THE RESPONSIVENESS OF THE PEAR
WHAT HAS BEEN DONE IS BUT THE BEGINNING
A correspondent who is seemingly prone to personify inanimate objects writes to ask which tree among our cultivated ones I regard as the most "human." And then, without awaiting reply, my correspondent supplies the answer: "The pear, of course," he says with full assurance. But when he goes on to state the reasons for this decision, I am not quite sure that his argument carries conviction. Perhaps the most striking bit of analogy that he offers is the fact that a pear tree sometimes fails to reach maturity until it is from fifteen to twenty years old, coupled with the cognate fact that the tree may continue to thrive for three score years and ten or even longer. He cites a good many other analogies, or supposed analogies, to be sure, the fact that the pear over-rides adversity, as it were, bearing abundantly in bad soils and when totally neglected; the fact that it grows by roadsides and in dooryards showing a domestic habit and as it were a friendly spirit toward man; and finally, the fact that it responds to attention and proves as receptive and responsive to good treatment as it is resistant to bad. But I am by no means sure that as to most of these traits, and for that matter in regard to any others that might be mentioned, the apple tree is not to be given a place quite on a par with that which the pear can claim. There is no occasion to dispute about the matter, however, for at best such comparisons have no great significance. Let it suffice that the pear and the apple, close cousins as they are, may very well be considered the two orchard trees that are friendliest to man, in the broad use of the word. They have been his associates probably almost from the earliest times when he learned that plants would respond to cultivation. They have gone with him on his chief migrations throughout the temperate zone and even well into sub-arctic regions. They have proved themselves adaptable to all soils and nearly all climates; and they jointly produce a variety of pulpy fruits that stand in a class by themselves and are quite without competitors, or were until the quince came under the hand of the plant developer in very recent times.
Which of the twain, pear or apple, was first adopted, no one can say, but it is certain that both were friendly with man even in prehistoric times. There is evidence from the ruins of remote civilization of the Lake Dwellers of Switzerland that the pear was known even in that day. Of course it was familiar to the Greeks and Romans from the earliest recorded periods of history. Long before that it had come out of its central Asian home-if, as is almost certain, that was its original habitat-and had become thoroughly domesticated about the Mediterranean. Other branches of the same race had migrated eastward until they found a home in China and Japan. And in these widely separated regions, at the extremes of the largest continent, the two descendants of the primitive stock developed, each in its own way, in response to soil, climate, and the diverse temperaments of the peoples, until the pear of Europe was in many ways a different fruit from the pear of the Far East. But there was one migration made by prehistoric man in which the pear, apparently, did not accompany him. This was the final stage of the eastward journey of our remote ancestors which carried them across a land bridge, now no longer in existence, between northeastern Asia and the present Alaska, and thus brought them to America. It seems a fair presumption that when prehistoric man made this final migration he brought the apple with him. At all events, with or without man's aid, the apple made its way across the bridge that joined the continents. Probably the fact that the seeds of the pear will not germinate when once dried may explain the failure of that tree to come with the forerunners of the Indian to the new continent. The seeds of all orchard fruits germinate far better if they have not been too thoroughly dried. But the seed of the pear is peculiarly susceptible to destruction through drying; and if the ancestral pear had the same quality, which we need not doubt, this fact may in itself have been instrumental in restricting the spread of a tree which, when introduced in America in modern times, proved thoroughly adapted to our soil and climate. We must not press this point too far, however, for the plum seed also dies if dried; yet the plum came to America in prehistoric times along with the apple. And, for that matter, we shall see elsewhere that there is another possible interpretation of the story of the prehistoric migrations of the trees. Be all that as it may, the pear retains to this day evidence of the inherent need, in the interest of its race, that the seeds borne at the heart of its fruit shall be preserved in a moist condition. The skin of the pear, except in the most recently modified varieties, is firm and thick. It is of a green or mottled yellow color calculated to protect it from the observant eyes of birds and animals rather than to attract them. It has been assumed that the eatable pulp that surrounds the seed was designed by nature-that is to say, developed through natural selection-for the purpose of attracting animals and birds, that these creatures may aid in disseminating the seed. But the case of the pear, in common with that of the wild crab apple, suggests that the chief purpose of the fruit-pulp is to keep the seeds moist through the winter. As a further aid to this, and in token of the moisture-loving quality of its seeds, the skin of the pear is fortified by a deposit of woody cells at its inner surface that give it a granular or even gritty texture. This unique quality of the fruit may even extend to the pulp itself, especially with the more primeval forms, giving the pear a texture different from that of any other fruit. This unusual habit of depositing wood cells in the fruits, aside from the seed case itself, is no longer of use to the cultivated pear; but the fact that it tends to be retained shows how important a part it bore in the struggle for existence of the pear's remote ancestors. But let us put aside theories as to the remote history of the pear and consider the fruit in its modern relations. The significant thing to bear in mind is that in our day the pear is represented by two races, obviously related, yet quite as obviously long separated, one of them finding its home in Europe and (since the Discovery) in America and the other being indigenous to eastern Asia, the two having thus migrated in opposite directions, circling the earth, and finally meeting on the Pacific Coast of America. And the fact that these two races of pears have thus diverged, yet still retain the capacity to hybridize, is an all-important one from the standpoint of the fruit developer. This fact is, indeed, the basis of the newest progress in the development of the pear, and it gives the augury of still more important developments probably to take place in the near future. It is only fair to recall, however, that the new beginnings in the development of the pear took place in western Europe independently of an oriental alliance.
