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The Scented Calla

HOW FRAGRANCE WAS INSTILLED IN A SCENTLESS FLOWER

Not long ago a young woman visitor who had learned that the function of odor in flowers is to attract bees and other insects made a remark at once naive and wise. "It seems wonderful," she said, "that bees and other insects should have the same taste in perfumes that we human beings have. The rose and the apple blossom are sweet to them as well as to us; whereas one might expect that they would care for something quite different, especially when we remember that cultivated people generally like more delicate perfumes than those that please uncultivated people." This remark, as I said, was at once wise and naive. It was wise because it showed a tendency to seek causes for things in nature instead of taking them for granted as most people are prone to do. It was naive because it quite overlooked the true significance of the function of odors in nature. A moment's further reflection would have shown the young woman that it is not at all a question of the bee liking the things that man likes, but a question of men having learned to like the things that the bee likes. The scent of the flower was not put forth to please or displease man, but to please and attract the insect. And man learned to like the odors that were constantly presented to him largely because they were constantly presented; just as you may learn to like a food-say, for example, olives-by repeatedly tasting it, though at first you do not care for it. The exception, of course, is the odor that is associated with unhygienic things, such as decaying vegetable and animal matter. These are attractive to the insects that feed on them because the substances that produce the odors are to these insects wholesome. But they do not attract the bee because they contain nothing on which that insect can feed; and they do not attract us because for us the substances that produce them are pernicious. But doubtless the carrion beetle finds the odor of decayed meat a much more attractive aroma than the odor of orange blossoms. And, to make direct application to the case in hand, unquestionably the flies and other insects that are useful to the calla in pollenizing its flowers would be quite unattracted by the sweet and pervasive odor that is given out by the new race of scented callas which I am about to describe.

HOW THE CALLA IS FERTILIZED

It was on smelling the perfume of my scented calla that the visitor made the remark I have quoted. And she followed it with this question: "If the odor of plants is of use to them in attracting bees, why do not all the callas have a perfume like this new one you have developed?" And here again a moment's reflection would perhaps have supplied the answer. The calla does not need to attract the bee, therefore the production of the chemical substances that give out a sweet perfume would have been a waste of energy for this flower. Perhaps there may have been a time in the past when the calla, like so many other flowers, depended on bees for cross-fertilization, and lured them with its scent; but nowadays the process of cross-fertilization in this plant is effected in a quite different fashion. If you closely examine the calla you will observe that what you would casually speak of as a single blossom is in reality a case or shield-in point of fact a modified leaf-twisted into a sort of cornucopia and adjusted about a central stalk or "spadix" on which many minute and inconspicuous blossoms are clustered. The object of this arrangement is doubtless in part to give protection to the flowers, but largely to supply a conspicuous signal to attract night-roving insects, in particular various species of small gnats and flies. In point of fact the white canopy of the calla affords a very convenient place of refuge for numerous small insects. Tests have shown that the air inside the calla "blossom," particularly toward its base, where the insects congregate, is perceptibly warmer than the outside air. It has been proved by recent experiments that the chemical processes associated with plant growth generate heat. Germinating seeds, for example, give out a measurable quantity of heat. So it is not strange, perhaps, that the partially confined air at the base of the tubular calla flower-case is at all times a little warmer than the surrounding atmosphere. In any event the insects find this a snug corner, the attractiveness of which is further enhanced by the presence of a certain amount of edible pollen. In short, for such insect tribes as like the particular fare which the calla offers, its beautiful white tube constitutes a highly attractive lodging-place and lunchroom. Meantime, while the insects are lodging at the base of the stalk on which the true flowers grow, these flowers shed their pollen and let it settle on the backs of the visitors. And when, in due course, the insects resume their voyaging, they carry the pollen with them and in time transport it to other calla blossoms; for when they enter the new flower they are likely to find the stalk at its center a convenient alighting place, and crawling down this are sure to leave some of the pollen in contact with receptive pistils. That the pistils shall be those of a different plant from the one that supplied the pollen is ensured by Nature's familiar device of having the stamens and pistils of the same flower ripen at different times.

