The White Blackberry
How A Color Transformation Was Brought About
To speak of white blackbirds or of white blackberries is to employ an obvious contradiction of terms. Yet we all know that now and again a blackbird does appear that is pure white. And visitors to my experiment gardens during the past twenty years can testify that the white blackberry is something more than an occasional product-that it is, in short, a fully established and highly productive variety of fruit. I doubt, however, whether there is record of anyone having ever seen a truly white blackberry until this anomalous fruit was produced. Nevertheless it should be explained at the outset that the berry with the aid of which I developed the new fruit was called a white blackberry. It was a berry found growing wild in New Jersey, and introduced as a garden novelty, with no pretense to value as a table fruit, by Mr. T.J. Lovett. He called the berry "Crystal White," but this was very obviously a misnomer as the fruit itself was never white, but of a dull brownish yellow. It had as little pretension to beauty as to size or excellence of flavor, and was introduced simply as a curiosity. When a white blackbird appears in a flock, it is usually a pure albino of milky whiteness. It may be regarded as a pathological specimen, in which, for some unknown reason, the pigment that normally colors the feathers of birds is altogether lacking. It is not unlikely that the original so-called white blackberry was also an albino of this pathological type. But if so, hybridization had produced a mongrel race before the plant was discovered by man, or at least before any record was made of its discovery; for, as just noted, the berry introduced by Mr. Lovett could be termed white only by courtesy. Nevertheless the berry differed very markedly from the normal blackberry, which, as everyone knows, is of a glossy blackness when ripe. So my interest in the anomalous fruit was at once aroused, and I sent for some specimens for experimental purposes soon after its introduction, believing that it might offer possibilities of improvement. Making use of the principles I have found successful with other plants, my first thought was to hybridize the brownish white berry with some allied species in order to bring out the tendency to variation and thus afford material for selective breeding.
CREATING A REALLY WHITE BLACKBERRY
The first cross effected was with the Lawton blackberry, using pollen from the Lawton berry. The Lawton is known to be very prepotent; it is of a very fixed race and will reproduce itself from seed almost exactly, which is not true of most cultivated fruits. Its seedlings often seem uninfluenced when grown from seed pollenated by other varieties. It was to be expected, therefore, that the cross between the Lawton and the "white" berry would result in producing all black stock closely resembling the Lawton; and such was indeed the result. But the Lawton also imparts its good qualities to hybrids when its pollen is used to fertilize the flowers of other varieties. As a general rule it is my experience that it makes no difference which way a cross is effected between two species of plants. The pollen conveys the hereditary tendencies actively, and so-called reciprocal crosses usually produce seedlings of the same character. That is to say, it usually seems to make no practical difference whether you take pollen from flower A to fertilize flower B, or pollen from flower B to fertilize flower A. This observation, which was first made by the early hybridizers of plants more than a century ago, notably by Kolreuter and by Von Gaertner,-is fully confirmed by my observations on many hundreds of species. Nevertheless it occasionally happens that the plant experimenter gains some advantage by using one cross rather than the other. In the present case it seemed that by using the Lawton as the pollenizing flower, and growing berries on the brownish white species, a race was produced with a more pronounced tendency to vary. Still the plants that grew from seed thus produced bore only black berries in the first generation, just as when the cross was made the other way. It thus appeared that the prepotency of the Lawton manifested itself with full force and certainty whether it was used as the staminate or as the pistillate flower. When the flowers of this first filial generation were interbred, however, the seed thus produced proved its mixed heritage by growing into some very strange forms of vine. One of these was a blackberry that bloomed and fruited all the year. This individual bush, instead of dying down like others, kept growing at the top like a vine or tree, and when it was two or three years old it was so tall that a step-ladder was required to reach the fruit. Its berries, however, were rather small, soft, and jet black in color. This plant, then, was an interesting anomaly, but it gave no aid in the quest of a white blackberry. But there were other vines of this second filial generation-grandchildren of the Lawton and the original "Crystal White"-that showed a tendency to vary in the color of their fruit, this being in some cases yellowish white. Of course these bushes were selected for further experiment. Some were cross-fertilized and the seed preserved. The vines that grew from this seed in the next season gave early indications of possessing varied qualities. It is often to be observed that a vine which will ultimately produce berries of a light color lacks pigment in its stem, and is greenish or amber in color, whereas the stem of a vine that is to produce black berries is dark brown or purple. A few of the blackberry vines of the third generation showed this light color; and in due course, when they came to the fruiting age, they put forth heavy crops of clear white berries of such transparency that the seeds, though unusually small, could readily be seen through the translucent pulp. These were doubtless the first truly white blackberries of which there is any record. But there were only four or five bushes bearing these white berries in an entire generation comprising several hundred individual bushes, all having precisely the same ancestry. From among the four or five bushes, the one showing a combination of the best qualities was selected and multiplied, until its descendants constituted a race of white blackberries that breeds absolutely true as regards the white fruit.
