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A visiting scientist who had seen my little preserving tomato and had learned its origin was curious to know just how I came to make the hybridizing experiment that resulted in its production. I found it difficult to answer the inquiry to his entire satisfaction. One does not recall all the details as to methods, let alone motives, after an interval of twenty-five years. But so far as can be recalled, I had no very definite object in combining the common tomato and the currant tomato except the one of general interest in the processes of nature, and a sort of all-inclusive desire to see what would happen when plants of such diverse character were united. My visitor felt that I must have had some definite idea in mind-some ideal tomato at the production of which I was aiming, and he seemed to feel distinctly dissatisfied when assured that in this particular case a result had been achieved that had not been forecast. The plant developer had been like a chemist putting together newly discovered elements. He knew that he would probably get something interesting, but just what that something was to be could not be predetermined.


I recall this incident by way of illustrating another phase of the plant developer's art than that illustrated by the development of the canning pea as detailed in the preceding chapter. In that case, it will be recalled, the plant developer was in the position of an inventor called upon to meet a precise set of specifications. He knew from the outset what was to be aimed at and, having acquired a certain craftsmanship, he knew how to set about securing it. A large number of inventions in the mechanical world have such an origin as this. When Edison started out to find a filament that would show just the right resistance to the electric current, and yet would not be consumed with its own heat, he knew just what he was seeking, and his problem of the development of an incandescent light bulb was comparable, in a general way, to the problem of producing a canning pea of just the right size and quality. But, on the other hand, a long list might be cited of inventions and discoveries of vast importance that were matters of accident. Perkins' discovery of the aniline dye; Nobel's discovery of nitro-glycerine; Rontgen's discovery of the X-ray; Becquerels discovery of radio activity-these are instances where a man found something for which he was not specifically looking. Of course he had to be in line of discovery. It was essential that he should be handling the right materials, and working in a laboratory having the right accessories, or the discovery could not have been made. Nevertheless, in each case, the discoverer found something for which he was not seeking; his experiment had results that he could not have predetermined. And here again, the analogy with that other type of experimentation through which, for example, the preserving tomato was developed will be obvious.


The point to be emphasized is that the plant developer is an inventor who works sometimes according to one method and sometimes according to another. HIe is dealing always with complex and intricate matters. Sometimes he has studied them so well that he knows what to expect of them in certain combinations. In other cases he is feeling his way, and has no very clear notion of what to expect. It might be said that he is looking for surprises rather than for anything definite; and in that event he is pretty sure to find what he is looking for. Such at least was my experience in the early experiments with the tomato that led ultimately to the production of the particular hybrid at the moment under discussion. These experiments had their origin at the very beginning of the period of my investigations in the field of plant development, a good while before I came to California. But in those days, notwithstanding one or two successes, I was only laying the foundation for my future work-learning how to handle the tools of my trade. So although there may have been interesting discoveries within reach, I did not always know how to grasp them. I had not learned, for example, the all-important lesson that the second-generation hybrid, rather than that of the first generation, is the one that must be looked to, in a large number of cases, for important development.