A GIFT OF NATURE

All this sufficiently explains the utility of the large white modified leaf or spathe which we commonly speak of as the calla's flower, and also, the normal habit of this flower in producing only the musty odor which is rather disagreeable to us, but which is obviously attractive to the particular insects which the calla needs as coadjutors. But it does not explain how it chanced that among a large quantity of seedlings of a tribe of calla known as the "little gem," I one day found a single specimen that not only lacked the disagreeable smell of the others, but had a mild yet unmistakable aroma that was distinctly pleasing. Explanations aside, such a specimen did appear among my callas, and it was by raising seedlings from this anomalous specimen and carefully selecting the best specimens for successive generations that I developed the perfumed calla. The first plants that grew in the first generation from seeds of my first scented calla showed no improvement over their parent in point of fragrance. But in the second generation, as so often happens, there was a marked tendency to variation, and from among the numerous seedlings of this generation I was able to select one that had a fully developed and really delightful perfume. By propagating this specimen as usual, by division, scented callas precisely like the mother plant were soon developed in quantity. Other races showing the quality of scent-production in varying measure were produced from the seed, but no one of the seedling varieties ever equalled the selected plant, and the finest fragrant callas in existence to-day are all the descendants, through the process of division, of the original second generation seedling. This new race of callas was named the "Fragrance." Fortunately it chanced to combine with the habit of perfume production the habit of abundant and constant blooming. Indeed, in this regard it probably excels all other varieties of calla.

THE NEW CALLA A "SPORT"

It thus appears that the perfumed calla was developed through selection and in the short period of two generations, from a perfumed individual that appeared "spontaneously" among some thousands of odorless seedlings. Using a term that is peculiarly popular in recent years, we might say that so marked a variation from the normal or usual form of calla constituted a "mutation." In the size and color and general appearance of its flower, as well as of its entire structure, the new calla precisely resembled its fellows. Yet we are surely justified in speaking of so very marked an anomaly as the production of a strong perfume as constituting an important departure from the normal. No one knows precisely what the chemical changes are that produce the perfume of a flower, or through precisely what transmutation of forces one flower is made to produce an odor quite different from the odor of other flowers. But for that matter no one knows just what are the conditions that induce the stimulus that we interpret as an odor of any kind. The sense of smell seems the most mysterious of our senses. But whatever these inherent conditions may be, they constitute changes in the intimate structure of the plant itself that must be admitted to be important in character, inasmuch as they have to do with the well-being of the plant, and may even determine-through their appeal or lack of appeal to insects-the perpetuation or the elimination of a species. In the case of my scented calla it was perfume alone that differentiated a particular individual from thousands of other individuals growing in the same plot. On this basis alone I selected out this particular flower, put it in a plot by itself, gave it every encouragement, and determined that its progeny should live and perpetuate the particular strain it represented; whereas but for this single feature of variation, that individual plant would in all probability have been destroyed along with hundreds of others. The development of the scented calla, then, through artificial selection based on the recognition of the value of fragrance as an addition to the attractiveness of this flower, represents in a small way and in epitome the history of the development of numberless races in nature through the operation of natural selection. In this particular case, natural selection probably would not have resulted in the production of a race of scented callas, because, as already pointed out, fragrance of this character has no value for this particular flower. It might even chance that the fragrance which to our senses is exquisite would prove unattractive or even repellent to the flies that normally frequent the spathe of the calla and aid it in perpetuating its species. In that case natural selection would certainly ensure the early destruction of the race of scented callas. It may well have been through such discriminative selection on the part of insects that the calla lost its scent in the past ages. For of course natural selection can operate quite as effectively in weeding out organisms that have undesirable traits as in perpetuating organisms that show favorable variations. One process is necessarily complementary to the other; they are two sides of the same shield. In another connection we shall have occasion to deal more at length with the processes of natural selection; and we shall see numberless examples before we are through of the way in which artificial selection is instrumental in developing new races of plants.

