Luther Burbank: His Methods and Discoveries (1914)
[VOLUME IX—CHAPTER I]
WHAT TO WORK FOR IN FLOWERS
AND HOW TO PROCEED
ONE of my plant developments that usually interests the visitor as much as almost any other has to do not with the flower or fruit of a plant but with the leaf.
The plant in question is a species of "wild geranium" known as Heuchera micrantha, a native of the western coast, and the anomaly of leaf that attracts attention is the curiously erected, crinkled, and corrugated condition that makes the foliage of this plant quite unlike that of any other member of the tribe that anyone has seen. Indeed the new variety is so changed from its ancestral type that it is considered entitled to recognition with the varietal name cristata added to its technical title. Were it found growing in the woods instead of in a garden, it would be pronounced a new species altogether.
The story of this anomalous geranium will serve as well as another to introduce our studies of the development of new varieties of flowers, even though the particular development under consideration has to do with the leaf of the plant, and not with its blossoms. The principle of development is the same in its application to each part of the plant, and we shall see plenty of illustrations of work with the flowers themselves before we are through.
The wild geranium, of which the plant with the strange leaf is a modified representative, is a plant that normally has leaves some of which are rather decorative because of their very slightly scalloped margins, but which in general are quite plain. Some of the leaves are flecked with brownish spots, but the surface is quite smooth, as much resembling an apple or geranium leaf as any other. Even botanists have never taken special notice of any variation in the form of the leaf.
There is, however, a marked tendency to variation in different specimens, especially in the brown spots on the leaves, and the crimson shadings in the fall.
A NEW LEAF BY SELECTION
Several years ago, in examining some of these plants growing wild on a rocky ledge over Mt. St. Helena, I observed one that had leaves slightly crinkled at the edges. This slight, almost insignificant, variation suggested the possibility that farther variation in the same direction might take place if the plants were educated in the right way. So I transferred the plant with crinkled leaves to my home grounds, and in due time gathered its exceedingly diminutive black seeds.
|A Metamorphosed Leaf|
|In the center above, an ordinary leaf of the Heuchera, or "wild geranium"; at right and left below, different types of crested leaves of the same plant developed by Mr. Burbank through selective breeding. The story of this modification of a leaf, as told in the text, has peculiar interest. Doubtless many other plants have similar possibilities of development, and many an amateur should be stimulated to emulate Mr. Burbank's example; for the results of this particular experiment are at once novel and striking.|
When the little plants that grew from these seeds next season were carefully examined, I observed that some of them had leaves slightly more crenated or crinkled than the others. So even before the plants made much growth I was able to weed out about half of them, as showing no evidence of progress in the desired direction.
When the plants were still larger, but before any flowers appeared, about half of the remainder were pulled up; and later in the season still others were discarded that had shown the crinkled condition at an earlier period but did not tend to carry it well as they advanced in age.
Of the many thousands with which I had started in the spring, only a handful remained toward seed time. And at last a single one among these was chosen as presenting leaves that from the point of view of the experiment were best.
This single plant was allowed to mature its seed.
The plants that grew from this seed, representing now the second filial generation from the original wild plant, were treated in precisely the same way. But it should be recorded that there was great improvement in this second generation. Now three-quarters of the plants showed leaves that were markedly crinkled. Each plant produces thousands of seeds, and progress was relatively rapid, as great numbers could be produced from which to select.
By process of elimination, the one best plant was again selected and its seed preserved.
In the next generation, practically all of the plants showed the curiously modified form of leaf.
In the fourth generation, as before, very large numbers of plants were raised that there might be wide opportunity for selection. Now all the plants presented the crinkled leaves, but there were of course individual specimens that excelled, and these were chosen to the exclusion of the others.
Their progeny bore uniformly crinkled leaves of the most pronounced type, and they constitute the new species Heuchera cristata as it grows to-day.
The remarkable crinkled and convoluted leaves are so interesting that they are sometimes preserved by electroplating, to be used as ornaments. They give the plant a very curious and individual appearance, and present a striking illustration of what may be done, by mere inbreeding and systematic selection, to develop and accentuate a plant characteristic.
Variation in Color As Well As Leaf Form
|These are different examples of Mr. Burbank's curiously crested "wild geranium"—a plant that has been so modified as to merit botanical classification as a fixed variety. It will be seen that there is marked variation in color as well as form. By selective breeding, the color variations might be fixed just as the corrugated condition of the leaf has been. The modification shown has been effected by selective breeding alone, without hybridization.|
No one who casually observed the old parent form of the plant and the new modified form growing side by side would be likely to suspect that the two belong to the same species. Yet an examination of the flowers would show that these are identical, for in making the successive selections I paid attention to the leaf exclusively, and did not seek in any way to modify other portions of the plant's structure.
