GETTING THE UTMOST VARIATION OUT OF A FLOWER
HOW THE CHINESE BALLOON-FLOWER WAS TREBLED
In illustrating the possibilities of flower development, the case of the Chinese balloon-flower (Platycodon) will answer as well as another. I had been for some time working with a bed of these flowers, with an eye to the increase of their beauty of form, their size, clearness and intensity of color, and the closer and more graceful placing of blossoms on the stalk. As to all of these matters, the existing balloon-flowers left a good deal to be desired. My method of work was that which I have already outlined so fully in connection with other flowers. The essentials of it, as the reader is aware, are first the careful scrutiny of the entire colony to discover the individual that is the very best of all as to the particular character in question. This individual is selected and its seed carefully preserved. If the three or four different qualities, improvement of which is desired, are not combined to best advantage in any single individual, then it is necessary to select an individual for each quality, and to carry forward three or four lines of experiment at the same time. It will be recalled that in developing a special variety of small sweet canning pea, with the qualities of uniform ripening, of small seed, and of seeds of uniform number and equal size in the pod, I was enabled to find these qualities exhibited in such combination that the experiment went forward rapidly, so that in the course of six generations I had developed precisely the variety of pea that was desired. But it will also be recalled that half a dozen other lines of experiment were carried forward at the same time, using the same group of peas, that led finally to the production of as many quite different varieties, characterized by large size of seed, by lentil-shaped seeds, and the like. And these secondary experiments were carried out without in any way interfering with the primary one. It was merely that, in searching among the different vines, I could not fail to notice individual plants that showed interesting characteristics, and nothing more was required than to mark these differently from the others and save their seed. So in such a case as that of the balloon-flower, where it is desired to increase three or four quite different qualities-in this case size, beauty of form, manner of placement of blossoms, and intensity and clearness of color-it does not so very greatly matter whether in the early generations one finds the different qualities combined in a single individual, or whether, as is more likely, he finds one individual that is most graceful, another that has blossoms placed on the stalk in the best manner, and a third that shows to best advantage as to intensity and clearness of color. It is much more probable, in practice, that the second alternative will be the one actually presented. Indeed, it is altogether unlikely, when new qualities, such as these, that have not hitherto attracted the attention of the cultivator of the plant, are in question, that one will find a single individual that surpasses all its fellows as to each quality. In point of fact, with the balloon-flowers, it was necessary to save seed of three or four individuals and search among their progeny in turn in the following season, and make additional selections that involved a number of individuals. But when selection has been carried to a stage where we have one race of balloon-flowers presenting plants that are uniformly of graceful and attractive form, and another race that has the flowers arranged in a satisfactory way on the stalk, and a third race that produces flowers of a brilliant white color, the materials are in hand for an amplification of the experiment along lines with which the reader is already familiar, through which the desired combination of these traits in a single race may be effected with almost absolute certainty.
THE COMBINATION OF QUALITIES
The method in question consists, of course, in cross-pollenizing the best individuals of the three new races. Of course, one cannot blend three strains in a single cross-pollenizing experiment. But one can cross-pollenize specimens of each one of the three with each of the others, making the cross reciprocal in all cases to make quite sure. Each of the new hybrid races will thus blend, in one way or another, the traits of two of the parent forms. Selection being made to find the best types among these two crossbred races, the ones selected will, of course, be inter-pollenized and their offspring, representing the second generation from the three parent forms, will combine all the hereditary factors of their three specialized ancestors. Among these second generation hybrids there will be found, in all probability-if large numbers of specimens are examined-some individuals that will combine in the superlative degree the qualities of gracefulness of vine of one grandparent with the satisfactory arrangement of flowers of the second grandparent and the brilliant whiteness of blossoms of the third grandparent. It is then an obvious procedure to save the seed of this individual, and while we must expect wide variation among the plants grown from that seed, there will almost certainly be some among them that will reproduce the combined good qualities of the parent, and further selection along precisely the same line-what I sometimes speak of as "line breeding"-will result in fixing of the type, so that we shall have the variety, hitherto existing only in our imagination, which we have all along been seeking to produce. Moreover, not alone shall we have produced a type which combines all the best qualities of the different members of the original balloon-flowers, but our new race will almost certainly present these characters in markedly accentuated form. The perfected balloon-flower will be more graceful in form than the most graceful one of the original colony. It will have its blossoms much more artistically grouped on the stalk than any balloon-flower that has hitherto been seen, and the color of these blossoms will be cleared and more brilliant than those of any individual member of the original colony, whether blue, white, or intermediate, as may readily be demonstrated by comparison if the original colony has been preserved, and is now represented by unselected progeny. Of course, in