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I am sometimes asked why I did not establish my experimental gardens in Southern California. My answer is that I chose the location somewhat by accident, but that I soon found reasons for not changing it. The chief of these is that I desired to produce fruits, flowers, and vegetables adapted for growth in the widest possible territories, and it was therefore desirable that I should be located in a region where the plants could have the test of relatively cold winters during the time of their development. Moreover nearly all our orchard fruits thrive and come to perfection in this part of the state better than almost anywhere in the southern part. But, on the other hand, the location has not been altogether without its drawbacks; for whereas I am able to experiment to better advantage with the hardy plants, I am somewhat handicapped in the attempt to deal with the more tender ones. This is notably true of the orange and its allies of the citrus family. These fruits very naturally interested me from the outset, not only because of their economic importance, but because the five familiar species of the family, namely the orange, lemon, lime, shaddock, and citron present inviting diversities of form and habit, and yet are so closely allied that they cross very readily, and thus give the plant experimenter precisely the opening that he is always seeking. It is probable that all these citrus fruits sprang from one original species growing somewhere in the region of northern India. But although the habitat of these plants has always been restricted to sub-tropical climates, yet they have become so diversified as to form fairly good species, and the different traits of the various members of the clan are fairly fixed. Not, indeed, that any of them may be raised advantageously from seed, for here they show the same diversity that is shown by the other orchard fruits. But all varieties of oranges, for example, differ quite radically from any variety of lemons, and the seeds of the orange will not produce the lemon, or vice versa, however widely the progeny may differ from the parent form within the limits of specific variation.


My attempts to cultivate the citrus fruits date back about a quarter of a century. I pursued the investigation actively for a time, securing everything that was to be had, including the small Japanese variety called the Kumquat, Kimkan, or Kinkit, Citrus Japonica. This is a small, lime-like fruit produced in amazing abundance, having acid flesh but a skin with sweet, pleasant, orange flavor. Wild oranges were sent me also from Central Africa, Australia, and South America, and the best cultivated varieties from Burmah, Ceylon, and various less distant regions. The object primarily in view was the production of a hardy orange; one that would grow in northern California, and in regions of the eastern United States well to the north of the present limits of growth of this tender fruit. My experiments were promising at the outset, and I soon had a variety of hybrid seedlings. But there came a series of cold winters that destroyed the entire citrus orchard, and after one or two other tentative efforts, I was compelled to admit that my farms are located in a region unsuited for development of the citrus fruits. The initial investigations through which the hardy orange is developed must be made in a more favorable locality. I frequently mentioned my belief that a hardy orange could be developed, however, and it is satisfactory to record that experiments along this line have more recently been undertaken under the patronage of the United States Government. The variety known as Citrus trifoliata, a wild form which had never been much cultivated, was known to be exceptionally hardy. This was hybridized with the sweet orange in the Government experiments just referred to, and the early results are thought to be very promising. "Among the seedlings observed," says Professor E. M. East, "several have proven valuable." They form a new class of citrus fruits and have been called Citranges. Three of these varieties have been named the Rusk, the Willits, and the Morton. The Rusk, which is a hybrid of orange crossed by trifoliata, is a small fruit with a bitter tang like the pomelo. It makes excellent marmalade and preserves. The Willits, coming from a cross of orange upon trifoliata, is a rough, but thin-skinned fruit, resembling an orange in appearance but a lemon in flavor. It is used as a condiment or for citrangeade. The Morton, coming from the same kind of cross as the Willits, is a large, juicy, almost seedless fruit, only slightly more bitter than the sweet orange. "Young trees of these three varieties have endured a temperature of eight degrees above zero, and it is thought that by the use of these, and of similarly obtained varieties, citrus fruit culture can be extended fully 400 miles north of the present region." Doubtless the orange will always remain a relatively tender fruit, for it is an evergreen that has never wandered far from the Tropics. But it is equally little to be doubted that it could be made much hardier than any existing race of citrus fruits, and the incentive for the production of such a hardy race is so great that there should be no dearth of experimenters in the field. The orange crop is occasionally blasted even in Florida by an unusual frost. In 1895, for example, the loss of the trees themselves was so great as to put a serious handicap on the industry for a term of years. So it is imperative that a race of oranges should be developed that will be capable of enduring occasional periods of cold. But, aside from the tentative experiments just noted, very little has hitherto been accomplished in this direction. The field is open for any experimenter who is located in a region that lies well within the present orange belt (preferably near its northern limits) and the reward that awaits the successful developer of a hardy orange is sure and significant.