NOW BREEDS TRUE FROM SEED
The descendants of this particular bush were widely scattered and passed out of my control. But subsequently from the same stock, I developed other races, and finally perfected, merely by selection and interbreeding from this same stock, a race of white blackberries that breeds true from the seed, showing no tendency whatever to revert to the black grand-parental type. This is, in short, a fruit which if found in the state of nature would unhesitatingly be pronounced a distinct species. Its fruit is not only snowy white in color, but large and luscious, comparable in the latter respect to the Lawton berry which was one of its ancestors. "Was there ever in nature a berry just like this?" a visitor asked me. Probably not; but there was a small white berry and a large luscious black one, and I have brought the best qualities of each together in a new combination.
THE ANOMALY EXPLAINED
Reviewing briefly the history just outlined, it appears that the new white blackberry had for grandparents a large and luscious jet black berry known as the Lawton blackberry and a small ill-flavored fruit of a yellowish brown color. The descendant has inherited the size and lusciousness of its black ancestor, and this seems not altogether anomalous. But how shall we account for the fact that it is pure white in color, whereas its alleged white ancestor was not really white at all? The attempt to answer that question brings us face to face with some of the most curious facts and theories of heredity. We are bound to account for the white blackberry in accordance with the laws of heredity, yet at first blush its dazzling whiteness seems to bid defiance to these laws, for we can show no recognized white ancestor in explanation. A partial solution is found if we assume, as we are probably justified in doing, that the original stock from which the so-called "Crystal White" berry sprang was a pure albino. It has already been suggested that such was probably the case. There is, indeed, no other very plausible explanation available of the origin of the anomalous berry. White is not a favorite color either among animals or among vegetables. Except in Arctic regions it is very rare indeed to find an unpigmented animal or bird, and white fruits are almost equally unusual. In the case of animals and birds, it is not difficult to explain the avoidance of white furs and feathers. A white bird, for example, is obviously very conspicuous, and thus is much more open to the attacks of its enemies than a bird of some color that blends with its surroundings. So we find that there is no small bird of the Northern Hemisphere, with the single exception of the snow-bunting, which normally dresses wholly in white. The exception in the case of the snow-bunting is obviously explained by the habits of the bird itself. And even this bird assumes a brownish coat in the summer. There are a few large water-fowl, notably the pelican and certain herons that wear snowy white plumage habitually throughout the year. But these are birds of predacious habits that are little subject to the attacks of enemies, and it has been shown that the white color, or bluish white, tends to make the birds inconspicuous from the viewpoint of the fish that are their prey. So in the case of the tiny snow-bunting and of pelicans and herons, the white color of the plumage is seen to be advantageous to its wearer and hence is easily explained according to the principle of natural selection. The same is true of the white plumage assumed by those species of grouse and ptarmigan that winter in Arctic or sub-arctic regions; and contrariwise, the pigmented coats of the vast majority of the birds and animals of temperate zones are accounted for on the same principle. But just why the fruits of plants should almost universally be pigmented seems at first not quite so clear. It is ordinarily supposed to be advantageous for a plant to have its fruit made visible to the birds and animals, that the aid of these creatures may be gained in disseminating the seed. And it must be obvious that a white blackberry would be as conspicuous in the woodlands where this vine grows as are the jet black berries of the ordinary type. Why, then, you ask, has not natural selection developed a race of white blackberries? I am not sure that anyone can give an adequate answer. Perhaps it is desirable to have the seeds of a plant protected from the rays of the sun, particularly from those ultra-violet rays which are known to have great power in producing chemical changes. Recent studies of the short waves of light beyond the violet end of the spectrum show that they have strong germicidal power. It will be recalled that the celebrated Danish physician Dr. Finsen developed a treatment of local tubercular affections based on the principle that ultra-violet light destroys the disease germs. And most readers have heard of Dr. Woodward's theory that very bright light is detrimental to all living organisms. Possibly too much sunlight might have a deleterious effect on the seeds of such a plant as the blackberry. Indeed, the fact that the berry quickly develops pigments under ordinary conditions, and develops them much earlier than the stage at which it is desirable to have the fruit eaten by birds, suggests that this pigment is protective to the fruit itself in addition to its function of making the fruit attractive to the bird. But be the explanation what it may, the fact remains that very few fruits in a state of nature are white; and no one needs to be told that fruits of the many tribes of blackberries, with the single exception of the one under present discussion, are of a color fully to justify the name they bear. Yet the experiment in breeding just recorded proves that, at least under the conditions of artificial selection, a race of berries may be developed which, though having the flavor and contour of the blackberry, is as far as possible from black in color. The fact that this race of white berries was developed in the third generation from parents one of which is a jet black fruit and the other a fruit of a brownish tint, seems at first glance to give challenge to the laws of heredity.