FOUNDATIONS OF NATURAL SELECTION

But for the moment I will consider a little more at length the question of the origin of the variation which resulted in giving this particular calla a perfume that was not normal to its race. In so doing, we shall gain a clue to the genesis of other types of variation or mutation through which various and sundry new races of cultivated plants have originated, and through which also, we have every reason to believe, numberless species of animals and plants in a state of nature have been evolved. The presentation of this subject puts us in touch with one of the newest and doubtless one of the most important aspects of the problem of evolution. Since Darwin we have fully understood that all evolution of organic forms must have its origin in variations. No two individuals even of the same species are precisely alike, and it is not at all unusual to find individuals of a species showing very considerable differences, even as regards the essentials of size and form and function. Indeed, a certain range of such variations is considered to be absolutely normal. One would never state, for example, that any particular bird has a wing or beak or tail of precisely a given length; instead of this the ornithologist records the average or mean length, or the limits of variation shown by different specimens. And it is universally recognized, since Darwin gave us the clue, that the building up of new species must be brought about through the selection of favorable variations. A bird with an extra long wing, for example, might be able to fly a little faster and secure its insect prey with greater facility than its fellows; and this slight advantage might be instrumental in saving the life of such a bird, and thus enable it to transmit its peculiarity to offspring that would constitute a long-winged, swift-flying race. Take the following incident as a tangible illustration: In the summer of 1904 it chanced that there was a severe drought in New England and there were entire regions in which the insects upon which the common house martin feeds failed to be hatched at the usual time. The result was that there was dearth of food for the martins, and a very large proportion of these birds died of starvation. In some cases forty or fifty birds would be found starved to death in a single bird-house. There are entire regions in New England today where the martin is a rare or unknown bird, although prior to 1904 it was abundant. Now we may reasonably assume that any individual martins that escaped were those that had either greater powers of flight or a stronger inherent tendency to make wide flights in search of food than their fellows. The few individuals thus saved furnish us a concrete example of the survival of the fittest through natural selection. And this illustration is cited at length because it makes tangible the fact, to which I shall have occasion to revert time and again, that the processes of nature through which species have been developed in the past are still in operation everywhere about us. Many people are disposed to think of natural selection as a principle referring to past times and to the development of organisms long since perfected. In point of fact past times are like present times in the operation of their laws. The reactions between organism and environment are now, what they always were. No race is perfected, no organism freed from the struggle for existence; although, of course, under the conditions of civilization the operation of "natural selection" may be modified through man's influence, and the conditions of life for a given organism radically changed by artificial selection.