NEW BEGINNINGS IN EUROPE
The pears of today, as known in the eastern United States, and for that matter most of the finest Californian varieties, are the bearers of an impulse to development that was given by a French horticulturist, Jean Baptiste Van Mons, and Andrew Knight of England about a century ago. Van Mons acted on a theory, now abandoned, that young plants produce the best progeny. But this led him "to sow, to re-sow, to sow again, to sow perpetually." And he selected his seeds with such care as to develop many improved varieties. In particular, he taught some pears to bear fruit in three years from the seed. Van Mons produced by selection about four hundred new varieties of pears, among others a dwarf variety that was a prolific bearer. Meantime, however, the pear was making its way in America, and one of the most famous varieties, the Seckel, originated in the early part of the nineteenth century on the farm of a man whose name it bears near Philadelphia. This was a "spontaneous" variant or mutant, the precise origin of which is unknown. At the time of its origin the Seckel was pronounced by the conservative London Horticulturist Society to be superior to any European variety of fall pear then known. Rather curiously it chanced that the next very notable step in the progress of the pear also took place on a farm near Philadelphia. The owner of the farm was Mr. Peter Kieffer. The thing for which he was responsible was the introduction of a pear bearing his name, which originated through the chance hybridization of a pear of European strain with the Chinese sand pear, which had been introduced as an ornamental garden tree not long after relations were established between America and the Far East. The oriental pear which thus at last came to mingle its racial strains with those of this remote relative, after the two had traveled around the world in opposite directions, was a graceful tree having large and attractive flowers and bearing fruit of a pleasing fragrance but of such consistency as to be almost uneatable except when cooked. In spite of the defects of its fruit, however, the oriental pear had certain qualities of hardiness and resistance to disease that made it a valuable mate for its European cousin. So the Kieffer pair soon gained popularity. So also did a number of other hybrid pears of similar origin, including the Le Conte, the Garber, and the Smith. These hybrids soon became standard pears in the Gulf States, where the European pears do not thrive.
MAMMOTH PEARS IN CALIFORNIA
The hybrid pears did not gain popularity in California, because the climate and soil of this state seemed to be peculiarly hospitable to the European pears, notably the Bartlett. By crossbreeding and selection these have been so developed, without hybridization with the oriental species, as to assume almost colossal proportions, and while differing widely in flavor from the original stock, to retain enough characteristics of the original to constitute a most valuable market fruit. The California pears, indeed, have quite outdone themselves. They have been described as "grand in size, delicate in color and aroma, and of unsurpassed richness." A specimen has been reported that was "nine inches high, sixteen inches around the base, and five pounds in weight." Pears of allied varieties show scarcely less notable tendency to grow to unprecedented size; for example, five Vicar of Winkfields are reported as weighing four pounds, eight ounces; nine Easter Beurre as weighing 24 1/2 pounds, the heaviest single specimen weighing 2 3/4 pounds, and the like. In the mere matter of size, then, there remains little to be desired; but there are other qualities as to which not so much can be said. In particular the pear is often susceptible to disease, and in general the extreme development of productivity has been more or less associated with a tendency to lose vigor, rapidity of growth and general vitality. For this and sundry other reasons it seemed to me that it might be desirable to make further experiments in the blending of the oriental and occidental heredities. So as early as 1884 I made importations of the seeds of the Japanese pear. In a shipment containing loquats, plums, chestnuts, persimmons, gooseberries, blackberries, peaches and raspberries, I received also twenty pounds of pear seeds. The seedlings were grown, but at first little use was made of them except as grafting stocks. The valuable developments that ultimately came from the introduction of the oriental heredities were not secured at the outset.