But when I came to California and found opportunity for expanding the work, I from time to time took up the old New England experiments where they had been left. In some cases I had brought seeds with me, and was able to complete under the new conditions experiments that had been begun in New England. In other cases it was necessary to start anew, but I had experience as a guide, and that constituted an asset that often proved a wonderful time saver. In the case of the tomato, experimentation was reopened on a comprehensive scale about the year 1887. It was at this time that I hybridized the common potato and the currant tomato and produced the interesting new form about which we have just spoken. The common tomato needs no description, but the currant tomato is much less familiarly known. It is a plant with long, slender, trailing vines and slender leaves and it bears racemos of small currant-like fruit. It occurred to me that it would be highly interesting to hybridize this trailing plant with a particularly tall, upright, compact variety of the common tomato. The cross was made reciprocally, pollen from each plant being used to fertilize the stigma of the other. The fertilization was effected without difficulty, and an abundant supply of seed was produced. The hybrids that grew in the next generation were many of them pretty clearly intermediate in form and appearance between the parents. But some of them were almost ludicrous in appearance. They took on twisted and contorted forms, and in particular their leaves were curled and twisted into fantastic shapes. As to fruit, some of the plants produced long clusters with tomatoes much larger than cherries; others furnished small fruit like that of one of the parents. And in some cases a plant that had retained the short stocky tree form of the common tomato bore clusters of small tomatoes in bunches similar to those of the other parent. The foliage varied astonishingly between the two types. In some there was an exact compromise that was very curious. The dark, blistered leaves of the ordinary tomato, combined with the long, slender leaves of the currant tomato, produced a most interesting effect. Other specimens showed every possible gradation between the parent forms. Here, then, was a case in which there was no conspicuous dominance of one parent or the other as regards any individual character that could be segregated and classified. Neither as to size and form of plant-stalk, nor as to leaf, nor as to the fruit itself, was there clear prepotency or dominance of one parent over the other. If there was an exception to this it was perhaps that the fruit tended to be borne in clusters, as in the case of the currant tomato, rather than singly or in small groups as with the ordinary tomato. Attention is called to these diversities because it is well to emphasize anew that the phenomena of the clear segregation of "unit" characters, with the exhibition of dominance and recessiveness-which the pea with which Mendel experimented manifests so beautifully, and which we have seen manifested in the characteristics of numerous other plants-is not a universal phenomenon that the plant experimenter may confidently expect always to discover and use as an easy and simple guide along the path of plant development. Different species of plants, different varieties, even different individuals show diversity as to the extent to which the so-called unit characters are segregated and mutually combined or antagonized, as the reader who has followed the story of various plant developments already outlined is clearly aware. We shall have occasion to revert to this subject more than once, and to point out various possible interpretations of the phenomena, various underlying harmonies that do not appear on the surface. But for the moment we are concerned with the story of the new tomato, and may be content to put forward the facts regarding it without great insistance on their theoretical interpretation. Suffice it that the progeny of the tree-like tomato and the trailing one were a varied company, giving the plant developer almost endless opportunities for selection. I chose, naturally, from among them those that bore the handsomest and largest fruit, and in planting these, was enabled, in the course of several generations, to secure a very handsome plant with attractive fruit of new type which came true from seed. It required about six years to produce and make sure of the new variety, which was announced in my first catalog of new plants, issued in 1893. The description there given of the new fruit was as follows: AN INTERESTING HYBRID "This distinct novelty and ornamental fruiting plant grows about twelve inches high by fifteen inches across. The curious plated, twisted, and blistered, but handsome leaves, sturdy, compact growth, and odd clusters of fruii will make it a favorite ornamental plant." Another account supplemented this by describing the fruit as "a small, round, scarlet tomato, borne in clusters, the individual fruits measuring only three quarters of an inch in diameter; of splendid scarlet coloring and unusually rich, sweet flavor." The comparatively rapid development of this curious form of plant, so widely divergent from the ordinary tomato, illustrates the possibilities and suggests the compelling interest of such experiments in hybridizing and selecting even our commonest garden plants. The work is of course no different in principle from that followed by the plant developer in the orchard, whose work has been detailed in earlier volumes. But there is this important practical difference: In experimenting with such a plant as the tomato, we get results quickly because the plant grows and fruits in a single season. The results of any given experiment may be known within a few months of the time when the seed is planted. This is quite different from the case of the orchard trees, which require, as we have seen, long periods of patient waiting, few of them bearing, even under forced methods of grafting, in less than two or three years, and some of them, such as the pear and fig, requiring a much longer period. On the other hand, there is one regard in which the orchardist has an advantage. It is not necessary for him to fix his new varieties so that they will come true from the seed, inasmuch as his plants will propagate by division. But in dealing with plants of annual growth, like the tomato, it is obvious that a new variety can have little value unless it will come true from the seed. (The tomato is really a perennial, that is treated as an annual.) So the task is not completed when a new variety is produced; additional experiments must be conducted to fix the variety. Even this may be accomplished, however, by careful attention to selection, in the course of a few years, as we have just seen illustrated in the case of the hybrid tomato.