my own experiments, the unselected members would usually have been destroyed, but the worker who experiments on a smaller scale may find it desirable to preserve the old colony, or some members of it, if for no other purpose than to find encouragement in making such a comparison as that just suggested. The results, as I have said, are sure to be encouraging if you have carried out the experiment in the way just outlined. Nothing more is required than the use of your eyes and reasonable judgment in selecting the best specimens; care in the preservation of the seeds; cultivation of the seedlings in the way we have elsewhere fully described; and persistency in following up the experiment. I have a good many times pointed out that in such experiments there may not be very much encouragement in the first generation or two. Some forms of plant, and in particular those that have not been very much under cultivation, or that are represented by only one or two species, may hold fixedly to their type and show at first only a slight range of variation. In such cases you must be content to go forward by very slow stages, taking but the shortest step ahead with each generation for the first two or three years. But even where progress is as slow as this in the beginning, the time will almost surely come when the effect of what I have several times referred to as the momentum of variation begins to be felt. Some season, to your surprise and delight, you will discover that the plants are varying much more widely than they have done hitherto. Instead of having to scrutinize your seedlings with the utmost care to determine which ones are largest and most vigorous; and then in turn scrutinize with equal care the blossoms-when they appear-to determine which are largest and most brilliant, you will find that some feew seedlings will jump ahead of the others as if they belonged to another race, bringing to your mind the familiar tale of Jack's Beanstalk, or the less familiar story of Darwin's Hero morning-glory, which appeared suddenly after several generations of selection. When the seedlings which thus practically select themselves have come to blooming time, your delight will be enhanced as you discover that the blossoms they bear are markedly larger and more brilliant than any you have seen before. Now all your disappointment and discouragement of the first day is forgotten. Now your enthusiasm is reanimated and accentuated. From this time forward you carry on the experiment with renewed zeal, and you feel confident at last that the coveted goal is within sight.
PLANTS THAT TEND TO VARY
Of course there are other plants that give encouragement from the very outset. Such is the case with almost any of the familiar cultivated plants, of which there are many species and varieties that have long been given attention by the horticulturist. Suppose, for example, that you were to plant all the seeds taken from the seed pods of a single dahlia. Perhaps you have done this on occasion, not with any thought of making new experiments or developing a new variety, but merely in the hope of reproducing the characteristics of the best and most beautiful dahlia among the number in your garden. In that case you have doubtless been subjected to bitter disappointment. For when the carefully nurtured seedlings came finally to blooming time, instead of presenting flowers closely similar to those of the parent form, they have shown, in all probability, the widest range of variation-not one of them perhaps has been closely similar to the parent. Nor, perhaps, were any two precisely alike. Among them you could discover resemblances to all the other dahlias in your garden and, indeed, to a large proportion of those that you had seen pictured in the seed catalogues. In a word, your dahlia seeds show that they contain the racial strains of a great variety of ancestors, and they present a variation that is truly disconcerting to the gardener whose sole desire was to produce a lot of dahlias of uniform character. In one case, recorded by Darwin, an experimenter listed no fewer than eighteen different varieties of the dahlia grown in the first generation from the seed of a single plant, and of course there were all manner of intermediate forms. In the listed eighteen only six corresponded pretty closely to certain named or catalogued varieties. It would perhaps more truly present the record if we were to say that there were not eighteen different varieties merely, but as many varieties as there were individual plants. But while such an experience as this is utterly disconcerting to any amateur whose only thought is to produce a bed of flowers of uniform color or character, the same experience would offer precisely the opportunity that the would-be developer of new varieties is seeking. Now it is not a case of hunting here and there throughout a company of seedlings for one that differs by a shade from the others. It is a case of selecting two or three or a dozen individual plants that present features that attract the experimenter; and selecting their seed to be planted the following year in individual plots, that the experiment may be carried forward, generation after generation, just as before so far as principles are concerned-but very differently as regards results, inasmuch as now there is the most striking departure in each successive generation from the characteristics of the parent form, how wide the departure may be within a few generations is well manifested by the dahlias, since these plants, as we have already learned, have all been developed in the space of about a century from wild originals. Moreover, by no means are many generations represented as might be supposed, inasmuch as the dahlia is propagated usually from the bulb, and it is only now and again that an experimenter has taken the plant in hand to raise it from the seed and separate out new varieties. That a plant which in its wild form is an ordinary sort of composite-not very different from the Black-Eyed-Susans and allied sunflower-like plants that abound by every roadside-could be developed in a comparatively short series of generations into the extraordianry flower with solid heads, and presenting the gorgeous and variegated colors of the dahlia of today, is in itself an object lesson in the possibilities of plant development that is nothing less than inspiring.