Everyone is familiar nowadays with the so called Navel Orange, which combines the very notable quality of seedlessness with large size and general excellence of quality. The seedless condition of this orange is not the result of skillful selection, but appeared as a "sport" in certain wild oranges of Brazil. There are almost numberless varieties of oranges growing wild in the region of the Amazon. A lady who was traveling through South America, was surprised to find among the oranges served at the hotel where she was stopping some that were seedless-a thing hitherto never conceived even as a possibility among cultivators of the fruit. The discovery was communicated to the Agricultural Department at Washington, and in 1870 the new variety was imported. Four years later specimens of the tree were sent from Washington to California and the fruit, which was subsequently christened the Washington Navel in recognition of its origin and its peculiar form, soon came to be extensively cultivated. This variety is subject to bud variation and a number of more or less distinct varieties have made their appearance. But there is still opportunity for improvement through further selection.


The orange is budded or grafted on roots of its own species or on those of the lemon or the shaddock, better known as the grapefruit. The process of budding is altogether similar to the budding of other trees and it presents no difficulties. Stocks may be grown from seed but, as already noted, seedlings cannot be depended upon to reproduce the parent forms, and all the best varieties of orange are propagated by grafting. The chief peculiarity of orange culture is that it is necessary to grow the fruit on irrigated soil. Water is, of course, essential to all plant life, but a tree like the orange, with heavy evergreen foliage, makes exceptional demands, and it is imperative, if the large juicy fruit is to be brought to perfection, that these demands shall be adequately met. It was the recognition of this fact by the old Moors more than a thousand years ago that made Valencia in Spain, thanks to the Moorish system of irrigation, the heart and center of the orange industry of the world. The irrigation system established by the Moors is still in successful operation, and Valencia remains the largest single shipping port for oranges anywhere in the world. It is only in very recent years that California fruit has challenged the product of the Spanish orchards. The absorption of water by the roots of the tree, and its elevation through the trunk to supply the deficit made by constant transpiration from the pores of the leaves is a phenomenon that has been perfectly familiar to botanists for a long time. It was demonstrated experimentally by Stephen Hales early in the 18th century. But the forces that lie back of the phenomenon have been very little understood. Very recently one of the most celebrated American botanists has declared that the cause of the rise of sap in trees remains perhaps the most interesting of botanical puzzles. It is, in effect, as some one has pointed out, a case of water running up hill, and many botanists have found it mystifying that the plant tissues are able to withstand the pressure that a column of water must exert, particularly in the case of tall trees.


In point of fact, however, it should be recalled that the sap in the tree is not carried in open tubes comparable to the arteries of the animal system. If it were in such tubes, doubtless no plant tissues could withstand the pressure that would be exerted by the weight of the column of water, carried, let us say, to the top of a redwood tree. For that matter, a column of water in even a relatively small tree like the orange would probably exert a deleterious pressure on the cellular structures. But in reality the water in the plant is contained largely in the cells of the plant tissue, and is passed on by osmosis or exudation from one cell to another. It seems probable that the laws of osmosis as developed by the Dutch physicist Vant Hoff, partly in response to questions raised by Professor deVries, give a clew to the entire subject of the rise of sap in the tree. According to Vant Hoff's theory, osmosis or the passage of water through a membrane from a weaker to a stronger solution, is due to the pressure of the molecules in the stronger solution which, in virtue of their greater numbers, beat against the cell wall and exert a pressure exactly comparable to the pressure of a gas. The push of the molecules against the cell wall suffices to squeeze water through the wall until there is an equalization of pressure on both sides. As the protoplasm in the cells of the rootlets of a plant is more concentrated than the watery solutions in the soil about it, osmotic action is established, which results in the cells taking up a certain amount of water. But cells that thus take in water at once give up a portion of it to their neighbor cells, and these in succession pass it on to their neighbors. Thus, through an endless series of reactions between the cells the water is carried up in the living wood next to the bark of the tree and ultimately to the leaves.


The process is not altogether unlike the activities of a fire brigade in the rural districts, where a line of men is formed from the fire to the nearest well, and buckets are passed from hand to hand. If the fire is in the upper story of a building, men on the ladder may similarly hoist one bucket after another from hand to hand. And in this case it is obvious that there is no question of a column of water to exert pressure. The water is transported in individual buckets each one independent of the others. And it would appear that the case of the water in the plant cells is closely comparable. Each pair of cells constitutes a system more or less independent of all the others. The forces of osmosis, operating between each pair of cells, are in command of the situation and so break the continuity that all semblance to a continuous column of water is lost. The full power of the molecular forces that, acting jointly, carry the water to the tree tops will best be understood when it is recalled that if a rubber tube is put tightly about the end of an amputated twig, water in this tube will be forced upward by the pressure of water in the cells of the twig. This experiment, first made by Hales in 1727, in itself shows how utterly different are the conditions of water in the tree from the mere mechanical condition of pressure that governs the water in a closed tube, or otherwise standing in a single receptacle.