To the person who has not had experience in plant development, probably the most remarkable feature of the entire matter is the comparatively short time required, and the few generations involved, in producing what is a remarkable transformation—the most conspicuous transformation in a leaf that has ever been produced. The nearest approach to this structure is seen in the leaf of the Rex Begonia called Erdody. It may seem further remarkable that a transformation of such significance could be effected in a few generations by selective breeding; without the aid of special experiments in hybridizing.
But this case is presented here at the beginning of our special studies of flower development, largely to emphasize the possibility of modifying even so fixed a structure as the leaf of a plant merely by selection of individual specimens that vary in a given direction for a few generations.
I would emphasize, however, the necessity of operating with a large number of specimens if one is to obtain the best results in the shortest practicable time. The account of the experiment just given makes it clear that by having large numbers to choose from, I was enabled to discard numberless specimens that would have answered the purpose fairly well in favor of the single specimen that showed the desired quality modified preeminently.
THE QUESTION OF HYBRIDIZING
This case, as was said, illustrates the possibility of producing striking results in plant modification by mere selection without hybridization. No effort was made to induce the plant to vary more rapidly, first because there seemed no necessity for stimulating it to further variation, and secondly because no plant was at hand which presented such a character as the one I wished to develop.
Yet it should not be overlooked that there was an element of pollenizing involved, even though the pollenizing was not done by the plant experimenter. This is almost axiomatic because of course the plant would have produced no seeds unless its pistils had been pollenized.
All that I had done, to be sure, was to transplant the original geranium to a bed where it was isolated from any other plants of its species. But such isolation in itself served to provide that the pistils of the plant should be fertilized with pollen from its own flowers.
In other words, by isolating this heuchera with crinkled leaves it had been determined that the pollen and ovules from the selected plant should combine to produce the seed germs for the next generation. And in so doing I made sure that both hereditary strains—that brought by pollen and that brought by ovule—should have the same hereditary factors, because they were borne on the same plant.
This, then, was a case of inbreeding, or "intensification" which has been mentioned previously. It was as far removed as possible from the hybridizing experiments we have witnessed in which species of widely different type, say the strawberry and the raspberry, were interbred. In such a case as that, the pollen and the ovule bring groups of hereditary factors that are widely divergent. And even in the usual cases of cross-fertilization within a species, where pollen of one plant is brought to the pistil of the flower of a neighboring plant, there is a certain opportunity for the mingling of diverse hereditary factors, inasmuch as no two plants are precisely alike.
But in the case of our heuchera, the flowers were self-fertilized or at most the pollen from one flower was transferred by an insect to the pistil of a neighboring flower on the same stalk, and thus it was arranged that both hereditary strains should be as nearly identical as is possible.
In the essential matter of the form of leaf, the hereditary factors brought by the pollen grains called for a leaf with crinkled edges; and the hereditary factors carried by the ovules had the same specifications. So there was the best possible chance that the offspring would reproduce or accentuate the parent character.
And yet the results show that there must have been a certain amount of diversity among the various pollen grains and ovules even of the single plant, inasmuch as the plants that grew from its seed were diversified in character.
About half of them, it will be recalled, did not present the crinkled leaf to any extent and were at once eliminated.
And the other half showed the character in varying degree.
Indeed, no two of them were precisely identical, so we are justified in the conclusion that no two pairs of pollen grains and ovules brought precisely the same combination of hereditary factors together.
When we consider the matter in this light, it will be evident that all pollenizing experiments are in a sense hybridizing experiments in one degree or another, inasmuch as they all of necessity bring together pollen grains and ovules that vary somewhat, even if only in very minor degree, in their hereditary factors.
But it remains true—and indeed is too obviously true to require comment—that the case of the pollen grains united with pistils on flowers of the same plant (the case, that is to say, of the heuchera under consideration) is that in which there is the least possible degree of variation between the two sets of elementary factors that are combined.
Therefore this process of so-called inbreeding introduces the least possible disturbing elements, and gives the largest probability of the reproduction of any given trait of the mother plant—which in this case is the father plant as well.
The practical results have been already illustrated in the production of this new race of heuchera with leaves crinkled and corrugated in unique fashion so that they differ fundamentally from the characteristic leaves of any other species or variety.