Among my later experiments with the tomato were some that had exceptional interest because of the material used. It chanced that when I left home in the east, many years before, I brought with me seed of several of the standard varieties of plants and of some crossbreed varieties; and, as has been pointed out, I was hybridizing tomatoes as well as beans and other plants even at that time. The lot of seed thus brought to California included some seeds of the tomato. As was customary in those days, this seed had simply been pressed out of the fruit, and dried on a piece of paper with the surrounding pulp still clinging to it. Nineteen years afterward I planted some of these seeds, being interested to see whether they retained their power of germination. Somewhat to my surprise, almost every seed germinated. But the majority of the seeds did nothing more than form cotyledons, lacking the central bud for further development. There were a few exceptional plants, however, among the large company-perhaps altogether two dozen-that continued their growth and in due course fruited. The fruit of some of these plants grown from nineteen-year-old seed was sent to an eastern horticultural journal, whose editor commented on the fact that seed kept for this long period still produced fruit quite equal to anything that had been developed in the intervening nineteen years. In planting the nineteen-year-old seed, I retained a certain quantity from the same lot for a further test. The following year it was planted in the same careful manner. But although a few of the seeds germinated and sent up cotyledons of a weaker type, not one had the power of developing beyond that stage. All of these seeds in the tewntieth year seemed to have lost the capacity to produce a central bud from which the plant stem could develop. Of course it may have been only an accident that a few seeds were able to take on mature growth after nineteen years, whereas not one could do so after twenty years. But I am inclined to think that the seeds had reached just about their limit of suspended vitality. The fact that germination began, but that it did not continue because of lack of a central bud, suggests that degeneration of part of the substance of the seed had taken place. Seemingly it was only the most resistent seeds that were able to stand this degenerative process, and retain unimpaired vitality to the end of the nineteenth year. The heredity of those that grew was preserved intact: the seeds producing exactly such plants and fruit as if they had been planted nineteen years before.


The interesting question arises as to whether the degeneration of germinal matter was confined entirely to the store of nutrient substance in which the germinating nuclei of the future plant are imbedded, or whether it included any portion of the germinating structure itself. The fact that failure to continue growth-in the case of the seeds that put forth cotyledons and then died was due to a lack of the central bud that usually puts forth between the cotyledons, suggests that the germinal substance itself was impaired. Of course this germinal matter is of tangible, even if very minute, size and there is no apparent reason why it might not be impaired as to a portion of its substance. Conceivably, the substance of the complex molecules making up the germinal protoplasm may undergo a gradual process of decay or disintegration through the throwing out of some of their atoms, somewhat as radium and its allied substances are disintegrated. This of course is a pure assumption, yet it is not altogether without plausibility. But whatever the precise manner in which the degeneration of the germinal plasm is brought about, the suggestion that one portion of its structure may be affected more than another raises a question as to whether, conceivably, such a deterioration of the germ plasm within a seed, in an exceptional instance where a seed is stored for a number of years before being placed under conditions proper for its germination, might not result in the production of a deformed or modified plant. Whatever differences of opinion may be held among biologists as to the possible transmission of modifications of the body plasm, all are agreed that modifications of the germ plasm become a permanent heritage and are passed on to the offspring. So it seems at least a possibility that we have presented, in the deterioration of the germ plasm within the seed, an explanation of the appearance of mutants or sports that may become the progenitors of new races. Attempts to produce mutants by treating the ovules of plants with chemicals, including radium, have been made by several experimental botanists, notably by Dr. D. T. MacDougal, of the Desert Laboratory at Tucson, Arizona, and by Prof. C. S. Gager. Prof. MacDougal's evening primroses, grown from seeds that were treated with chemicals while in embryo, sometimes differ markedly from other plants of the species. Prof. T. H. Morgan has made similar experiments with the eggs of a fly, treating them with radium, and thus producing individuals strikingly different from their parents. These experiments, then, although they mark merely the beginning of a new line of research, are interesting in their suggestiveness. And it occurs to me that the case of the nineteen-year-old tomato seeds may have a bearing on the same subject. It would be well worth while to conduct a systematic line of experiments in which seed of a fixed species is stored in large quantity, and a certain proportion planted each year, careful record being made of the characteristics of the successive groups of plants, with an eye to any modifications that may occur when the seed approaches the limit of the term through which it can maintain vitality under the conditions given it. It is said that there are records of wheat germinating after it had been preserved for centuries in the tombs of Egypt, although there is no proof of this; but most seeds have far more restricted capacities for maintaining vitality. My experiment suggests about twenty years as the limit for the tomato seed under fairly good conditions. So the seeds of some fixed type of tomato might very well be among those selected for such an experiment as that just suggested. My own observations in the matter are chiefly confined to what has just been related to my nineteen and twenty-year-old tomato seeds; and I must leave further investigation along this line to younger experimenters.