TRAITS OF THE ORIENTAL PEAR
About 1890 I imported from Japan large quantities of the seeds of the Chinese sand pear. The seedlings proved extremely variable. Some of them grew six or seven feet the first year, while others from the same lot of seed, under exactly the same conditions, grew only a few inches; and a corresponding rate of growth characterizes the seedlings as long as they live. But, although the seedlings themselves proved so variable, their fruit was singularly uniform in size and quality. As to shape, the fruit of the oriental pear is usually oblate, approaching the globular. This raises a rather curious, if not very important, question as to whether the European pear owes its very characteristic shape to artificial selection. The ordinary pear, as everyone knows, has a form that is so individual and so little duplicated, that no single word of familiar usage describes it. In this regard, as in a good many others, the pear is unique. One would not commonly think of describing anything as "apple-shaped," or "peach-shaped," or "plum-shaped," but "pear-shaped" is a cognomen that is at once convenient and definitive. So, as I said, the fact that the oriental pear has not assumed this shape has a certain interest and suggestiveness. The hybridizing experiments that were begun as soon as I was in possession of the oriental seedlings called for more patience, perhaps, than almost any other tests that the fruit experimenter can make, for the very obvious reason that the pear is the slowest to mature of all the fruits grown in temperate climates. It often requires from ten to twenty years for seedlings of the pear to come to their first fruiting. The matter may be forced a little by grafting the pear cions on quince stock, but while this makes them fruit earlier, it also tends to dwarf them, and I do not recommend this as a general practice, though highly desirable for special purposes. Whoever has not patience to wait had best not undertake experiments with the pear. With a tree of such slow development, it is peculiarly desirable to make no mistakes in selecting seedlings for preservation. Judgment as to the future tree must be based, as with other fruit, largely on its growth, and the appearance of the foliage. Pear seedlings that have an abundance of large leaves, and strong, thick, short-jointed wood, and thick, fat buds, are those to be selected. But this is not by any means as sure an indication of superior fruit in the pear as in most of our cultivated fruit, for the reason that Van Mons and other workers in this line have mostly sought early-bearing and fine quality of fruit, neglecting the foliage and growth of the tree almost fully.
THE EFFECT OF NEW BLOOD
I grew great quantities of pear seedlings from seed imported in 1884 from Japan. The selected seedlings of this original stock have enormous, glossy leaves, some of which for weeks after the first frost show varied and brilliant colors almost like the autumn foliage of oaks and maples of the Northeast. Many of the best of these were distributed for planting as ornamental trees. Very early in the experiments I found among many seedlings of a cross between the Bartlett and the hybrid Le Conte one that seemed to have exceptional qualities. This proved to be astonishingly productive of fruit of the largest size and best quality, and the tree had extraordinary vigor of growth and was apparently immune to the blight. But only one was selected as showing good promise as a fruit bearer. Through further hybridization and selection, during a period of nearly a quarter of a century, the hybrid progeny of this Japanese pear developed a variety that was introduced in 1911 as the "Test." Year after year it had produced two or three times as much as any other pear that I had ever grown. The fruit averages rather larger than that of the Bartlett, and it appears about four weeks later. The flesh is similar to that of the Le Conte but superior to it in quality, although hardly comparable to that of the Bartlett except when cooked. Although I have raised and fruited numberless seedlings from a great variety of crosses, and have noted many variations, the Test is the only one that I have thus far thought worthy of introduction. Several hundred three-year-old seedlings of this new pear, grafted on quince stocks, give great promise by their vigorous, compact growth, heavy foliage and full, round buds. Among those that have fruited are some mammoth pears of exquisite quality when cooked; and a few are good when fresh. There is unusual variation in growth of wood, foliage, season of ripening, form, size, and quality of fruit. Some of the hybrids have a smooth, polished skin with red cheeks; others are russet throughout. The varying qualities of the hybrids are doubtless due to the releasing of latent characters brought about by the commingling of the two widely diverse strains. lt was necessary thus to hybridize and select through successive generations, because the oriental pear brought to the combination very undesirable qualities of fruit as to texture and flavor. Only when these were eliminated from later generations, and the qualities of the Bartlett and its allies substituted, did the hybrid pear become a commercial possibility. But, along with its undesirable qualities of fruit, the oriental pear brought other qualities that were pre-eminently desirable. First and foremost it had fundamental vigor of constitution that promised to supply precisely what the European pear most lacked. This was manifested not only in the vigor of its growth, but in its seemingly almost entire immunity to the attacks of the disease that has been the scourge of the pear growers of America for more than a century, and which made its appearance in California about ten years ago, the disease known as the pear blight.
