Planning a New Plant
THE FIRST STEPS IN PRACTICAL WORK
Someone has said that a painter is a man who can see the picture in the landscape. In similar fashion it may be said that a successful plant experimenter is one who can see new varieties of future plants when he looks at old existing varieties. But of course the painter, whatever his constructive imagination, does not always see at first glance every detail of form and color that will ultimately appeal to him. Nor can the plant experimenter claim, by any manner of means, to know always from the outset just what his new plant creations will be like. There are numberless instances, indeed, in which a plant experimenter who operates on a large scale may make hybridizing experiments, merely to test the possibilities of crossing certain species, without having any precise and definite goal in view. But, on the other hand, it is necessary in the pursuit of practical plant developments to have a tolerably precise idea in mind as to the particular direction in which progress is desirable. Lacking such an ideal, the breeder of plants would be about as likely to produce new creations of value as an architect would be likely to construct a fine building by putting materials together at random without a carefully preconceived plan. "In what percentage of cases have you achieved the ideal at which you aimed in the production of new varieties of flowers or fruits?" a visitor asked me. The question is almost impossible of definite answer. When I first commenced, doubtless a very small proportion of my experiments came out as I expected. But now, with years of experience to guide me, I may say that I practically always get something not far different from what I desire. In many cases, the result comes just about as I expected. But this is because I am working with plants that I have previously tested. With a new plant I am still sometimes in doubt. But if it is a case of poppies or walnuts or any one of a score or so of other plants that I have fully tested, I know just about what to expect. At best, however, I am very often reminded that each species has its own individuality and that even the most familiar plant may hold surprises in store for us.
THE ROUGH SKETCH
"But just how do you start out when you are seeking to create a new form of plant life?" I am constantly asked. And here again the answer is difficult. Everything depends upon the ultimate object. If I am seeking merely to test the possibilities of making certain crosses, or as it were feeling my way along new channels, I am more or less like a person groping in the dark. This form of vague experimentation is often full of interest. I have already given some instances of what may come to pass when we hybridize plants of widely separated species or of different genera. The reader will recall the case of the petunia with the tobacco habit and of the dewberry crossed with such remote cousins as apple and pear and mountain-ash. These experiments were made without a clearly defined object-except to ascertain whether it was possible to hybridize plants of such diverse character. And the results of the experiment, while of very great scientific interest, were not practically successful in a commercial sense. I recall reading an address by the late Professor Newton, a distinguished American astronomer, on the subject of "dead work," in which he emphasized the fact that the main bulk of the experiments which any scientific worker must make will lead to no definite goal. A large part of the time of every experimenter must be given up to following trails that lead nowhere in particular or that end in blind cul de sacs. The work of the plant experimenter is no exception, but there is always an incentive to further effort in the knowledge that a path that seems to lead only into impenetrable mazes may presently bring one out into the light. To make the application to one illustrative case among many, I recall that for twenty-four successive seasons I attempted to hybridize certain species of Solanum before I finally succeeded in effecting a cross that gave me a single seed from which sprang the new race of Sunberries. But it must not be understood that the main bulk of my experiments are made in any hap-hazard manner. On the contrary my most important results have been attained by continuing the experimentation along rigidly predetermined lines and by methods of hybridizing and selection that my earlier work had fully established. Having served a long apprenticeship and tested the usual limits of making new plant combinations, I was presently able, like any other trained technician, to apply the knowledge thus acquired toward far more definite results than were at first possible. In the case of the Shasta daisy, the plans were all laid out beforehand as to just what type of flower I wished to produce. The ideal of a white blackberry was also, of course, a perfectly precise and definite one. Obviously the scented calla, the stoneless plum, the early bearing cherry, the sugar prune, and the spineless cactus are other instances in which the ideal pursued was as clearly conceived and as definitely outlined in advance of my earliest experiments as a cathedral is outlined in the mind of the architect before he commences his preliminary drawings. In one case as in the other the details may be modified as the work progresses, but the general idea of the structure aimed at-be it new fruit or new building-must be conceived with a good deal of definiteness from the outset. My original conception of a new plant creation, in the cases outlined and in a large number of others, certainly bore as close a resemblance to the final product achieved as the first rough drawing of the architect bears to his finished plans. "But how do you begin? What is the very first thing?" a visitor insists. The "very first thing" I have already described-it is the conception of an ideal, a mental picture of the new plant form desired.
