAN HAD SAID COULD NEVER BE MADE

Several years ago a party of noted scientists from various parts of the world were visiting my nursery. I asked one of them, an American, even then well known to the public as an authority on horticultural subjects, to come over to another part of the grounds and see one of my crosses between the plum and the apricot; one of my first crosses then just ripening. "There can be no such fruit," my visitor declared. "The two species are wholly different in all respects. Everybody knows it is impossible to cross two trees of such widely varying types as the plum and the apricot." I was not surprised to hear him make this statement. For at that time very few biologists, and in particular few technical botanists, had quite given up the notion that there are hard and fast lines between the different species as commonly classified. This belief has undergone a radical change, in recent years, and the many combinations of widely different species made on my Sebastopol grounds have had at least a share in broadening and clarifying the views of the classifiers. "Well, what kind of a tree do you think this is?" I asked a moment later. "Why, a plum, to be sure." "Please examine more closely, professor," I requested. "This leaf looks to me more like an apricot than like a plum!" "Yes-yes. I see now it is; it is surely an apricot-the leaf, though differing from most of the apricots, is certainly an apricot leaf." "Now look again, carefully-look at the foliage, bark, branches; and now let us examine the fruit. Then tell me what you really think it is." After a long and thorough examination, I heard the reluctant decision: "Well, it surely is what you claim it to be-a cross between the plum and the apricot. I never thought it could be made." I told him I had hundreds of others of different sizes, shapes, and qualities. "Show me another-quick! quick!" And I showed him not merely one other, but a score or two, to his added mystification. When the apricot and plum were crossed to produce an intermediate fruit, the accomplishment was thought by some botanists to savor of a violation of the laws of Nature.

BREAKING DOWN A BARRIER

Notwithstanding the general acceptance of the idea of evolution of species, a reminiscence of the old special-creation point of view lingered. Even if existing species have evolved in the past, they were thought to be fixed in the present; or at any rate to be separated by impassable hereditary gulfs. If, by a rare chance, species did interbreed, it was supposed pretty generally that the offspring must necessarily be sterile. Therefore, when the statement was made that I had crossed the plum and apricot, and produced a healthy and vigorous new fruit, it was met with profound skepticism from most quarters. But it was only necessary to bring the skeptics to the trees themselves and introduce them to the new fruit to convince them that what they considered impossible had really been accomplished. The Plum-apricot hybrid attests its heritage convincingly to any competent observer. As we have elsewhere seen, the apricot has been found difficult to improve, because of its lack of adaptability-pliability, as it may be called. The tree thrives, blossoms well, but rarely fruits in this region, chiefly because of the tenderness of its blossoms. Partly because the climate here made it difficult to attempt the improvement of this tender plant, I decided to try crossing the apricot with the plum, which thrives unusually well in this locality. Had I known how much time and labor and patience these experiments were to demand, they might never have been undertaken. Plant improvement of any kind tests purse and patience; but the improvement of tree fruits strains both to the breaking point. Working with vegetables or flowers, it is possible to get valuable improvements well under way in from three to five years-after which, continued selection makes progress more rapid. With tree fruits you have only just begun after a dozen years of crossing, growing, testing, and selecting. Nevertheless it was with pleasurable anticipations that I began these experiments which later were to produce the plumcot. It was like entering an unexplored country. Apricot flowers were dusted with plum pollen and plum flowers with apricot pollen. But for a long time the experiment failed. Finally, however, when I about despaired of success, several crossbred seedlings were found among a lot grown from the seeds of a Japanese plum that had been pollenized with various apricot blossoms. The young seedlings could be early distinguished from the uncrossed seedlings by the foliage, bark, buds, and general appearances; differences being noticeable while the seedlings were still less than a foot high. The combined characters of the plum and the apricot were to be noticed in the bark, the leaves, the buds, and especially the roots. The apricot root is bright red while the plum root is yellow, pale yellow or almost white. The hybrid seedlings had red roots.

