EXPERIMENTING WITH THE RESPONSIVE DAHLIA
AN INFINITY OF VARIATION WHICH HAS ONLY BEEN TAPPED
If you have seen a Navajo blanket you are aware that the Indians of the Southwest are lovers of vivid colors-in particular of glaring reds. It would appear that the insects of the same region have acquired similar tastes; for they have aided in the development of a good many flowers that advertise their wares with the most brilliant hues. The cactus furnishes a familiar instance. Another example is supplied by the even more familiar dahlia, which in its native Mexican form had florets of bright red with a yellow center supplying the basis for the modified color schemes of the dahlias now under cultivation everywhere. The original red dahlia so attracted the eyes of the Spanish conquerors in Mexico that they sent the plant to Europe, and its reception there suggests that barbarian and insect have no monopoly of the color sense to which red appeals. For the Mexican composite flower was taken into the European gardens, and made to feel quite at home in its new habitat. The new exotic came, as a matter of course, under the eye of the great classifier Linnaeus. And he thought so highly of it that he was moved to name it in honor of his friend and pupil, Dr. Andreas Dahl. The great Swedish classifier spoke with final authority in that day, and "Dahlia" the plant became in all languages and wherever grown-except, of course, in its native habitat; and what it might be called there, if anything, did not greatly concern the civilized world. The scientific generic name Dahlia seemed to serve as well as another for the popular name also. So the name of the friend of Linnaeus has been perpetuated as a household word, familiar almost as the words rose or violet; but of course the great majority of people who pronounce it give no thought to its origin, and are quite unaware that they are paying tribute to a man, and commemorating a friendship, when they speak of this familiar garden flower. So entirely has the origin of the word been overlooked, indeed, that the name dahlia, which should obviously be pronounced with the broad a, is universally pronounced with the long a in England and with the short a in America, each branch of the Anglo-Saxon race seemingly trying to get as far away as possible, in different directions, from the natural pronounciation suggested by the derivation of the name, and its spelling-if indeed the spelling of a word in our language can be said to have any particular association with pronunciation.
EARLY DEVELOPMENT OF THE DAHLIA
All that, however, is of no great importance. A dahlia by any other name or pronunciation would be equally attractive. What is important is that this flower, brought from its sub-tropical home, proved wonderfully adaptable to its new surroundings, and showed a responsiveness to good treatment that presently transformed its general appearance, and gave it secure place in the group of three or four most popular flowers. There are several species of dahlia, all natives of Mexico or the regions a little farther south. But the species that is chiefly responsible for the development of the new races, or at any rate those that first gained recognition in Europe, is one that because of its tendency to vary even in a state of nature was named Dahlia variabilis. This flower, which was introduced into England in the year 1789 by the Marchioness of Bute, has the general form of a very large daisy and it resembles numerous familiar wild sunflowerlike composites, except that its floral envelope is dull scarlet with a yellow center, instead of being yellow or white. We have seen a good many illustrations of the effect of transplanting a plant from one region to another. The dahlia furnishes yet another example. Brought from sub-tropical Mexico to the relatively cold climate of England, it soon showed the effects of altered climatic conditions. The tendency to vary was accentuated, and when in due course the plant was hybridized with other species brought from the same region, the hybrids took on such modifications as presently to produce races of dahlias so utterly divergent from the parent forms as to be almost unrecognizable. Not even a botanist would associate the wild composite with its eight flat florets of ordinary shape and appearance, with the relatively gigantic rose-shaped flower made up of an infinite number of tubular florets packed together into a solid head. The colors of the flower have been correspondingly modified, although the original red and yellow of variabilis, together with the white and crimson of certain other species, form the basis of the coloration of all the cultivated varieties. And as to size of stalk, whereas the original species rises to a height of seven or eight feet, there are dwarfed cultivated races that are only eighteen inches high. In habit, there is a corresponding range of variation, some cultivated species requiring a large amount of moisture, whereas others thrive in a dry soil. Even the seed is of altered shape, and the time of blooming, which in the early part of the nineteenth century was said to be from September to November, has been so extended that some of the modified dwarfed forms are now in full flower in June. In quite recent years a type of dahlias has been introduced in which the petals are less tubular but have a typical and characteristic tapering form. This is known as the cactus dahlia, partly because of the shape of its flower, and partly because of its brilliant scarlet color. The original flower of this type was found in Mexico about 1879, and was named Dahlia Juarezii, after President Juarez, "the Washington of Mexico." The precise origin of the plant is unknown, but it is believed to be a variety of the original Dahlia variabilis. In any event the new type has been crossed with other races, and it now appears, like the others, in practically all colors, with the single exception of clear blue, this color alone being seemingly unwelcome to flowers of the tribe, just as it is to the poppies and the gladioli, both of which tribes show a range of coloration strikingly similar to that revealed by the dahlias.