EVOLUTION THROUGH MUTATION

But let us not forget our theme. With the case of the scented calla to furnish our text, I was about to speak of those variations from the normal on the part of any given organism which lie outside the ordinary range of variation and which therefore constitute so definite and pronounced a departure that they have long been spoken of as "sports." To these the present day evolutionist, following Professor Hugo de Vries, gives the name of "mutations." It has already been said that the appearance of a scented calla constitutes such a change. But of course the anomalies that are usually listed as mutations are as a rule of an even more noticeable character. A classical illustration was given by Darwin himself in the case of the Ancon ram, which was born with legs only half the normal length, and from the progeny of which was developed a short-legged race of sheep. But the word mutation had not come into vogue in Darwin's time, and the idea of evolution through such marked departures from the normal was subordinated, in Darwin's interpretation of the origin of species, or at least in that of his immediate followers, to the idea of advance through the preservation of slight variations. So when, just at the close of the nineteenth century, Professor Hugo de Vries came forward with his "mutation theory," it had all the force of a new doctrine, and was even thought by some enthusiasts-though not by its originator-to be in conflict with the chief Darwinian doctrines. The observations that led Professor de Vries to the development of this theory were made on a familiar American plant that had found its way to Europe and was growing in profusion by the roadside near Amsterdam. The plant is known as the evening primrose. Professor de Vries noted a hitherto undescribed variety of this plant in a field near Amsterdam. He took specimens of the plant to his famous experimental gardens and carefully watched the development of successive generations of seedlings. To his astonishment he produced in the course of a few generations more than a dozen divergent types of evening primrose, all descended from the original plant, each of which bred true to the new form suddenly assumed. Professor de Vries spoke of these sudden and wide variations from type on the part of his evening primrose as constituting "mutations." He conceived the idea that similar mutations or sudden wide variations had probably constituted the material on which natural selection had worked in the past. Such mutations being observed to occur in the case of the evening primrose, it is not unnatural to argue that similar mutations must occur in the case of other organisms; and it requires no argument to show that such wide variations offer better material for the operation of the laws of natural selection than could be offered by the minute and inconspicuous variations that had hitherto been supposed to constitute the basis of evolutionary changes. There were many reasons why the mutation theory appealed to contemporary biologists, thus accounting for its very cordial reception. For example, there are numberless instances in nature where the development of a useful organ is exceedingly hard to explain on the basis of natural selection, because the organ in its incipient stages could have no utility. Similarly a modification in the location of an organ-say the shift in the flatfish's eye until both eyes are on one side-is difficult to explain as a process taking place by infinitesimal stages, on the basis of natural selection. A slight shift in position of the eye of the flatfish would have no utility whatever. It is only when the shift has become sufficient to bring the eye on the upper side of the fish that the creature would have any advantage over other flat fish whose eye is on the under side. If we imagine a mutation in which a fish appears with an eye distorted in location sufficiently to be usable while its owner lies flat on its side in the mud, we can readily understand how such a mutation might be favorable to the individual and thus might furnish material for the development through natural selection of a race of flatfish having both eyes on one side. We have every reason to believe that the races of flatfish now existing have recently-in a geological sense-developed their observed condition of having the eyes thus located; indeed proof of this that amounts to demonstration is furnished by the fact that the young flatfish even to this day is born with its eyes located like those of other fishes, the migration of the eye, so to speak, taking place as the individual develops the racial habit of lying on its side. But as I said, it is unquestionably difficult to conceive how the useful distortion came about unless it began suddenly as a "sport" or mutation. This is one instance among many. And so Professor de Vries' observation, which proved that mutations do sometimes seemingly occur "spontaneously" in nature was seized on as affording a solution of one of the puzzles of evolution, and the mutation theory was pretty generally regarded as a valuable supplement to the Darwinian theory of evolution. It should be clearly understood, however, that neither Professor de Vries himself nor anyone else speaking with authority, has thought of the mutation theory as in any sense contradicting the Darwinian theory of natural selection. On the contrary, it is to be regarded as supplementing and supporting that theory. If creatures are subject to large variations in a single generation, such variations afford peculiarly good material for the operation of natural selection. Moreover, evolution by mutation would presumably be much more rapid than evolution that depended for its leverage upon minute variations.

WHAT CAUSES MUTATION?