BATTLING HEREDITIES

With the recognition of characteristics began the great work of selecting and discarding. Moreover, fresh hybridizing tests were made, and in due course other hybrids were produced, some having the plum and others the apricot for the seed parent. Where cross-fertilization could be effected, it made no difference which way the species were crossed. But the conflict of hereditary tendencies was at once apparent. Hybrids appeared that departed widely from the traditions of either parent. Moreover, there was the tendency to sterility that threatens the offspring of every wide cross. One of the first plum-apricot hybrids produced did not have a stamen on the whole tree. It was evidently a cross of the plum and apricot, but in the combination the means for perfect reproduction was overlooked. Experiments were made by applying pollen to the malformed blossoms. But few ripened-the majority remaining dormant. The cross brings out this striking malformation, but there are doubtless almost numberless tendencies striving for mastery that remain submerged, seemingly neutralizing one another-perhaps destined ultimately to come to the surface under influence of a changed environment. At every stage of the development and improvement of a plant short cuts must be introduced, where time and expense can be saved. Instead of waiting years for a seedling to bear, it is possible to save much of that time by the application of well known methods of grafting, elsewhere described. Some of the most vigorous and best growers of the hybrid seedlings were grafted into older plum trees. After two or three years several of them began to bear fruit abundantly. The grafts showed that fruit would actually be produced-fruit of fine quality; this much was assured. And it was a fruit of a new order-neither apricot nor plum. In view of its origin, it seemed appropriate to christen the new fruit the Plumcot.

PARENTAL RESEMBLANCES

The new fruit is similar to the plum in its firmness and color. In form also the cross usually follows the plum parentage, for every shape that is seen among the many thousands of varieties of plums is also seen among the plumcot seedlings. But there are varieties also that closely resemble the apricot in form. The stones vary widely, some of them almost duplicating the apricot stone, and others being similar to the plum stone. A few varieties have stones which resemble the peach stone in many respects, especially in the corrugated and honey-combed appearance and in thickness of the shell. There is no uniformity in the color of the stones. Some of them are almost white, others yellow; a few are wine colored; and there are browns of various shades. The sharp, knife-like projection from one edge of the stone-a characteristic of the apricot-is found in many of the seeds of the plumcot. Notwithstanding these extreme variations, however, it is usually not difficult to distinguish between the plumcot seeds and those of the plum or apricot. They are usually plumper than those of the plum, and have an individual appearance that would be noticed by anyone who examines them. Some stones are attached to the flesh, while others are free, some are smaller than the stones of either the plum or apricot, while some are much larger, comparable to the peach stone. The flesh of the new fruit is-the flesh of a Plumcot. As great production as could be desired, combined with large size and other good qualities, had not up to that time been produced. This lack, while discouraging for the time, was by no means an insurmountable obstacle to the production of a fruit comparable in its relative perfection to our other standard fruits. When it is possible to add to the most stubborn plant, practically any desired element-color, hardiness, earliness, or any other it may lack-the plant improver may be assured that productiveness can also be added. In order to give an idea how a number of seedling plumcots proved up, the following test records of some of the plumcots produced are listed. It is to be remembered that these are some of the results of earlier experiments. On consulting my record books, I find that the earlier Plumcots were usually listed as poor to medium growers, and almost without exception as poor bearers. Such records as these are typical: "No. 10-Poor grower; fruit small. No. 14-Strong grower and poor bearer. No. 16-Poor grower and poor bearer. No. 18-Medium grower and poor bearer." This is not as discouraging as it might seem on the face of it. All of the trees represented by the above numbers bore regularly; they produced a fair crop every year. Moreover, there were others that were listed as "medium" bearers, and even as "heavy" bearers. One of these now fruiting produces such an enormous quantity of fruit that it would seem impossible for the tree to hold it; the branches are literally crowded with plumcots from base to tip. Quality also is good. So this variety gives a good basis for more seedlings and for crosses that will produce regular and abundant bearers of fruit of superior quality. The plumcot was at first slow of improvement owing to the comparatively few seeds available, and the time It took those to come again to bearing, yet a number of varieties which combine the pleasing quality of the apricot with the hardiness and productivity of the plum are already in existence. The larger proportion of the successful crosses between the apricot and the plum have been made with the Japanese plums. Few seedlings have been raised from the apricot trees pollenized with the Japanese pollen, the seeds generally being produced on the plum tree. The seedlings of the second generation show an astonishing number of variations. Although both trees and fruits of these variations usually resemble both parents in various respects, yet we are so unaccustomed to seeing such combinations of characters that they appear to be new. In fact, the combinations are new, though the characters exist in the heredity of one parent or the other; but these are often greatly intensified in certain individuals.