The lesson to be drawn, then, from this experiment is that when we wish to modify a plant as to some particular feature of its anatomy, we shall proceed to best advantage if we (1) select an individual that shows the most marked departure from the normal in the desired direction of any that can be found; (2) isolate this plant so that its flowers shall be self-fertilized, or else hand pollenize them; and then (3) follow out a similar course of selection of the best individual and selffertilization of its flowers through successive generations until the maximum amount of variation in the desired direction that is attainable has been produced. It sometimes hastens the process to combine two or more of the best plants by crossing rather than to depend on a single one.
We shall see in other connections, as indeed we have previously seen in our studies of many plants, that it is frequently desirable to stimulate variation by hybridizing plants that are divergent, even plants of different species. But when an individual plant presenting an approach to the desired variation or modification has been found among the hybrid progeny, the successive steps of inbreeding and selection, through which the character is accentuated and fixed, will be carried out precisely as in the case of the little heuchera just cited.
Indeed, had we been able to take up the story of our little heuchera a generation or two earlier, we should have found, in all probability, that such a crossbreeding experiment as has just been suggested had been performed for us by Nature. It is highly probable that the original specimen with the tendency to crinkled leaves that was found in the woods was the product of a cross between plants, perhaps of the same species, that were individually widely variant from one another. The plant grew on a cliff where very dry, very moist and very unusual conditions of the sun, the shade, moisture and soil prevailed, thus having current in its heredity a tendency to vary more or less, since heredity is only the visible effect of near and far environments.
Whatever the individual peculiarities of the parents of this particular plant, the individual that I found had leaves that were somewhat highly accentuated in a certain direction, being thus proved to be the possessor of a somewhat unusual combination of hereditary factors for leaf formation.
In a word, then, whereas the experiment with the little heuchera may be described as consisting exclusively (so far as the plant developer was concerned) of a series of selections, it really involved also the principle of the inducement of variation by crossbreeding and the fixing of characters by inbreeding.
And these fundamental principles of plant development must be involved, in one degree or another, in all successful experiments in the development and fixing of new types of plant form or leaf or flower or fruit.
Let us now witness the application of the same principles to the flower of the plant with reference to the different characteristics of size and color and odor and modified petal or stamen or pistil that may be involved.
PRODUCING A DESIRED FRAGRANCE IN THE FLOWER
Probably no other characteristic of the flower is more highly prized than its odor.
The rose and the carnation owe their popularity as much to their fragrance as to their color and form, yet there are numbers of very beautiful and popular flowers that are quite without attractive fragrance. There is no line of experimental work with the flowers that should be more attractive to the amateur than the development of fragrant varieties of some of these odorless flowers.
And fortunately it is an undertaking that may be expected to produce very satisfactory results—as immediate, as striking, and as valuable results as from any other plant experiment. In any group of odorless flowers, you may have the good fortune to detect, if you search carefully enough, one that differs from its fellows in having at least a suggestion of fragrance. And if you will work in the right way with this individual, you will probably be able to produce a race of perfumed flowers—supplying you, therefore, with a flower different from anything in the garden of your neighbor, and adding the finishing touch to a blossom which, however attractive otherwise, could not be considered perfect so long as it lacked this finishing quality.
In an earlier volume we have heard the story of the fragrant Calla.
The reader will recall that this anomalous variety, known now as the Fragrance, was developed by simple selection, along the lines just illustrated in the case of the little heuchera, with the difference merely that the characteristic borne constantly in mind was fragrance of the Calla blossom instead of a peculiar conformation of leaf.
By "line-breeding" and careful selection, I was enabled in a few generations to isolate a calla that has delicious fragrance while retaining all the other qualities of the flower unchanged.
The seedlings of this selected calla are not invariably fragrant. By careful inbreeding the fragrant calla could without doubt be made to breed true to the quality of fragrance. In the particular case of the calla, this is of no special importance, as the plant is propagated by division.
But in plants that are propagated solely by seed, the fixing of the quality of fragrance would be essential.
Fortunately it presents no special difficulties once a fragrant variant has been found.
In a later chapter we shall learn of other experiments in producing fragrant flowers, and details will be given of the story of my fragrant verbena which was introduced under the name of Mayflower. The amateur who wishes to experiment along these lines may begin with almost any odorless flower in the garden. It is only necessary to search for delicate traces of fragrance, and to learn to recognize nice shades of distinction among odors. Anyone can readily detect the difference in fragrance in several varieties of the violet, roses, or carnations, for example; and a still more highly cultivated odor-sense enables one to notice differences in the fragrance of apple, peach, or almost any other blossoms from different trees or plants.
So it is not necessary to confine one's experiments to flowers that lack fragrance altogether. Interesting results may be obtained by selecting among fragrant flowers those that have the most pleasing perfume, and developing those races that are especially notable for their fragrance.