Doubtless among my most interesting experiments (to the general public) with the tomato have been those in which this plant was grafted on the stalk of the potato; together with the complementary experiments in which the potato was grafted on the stalk of the tomato. The grafting of herbaceous plants such as these presents no complications as a mechanical procedure. The fact that the stem is succulent throughout makes such grafting a less delicate process than the grafting of twigs of trees, for example, in which, as we know, it is necessary to bring the cambium layers of the bark in accurate contact. With herbaceous plants like the potato and tomato, the stem may unite at any portion where the cut surfaces come in contact. To make a neat and thoroughly satisfactory graft, however, it is of course desirable to select stems of exactly the same size. The splice graft, elsewhere described, is the best one to use, and if the incisions are made with care, so that the incised surfaces fit accurately together, it is only necessary to tie a piece of cloth about the united stems for a few days until union has taken place. It is not necessary to use grafting wax, if protected from winds and too hot sun. The operation is preferably performed in the greenhouse. With this method, I grafted the tops of young tomato plants on the main stalks of potato plants, at a time when the stems were about one-quarter of an inch in diameter. The reverse operation, grafting amputated potato tops on tomato roots, was performed at the same time. Of course the tomato and potato belong to the same genus, and it seemed reasonable to suppose that such grafting might be successful. But, on the other hand, numerous attempts have been made to hybridize the two plants by cross pollenation, and these have always resulted in failure. I have tried it many times, and have never been able to fertilize one plant with pollen of the other. We know that, as a rule, plants that cannot be cross-pollenized cannot be mutually grafted. The same barriers usually exist in one case as in the other. The potato and tomato grafts, however, proved very notable exceptions to this rule. In both combinations, the union between the foreign stems took place quickly, and resulted in a stem as strong as the ordinary stem of either plant. Growth continued, and the plants came to maturity at about the expected season. But the results of the strange alliance were interesting to the last degree. They must be considered in detail because they have a bearing on one of the most interesting open problems of plant development-the question of sap hybridism.