FURTHER CHARACTERISTICS OF TREE AND FRUIT

The foliage, growth and general appearance of the plumcot trees most often combine the characters of the two species in such a way that it is impossible to classify them either as plums or apricots. There are, of course, many gradations, so that some trees much resemble the plum, while others closely resemble the apricot. Several varieties of the new plumcots were exhibited at the Pan-American Exposition at Buffalo in 1901. The exhibit aroused interest-both for its novelty and beauty and because of its promise of a new fruit for the orchardist. As announced in The California Fruit Grower of May 24, 1903, a special gold medal was struck as an award-though no award had been scheduled, or could have been for any such exhibit. Such fruit had probably never been thought of by the board of awards or any one else. Such recognition was pleasing. Yet the plumcot in 1901 was far from being a perfect fruit. It was rather in the experimental stage. Further work in cross-breeding and selection was requisite for its perfecting. The first one of these plumcots introduced was sold to John M. Rutland of Australia. Mr. Rutland came from Kiewa, Australia, and lived near my Sebastopol proving grounds for several years in order to study these new fruits, as well as the cactus and other of my productions. When he saw this plumcot, he thought it good enough for introduction. Accordingly, in July, 1905, he purchased the right of distribution in the Southern Hemisphere, including all of Africa. He named this variety the Rutland. The following year the new fruit was introduced in the Northern Hemisphere by George C. Roeding of Fresno, California. The Rutland has long, slender branches and long, slender leaves. It is a completely balanced combination of the Satsuma plum and the apricot. The exact pedigree of the Rutland is inferred rather than known. The crosses were so numerous and so complicated at that time, that no attempt was made to keep an exact record of all of them. There is little doubt, however, that Satsuma is one of the parents, because the flesh of the Rutland is red, and the Satsuma was the only plum which had red flesh that I was using for crossing at that time. The fruit of the Rutland is large, globular, cling-stone; both the flesh and the skin are of a deep crimson color. The flesh has an acid flavor until mature, and when fully ripe resembles the Satsuma in its acid qualities. Its principal value is for jam and jellies. There are a dozen or more bearing trees of this variety on the Sebastopol place, and they have never failed to produce a crop each season. The amount of fruit, however, is too small to make the trees valuable commercially in this climate. The Rutland was a fruit of unusual scientific interest, and was introduced partly under that consideration-not merely as a commercial fruit. It was sent out as a curiosity, the type specimen of a new kind of fruit and the forerunner of numerous good varieties that will follow.

FIXITY OF THE NEW SPECIES

It might be thought that seedlings from plumcots would revert to the type of plum or of apricot, but they do not. The combination is complete and permanent. Among the many thousands of seedlings which have been grown, not one has produced either true plum or true apricot. All are plumcots. It is therefore plain that the new fruit is fixed as a species. Of course it is not expected to fix any of the varieties so that they will come true to seed, any more than any variety of plum or apple or pear will come true to seed. Nevertheless, the mixed heritage of the new fruit is not altogether obscured. The tendency to segregation of plum factors and apricot factors in the second and succeeding generation is variously manifested. It would probably be feasible to select specimens that by inbreeding and selection could be made to develop races fairly duplicating each of the parental stocks. Such an experiment would have scientific interest rather than practical value. The plumcots are still new; they have not been introduced to the general trade long enough to be fully tested in many parts of the world. It was hoped from the outset that among the new varieties some would be found bearing fruits equal to or better than the apricot in flavor, on trees at least as hardy as the standard varieties of plums. This expectation has been realized in a variety of plumcot that has been named the Apex. This makes it possible to raise delicious apricot-like fruits in many localities where the apricot cannot be grown.