THE PEAR TREE SCOURGE
To appreciate the importance of this element of resistance to disease, as manifested by the oriental pear, it must be understood that the blight is a malady of such virulent nature that when it attacks the pear tree it very commonly results in killing it outright. This suggests, obviously, a peculiar susceptibility on the part of the pear. Such susceptibility is manifested, unfortunately, in exceptional measure by the best European varieties, including the Flemish Beauty and the Bartlett. This, presumably, is the penalty of over-specialization in a certain direction, or unbalanced selection. Until very recently the cause of pear blight was much disputed, but the agricultural experiment stations have now furnished conclusive proof that it is a bacterial disease, due to the presence of a germ that has been named Bacillus amolovorus. This germ has close cousinship with the various tribes of bacilli that cause the contagious human maladies. And there is a curious resemblance between the assault of the microbes on the pear tree and the corresponding assaults of certain bacilli, for example the diphtheria bacillus, on the human organism. In one case as in the other, the bacilli, once they find a lodging place, multiply inordinately and give out excretions that are virulently poisonous. Located on the flowers and fruit of the pear, or finding their way to the inner bark or cambium layer of the tree, they multiply prodigiously and exert a malignant influence that withers blossoms, blights the fruit, and causes the leaves to take on a bronzed red hue that is often premonitory of the death of the tree. If they find lodgment in the cambium layer of the trunk, they may spread rapidly in every direction, until they girdle the tree, shutting off its supply of sap as effectively as if it had been girdled with an axe. Wherever lodged, the colonies of bacilli may be located by the oozing out of a milky or dirty brown sticky liquid when the spring rains come. This liquid is attractive to insects, and as the feet and bodies of these marauders become covered with the germ-laden fluid, the transfer of the germs to other trees and to flowers and fruit even fairly remote is thus assured. Not merely flies and gnats, but the bee itself may have a share in thus transporting the contagion from one tree to another till it infects every tree in the orchard. The nectary of a pear, which the bee may inadvertently inoculate, furnishes a most favorable medium for the multiplication of the bacilli. Thence they work their way from the fruit buds to the limbs. Once they gain access, through the links in the tree's armor furnished by the buds, to the cambium layer of the inner bark, there is nothing to prevent the indefinite extension of their colony. A tree thus inoculated may soon take on the appearance of a tree scourged by fire. Indeed, the malady is sometimes spoken of as "fire blight."
ANTISEPTIC SURGERY IN THE ORCHARD
The measures taken by the horticulturist to save his tree when thus attacked are curiously suggestive of the methods of the modern surgeon. Infected limbs must be amputated; local areas of infection in the bark or trunk or large branches must be thoroughly excised, including a goodly portion of healthy wood and bark to make sure of the removal of every microbe. Large wounds are then carefully disinfected with a sponge or bunch of waste soaked in kerosene or in a solution of corrosive sublimate, one part to the thousand. It is merely antiseptic surgery applied to the tree to combat a microbe closely similar to the ones that are man's most malignant enemies. But, of course, such measures as these, however necessary, can by no means be regarded as solving the problem of the pear blight. Just as the surgeon of today attempts to prevent the intrusion of the germs, rather than to depend on killing them after they appear, so the orchardist must hope to find a means of preventing the blight instead of being obliged to practice such heroic and wasteful curative measures. One measure looking to this end that has been suggested is the destruction of old hawthorne and wild crab apple trees and of abandoned pear and apple trees in the neighborhood of the orchard, since a single infected tree would prove a source of danger to every tree within a radius of a mile or more. Such measures are important; but they do not go to the root of the matter. The real solution must come through making the tree immune to the attacks of the germ. This is the keynote of preventive medicine with the human subject today, as illustrated by the vaccine treatment, of which the most familiar example is Sir Almroth Wright's inoculation for the prevention of typhoid fever. It is at least within the possibilities that a not dissimilar inoculation may give the tree immunity by developing its powers of resistance, quite as the human subject is given immunity. Of course the tree has no arterial system that can be inoculated with hypodermic syringe as the human subject is inoculated. But the life of the tree is dependent on the circulation of fluids within its tissues none the less. These fluids are taken in by the roots, and they find their way to the uttermost leaf. So it is conceivable that by proper treatment of the soil about the tree, the tissues of the tree itself might be so altered as to become resistant to the attacks of the bacterial enemies.