NEW FORMS AND COMBINATIONS
My own experiments with the dahlias have largely had to do with flowers of the cactus type. I have raised these by the hundred thousand, and have produced some really fine forms that have been introduced by Vaughan, Burpee, and others. The modifications introduced have been numerous, and some of them at least have constituted rather notable improvements, notwithstanding the elaborate development of this plant by many earlier workers. In the course of my experiments I have endeavored to give a new impetus to variation and renewed vitality by hybridizing the cultivated forms with the species imported directly from Mexico. To be sure the dahlias originally in hand are so hybridized-to say nothing of the original tendency to variation-that there is plenty of material for selection in any lot of seedlings. Still I have thought that I might gain some new combinations by the use of wild strains, and in this my expectations have been realized. One of the faults of the dahlia, even in the best varieties, is that there is a tendency to expose the center of the flower, owing to the fact that not all of the stamens have been transformed into florets even in the most developed varieties. The result is that in a dry summer, or toward the end of the season, even good varieties may fail to show the fully rounded head that is prized by the connoisseur. I succeeded through selection in overcoming this defect, causing the heads to fill out altogether, so that they were double to the very center, even at the end of our dry California seasons. A number of varieties were thus perfected, and these were, I believe, the only entirely double dahlias that were ever produced. As the ideal sought was approximated, the flowers produced less and less seed, and the perfectly double ones produced none at all. So the races thus developed must be propagated altogether from the bulbs. This, indeed, is not an insuperable objection, inasmuch as this is a common way of propagating the dahlia. But of course there is always an added merit in a garden flower that can be produced from the seed. It is well-known, however, that even the best-fixed races of dahlias are not expected to breed true from the seed. Like other specialized flowers they carry too many hereditary strains in new combinations to be expected to breed true to any single type. So while the dahlia is constantly raised from the seed, it is always to be expected that the seedlings will show a wide range of variation. It is only in specimens grown from the bulb that any certain prediction can be made as to the precise characteristics of the prospective flowers. One of my beautiful yellow double dahlias has shown a curious responsiveness to the diverse conditions of soil in the gardens at Santa Rosa and at the experiment farm at Sebastopol only seven miles distant. At Santa Rosa the plant grows to a height of about three feet, and resembles the common types of dahlia as to its general maner of growth, though an unusually profuse bloomer. But at Sebastopol the plant is a dwarf, not exceeding two feet in height; and as it retains its habit of profuse blooming the dwarfed form looks like a solid bouquet of cut dahlias. Similar modifications in the size of plants, but less striking in degree, are of course common enough under differing conditions of soil, and in particular with varying moisture. But of course such variations do not affect the heredity of the plant appreciably. They have no relation with the production of dwarf and gigantic varieties in the same fraternity through hybridizing, of which we have seen examples among various races of plants. With all its attractive qualities, the dahlia is not quite a perfect flower because it lacks fragrance. This defect also I have sought to remedy, and as regards the mere matter of production of a fragrant dahlia, have been entirely successful. Unfortunately the new fragrant races have not hitherto combined odoriferousness with the qualities of size and form and color that enable them to compete with the best standard varieties. Still, enough has been done to show that with further effort the dahlia may be given a perfume that will greatly enhance its attractiveness.
RACES OF FRAGRANT DAHLIAS
In endeavoring to develop a race of fragrant dahlias I followed the same rules of selection that have been repeatedly outlined. The first thing was to find an individual that revealed even the faintest pleasing aroma. In general, dahlias have either no odor, or a slightly disagreeable one. The tribe of composite flowers to which the dahlia belongs depends as a rule upon the conspicuous floral envelope to attract the pollenizing insects, and has not developed fragrance. But it is probably true with regard to fragrance as with regard to combinations of colors that there are unrevealed hereditary factors in the germ plasm of almost every flower. The production of odoriferous oils and essences is so characteristic a phenomenon with plants in general, that we can hardly doubt that every tribe has in its ancestral strains very complex elements for the production of odoriferous compounds. Odors appear to play a very important part in plant life, not merely in the attraction of insects to facilitate cross-fertilization, but also in giving plants protection. Otherwise it would be hard to account for the almost universal prevalence of odors of one kind or another in connection with the various tissues of the plant. Moreover there is a far closer relationship than is commonly supposed between agreeable and disagreeable odors. Ottar of roses, properly diluted, has a delicious fragrance; but the same essence in its concentrated form is positively disagreeable. Also the combination of disagreeable odors sometimes produces a delightful fragrance in the hands of the perfumer. This may give the clue to the rather puzzling fact that even among fragrant flowers there may be found occasional blossoms that have a more or less disagreeable odor. By eliminating these, the quality of the odor of a bunch of flowers is greatly bettered. Yet many persons gather flowers indiscriminately without realizing why some bouquets have more agreeable odor than others. Making application of a knowledge of this affinity between disagreeable and agreeable odors, I searched diligently among dahlias of various races for a long time, hoping to find one in which the disagreeable odor was supplanted by an agreeable one. And at last the search was rewarded. I found a dahlia that had a faint but very pleasing fragrance comparable to that of magnolia blossoms. Of course the seeds of this plant were saved, and in the following season the most careful search was made among the plants that grew from them for fragrant flowers. And, as might be expected, a certain number of these were found. By repeated selection, always searching for the most fragrant flowers, and carefully saving their seed, a race of dahlias was developed many of which had a very agreeable perfume. Rather I should say that there were several races, for the quality of fragrance was associated sometimes with one set of characteristics of size and form and color, and sometimes with another. Selection being made in this case for fragrance alone, as was necessary in order to intensify this evasive quality, it was necessary mostly to ignore the other qualities, and as usual in such cases, it resulted that the new fragrant races of dahlias, while having perfume that recommended them, were somewhat lacking in the other qualities. The great popularity of the flower has led to such perfectionment of its various characteristics in recent years that the standard of competition is very high, and it would be useless to introduce a new variety that did not measure up in all regards to the existing varieties. So up to the present time the fragrant dahlias have not been introduced, except three or four, which were purchased by Vaughan, of Chicago. Further experiments in selective breeding, aided probably by hybridization, will be necessary before the quality of fragrance is combined with satisfactory qualities of size and form and color. But, as I said, there is every probability that these combinations will be effected in due course, and that races of dahlias which combine all the qualities for which the flower is now prized, with the added quality of pleasing aroma, will be available.