The tomatoes that grew on the root-stalks of the potato developed much as other tomato vines do, although in some cases it seemed that the vines bore closer resemblance to potato vines than is usual. But the fruit that appeared in due season was a tomato differing in no very obvious respect from other tomatoes of the same variety. They, however, were not of as good quality. Meantime the potato roots, which supplied water and mineral salts to the tomato vine above them, and which in turn must receive material for the growing of their tubers from that vine, showed quite unmistakably the influence of the foreign system of leaves with which they were associated. Instead of being smooth and symmetrical like ordinary potatoes the tubers were small and ill-shaped, and some of them had rough and corrugated scale-like surfaces, suggesting the skin of a lizard rather than that of a potato. Moreover, they were bitter in flavor and utterly unlike the ordinary potato in taste. They further showed their departure from the traditions of their kind by manifesting a tendency to sprout even while the tomato top was still growing vigorously. Perhaps these results, as regards both the relative normality of the tomatoes borne by the grafted vine, and the abnormality of the potatoes grown by the roots, might have been expected. At least they seemed quite explicable. It will be recalled that the conditions of plant growth were detailed somewhat at length in the first chapter of the present volume, and that it was there pointed out that the plant roots absorb from the soil about them mineral salts in solution that are carried up to the leaves of the plant before they are transformed into organic matter by combination with carbon drawn from the air. It was noted that the organic compounds thus manufactured in the leaves of the plant must be sent back down the stem of the plant to be deposited, in case of a tuber-forming plant like the potato, in connection with the roots in the ground. It follows, then, that the tomato plant, even though its source of supply was the root system of a potato, merely gained from these roots part of the raw inorganic materials with which its leaves were to manufacture the special compounds that go to make up a tomato. Inasmuch as the tomato leaves were themselves unmodified, there was no reason why their product, the tomato, should be greatly modified. In receiving its supply of raw material from a foreign root, the tomato top was in no different condition from the ordinary cions in a fruit orchard, which, as we have seen, are habitually grafted on roots or branches of a foreign species. But the case of the potato tubers is obviously quite different. Their substance is made up of material that came originally, to be sure, in part from material gathered by potato roots; but this material had traveled up to the leaves of the tomato plant and had there been transformed; so when it returned to be deposited and form tubers it was a tomato compound and not a potato compound. It was not absolutely different in material from the material of the ordinary potato, because the tomato and potato are cousins. But the modification had been great enough to transform the tuber, and make it a deformed and perverted thing, more or less comparable, doubtless, to the tubers of some ancestral race from which both the tomato and potato have developed. The extraordinary thing, perhaps, was that the tomato should have manufactured starch in such quantity as to have supplied material for even these dwarf tubers, inasmuch as the normal tomato plant produces no tubers of its own. But seemingly the buds designed to produce tubers on the potato roots made an incessant appeal that the vine above could not resist, even though it was able to fulfil but imperfectly the specifications for a potato tuber.


Meantime, what of the potato tops that were grafted on the stem of the tomato? How did these prosper? Here, it is obvious, were complications of a different order. The tomato vine obviously could bear no tomatoes because it had no tops. Meantime the potato vine was equally handicapped as to the production of subterranean tubers since it had no roots of its own kind. But the tomato roots of course sent up their supply of water and salts in solution, and the potato leaves set to work as usual developing material for the manufacture of tubers. When, however, the effort was made to send this material for tuber formation back to the roots, there was an embargo put on such transportation because the tomato roots have no knowledge of the art of tuber making. In this dilemma the potato crop, under spell of the compelling instinct of tuber formation, made the only compromise possible by growing aerial tubers at the joints where the leaves appear from buds springing from the point of union with the leaves of the stein. What would ordinarily have been leaf-bearing branches were terminated with small potatoes, which, because of exposure to the sunlight, generally took on a greenish tint, those in full sunlight sometimes being thoroughly green, while those that were shaded by leaves were of a lighter color. The potato vine growing on a tomato stem and bedecked with aerial potatoes, like some strange form of exotic fruit, was certainly one of the most curious forms of plants ever seen. It is perhaps needless to add that the potato vine produced no fruit that gave any suggestion of the influence of the tomato. The tubers it grew were potatoes and nothing else; their modifications in form and color were obviously due to the lack of their natural protective soil covering. But the fact that the vine, handicapped by lack of roots of its own kind, should have been able to transform leaf buds into tuber-growing aerial rootlets furnishes an interesting lesson in the metamorphosis of parts. How the great poet Goethe, who first expounded the theory of metamorphosis of parts, and clearly recognized the fundamental unity of stem and leaf and flower, would have enjoyed the viewing of a spectacle like that!