CLUES TO BE FOLLOWED
It has occurred to me, for instance, that the cherry crop is not what it might be. I have learned that there is a steady market for early cherries and that a difference of a few days in the time of marketing may make a difference of more than one hundred percent in the price. And so I ask myself, why not create a new cherry that shall be ready for shipping at least two or three weeks earlier than any cherry now in the market? Of course, I reflect that my early cherry must have a number of other desirable qualities-large size, rich color, lusciousness of flavor. I know at the outset, or I presently learn, that it is desirable also, from the standpoint of the shipper, that my cherries shall grow on short stems. I know that the tree producing them must be hardy, capable of withstanding both cold winters and dry summers, and that it must have an inherent vitality that will make it resistant to the attacks of insects and fungoid pests. Next I ask myself what warrant there is for supposing that I can build such a fruit-structure as I have conceived. And here the answer is supplied solely by the use of imagination in connection with the inspection of existing races of cherries. I examine the best fruits already in the orchard and find that there is a large measure of variation between the cherries grown on different trees, as well as between the individual specimens on the same tree. In imagination I look back far into the past and inquire as to the racial history of this fruit. I am led to believe that certain among the ancestors of the cherry have grown in semi-tropical climates, and I know that even in the present day there are species, doubtless sprung from the same original stock, that grow far up into Canada. I ask myself why it is that the cherry shows such a propensity to vary, and I find an answer in the assumption that the existing cultivated races carry in their veins, so to speak-that is to say in their germ plasm-hereditary tendencies drawn from varied strains of a mixed ancestry. And I feel well assured that it should be possible, by accentuating the tendency to variation through further hybridizing, and by careful selection, to combine and bring out in a more or less remote generation of progeny of the existing cherries, a race that will furnish extreme examples, through reversion, of the limits of variation in each direction-and as regards each particular quality of fruit-that any ancestor reached. It will be obvious, then, that I am not preparing to make bricks without straw. I am counting well the materials with which I must work, just as the architect from the first stroke of his pencil bears in mind the materials of the future cathedral. I do not imagine that I can produce an apple from my cherry stalks, any more than the architect assumes that he can build a marble cathedral out of bricks. I know that there are sharply defined hereditary limitations beyond which the cherry cannot be made to go within any such period of time as that limiting my experiment. In other words, I do not ask the impossible, altough it has often seemed to my critics that I have asked the highly improbable. But the results I have attained are in themselves sufficient answer to the critic. If my vision has in some cases been the clearer, it is merely that my knowledge of plant life, drawn from the school of experience, has been wider. To the uninitiated observer it may have seemed that I set no limits to the transformations I attempted. In reality, my plan has always from the outset recognized most definite limits-although often enough the limits as I conceived them were quite different from those that had been set by theoretical botanists.
