THE TIGRIDIA AND SOME INTERESTING HYBRIDS
NEW CHARMS IN FAR AWAY FLOWERS
About a quarter of a century ago I commenced cultivating and crossing all the Tigridias, or Tiger Flowers that were then offered by any seedsman or nurseryman anywhere in the world. I also secured all the species of the allied genus Ferraria that I could obtain and cultivated them for the purpose of hybridizing them with the tiger flowers. The Tigridias are natives of subtropical and tropical America, ranging from Mexico to Peru and Chile. The Ferrarias are from the Cape of Good Hope, and are represented by a number of species. Both tribes belong to the Iris family, and the two forms are so closely related that by some botanists they are regarded as properly falling within the same genus. My own experiments, which show the ready hybridization of the various Tigridias and Ferrarias, suggest that they are closely related. Yet the fact that they are indigenous to different continents shows that they have been separated for a very long period of time, although doubtless of common ancestry. The students of geological botany tell us that there must have been a great mass of land in the southern hemisphere at one time on which races of plants developed that subsequently were isolated on the land masses that are now known respectively as South America, Africa, Australasia, and New Zealand. At that remote period the Tigridias and Ferrarias were doubtless of one stock, and the fact that their descendants of today retain such elements of affinity as to puzzle the botanists and to serve well the purposes of the hybridizer gives another illustration of the wonderful pertinacity with which the characteristics of a plant are sometimes transmitted through almost numberless generations without radical transformation. It is little wonder that the earlier biologists, before the coming of Darwin, when confronted with such observed cases of affinity between races that must have been separated for countless thousands of years, were strong in their faith in the fixity of species. Yet the facts of variation, even within a few generations, are too obvious to escape attention. And the compromise has been found, as everyone knows nowadays, in a recognition of the fact that time is long, and the further fact that natural selection may be instrumental in maintaining the fixity of a race, provided the environing conditions are unchanged, just as it may be instrumental in somewhat rapidly changing the form of a race when the environing conditions have altered.
HYBRIDIZING THE TIGER FLOWERS
From the outset I found that the various tiger flowers thrive in my gardens, particularly in the sandy land at Sebastopol and in sandy beds especially prepared for them at Santa Rosa. As I have already said, I began at once crossing and hybridizing the various species and varieties, and of course carried out selection among the seedlings and made new crossings, according to my usual custom. The type species with which the experiments began was the Tigridia pavonia, of which there are numerous varieties. Another form known as the Conchiflor or Shell flower was utilized, and subsequently the T. buccifera, a form more recently introduced from Mexico. An especial effort was made to introduce also into the combination the strains of a plant of yet another genus, the Herbertia platensis. This is a tall-growing plant bearing close resemblance to the Tigridias, and by some botanists classified with them. It has pale blue flowers marked with yellow, and the specimens are of a somewhat different structure from those of the Tigridia, though the bulb and general growth of the plant are similar. I particularly desired to introduce strains of the Herbertia platensis, because this is a very strong-growing plant, and its vigor and health would be of great service in giving hardiness which is the one thing that the Tigridias more especially lack. In particular, the bulbs of the tiger plant are difficult to keep over winter, and especially subject to decay from exposure to air and to the attacks of aphids when stored. But much to my disappointment I was never able to effect hybridization between any of the Tigridias, either pure bred or hybrid, and the Herbertia. The experiment was made over and over, and in every case it was without result. Meantime, however, there was no difficulty whatever in hybridizing the ordinary cultivated strains of Tiger Flowers among themselves and with some of their South African relatives. And the results of such hybridizings were manifest almost from the outset. One of the most striking modifications shown by the hybrid Tigridias was the development of varieties having striped flowers. It might very well be expected that a "tiger flower" would be striped. But in point of fact the native Tigridias are spotted and never striped. They might with much greater propriety have been named after the leopard or panther, or better yet, considering their origin, after the South American jaguar. But the botanist who originally named them seemingly had rather vague notions as to the markings of the coat of the tiger, or else considered it sufficient that the flower itself wears a yellow mantle with dark markings. In any event, there is something about the aspect of the flower that makes the name "tiger flower" seem not inappropriate. And the propriety of the name becomes quite beyond challenge when my new hybrid varieties are under observation. For these are striped in a way that is very striking. Quite aside from its suggestions as to one feline or another, however, the new hybrids are flowers of great beauty and interest and differ conspicuously from any of the parental forms. Not only are the markings thus conspicuously altered, but the flower itself is greatly increased in size. The tendency to freedom of bloom is accentuated. Moreover the hybrid plants have gained greatly in vigor of growth, in hardiness, and in resistance to disease. The colors of the new flowers are conspicuously brightened. The striping is usually crimson on white, crimson on yellow, or yellow on crimson. In addition to presenting these stripes, which are quite unlike any marking of the native Tigridias, the hybrid flowers generally retain the dotting at the center that characterizes the tribe in its original form. But these dottings are greatly increased in size. In some instances, on the other hand, the dottings are partially or entirely eliminated. The original types of these very striking new forms of Tiger Flower were readily fixed so that they breed absolutely true from the seed. It was possible, however, to increase the size of the flower by selection, and this increase in size was a permanent acquisition; also to add brilliance with new combinations of colors. And of course the hybrid plants thus perfected exceed greatly the size of any plants that could have been developed by mere selection without crossing.