ATAVISM AND UNIT CHARACTERS
Even though we assume that a remote ancestor of our newly developed white blackberry was a pure albino, the case still seems mysterious. Similar cases of reversion to the type of a remote ancestor have been observed from time to time by all breeders of animals and by students of human heredity, and it has been customary to explain such cases of reversion, or at least to label them with the word "atavism." If this word be taken to imply that all traits and tendencies of an ancestral strain are carried forward from generation to generation by heredity, even though unable to make themselves manifest for many generations, and that then, through some unexplained combination of tendencies, the submerged trait is enabled to come to the surface and make itself manifest, the explanation must be admitted to have a certain measure of tangibility. Nevertheless there is a degree of vagueness about the use of the word "tendencies" that robs the explanation of complete satisfactoriness. Meantime the human mind is always groping after tangible explanations of observed phenomena. It is always more satisfactory to be able to visualize processes of nature. It was for this reason that Darwin's theory that natural selection is the most powerful moving factor in the evolution of races gained such general recognition and still remains as the most satisfactory of all hypotheses of evolution. And it is for the same reason that a tangible explanation of the phenomena of atavism or the reversion to ancestral types has gained a tremendous vogue in recent years. The explanation in question is associated with the name of the Austrian monk Mendel, who made some remarkable experiments in plant breeding about half a century ago, and who died in 1884, but whose work remained quite unknown until his obscure publications were rediscovered by Professor Hugo De Vries and two other contemporary workers, and made known to the world about the year 1900. Since then a very large part of the attention of the biological world has been devoted to the further examination of what has come to be spoken of as Mendelian principles. And, as is usual in such cases, unwarranted expectations have been aroused in some quarters as to the real import and meaning of the new point of view; also a good deal of misunderstanding as to the application of the so-called Mendelian laws of heredity to the work of the practical plant developer. In view of the latter fact it is well to bear in mind that such experiments in plant breeding as those through which I developed the white blackberry and hundreds of others were made long before anything was known of Mendel and his experiments, and at a time when the conceptions now associated with Mendelism were absolutely unknown to any person in the world. It is well to emphasize this fact for two reasons: first, as showing that practical breeding, resulting in the bringing to the surface of latent traits, for example, whiteness in the blackberry, could be carried to a sure and rapid culmination without the remotest possibility of guidance from "Mendelism"; secondly, because from this very fact the interpretation of my experiments has fuller significance in its bearing on the truth of the Mendelian formulas than if the experiments had been made with these formulas in mind. This is true not alone of the creation of the white blackberry, but of the similar development of the Shasta Daisy and of a host of other new forms of plant life that will find record in successive chapters of the present work. But while I would thus guard the reader against the mistake, which some enthusiasts have made, of assuming that the Mendelian formula about which so much is heard nowadays must revolutionize the methods and results of the plant breeder, I would be foremost to admit that the remarkable work of Mendel himself, together with the work of his numerous disciples of the past ten years, has supplied us at once with several convenient new terms and with a tangible explanation or interpretation of a good many facts of plant and animal heredity that heretofore have been but vaguely explicable, even though clearly known and demonstrated as facts. The case of the white blackberry with which we are at the moment concerned, is a very good illustration in point. My experiments in the development of that berry, might be interpreted in the older terminology something like this: The big, luscious, black Lawton blackberry proved prepotent when crossed with the small brownish "Crystal White," and the offspring were therefore all large luscious blackberries closely similar to the prepotent parent. But the qualities of the other parent were latent in these offspring, and, the tendency to variation having been stimulated by the hybridizing of these different forms,the offspring of the second generation showed great diversity, and a tendency to reversion to the traits of the more obscure or less prepotent of the two grandparents. In the still later generations, the conflict of hereditary tendencies continuing, an even more striking reversion, according to the principle of atavism, took place in the case of a few of the many progeny, bringing to light the pure white berry as an inheritance from a remote and long forgotten ancestor.