Many boys have made the experiment of bursting a barrel by the pressure of water in a small iron pipe projecting upward from the barrel. Whoever has seen the experiment will not doubt that the physical laws governing the water in the trunk of the tree are quite different from those that govern the water in the iron tube. And the difference is due, the physicists assure us, to the interposition of the molecular forces. Whether or not the laws of osmosis, above outlined, as discovered by Vant Hoff, give full explanation is matter for the physicists to decide. As yet they are not quite sure about it. But that the osmotic forces are at least partly instrumental in lifting the water, all are agreed. Meantime, referring specifically to the orange, it requires no great powers of observation to discover why this tree stands in such pre-eminent need of an exceptional water supply. It is only necessary to recall that the bulk of the fruit is juice, each orange containing four or five ounces of water, to discover what the tree does with the liquid it imbibes so freely. A well-laden orange tree, with say a thousand mature fruits, is carrying the equivalent of thirty or forty gallons of water in its globular buckets; and of course there is constant transpiration of moisture from the leaves which in the aggregate is far greater.


And all of this, of course, applies not merely to the orange but to the allied citrus fruits, in particular to the grapefruit and the lemon. Indeed, the entire company of citrus fruits is characterized by exceeding juiciness of pulp, the bulk of the fruit being made up of water-with delicious acids and sweets instilled therein-merely intermeshed with enough thin fibrous tissues to give stability to the fruit structure. These fruits are further characterized by the unique quality of the fruit-covering, which is painted with marvelous hues that are so unique as to have given their names to prominent pigments of the painter's color box; and incorporate curious series of minute oil wells laden with essential essences of no less individual quality. These traits, among others, mark the citrus fruits as constituting a highly specialized and isolated group of plants. It is not to be expected that any one of them could be hybridized with a member of any other family. But, on the other hand, within the bounds of the citrus family there is full opportunity, as I have already pointed out, for cross-fertilization. I am confident that many interesting developments would have resulted from the hybridization of oranges and lemons and limes and citrons in my orchard had not the frost treated the tenderlings so harshly. Not unlikely there would have been developed new citrus fruits differing from any existing one as markedly as the plumcot differs from apricot and plum. This, of course, is only matter of conjecture for the experiments were cut short, as already told, before they passed beyond the early stages. Still the fact that I was able to effect hybridization between the various citrus fruits is highly suggestive and should prove stimulative to other workers. Here is a field as yet scarcely entered and one that offers almost unbounded possibilities. The orange industry is the great fruit industry of California today, as it is of the Gulf States. In both of these regions experimenters should take up the work. It is at least possible that new and strange citrus fruits may thus be brought into being. As a single hint suggestive of possibilities, let me recall that the very earliest plum in existence today is probably the one that I developed by successive hybridizations which ultimately introduced and blended the strains of six of the latest plums. Possibly, then, the problem of developing an orange resistant to cold-one that may be grown not merely along the Gulf but along the Great Lakes as well-may be solved in similar fashion. It seems paradoxical to suggest that the blending of oranges from half a dozen tropical and sub-tropical climates-India, Arabia, Northern Africa, Brazil, Florida, Southern California-might produce a fruit adapted to the climate of, let us say, Missouri or Ohio; yet the case of my early plum, descended from late ancestors, suggests that this idea is not altogether chimerical. This work will be greatly simplified by the fact that we now have an orange, before mentioned, which, without special selection for this purpose, is now hardy as far north as Philadelphia.


And a similar suggestion may be made regarding a considerable company of other fruits that have come to us from tropical and sub-tropical regions. The olive, the fig, the persimmon, the guava, the alligator pear, the banana, the pomegranate, the pineapple-these are but a few of the more familiar members of a varied company of fruits, not in themselves related except that they all had their original home in the Tropics and for the most part have proved indisposed to migrate extensively into temperate zones. One or two of these, to be sure, have shown a tendency to follow the example set by the plum, the pear, and the apple, and try their fortunes in regions lacking the perpetual summer of their original habitat. Most notable among these, perhaps, is the persimmon, which made its way to Japan on one continent, and to the south central regions of the United States on the other. This fruit has been cultivated to best advantage in Japan, where the secret was first discovered that its astringency is lost when the fruit is packed closely in air tight receptacles. In this country it was discovered by Mr. Geo. C. Roeding of Fresno that the secret of the Japanese persimmon is no more mystifying than this: It is merely necessary to pack the fruit in tubs from which Saki or Japanese "rice beer" has been recently removed. It appears that carbonic acid in the absence of oxygen produces in the fruit precisely the chemical changes necessary to transform it from an astringent and inedible fruit to a highly palatable one. I have raised vast numbers of seedlings of the Japanese persimmon and have attempted to produce new varieties by crossing this with the American persimmon; but as yet I have not succeeded in effecting this hybridization-chiefly, perhaps, because the American species is such a shy bearer that I have had few good opportunities to cross-fertilize the two. Now that the good qualities of the persimmon are beginning to be more generally recognized, further experiments in this direction will probably be carried out, and there is every reason to expect, arguing from analogy, that new and greatly improved races of persimmons may thus be developed. Whoever will contrast the hybrid Japanese-American plum of today as developed in my orchards at Santa Rosa and Sebastopol with the best plums of thirty years ago will see at least a suggestion of new possibilities in the prospective union of the Japanese and American persimmon. For the best existing varieties of persimmon-the Japanese races are incomparably superior to the American-have such qualities as furnish a secure foundation on which to develop a really notable orchard fruit.