UTILITY AS WELL AS BEAUTY
The new tiger plants, although still lacking something of hardiness, were greatly improved in this regard over their ancestors. Most of the old tigridias, as I have said, are quite subject to insects and disease. The hybrid forms are much more resistant. There is also a greater power on the part of the new plants to stand sunshine. The old tigridias sometimes withered under the influence of the sun. This might not at first thought be expected of a tropical plant, but it should be recalled that the growth of vegetation in tropical regions is so luxuriant that low-growing plants of this order are not usually subject to the direct rays of the sun throughout the day. It goes without saying that the bulbs of the new tiger plants were improved in proportion to the stalks and flowers. The bulbs of the tiger plant are elongated and tunicated, and multiply by division somewhat after the manner of the hyacinths, tulips, and the allied races in general. The bulbs of the new hybrid tigridias were doubled in bulk, and in some cases quadrupled, as contrasted with the parent forms. Like the somewhat similar bulbs of the gladiolus, they may best be kept in the ground over winter here in California, instead of being taken up and stored as is necessary in colder climates. The development of the bulbs of the tigridias has not been at all a matter of accident. At all stages of the experiment in hybridizing and selection, I have paid the most careful attention to the condition of the bulbs, selecting always those that were largest, firmest and soundest. And the reason for this was not merely that such bulbs usually produce the best flowers, but also that it is worth while to improve the size and quality of the bulbs quite on their own account. The particular reason for this is that the bulbs of the Tiger Plant are edible. When cooked like potatoes, or made into a stew, they constitute a really delicious vegetable. To my taste the bulb of the tiger plant is at least the equal of any vegetable under cultivation. It is also highly nutritious. I am not sure that it has an equal among the vegetables of our gardens in its combination of nutritiousness and appetizing flavor. These very qualities lead to its destruction by all kinds of animal and insect life, like the Lilium Brozvnii, which has no bitter principle, containing sweet and nutritious matter, and which also is attacked and appropriated by insects and other creatures. As yet the tigridia is too tender to gain a place in the vegetable garden on a footing with the potato and allied bearers of bulbs and tubers. But when through further breeding experiments, it has been rendered more amenable to general cultivation, its bulb being at the same time still further increased in size, the tiger plant may come to be valued for its edible bulb quite as highly as for its beautiful and spectacular flower.
MULTIPLICATION BY BULB DIVISION
The habit of storing nutritious matter in its bulb, and the further habit of producing collateral bulbs from which new stalks will grow, so that the plant multiplies indefinitely in this way, is characteristic, as everyone knows, of a large number of plant families, many of which have come within the scope of our studies. The phenomenon of bulb division, indeed, is so familiar to everyone who has experimented in the vegetable or flower garden as to take its place among those familiar matters of fact that call for no comment. Yet if we consider the matter thoughtfully it will be clear that this habit of putting forth offsets from a bulb as the basis for the development of new plants is an altogether extraordinary phenomenon-quite as mysterious, indeed, as the production of the seeds that bear the complex hereditary factors and transmit the qualities of a race of plants from one generation to another. There is, in point of fact, no fundamental difference between the production of new plants by bulb division and their production by seed, except that in the latter case there is opportunity for the union of two different racial strains, one borne by the pollen and the other by the ovule. This, to be sure, is a difference that has very important practical bearings, inasmuch as the union of two different hereditary strains gives opportunity for the blending of hereditary factors and their recombination, thus compelling variations that furnish the basis for natural or artificial selection, through which new races are developed. All this needs no explication here, as our earlier studies have made it perfectly familiar. But what I wish now to emphasize is the fact that the bulb that produces a new plant carries the hereditary factors of the parent plant substantially as they are borne by the ovule or the pollen grain that the same plant puts forth on its aerial stalks, and exactly as the bulb of any plant-in fact, the bulb of any plant is only a fat, immature, underground bud. If the ovule could develop without being fertilized, or if the pollen could grow into a plant, the result in either case, we may reasonably assume, would be a reproduction of the plant closely similar to the parent form, just as the aphids and the bees when parthenogenetically produced, and in a few instances of plants, for example, the violet. Yet there are differences between the different pollen grains and between the different ovules of the same plant, as demonstrated by the fact that flowers, for example, of different hues may