AID FROM GALTON'S LAW
In attempting to estimate the possibility of improvement in a given form of plant life, it is of value to recall the formula put forward by the late Sir Francis Galton; a formula often spoken of as Galton's law. According to this estimate, the hereditary traits of any given organism are so intermingled that we may assume as a general rule that offspring of a given generation will inherit about half their tangible traits from their parents, one-quarter from their grandparents, one-eighth from their greatgrandparents, and so on in decreasing scale from each earlier generation. Stated otherwise, according to this rule, we should be able by observation of the parents of any given organism, to see presented half of the traits of the offspring; but we may expect that the offspring will manifest, as the other half of their inheritance, traits that have come to them, through the process of reversion or atavism, from remoter generations of the ancestral strain. And this obviously gives opportunity for the appearance of an enormous variety of traits in any given generation that were not manifested in the preceding generation. Thus any given individual has normally, as a moment's reflection will show, four grandparents, eight greatgrandparents, sixteen ancestors in the generation before that, then thirty-two, sixty-four, one hundred and twenty-eight, and so on in a geometrical ratio with each remoter generation. So the normal ancestral clan of any one of us numbers more than a thousand different individuals within the relatively limited period of time compassed by ten generations. And, according to the estimate of Galton, to which numberless cases of atavism give force, certain traits and tendencies of each and every one of these ancestors may make themselves manifest in the personality of any given descendant. Galton's studies, upon which his formula was based, were chiefly made with reference to human beings, but we now know that the laws of heredity apply with equal force to all kinds of living organisms, including plants; and whatever the limitations of Galton's law as a precise formula, there can be little question as to the general truth of the principle that he invoked. Hence the value of that search in imagination for the ancestors of our cherry in their widely separated habitats and with their widely diversified traits and habits. But of course in making practical studies for the development of the mental blue print with its forecast of qualities of our new cherry, we must perforce be guided largely by the observed qualities of the parent stock with which we deal. Precisely what were the qualities of the remote ancestors, we can only infer. But we can see for ourselves what are the qualities of the fruit before us. We know, then, pretty definitely what we may expect as to one-half the traits of a hybrid that will result when we cross two varieties of cherries in our orchard. The other half must be somewhat matter of conjecture, to be revealed by the actual product or, as is practically the case, by succeeding generations. What we actually do, then, in practice, is to take flowers from a cherry tree that has been observed to bear fruit somewhat earlier than neighboring trees, and with this pollenize flowers of another tree that has been observed to produce fruit of exceptionally good quality. Pollenation accomplished, by the method elsewhere described, we can only mark the branch for future identification, and await results. The seed thus secured will be planted next season, and in due course we shall have a seedling which, when grafted on another tree to speed its maturing, will come to blossoming time-after another period of waiting-and finally show us the first fruits of our experiment. From this fruit we shall raise a new generation of seedlings which will reveal to us beyond peradventure a varied assortment of ancestral traits that the parental forms of our first hybridization did not show. And from among these diversified forms, we shall be able, by a long series of selections and new hybridizations, to make our way toward the attainment of our original idea. The precise steps and the varying details through which this may be attained, will be discussed in other chapters. Here we are concerned only with the general outline, and, this having been presented, we may leave our cherry in this interesting stage of partial construction. To be sure we have not seemingly advanced very far toward our ideal in these two generations; but in this our case is only comparable, after all, to that of the architect, who, when he has planned a building that shall ultimately tower toward the skies, must be content to see the workmen first begin digging in the opposite direction, to lay foundations far beneath the earth's surface. This matter of the very doubtful result of the first stages of a hybridizing experiment should be emphasized, because otherwise the amateur is pretty sure to become discouraged at the outset and to proceed no farther. Many an experimenter has given up a quest because when the two varieties of plant were crossed the offspring seemed inferior as to the desired quality to either of the parents. But the experienced plant breeder knows that this is very often to be expected and that he should not be in the least discouraged by this result. It is necessary to go on to the next generation before we can hope to discover the real possibilities of the experiment. The simple fact is that, where varieties or species of plants that differ markedly as to certain qualities are hybridized, the offspring very frequently seems to present what has been spoken of as a mosaic of characters rather than a blending. It may and very commonly does manifest, as regards any given quality, the influence of one parent seemingly to the exclusion of the other. A familiar illustration of the same rule may be observed when a person having black eyes marries one having blue eyes. It is obvious that no individual child of this union can have both black eyes and blue eyes. In