And for the modern plant developer, the strange compound vines have no less interest, for they suggest a number of questions that are much easier to ask than to answer. How, for example, was the leaf system of the potato that grew the aerial tubers to know that tubers were not being formed about its roots in the ordinary way? It did know this, obviously, else it wvould not have adopted the unprecedented expedient of growing tubers in the air. It is easy to speculate, and to suggest, for example, that the potato plant producing an excess of sugar and starch in the usual way, must find some place to deposit it, and that as no demand came from the roots, the only available buds were made to do vicarious service. But the explanation obviously lacks a good deal of complete satisfactoriness. For the moment, we perhaps must be content to recognize in this another illustration of the fact of communication between the different parts of a plant, and of the harmony of purpose through which the plant as a whole is made to respond to the conditions of the environment in the way that best meets its needs. But we are forced to recognize, through such an illustration, a greater capacity for adaptation, seemingly almost of a reasoned character, than we are commonly want to ascribe to the plant. The case of the tomato plant growing on the potato roots, which so perverted the character of the tubers that it supplied, has practical interest for the plant breeder, and in particular for the orchardist, because it demonstrates the effect of a cion on the stalk on which it is grafted. Of course the ordinary fruit tree does not develop a system of tubers, and so it does not call for such a supply of starch, for example, as that which the tomato vine was induced to produce for the tomato roots. But the root system of any tree requires nourishment if it is to develop, and this nourishment, as we have seen, must be supplied by the leaves of the tree above it, even though the roots themselves first collect part of the materials. It follows that the root system of any tree, while it is absolutely essential to the leaf system above it, is also very largely dependent on that system. In other words, there is the closest reciprocal relation between root system and leaf system. This relationship, which many orchardists overlook, I have long recognized and have repeatedly referred to. But the case of the tomato on the potato root emphasizes the lesson in such terms that no one can ignore it. With this illustration before us, we can scarcely doubt that the root system of any stock on which a foreign top is grafted (as is the custom in most orchards) is modified in some measure by the cions it bears. The foreign leaves cannot supply precisely the same quality of nourishment to the root that leaves of its own kind would have supplied. In the main, no doubt, the protoplasm of the root assimilates the nourishment that comes to it, and makes it over into its own kind of protoplasm. But we know that the flesh of animals varies in quality with the food given the animal, and we cannot well doubt that the protoplasm of the root of a plant must similarly be modified by the character of its food. And this line of thought suggests the further possibility that when more cions than one are grafted on the same branch or on the same trunk, there must be a certain intermingling of the sap from the different leaf systems in the course of the journey to the roots of the tree; and that it might very conceivably happen that a sufficient blending would take place so that the modified sap might find its way to the fruit buds of a given cion, and effect the character of the fruit in a way not altogether unlike the effect of hybridizing. This would account for the case narrated at length in an earlier chapter, in which a cion of the purple-leaved plum grafted on the stem of a green-leaved Kelsey plum tree, appeared to influence the fruit of a neighboring stem so that the seedlings that grew from that fruit bore purple leaves. As before stated, such a striking instance of evident "sap hybridism" is exceedingly rare; but can we be sure that influences of a less tangible character are not constantly exerted by engrafted limbs? May it not be possible, even, that the influence of cions from many sources on one another, when they are placed together in large numbers on a single tree, as in the case of my colonies of plums and cherries and apples, may be very notable indeed, even though of such character as not to be demonstrable? Is it not at least possible that the improved quality of the new and splendid varieties that appear on the various cions of these multiple trees, is in some minor part to be ascribed to the mutual influence of cions of many different strains of past generations, one on another? If this thought be permitted, we must recognize in such fruit colonies as those in question an influence exercised by the community for the benefit of the individual that is comparable to the intangible influences through which a community of human beings affects the moral character of its individual citizens. All this carries us somewhat afield from the case of our grafted tomato-potatoes, but only to the extent of a natural application of principles clearly suggested by the phenomena exhibited by these extraordinary plants.


Let us repeat that the grafting of these two plants is not a difficult procedure. It is well worth the effort of any amateur to repeat these experiments (so far as I know, this has not been done until recently, and its significance has never been fully appreciated), and to observe for himself the curious phenomena that will result. Possibly the results of my own early experiments might not be exactly duplicated. But there is little doubt that interesting and encouraging developments would result.

This text is from: Luther Burbank: his methods and discoveries and their practical application. Volume 7 Chapter 4