be borne on plants grown from a single seed pod. So if we are to present the matter quite in its true light we should say that the aggregate pollen-product and ovule-product of a plant must be considered as representing the personality-that is to say, the hereditary complex-of the plant. No single pollen grain and no single ovule contains representatives of all the types of hereditary factors that are present in the germ plasm of the plant as a whole. Stated otherwise, the pollen grains and ovules are very specialized and concentrated portions of matter, each of which contains a similar equipment of the most fundamental of the hereditary factors, but each of which contains a somewhat different assortment of the less fundamental ones. All the plants that grow from the seed of a tigridia, for example, will be unequivocally tigridias in stem and leaf. But there will be minor differences among them as to details of size, as to freedom of flowering, as to precise size of flower, and as to the exact distribution of the color markings. As a matter of course, when the seeds are the product of cross-pollenizing, the offspring, representing now two parents, will show a still wider diversity of hereditary traits. Meantime, turning attention again to our bulb, we find that this structure represents the parent form with much greater fidelity. As nearly as there can be identity between two different living things, the plant that grows from any offset of the bulb of any given Tigridia will be identical with the parent form. A certain amount of diversity there must always be, because no two living organisms are absolutely identical. But for all practical purposes it may be said that the different plants grown from offshoots of an original bulb are identical. The process of bulb division can be repeated a thousand or a million times, until the original bulb has been so multiplied that its descendants people the earth. But from first to last, one bulb will be substantially like another, and all the myriads of plants that have thus arisen may be said to constitute a single personality. All this is such familiar matter of fact as to excite no comment. Yet, rightly considered, it is a fact of the most mystifying kind and one that must excite wonderment on the part of anyone who gives it serious consideration. That the multitudes of hereditary factors that exist in the germ plasm of so complex an organism as a flowering plant should be grouped in each successive bulb that develops as an offshoot of the original bulb of that plant in precisely the same combination, pre-determining the production of a future plant identical with the original, is a fact that becomes increasingly mysterious the more carefully we consider it. In particular, the mystery is great if we have kept in touch with modern ideas as to the segregation of the germ plasm and the body plasm of the living organism. There is a current notion, supported by high scientific authority, that the very earliest division of a fertilized egg cell, in the case of an animal or plant, results in the separation of the infinitesimal fleck of protoplasm into two different parts thatt are fundamentally different, one carrying the body plasm from which the structure of the new individual is to grow, and the other carrying the germ plasm that is to convey the potentialities of future offspring. New studies in the biological laboratory have shown that this idea, that the first division of the egg cell results in such a segregation of body plasm and germ plasm, cannot be fully accepted. Nevertheless it is obvious that in the case of all higher organisms, whether vegetable or animal, the germ plasm may be a thing apart. As finally segregated, for example, in the ovules and pollen grains, it constitutes a concentrated aggregate that transmits the hereditary factors from generation to generation in a sense independently of the bodily characteristics of the individual plant. You may, for example, determine that a given flower and the seed that grows from it shall be of exceptional size and vigor by cutting off all other flowers so that the energy of the plant shall be concentrated on a single one. But in so doing you merely give added vigor to the new generation; you do not alter its fundamental hereditary characters. These are pre-determined by the factors in the germ plasm that have been brought from earlier generations and of which the individual plant is only the carrier. All this, then, suggests the isolation of the germ plasm; and the newest theories of heredity have tended to emphasize the idea that germ plasm and body plasm are things of a somewhat different order. Yet the phenomena of reproduction by root division or by the putting out of new bulbs, furnish a striking demonstration that the germ plasm which predetermines the form of the future plant is present not alone in the pollen grain and the ovule, but also in the bulb. Even from the single bud of a bulb, as we have seen illustrated in several cases, a new plant will grow that will duplicate absolutely-in the interpretation just given-the qualities of the parent plant. And when we were studying the fruit trees we saw that the same thing is true of any aerial bulb if grown even on a foreign branch. Root bulb and aerial bulb alike contain the essential germ plasm of the individual of which they are a part. They nurture potentialities of a new individual that will duplicate the parent form.