Another experiment that I have tried, as yet unsuccessfully, with sub-tropical fruits, is the hybridization of the fig and the mulberry. The fig, as is well known, grows abundantly in California. Nearly every one has learned that for many years after it was introduced, the fig was a very poor bearer, blossoming abundantly but failing to ripen satisfactory fruit. The trouble, as was presently discovered, was that the peculiar minute species of wasp which is the sole bearer of pollen from the male or so-called Capri fig to the pistillate flowers, was not found in California. So soon as this insect was imported from Italy, figs of good quality were borne in abundance by hitherto barren trees. The fig has been under cultivation perhaps as long as any other fruit, and it is exceedingly variable when grown from seed. I have grown seedlings in abundance, but 99 out of 100 produce worthless fruit. You plant seeds of the white fig and you are quite as likely to get black or brown figs as white ones. This is probably because the Capri fig has never been cultivated for color; in fact very little attention has been given to it, even for the development of vigor and productivity. About the only attention paid it by the fruit grower has had reference to the early or late time of blooming. This is important merely because it is necessary that staminate and pistillate plants should bloom at the same time, else the fig wasp obviously cannot perform its pollenizing service. A pound of European figs, grown from flowers fertilized by the Capri insect (otherwise the seeds would be infertile) will produce perhaps ten thousand seedlings. But it requires patience to wait fifteen or twenty years to test the fruit, and it cannot be fairly tested in less time. It is difficult to hasten the process by grafting because the fig cion does not take kindly to being transplanted. Doubtless a satisfactory method of grafting might be developed, however, were sufficient attention given to the subject. Perhaps nothing more would be necessary than to protect the cion carefully against drying, by covering it with a paper bag until union has taken place, as is done in grafting the orange and various other fruits, and the walnut. As just stated, the attempts to hybridize the fig with its relative, the mulberry, did not prove successful. But this was probably because I did not give enough time and patient attention to the effort. The two fruits are botanically related and I sometimes think of the fig as a mulberry turned outside in. It should be possible to effect hybridization between the two species, and perhaps greatly to improve one or both of them; possibly even to develop a wholly new fruit through this union like the plumcot.


We need not enter into further details in connection with the subject of tropical fruits because I am chiefly concerned in this narrative to tell what I have accomplished in the way of plant development rather than to dwell on unrealized possibilities. But I cannot refrain from urging upon others who are geographically so located as to bring the tender fruits within the range of their experiments, the desirability of undertaking extensive series of investigations in this practically untrodden field. It should be recalled that all of our fruits, even the hardiest ones that now penetrate to the Arctic Zone, must have come originally from the Tropics. The fact that the plum and pear and apple have become hardy enough to resist winters of almost Arctic severity is in itself an all-sufficient evidence of the adaptability of the fruit bearers, and should be an inspiring object lesson to the experimenter with fruits that still retain the tropical and sub-tropical habit. It requires no very great powers of prophetic vision to forecast a day when a large number of fruits that now are known only in sub-tropical zones will have made their way, under guidance of the plant developer, across many degrees of latitude that at present seem like impassable barriers. The Feijoa (pronounced fay-zho-a) or fig guava (Feijoa Sellowiana) from Brazil, a vigorous fruiting shrub; the Cherimoya (Anona cherimolia) from the Central American highlands, which has been classed with the pineapple and the mangosteen as making up the trio of the world's finest fruits; the Australian Macadamia (Macadamia ternifolia), prized for both fruit and nut; the Natal Plum (Carissa grandiflora) from South Africa, with its fragrant flowers and scarlet fruit; and the White Sapote (Casimirva edulis) from Mexico with quince-like fruit of unique flavor-these are among the tropical and sub-tropical products that have come to us within recent years and that promise to make secure place for themselves among well-prized fruits of orchard and market. And there are others yet to come. Meantime I should not like to predict as to which among the fruits that now are confined solely to the region of the Gulf of Mexico and to Southern California as their northern limits, may not within a century be growing and bearing luxuriantly in the region of the Great Lakes.

-It should be recalled that all of our fruits, even the hardiest ones that now penetrate to the Arctic zone, must have come originally from the tropics.

This text is from: Luther Burbank: his methods and discoveries and their practical application. Volume 4 Chapter 9