Not only may plants be led along the line of some desired variation, but there is an element of chance in the enterprise that adds very greatly to its interest. There is always a certain allurement about the happening of the unexpected. It is highly gratifying to select a plant for some desired quality and to have it respond to selection in such wise that a variety presenting this quality is finally produced. But it is doubly gratifying to see here and there, quite unexpectedly, the putting forth of a flower of an unpredicted color, or the development of a form of which one hitherto had no conception. In a field of cultivated poppies, for example, where there were millions of specimens, all of substantially identical color, so that the field made a blazing sheet of yellow, I have come upon a single blossom of the purest white. To find this white blossom, isolated among the millions, is an experience that repays one for years of earnest effort and makes amends for almost any antecedent disappointment. It was such a chance discovery as we have seen that gave the world the wonderful new race of white Watsonias. Quite possibly the white flower that Mr. Arderne found among the colony of reddish pink ones may have been the only one of its color among a million, or perhaps ten million, of its fellows for miles around. But this single atypical individual chanced to be discovered, and its progeny today are found by thousands, even by hundreds of thousands, in the gardens and greenhouses, not alone of its native home in South Africa, but of all parts of Europe and warmer regions of America. I myself, as the reader will recall, have raised these white Watsonias by hundreds of thousands. Their strains were mingled in the germ plasm of the quarter million bulbs of this species that I was obliged to destroy in a single season. Such are the possibilities of multiplication of a plant. Such is the geometrical ratio at which the offspring of a single individual increase if given encouragement. Boundless, then, are the possibilities that lie before the plant developer who discovers a single specimen of an aberrant type. One white poppy among the million yellow ones might be the progenitor of a race that would displace entirely the whole race of yellow poppies. What I wish to illustrate at the moment, however, is not the possibilities of multiplication of the plant but the interest that attaches to the development of unexpected variations. And I repeat that the possibility of finding a new form in your flower garden almost any morning will give perpetual interest to your task, and will come to be a compelling incentive that will take you to the garden as steel is drawn to the magnet. To illustrate the possibilities from the case directly in hand, let us recall the new race of balloon-flowers, the evolution of which we have just traced. I have said that the experiment began with the ideal of a balloon-flower of better form, more graceful placement of flowers, and individual blossoms larger and of more brilliant color. I have said also that these ends were in due course attained, and have traced briefly the steps through which the new race of perfected balloon-flowers was evolved. Now it remains to add that when the experiment was approaching completion, and a new race of balloon-flowers in many ways satisfactory was actually in being, I discovered one day among the blossoms one that had a perfectly regular second row of petals, instead of the usual single row, or the irregular so-called double, which had sometimes appeared. Here was an unexpected variation, which was something that I had not counted on or considered. But, needless to say, I hailed the new arrival with delight, and marked it for further education. If we ask why this second row of petals appeared, the answer can be only a conjecture. Doubtless some condition of altered nutrition stimulated the plant to this abnormal production. It is customary to speak of such an anomaly as a "sport" or mutation. But doubtless these words beg the question. They name a condition, but do not in any way explain it. It is an observed fact, however, that sudden variations analogous to this may be stimulated by a change of climate or a change of soil, as when a plant is brought from another hemisphere, or by a surplusage or a shortage of food. It is familiarly known that in a beehive the larva that would otherwise grow into an ordinary worker may be made to develop into a queen, that is to say, a mature female, by forced feeding. In somewhat the same way a plant that has an excess of nourishment may tend to take on exceptional growth, and one manifestation of this might be a disturbance of the equilibrium of the floral envelope, with the production of an unusual number of petals. It is known, on the other hand, that a shortage of food supplies or disadvantageous conditions of climate may hasten the maturing of a plant, and cause it to fruit earlier than it otherwise would do. And any disturbance of equilibrium of this sort may lead to anomalies in the precise character of the flower. Possibly the reason why the petals of the flower are most likely to be altered as to number, and also as to color, is the fact that these are about the newest of all the plant structures. We have seen that the petals are not themselves essential to the fertilization of the plant-they are only advertisements to attract insects. They were developed late in the evolutionary history of the plant, and their variability is an additional evidence of their modernity. The fact that so many of our cultivated plants have become "double" is in itself sufficient proof of the tendency of the petals to be modified under conditions of change of climate and nutrition to which the cultivated plant is subjected. But from our present standpoint, what perhaps is of greatest interest is the fact that when petals have once shown a tendency to such modification, this propensity is heritable, and the progeny of the plant will reveal some members at least that show the same characteristic. Moreover, the "momentum of variation" to which I have so frequently referred will make itself felt in the tendency of these variants to take on still wider variation. In other words, the plant that has developed an extra petal or row of petals has in its germ plasm factors that will tend to urge it to the production of still greater modifications of the floral envelope. In the case of the balloon-flower, the plant that had developed a second row of petals, when its progeny were carefully scrutinized, was found to have transmitted the anomaly to a certain number, and among the progeny of these there presently appeared one that had a third row of petals. So in the course of comparatively few generations there had been produced a race of balloon-flowers that had trebled the number of petals that hitherto had been the recognized complement for flowers of this race. Multiplication of petals may result, as we have already noticed, from the transformation of stamens into petals, or it may come about from the springing into being of new petals de novo, rather than as modifications of any pre-existing part of the flower. The latter appears to be the case with the new rows of petals of the balloon-flower. Whether the modification will continue until the balloon flower has a large number of rows of petals, comparable to those of the double roses, for example, remains to be seen. But at the present stage the flower has a triple corolla, constituting a very striking modification. The ultimate limits of its variation can be determined only by further series of experiments.