THE BEST OF THE PLUMCOTS-UP TO DATE

The best of the plumcots so far produced is that just mentioned, the Apex, a final selection in 1911. It ripens with the very earliest of the early plums, about June 10. This means that its season is about three weeks earlier inland. It has not been fruited sufficiently long in other localities to know how it will yield elsewhere. The tree is a strong, upright grower, and has never failed to bear a full crop, even where apricots are failures. In some cases the Apex has borne a full crop of fruit even when the plums were a short crop on account of unusual weather conditions. This fruiting capacity is unusual in plumcots of such superior quality, and marks the beginning of a new race of plumcots as productive as the plum and as valuable as the apricot. The fruit of the Apex is extremely handsome, and very large for an early fruit, being 5 1/2 to 6 inches in circumference. It is globular, and pink, or light crimson in color. The flesh is honey yellow, firm, rich, aromatic, resembling that of the apricot, and sweet and delicious to the taste. The Apex tree is a much stronger grower than the Rutland, and produces perhaps ten times as much fruit. The fruit is larger and much earlier. It has yellow flesh instead of crimson, making it one of the most valuable market varieties. The Apex resembles the apricot very decidedly in form, size, and quality of fruit, while it is more like the plum in foliage, upright growth, productiveness, and smooth-skinned fruit. It thus illustrates the tendency to segregation of unit characters along Mendelian lines to which reference has been made. The Apex is the only plumcot yet introduced which has promise of becoming a standard market variety, though there are others equal or superior to it to follow. Its ability to withstand the requirements of long shipping have not been thoroughly tested, but its firm flesh and tendency to ripen slowly are strong indications of its value for transcontinental shipment. The exact parentage of the Apex is not known. The crosses have been so extensive and complicated that a complete record was thought of less value than the production of a fruit that would feed the millions. It is certain, however, that the Apex, like the Rutland, carries blood of the Japanese type of plum combined with that of the apricot. The Triumph plumcot was introduced by myself in 1911, having been, like the Apex, previously tested for several years. It is fairly productive here, the fruit ripening about August 1. It is of apricot form, is six inches around, with velvety purple skin, thickly dotted and mottled with scarlet. The flesh is firm and apricot-like in texture. It is not so promising as a shipping fruit as the Apex because of its deep crimson flesh and lateness of ripening. The Triumph is primarily a home fruit, and is valuable because of unique combinations of the apricot and plum qualities. During the several years this variety has borne fruit, the trees have never failed to bear at least a medium crop. Another plumcot introduced at the same time as the Triumph is known as the Corona. It is a strong, upright growing tree, bearing beautiful, large, golden-yellow fruit with a velvety skin. The fruit usually develops a red cheek when perfectly ripe. It is firm, sweet or subacid, and delicious. The Corona is a cling-stone. It ripens July 25. It is an unusually rapid-growing tree, but it is not so productive as the Apex. It will probably be grown only for home use. It is possibly hardy enough to be grown in many localities where the apricot does not fruit, and may be appreciated there because of its resemblance to the apricot. Besides the varieties that have been introduced, I have some thirty other selected varieties that have been given temporary names, for further testing. Some of these will doubtless be introduced if, as expected, they prove of value. Hundreds of other seedlings are being tested but have not developed sufficiently to give a very definite idea of their qualities.

HYBRIDIZING THE PLUMCOT

Now that the plumcot race has been thoroughly established, it is necessary to make further crosses. The obvious way to obtain improved varieties is to cross the best seedlings of those already produced. This is being done every year. Seeds of all of the plumcots grown on my place in 1912 were saved and planted; possibly two thousand of these seedlings being grown. One of my named varieties that has not been introduced is perhaps the most prolific fruit tree ever produced. The seeds from this are being saved separately. It is probable that the seedlings grown from this variety will be remarkable producers. By crossing some of the plumcots with the Prunus pissardii plum, some purple leaved plumcots have been secured. This characteristic of dark foliage is readily transmitted in the plumcot cross as it is in the plum crosses. It is expected that by this cross one or more varieties of plumcots will be secured that are valuable both for fruit and foliage. The purple-leaved plum trees have proved of great value for decorating lawns, and the plumcot trees are considered of even more value by some, because of the unique combination, and the brilliant color of the foliage. From a study of the plumcots already produced, it is safe to say that this new fruit will become known and grown in all climates where deciduous fruits are found. Numerous improvements must be made before the plumcot will become as popular as either of its parents. But only time and patient selection are required to effect these improvements. It is quite possible that in many regions the plumcot may in time replace the apricot as well as many of the plums. But more important by far than the quality of the plumcot as an orchard fruit is, the lesson it has taught as to the possibility of producing new fruits by hybridization. The plumcot stands as the first addition to the list of orchard fruits that has been developed within historical times. Apples, pears, plums, peaches, cherries, apricots, quinces-all were known to the Romans and Greeks and to their forbears of Oriental antiquity. The plumcot is a new species that originated just at the close of the 19th century. Its production forecasts a new era in fruit development.

-Plant improvement of any kind tests purse and patience; at every stage of the work shortcuts must be introduced in order that time and expense may be saved.

This text is from: Luther Burbank: his methods and discoveries and their practical application. Volume 5 Chapter 9