IMMUNITY THROUGH TREATMENT AND BREEDING
Nor is this idea altogether theoretical. Experiments have already been made that look to the checking of the growth of the tree by withholding fertilizers and water, that the development of the tender buds and shoots, which are the usual points of attack of the enemy, may be made to take place slowly and thus to present tissue of a less succulent order. Such hardening of the wood by withholding water has proved effective in the case of some pear orchards in Colorado, where it appears that the pear does not really need so much water as it ordinarily receives. But the effort to give the tree immunity must go even deeper. Induced immunity is valuable, but the ideal condition is that of inherent resistance, bred in the tissues. Physicians tell us that the all-important thing in warding off bacterial infections in the human subject is the inherent vitality and resistance of the patient himself. In the last analysis, this is the prime essential. A thoroughly rugged organism may be immune to almost every type of bacterial disease. We are told that almost no one escapes infection with the germs of tuberculosis. The ones who show no evidence of the disease are simply those whose tissues are so resistant that the attacks of the bacilli are thwarted. The horticulturist must take a lesson from the experience of the physician, in particular with regard to the malady we are now considering; for, as we have just seen, the analogy between the pear blight and human infections is almost perfect. So the ideal at which the plant experimenter must aim is the development of a tree that will be immune to the attacks of the bacillus, however freely the germ finds access to it. My new hybrid pear, thanks to its Oriental heritage, seems to fulfil this condition. The same thing appears to be true, at least in some measure, of the other hybrids that have the Oriental strain. So there is every reason to hope that we shall be able to develop races of pears, having all desirable qualities of fruit for the different markets, that will be free from the pest that hitherto has made the raising of this fruit a more or less precarious industry.
IDEALS AND POSSIBILITIES
As to the other needs and possibilities of pear development, not much need be said. Reference has elsewhere been made to the desirability of giving the pear a brilliant color; but this can doubtless be accomplished without great difficulty. It has also been noted that as to size of fruit, as well as in the matter of form, there is little to be desired by way of change. There is, however, one quality that the specialized pears have markedly lacked. They will keep for a time if plucked while green, and will ripen off the tree. But if allowed to ripen on the tree they decay very quickly after picking. It is obviously desirable that the pear should be given keeping qualities. But here, as in case of immunity to the blight, the solution is already in sight. Among the varied fruits of my hybrid seedlings, there are some that produce winter pears that keep quite as well as ordinary winter apples. These furnish the foundation for future hybridizing and selecting experiments, through which, without question, it will be possible to produce races of pear having all the qualities of flesh that have hitherto made the fruit popular, and with the added property of keeping over winter. Other possibilities of pear development lying a little farther in the future and therefore somewhat more vaguely outlined, have to do with the hybridization of the pear with the allied fruits of related species. It is well-known that the pear shows, in this regard, a strong disinclination for entering into such an alliance. The pear may be grafted on the quince but it is usually considered impossible to graft it on the apple. I successfully carried out such a grafting experiment, however, when I was a boy in Massachusetts, the cion being a Seckel pear. But although this grafted cion bore fruit for two seasons, it then died, probably because of the uncongeniality of the alliance. This experiment shows that there is not complete antagonism between the two species; and the same thing is further demonstrated by the well-known fact that the apple may be grafted on the pear stock; although here also the alliance is not likely to prove fruitful and satisfactory. But of course grafting is only an incidental adjunct of the work of the plant developer. The impulse to progress must come through hybridization and selection. Here, it appears to me, there are great possibilities. I have hybridized the pear and the apple; also the pear and the quince. The seedlings from these unions have sometimes seemed thrifty, but were always infertile. They were highly interesting none the less. The most successful cross was obtained by using the pollen of the Bartlett pear upon the Gravenstein apple. The seedlings from this cross were divergent in appearance, and variable as to growth. One of the seedlings grew fully as fast as the ordinary apple seedling, but most of them had a sickly, dwarfed appearance, and some died after having made a foot of growth. Three or four of those that lived were grafted on an apple tree. They maintained moderate growth for several years, but were never healthy or vigorous, and never gave any intimation of blooming. The results of the crosses between the pear and quince were closely similar. From these hybrids also I failed to secure fruit. Some grew with great vigor for years, while others almost refused to grow at all. In general appearance, and especially in foliage, the hybrids bear a closer resemblance to the pear than to the quince. But many appeared to be fairly good composites of these widely differing plants. As there are many varieties both of pears and quinces, each having individual characters and diverse hereditary tendencies, an inviting field is open to the careful and patient experimenter in crossing these distinct yet related species. If the right combination can be effected, the results undoubtedly will be profoundly interesting and valuable. Precisely what these results will be, no one can predict. But that new fruits, making most valuable additions to the dietary, may ultimately be thus developed, there is no reason to doubt. The pear and its cousin the apple may well be considered the two orchard trees which are friendliest to man.
This text is from: Luther Burbank: his methods and discoveries and their practical application. Volume 4 Chapter 4