GERM PLASM AND BODY PLASM
From all of which it follows that the germ plasm of the plant cannot be thought of as isolated from the body plasm. It may well enough be segregated within the substances of any given cell. But that it is present in connection with the living cells of the plant everywhere, from its roots to its remotest stem, is clearly demonstrated by the every-day methods of propagation employed in orchard and garden. Such being the case, it is difficult to avoid the conviction that the germ plasm that is part and parcel of every cell of the body plasm of the entire plant is more or less subject to the environing influences that effect the body of the plant. And from this it would follow, at least as a reasonable inference, that environing influences that modify the structure of the plant body must have an effect in modifying also the germ plasm in a way to influence the character of the future plant that develops from that germ plasm. And as much as this, it should be added, is admitted by all experimenters, even by those who deny the possibility of the transmission of acquired traits in the older interpretation of that phrase. That altered conditions of nutrition may modify the condition of the germ plasm in such a way as to modify the state of the offspring has been shown by experiments in many fields, both with animals and vegetables. Such being the case, the question of the transmissibility of acquired traits is reduced, as I have elsewhere quoted an authority as saying, to a matter of definition. Nevertheless, for practical purposes, it is unquestionably true that the germ plasm is enormously difficult to influence, and that under all ordinary circumstances it will convey its hereditary factors unchanged, or not appreciably changed, from one generation to another. In attempting to modify the forms of successive generations, the method that has hitherto proved successful, has been, not the modification of the individual germ plasm, but the bringing together of different germ plasms from diverse organisms through hybridization. For such union of germ plasms there is obviously no opportunity in the case of the new plant grown from the bulb. Hence the fixity of type of plants propagated in this way-a fixity that is often of the utmost practical importance, as in the propagation of a new race of vegetables or flowers, but which, by the same token, puts the plant thus propagated outside the field of the plant experimenter.
COMPLEMENTARY MODES OF PROPAGATION
Thus the two methods of propagation that are available for such a plant as the tigridia and for countless others of its ilk, are in a sense antagonistic or complementary in their influence on the history of the plant itself. Propagation by bulbs insures spread of the race, but insures also maintenance of the racial fixity. Should environing conditions change, it is unlikely that plants thus propagated could change rapidly enough to adapt themselves to these conditions. But at the same time that the plant is producing new bulbs it may also, year by year, produce seeds that are the result of cross-fertilization. And this method of propagation is a perpetual bid for such variation as will make possible a relatively rapid change in adaptation to a changing environment. That vast tribes of plants should have found it necessary to adopt both methods of propagation is in itself an evidence of the struggle for existence that is the basis of natural selection. In another way, also, the bulb perhaps evidences the hardness of the struggle for existence, particularly in tropical climates. Everyone knows that vegetation is exceedingly luxuriant in the tropics, and it is a matter of observation that the habit of developing tubers and bulbs is especially common among the herbaceous plants of tropical and sub-tropical regions. Perhaps one explanation is that the storing of food-supplies in the bulb enables the young plants to shoot up rapidly without waiting for the development of a large root system. By so doing they may stand a chance of competing with the surrounding vegetation and thus have a far better chance of reaching maturity than if they had grown from tiny seeds. It is probable, therefore, that the generality of bulbous plants that one would find in any given locality in their native haunts would have developed as offshoots of the bulb of an original plant or as inbred or close-bred. So the bulb has very fundamental importance in the plant economy. And it is interesting to reflect that it is correspondingly important from a human standpoint, inasmuch as bulbs furnish us some of our most important food products. We have seen that a good many of the plants that are propagated solely from the bulb or tuber, of which the potato is the most familiar example, may give up the habit of seed production altogether under cultivation. But, on the other hand, it is observed that plants that produce comparatively small bulbs in the state of nature may be stimulated to the production of far larger bulbs and more abundant offshoots under cultivation. And the inherent reason for this is obvious when we consider the share the bulb must play in perpetuating a species and aiding it in the struggle for existence under widely varying conditions and in competition with other plants. Making application to the particular case of the tigridia, it has already been recorded that I have found no difficulty in doubling or even quadrupling the bulk of the bulb of that plant, as well as greatly increasing the tendency to the multiplication of bulbs. I repeat that it will probably be found desirable to cultivate the plant further along these lines until it finds recognized place in the vegetable garden as the producer of a food of the finest quality, while at the same time retaining value as the bearer of beautiful flowers.
This text is from: Luther Burbank: his methods and discoveries and their practical application. Volume 10 Chapter 3