The modification of the balloon-flower has somewhat exceptional interest, because there is only a single species of the genus Platycodon, to which it belongs, anywhere in the world. In other words, this genus is what is called a monotype, and it is a well-recognized fact that flowers belonging to a genus having only a single species, and even to genera having half a dozen species, are relatively little subject to variation. Rightly considered, this is almost axiomatic; because the very fact that there are many species in a genus proves that the representatives of that genus have been variable; else they would not have developed so many different forms, since all members of a genus have sprung from the same ancestry within comparatively recent times. The balloon-flower has seemingly been isolated under climatic conditions that have not greatly changed for a long period and hence it has maintained its specific identity, and the type has become thoroughly fixed. And this fact, as I said, gives added interest to such an experiment as that just outlined, which shows how marked may be the developments that can be produced by selective breeding, even with a flower that tends very strongly to maintain fixity of type. But, in point of fact, as we have emphasized again and again, no flower is so fixed that it does not vary to some extent; and in the case of the balloon-flower, it appears that there are modifications in the type of the plant as it appears in China and in Japan, that are sufficiently divergent to be recognized by the botanist as established varieties. A form from Manchuria also has been modified, particularly in the matter of the time of blooming, which is much later than that of the type species. Also in the matter of color-that most variable of traits-there is modification, as some varieties are blue, some bluish white, and some variegated, in addition to the pure white form. There was, however, no other color until last season, when a plant bearing large red blossoms appeared among a few thousand seedlings which had been grown from my long-selected varieties. There is material at hand, then, through which cross-fertilization may be practiced, with the possibility of giving the flower still greater impetus to variation. Until such cross-pollenizing has been practiced, using varieties of the plant imported from the most widely spread regions-let us say races grown in China, in Manchuria, in Japan, in Europe, and in California -we shall not have tested fully the possibilities of variation of the balloon-flower. And indeed, even when these crosses have been made, there will still remain possibilities to invite the plant experimenter. For although the balloon-flower stands in a genus by itself, there are of course other genera that are not very distantly related in the Campanula family, to which the flower belongs. The balloon-flower is often spoken of as the Chinese bellflower, and with entire propriety, inasmuch as its nearest relatives are the European and American bellflowers, of which there are several familiar species, the best known, perhaps, being the one called popularly the harebell or bluebell, and the Canterbury bell. It is quite supposable that it might be possible to hybridize the Chinese flower with one or another of these European or American bellflowers. And in that event it is not to be doubted that the hybrid race would show new possibilities of variation and, by combining ancestral traits that have not been blended since remote geological periods, if at all, we should develop among the progeny of the balloon-flower races that would, in all probability, differ so radically from the parent form as scarcely to be recognizable as having any relationship whatever with the plant with which our experiment began. All of this, of course, is taking liberties with the future. In the case of the balloon-flower, such hybridizations have not as yet been successfully carried out. But in suggesting the possible results of such potential hybridization, we are merely drawing analogies from almost numberless experiments with other races of flowers, and we have every warrant for drawing such conclusions as those just suggested. Certainly we are justified in the conclusion that we have not tested to the fullest the possibilities of variation-that we are not by any means "getting the utmost variation out of the flower"-until we have supplemented the method of selection with that of hybridization. I may add that there are yet other possibilities of stimulating variation by chemical treatment of the developing ovaries of the flower itself; or by subjecting the plant to unusual conditions of hot-house temperature; but experiments of this type, reference to which has been made in an earlier chapter, have not fallen within the scope of my own work, and as yet have been carried out only tentatively by others. So I mention them here only as suggesting that there are other possibilities so various and so complicated as to give full assurance that no single line of investigation will ever reach a stage where it loses interest because it has brought the investigator to the lend of the road.
This text is from: Luther Burbank: his methods and discoveries and their practical application. Volume 10 Chapter 1