The failure to give attention to the matter of fragrance sometimes leads to the cultivation of a special variety of fragrant blossom that has altogether lost its perfume. An illustration of this came to my attention not long ago when visiting the seed farm of the best known seedsman in America. He showed me his new varieties of sweet-peas with great pride; and when I called his attention to the fact that a number of them were totally lacking in fragrance of any kind, he was not a little surprised.
He was breeding sweet-peas for immense size and had succeeded, through selection, in producing very striking varieties.
But he had taken it for granted that all sweetpeas are fragrant, and had before failed to observe that these particular ones had no perfume whatever.
Yet this seedsman is an expert who has been for nearly forty years in the business of growing flowers. Like perhaps most others, he had taken it for granted that all varieties of fragrant flowers are fragrant. Series of experiments in crossbreeding would be necessary to reintroduce the perfume to these varieties that have lost this finishing quality.
This case is mentioned to illustrate the fact that a given quality may be dropped out of a strain of flowers while another quality is being bred in. Also to emphasize the point that it is usually well to consider more than a single quality in any breeding experiment.
At least it is desirable to see that the qualities already present are not lost in the process of gaining new ones.
PRODUCING NEW COLORS
I am disposed to think that all shades of all colors that can be produced by blending of the primary colors are within the possible attainment of any flowering plant.
The obvious fact that certain species, and in some cases whole genera, produce only red flowers, others only blue ones or yellow, does not by any means prove that the plants in question have not the capacity to produce flowers of quite different color.
We have seen that the colors of wild flowers have been given them by insects. We have noted that the bright colors—reds, orange, blues—have been assumed by flowers that flourish in the daytime and seek association with the bees; and that the flowers that consort with night flying insects, such as moths, are almost universally decked in white or pale yellow—hues that make them far more conspicuous in the twilight than the most brilliant scarlet flower would be.
Most wild flowers of a given species are of a single color, or of a definite arrangement or combination of colors. Bees and other insects have learned to distinguish this characteristic color or combination of colors, and to go with certainty from one flower to another of the same species, thus unconsciously serving the flower well by cross-pollenizing its blossoms.
I have often thought how confusing it must be to the bees on coming to our gardens to find flowers that perhaps are familiar to perfume and form, now arrayed in a dress of unfamiliar hues. But bees, like flowers, can adapt themselves to their environment. They soon adapt themselves to the new colors and combinations of colors that man has given the flowers, and they go about their task with undiminished celerity and certainty.
Recognition of the fact that wild flowers have been given their colors by the insects through the slow process of natural selection (in which flowers that lack the color were not visited by the bees and hence produced no offspring; whereas the flowers that did produce the color were fertilized, and perpetuated their kind, and reproduced their qualities in abundant progeny) gives us the clue to the way in which we may go about the development of a new color or color-combination in a flower.
Suppose, for example, we desire to change the flower from white to yellow. How shall we go about it?
First of all, we must produce thousands of seedlings from our white flower. Let them blossom, and then search among them with the keenest eye to detect a trace of yellow color—which is found more or less in all white flowers—in the flowers of any single plant.
You are almost certain, if your scrutiny is sufficiently keen, to detect some plant that varies an infinitesimal shade from its fellows, showing at least a trace of yellow; for a really pure white is extremely rare in Nature.
Select the seed of this plant; sow it next season; and repeat the process of searching.
You will almost certainly be rewarded, if not in the first season, then in the second or third or fourth, by finding flowers that show very much more marked traces of yellow than the original flower. And even if the variation is not very striking at first, you will probably find that it tends to be accentuated after a few generations, especially in certain individuals. Each year you will discover flowers that are yellower than any of the preceding season; and presently you will have a blossom that is as yellow as you could desire, and a new race of plants that will breed true from seed. Placed side by side with the white flowers that were their ancestors, your new race will present a striking contrast.
The fact that you have thus been instrumental in virtually creating a new type of flower can scarcely fail to give you real satisfaction and pleasure.
The fact that you have a flower such as perhaps no one else in the world possesses, and that this has been produced by intelligent and persistent effort, must be a source of quite justifiable self-gratulation.
In subsequent studies we shall see that there are methods of stimulating the production of new colors and color combinations through hybridization. But in this introductory chapter I am dealing chiefly with the simpler cases, and suggesting experiments that the amateur may undertake at the outset.
The more complex cases will command his attention in due course.
Meantime it should be stimulative to reflect that, by mere selection, demanding no knowledge of botany, no expert knowledge of horticulture, but only the possession of reasonably acute vision and the exhibition of patience and persistence, it is possible to develop in the most commonplace flower-garden blossoms whose color is at once unique and of enhanced beauty.