point of fact, it is a matter of common observation that the offspring in such a case will have dark eyes. But it has also been observed that the blue eyes of one of the parents may reappear in the second generation. The tendency to blue eyes was entirely subordinated or submerged in one generation, yet it was by no means eliminated, as its reappearance in the next generation clearly proves. Similar instances without number may be studied from our plant experiments; for example, the case of the white blackberry. If flowers of this kind are fructified with pollen from flowers of a blackberry of the usual color, the hybrid progeny of the first generation will all bear black fruit. The quality of blackness has proved prepotent or dominant, and the opposed quality of whiteness has been totally subordinated so far as this generation is concerned. But if these black hybrid blackberries are cross-fertilized, from the seed thus produced there will spring a generation of brambles, some members of which will in due season produce white fruit precisely like that of the maternal ancestor. Such, it will be recalled, was indeed the experience in the development of my new race of white blackberries. To instill good qualities of fruit into the inferior original berry, it was necessary to cross with the large and well flavored Lawton blackberry. The immediate result was seemingly to obliterate the white-fruiting tendency altogether. But a wide experience of similar instances led me to continue the experiment, which for the moment seemed to be carrying me away from my ideal of a white blackberry; and the principle of reversion came to my aid in the next generation and gave me, as will be recalled, a berry that combined the light color of one of its grandparents with the size and flavor of the other. I have already suggested that it aids the memory, and helps to give tangibility to the facts, to recall the Mendelian phrase which speaks of blackness versus whiteness in such a case as constituting a pair of unit characters; naming blackness as the dominant and whiteness as the recessive feature; and which gives us assurance that a fruit which shows the recessive character of whiteness in the second generation will thereafter breed true, thus affording us evidence of definite progress toward the ideal of our experiment.
AID FROM UNIT CHARACTERS
As the principles that govern these cases are of very wide application, it follows that there is very great advantage from the standpoint of the plant developer, in the discovery of pairs of unit characters and the demonstration of their relation toward each other as regards dominance and recessiveness. An interesting illustration of this is afforded by the experiments made by Professor R. F. Biffin, of Cambridge University, in the successful attempt to develop a new race of wheat. Professor Biffin through a series of experiments showed that when beardless ears of wheat are crossed with bearded ones, the beardless condition proves dominant, so that all the offspring are smooth-eared; but that the recessive quality of bearded grain reappears in the second generation. The same thing held true for various other pairs of unit characters, such as red chaff versus white chaff, red grain versus white grain, hollow stem versus solid stem, and the like. Professor Biffin was able to make an immediate practical application of his experiments through which he developed a new race of wheat that is proving of great economic importance. It appears that the best races of British wheat have been peculiarly susceptible to the fungous pest known as rust. There are, somehow, certain races of wheat that are immune to the pest; but unfortunately these produce a very poor quality of grain. Professor Biffin found that susceptibility and immunity to rust constitute a pair of unit characters, in which susceptibility is prepotent or dominant. When he crossed the susceptible grain with the immune one, he therefore produced an entire generation of susceptible grain. His experiment had seemingly gone backward, quite as in the case of my first generation of white blackberries. But in the ensuing generation the recessive character of immunity reasserted itself; and, combined with this desired character, in a certain proportion of the progeny, there appeared the other desired quality of a good head of grain of fine quality. So by the application of this principle of the segregation and recombination of unit characters Professor Biffin produced a new race of wheat in two or three generations, and this new race of wheat breeds true. We shall see this principle illustrated over and over in connection with the long series of my plant experiments. In case of the wheat, as in that of my white blackberry, the process was relatively simple because we were dealing only with two pairs of unit characters. Moreover, the case of wheat is further simplified by the fact that this plant is self-fertilized and under conditions of cultivation has become a very fixed race, little subject to variation. When we deal with races of fruits that tend to vary almost indefinitely, and when further we are concerned with ten or a dozen unit characters, the matter becomes vastly more involved, as we have previously seen illustrated. But the amateur will do well to begin his experiments with simple cases, dealing with only a single quality, say a particular color of flower, that he may thus learn to distinguish the principles here enunciated. In due course he may go on to apply these principles to more complicated experiments in plant hybridization. But unless he learns at the outset that certain characters that are submerged in the first hybrid generation will inevitably reappear in the second, he will constantly blunder in his interpretation of tentative results. On the other hand, when he has learned to gauge his second-generation hybrids correctly, he is on the highway to success as a plant experimenter.
This text is from: Luther Burbank: his methods and discoveries and their practical application. Volume 3 Chapter 1