THE MENDELIAN EXPLANATION
Now this, as I say, would fairly explain the case of the white blackberry in such terms as were universally employed at the time when this interesting fruit was developed. But the evolutionist of today, considering the same facts, would be likely to offer an explanation in Mendelian terms that would have the merit of adding a certain measure of tangibility to the mental picture of the actual processes involved in the hereditary transmission of traits through which the white blackberry was developed. And there can be no question of the convenience of these terms and of their value in aiding to conjure up such a picture, provided it be not supposed that the presentation of such a formula is to clarify all the mysteries of heredity and to do away with the necessity in the future-as some misguided enthusiasts have assumed of laborious and patient experiments akin to those through which the triumphs of the plant developer have been achieved in the past. In a word, the Mendelian formulas, if accepted at their true valuation and for their real purpose, may be regarded as placing new and valuable tools in the hands of the plant experimenter, just as did the formula of natural selection as put forward by Darwin; but we must in one case as in the other guard against imagining that the phrasing of a formula may properly take the place of the practical observation of matters of fact. Bearing this caution in mind, let us note the changed terminology in which the Mendelian of today interprets the observed facts of the development of the white blackberry. His explanation would run something like this: When the Lawton blackberry is crossed with the whitish berry, all the offspring of the first filial generation are black because blackness and whiteness are a pair of "unit characters," both elements or factors of which cannot be manifested in the same individual; and blackness is the "dominant" character of the two, whiteness being "recessive." But the hereditary factors or "determiners" that make for whiteness, though momentarily subordinated, are not eliminated, and half the germ cells produced by the hybrid generation in which blackness is dominant, will contain the factor of whiteness, whereas the other half contain the factor of blackness. And when in a successive generation a germ cell containing the factor of whiteness unites with the germ cell of another plant similarly containing the factor of whiteness, the offspring of that union will be white, their organisms inheriting no factor of blackness whatever. It may chance, however, that for many successive generations a germ cell containing only the factor of whiteness fails to mate with another similar germ cell and so no white-fruited progeny is produced. In such a case generation after generation the white factors continue to be produced in the germ cells, but the union with a germ cell containing the black factor obscures the result just as in the case of the first cross, because the factor of blackness continues to be dominant. But, however long delayed, when a cell containing the white factor or determiner does mate with a similar cell, the offspring is white and-in the older terminology-reversion or "atavism" is manifested. A very simple and tangible illustration of the phenomena in question is furnished by the experiments in animal breeding made by Professor William E. Castle of Harvard. These experiments furnish a peculiarly appropriate illustration in the present connection because it chances that the animals experimented with are comparable to our blackberries in that they are respectively black and white in color. The animals used in the experiment are guinea pigs.