Nor need attention be restricted to mere matters of fragrance and color.
I have already suggested that it is usually well to consider more than a single quality. Cases like that of the heuchera leaf, in which for a special purpose a single quality alone is considered, are exceptional. As a rule, you may advantageously bear in mind, at the same time that you are developing a new fragrance, the question also of color of flower, and size, and form.
At all events, so soon as your experiment has reached the stage at which you have a number of fragrant flowers from which to select, all of which have about the same excellence of perfume, you will, as a matter of course, choose among these the one that combines with fragrance the most desired qualities of color and form and size of blossom.
DOUBLING THE PETALS AND INCREASING SIZE
As to the matter of size, it is obvious that not much need be said. A glance shows which plant bears the largest flowers. And it may confidently be expected that the offspring of this plant will tend to produce flowers of exceptional size, and that some among these will exceed the parent plant in this regard.
Precisely the same method of selecting, generation after generation, with size of flower always in view, will lead to the production of a race of plants that tend to produce uniformly, under the right conditions of nourishment and care, flowers of a far larger size than those of the ancestral form.
The matter of producing double flowers from a single variety—that is to say, flowers having two or more rows of petals instead of a single rowmay present greater difficulties. Not, indeed, that any new principle is involved, but merely that a longer series of experiments may be required to produce the coveted double flower. The start must be made here just as in the other cases, by searching among the hundreds or thousands of plants for one that bears flowers having even a single extra petal.
Seed of this plant being sown, it is likely that among the offspring there will be some that produce not merely one extra petal, but possibly two or three.
THE THREE REQUISITES
Now you are on the road to success. Thenceforward it is only a matter of time, skill, and patience—the three essential requisites of plant development combined with the dealing with large numbers of individuals.
Exceptionally there may suddenly appear a seedling producing flowers that are fully double. In such a case, if the truth could be known, it would probably appear that some of the ancestors of the seedling had produced—perhaps generations back—a double or partially double flower. Breeding from a double rose or carnation, almost all the seedlings revert to a single or semi-double form.
But in any event, once you have singled out a strain of flower that has the tendency to produce extra petals, you will probably find this tendency accentuated, manifesting what I have elsewhere referred to as the momentum of variation, and giving you results that are more and more encouraging each season.
ASKING TOO MUCH
Should you attempt to produce a double flower coincidently with the attempt to improve the scent and color and size of the same flower, you may presently discover that you are asking rather too much.
The flowers that improve in odor and color and size may not be the ones that show the increased tendency to doubling of petals.
In such a case, you may segregate the two groups, and carry forward the two lines of experiment coincidently in neighboring plots; and when you have attained a fair measure of success in giving one race of flowers perfume and color and size, and the other race a double or triple or quadruple row of petals, you may readily make a crossbreeding experiment through which you may combine all the desired qualities in a single hybrid offspring.
Even if the first-generation seedling of such a cross does not give you just the combination you are seeking, the second generation offspring or a subsequent one are almost sure to reveal some plants that meet your expectations.
So your simple experiments that began by mere selection will probably lead you to experiments in crossbreeding.
THE TWO BASIC ELEMENTS
Thus by natural stages you will have learned how to handle the essential tools of the plant developer. You will have learned that the two forces of heredity and environment are everywhere operative, and must everywhere be your sole dependence. But you will have learned also that your wishes become an important part of the environment, when you determine which flowers shall be permitted to reproduce their kind; and that you also take a hand at determining the line of action of hereditary tendencies when you crossPollinate the flowers, and decide which strains of heredity shall be blended.
Let me in concluding this preliminary chapter name two or three common flowers with which the amateur may advantageously begin his work in selective breeding.
The rose and the carnation naturally suggest themselves, but they have been so much worked on that they do not leave so much opportunity for wide improvement as some less popular flowers, though offering grand opportunities for immediate but less unique results.
The tulip is inviting, but calls for a good deal of patience.
Perhaps the four o'clock would serve the purpose as well as any other common flower. Also the hyacinth, the scylla, and the gladiolus are peculiarly good flowers on which to work. There are many beautiful varieties of all of these but new sorts could readily be produced. Moreover, they are grown from bulbs, so any new varieties may easily be perpetuated—a consideration that is by no means without significance to the amateur who wishes to obtain striking results with the least expenditure of time.
Details as to numerous other flowers, including both very common ones and those that are less usual, and varying from the simplest to the most complex, will come to our attention as we now take up in succession the records of my own work during the past four years in the development of new races of flowers.