Incidentally the idea of relatively rapid evolution, thus given plausibility, answered the objection of certain geologists who had questioned whether the earth had been habitable long enough to permit the evolution of the existing forms of life through the cumulative effect of slight variations. The mutation theory is thus in many ways acceptable. But to give the theory finality it is obviously necessary to proceed one step farther and ask this question: What causes mutation? And it is equally obvious that the question must be hard to answer. Professor de Vries, to be sure, made the assumption that the changes in his evening primrose were probably due to altered conditions of nutrition incident to the growth of the plant in a new soil. He further developed a thesis that probably all species are subject to mutation periods, which recur at more or less regular periods of their life history, and which thus ensure a degree of variation that will make racial evolution possible. The authority of de Vries sufficed to give wide vogue to his theory; yet it must be admitted that the explanation offered lacks tangibility and at best amounts to little more than begging the question. To say that altered nutrition produces variation in a plant is in effect to state the fundamental truth that all plants are more or less responsive to their environment. But there is nothing specific in the case of the primrose that explains in any precise way the relation of the change to the particular differences, let us say, between the soil of the original home of the primrose and the soil of Holland. Moreover in numberless other instances plants have been transplanted from one region to another without showing any such pronounced tendency to develop new races. It was recognition of the difficulties thus presented, undoubtedly, that led Professor de Vries to devise the rather visionary hypothesis of periods of mutation with which his theory was cumbered. But it is a well recognized law of logic that one should never seek remote and obscure explanations of observed phenomena unless all explanations of a more tangible character have been proved untenable. And it has seemed to me from the outset that in the case of the evening primrose a very much more plausible explanation is at hand than the one devised by the originator of the mutation theory. In a word, the varied tribes of evening primrose which Professor de Vries developed in his gardens at Amsterdam were overwhelmingly suggestive of various and sundry new forms of hybrid plants that I myself have developed year after year in my experimental gardens at Santa Rosa. The primus blackberry, the phenomenal berry, and the sunberry, are, if you wish so to consider them, instances of pronounced mutation, inasmuch as they are fixed forms of plants that vary widely from the parent forms. In a single row I can show walnut trees six inches high that are of the same age with others six feet in height, both grown from seeds of the same tree. The Shasta daisy and the white blackberry are mutants in the same sense. And as the reader will discover in due course, the list of such anomalies might be extended to tiresome lengths. In a word, it is perhaps not too much to say that my entire work has consisted in dealing with mutations in plant life. My chief work might be held, and I believe justly held, to be an exposition of the truth of the theory of mutation insofar as it applies to the explanation of the origin of species. Over and over again, hundreds of times in the aggregate, I have selected mutants among my plants, and have developed from them new fixed races. But in the vast majority of cases I knew precisely how and why these mutants originated. They were hybrids; and they were mutants because they were hybrids. And so from the outset I have believed that Professor de Vries' celebrated evening primroses had the same origin. It is true that the parent form was not known to be hybridized, and that there was no known form of evening primrose at hand through which hybridization could have taken place. But the precise origin of the original plants found near Amsterdam is entirely unknown; and the curious conformity of their offspring, under Professor de Vries' observation, to the habitual variation of hybrid races in the third and subsequent generations is so pronounced that it cannot well escape the observation of anyone who has had large experience with such races. This fact was at first overlooked by most biologists, largely because they lacked such experience. But now there is a growing tendency to take this view of the case. Attempts have even been made in very recent years to produce a similar series of mutational forms of evening primrose by direct hybridization of existing forms. And while the results have not been absolutely definitive, they are unquestionably suggestive; and there is without doubt a growing appreciation of the fact that plants may be made to take on the notable changes which we described as mutations by the hybridizing of allied races; and that this explanation of the origin of mutation has full validity, whether or not it be accepted as the sole explanation. We shall see the truth of this contention illustrated in scores of cases in the course of these studies.

THE FINAL INTERPRETATION

Meantime for the purposes of present illustration it is necessary to revert to the case of our scented calla. After what has just been said it will be obvious that I would explain this mutation as a reversion due to cross-fertilization. In other words, some remote ancestors of the calla may have been scented, and a chance mingling of ancestral germ plasms in the course of the production of thousands of seedlings of the calla, may have led to such a union of submerged hereditary factors as enabled this latent propensity to make itself manifest. According to this view, the case is comparable to that illustrated by an experiment in which Professors Bateson and Punnett hybridized two white-flowered peas of different strains and produced offspring bearing flowers colored blue and pink and purple. The white parent forms were so nearly identical as to be entirely indistinguishable except that a magnifying-glass showed the pollen grains of one form to be round and the pollen grains of the other form to be oval. This insignificant difference, however, is full proof that the plants belong to different strains. The union of the divergent strains seemingly brought together pairs of hereditary color-factors-if we hold to the Mendelian explanation-that had been separated and hence had gone unmated for an indefinite number of generations. In the same way, we may suppose, I had brought together, through a happy chance, in the course of these breeding experiments with the calla, two strains that bore complementary odor-factors, the union of which released and made tangible the latent quality of perfume-bearing, which, in all probability, no calla of either strain had outwardly manifested for hundreds or perhaps for thousands of years.

-No race is perfected-no living thing is freed from the struggle for existence.


This text is from: Luther Burbank: his methods and discoveries and their practical application. Volume 2 Chapter 3