AN ILLUSTRATION FROM THE ANIMAL WORLD
Professor Castle shows that if a black guinea pig of a pure strain is mated with a white guinea pig of a pure strain, all the offspring of the first generation will be black; and it is therefore said that blackness is prepotent or dominant, and whiteness recessive. But if two of these black offspring are interbred, it is an observed fact that among their progeny three out of four individuals will be black like their parents and one of their grandparents, and the fourth one will be white like the other grandparent. The Mendelian explains that the factor of whiteness was submerged, dominated by the factor of blackness, in the second generation; but that half the germ cells of these black individuals contained the factor of whiteness, and that by the mere law of chance the union of these germ cells brought together about one time in four two of the cells having the recessive white factor; such union resulted in a white individual. Meantime by the same law of chance the other three matings out of the four brought together in one case two black factors and in two cases a mixture of black and white factors. As black is dominant, these individuals having the mixed factors would be individually black (just as those of the first cross were black); but their progeny in due course will repeat the formula of their parent by producing one white individual in four. It should be explained that the Mendelian, in expressing this formula, usually substitutes for the word "factor," as here employed, the newly devised word "allelomorph," although the less repellent equivalent "determiner" is gaining in popularity. He calls the body substance of an animal or plant a "zygote," and he describes an individual that contains factors of a single kind, as regards any pair of unit characters (say only for blackness in the case of our blackberries or Professor Castle's guinea pigs) as a "homozygote"; contrariwise a body having both types of factors (blackberries or guinea pigs of the second generation, for example) as a "heterozygote." But these big words, while it is convenient to know their meaning, need not greatly concern us. It suffices to recall the convenient terms "dominant" and "recessive"; to recognize that a good many antagonistic traits may be classed as unit characters; and to welcome the conception of the division of the factors or determiners of such a pair of unit characters in the germ cell, as enabling us to form a tangible picture of the modus operandi through which the observed phenomena of heredity may be brought about.
MIXED HERITAGE OF THE BLACKBERRIES
It remains to be said that the case of our blackberries is a little more complex than the case of the guinea pigs just referred to, because there is a second pigment involved. The "Crystal White" berry, it will be recalled, was not white but brownish in color. There were thus transmissible two pairs of unit characters involved as regards the; matter of color, namely (1) black versus white, and (2) yellow or brown versus white. The black factor or determiner dominated absolutely in the first generation; but in the second generation a certain number of germ cells were paired in such a way as to eliminate the black but retain the yellow factor. It required a third mixture of the germ-cell factors to produce a union in which neither black nor yellow factors appeared, the offspring of this union being of course the pure white blackberry. The presence of the yellow factor accounts for the further fact, to which reference should be made, that there were various intermediate types of berries, neither black nor white, which appeared in successive generations but which are eliminated by selection as they did not fall in with our plan of development of a white race. The explanation just given makes it clear that, once a union of germ-cell factors having only the white element was effected, the black and the yellow factors being entirely eliminated from that particular individual, the germ cells arising from that individual would necessarily contain only the factor of whiteness; hence that all the progeny of that individual would "breed true" and produce white berries. Such is indeed the observed fact with my developed strains of white blackberries. Grown from the seeds, these breed far truer to their parentage than is the case with most cultivated fruits. As to certain other qualities they may vary, but all are white. The Mendelian explanation obviously cannot add any force to this observed and long ago recorded fact. But it does serve to explain the observed fixity and permanency of the new and anomalous breed. It enables us in a sense to understand the paradoxical fact that a berry having a whole galaxy of black ancestors may have no strain of blackness, no tendency to reversion to the black type, in its composition. But we must not put the cart before the horse by supposing that the new explanation adds anything to the force of the previously observed facts. Hypotheses are for the interpretation of observed phenomena, not phenomena for the interpretation of hypotheses. One other word in this connection. To would be plant experimenters who ask my opinion of matters connected with the old versus the new interpretations of heredity, I am accustomed to say: "Read Darwin first, and gain a full comprehension of the meaning of Natural Selection. Then read the modern Mendelists in detail. But then go back again to Darwin." Bear in mind Professor J. M. Coulter's comment that "Mendelism has extended from its simple original statement into a speculative philosophy," and try for your own satisfaction to separate the usable formulae from the intricate vagaries of the new creed of heredity. Let me cite a recent assertion of Professor William E. Castle, himself one of the foremost experimenters along the lines of the newest theory: "As to how a new race is begotten we have not got much beyond Darwin; indeed many of us have not got so far." The man who has got as far as Darwin in the matter of understanding racial origins,-to say nothing of getting beyond him-even in our day, is no tyro in the study of heredity.
-Read Darwin first; then read the modern Mendelists; and then-go back to Darwin.
This text is from: Luther Burbank: his methods and discoveries and their practical application. Volume 2 Chapter 2