WIDER HYBRIDIZATION ATTEMPTED
We have seen that the experiments through which the original wild dahlias were transformed into gorgeous double flowers of a characteristic type utilized the principle of hybridization at all stages. In my own experiments, I have attempted to extend the principle, not merely to all the flowers of the genus, but also to those of allied genera. According to the estimates of the botanist, the dahlias have fairly close relationship with plants of the genus Bidens. Indeed, a familiar species of the genus, known as Bidens atrosanguinea, a tuberous variety with dark purple flower, is often spoken of as the black dahlia. Its tubers and foliage strongly suggest the common dahlia in miniature. For four or five years I worked extensively with this so-called black dahlia, not only by way of improving the flower itself, but also in the attempt to hybridize it with the dahlia proper. I succeeded by selective breeding in enlarging the flower to about twice its original size, in making the petals much rounder and fuller, in adding extra petals, and in changing the color of the petals from the usual dark purplish crimson to a light crimson approaching scarlet and in a few cases to a pale pink approaching white. The bush itself was also made more compact. All these changes were produced by selection and re-selection, working constantly toward the new colors desired, and toward increase of the size of flower, and modification of form. The species worked with was a Mexican form. There is an aquatic form with large, brilliant, yellow flowers, closely related to the species known in the east as "pitchforks." For two or three generations, the flowers seemed fixed. I could see no change whatever; no tendency to break into new forms. I attempted to hybridize the two species of bidens, but did not succeed, so it was necessary to depend upon selection alone. The plants were grown in large quantities. After several years, slight variations appeared; and then, as in so many cases, the tendency to variation became somewhat accentuated. I became convinced that the black dahlia and other species of bidens are well worth cultivating, and that some other valuable tuberous flowering plants could be developed from them that would be welcomed by flower lovers in general. But other engagements made it impossible for me to carry the experiments beyond the early stages. And as to the matter of crossing the bidens with the dahlia, in which I had been especially interested, the result was altogether negative. Repeated efforts failed to fertilize either species with the pollen of the other. Notwithstanding the outward similarity of the plants, it would appear that their racial strains have diverged beyond the point of ready commingling. Still it is possible that a more extensive series of experiments might have met with better results, and further efforts along the same line are at least worth making. Could a cross be effected, we might reasonably expect some very interesting modifications in the hybrid product; notably, perhaps, an accentuated capacity for growth that would possibly give us dahlias rivalling the largest crysanthemum in size, as they already rival it in form and flexibility of petal-like florets.
CROSS-POLLENIZING THE DAHLIA
Among themselves, the dahlias cross very readily, it being, indeed, difficult to keep them from crossing when they are grown near together. Yet, as in the case of all composite flowers, the hand-pollenizing of the dahlias presents certain difficulties. The method of hand-pollenizing, with special reference to the washing off of the pollen from the pistillate flower before applying the foreign pollen, has been detailed in its application to composite flowers in general in the chapter on pollenization. It may be added that it is sometimes possible to blow the pollen away, if water for washing it off is not available. The use of a strong magnifier to inspect the receptacle and make sure that all pollen has been removed will give added certainty to your experiment. After the pollen has been thoroughly removed by washing, apply the head of the flower that is to be used as the pollen parent, rubbing it gently against the pistillate head while it is still wet. But to complete the experiment, it is desirable to mark the flower, and to repeat the manoeuvre on several successive days. This is necessary because not all the flowers in the head mature at the same time. The outer come to perfection first, and the process of maturing advances towards the center of the flower. So the first pollenizing must be done just at the right time, and successive pollenizings day by day until the entire flower has come to maturity, if all the pistils are fertilized. It is obvious, then, that the crossing of dahlias, while it presents no real difficulties, and is tolerably sure in its results, is a somewhat tedious and laborious process where the field of operations is wide. But, as already pointed out, it is not necessary for the experimenter who is seeking merely to modify existing varieties to resort to hand-pollenizing. The varieties that will appear among any ordinary lot of seedlings will afford him ample opportunity for selection. On the other hand, the experimenter who wishes to develop new types of striking individuality will of course crossbreed the old ones, using species or varieties as widely separated as possible. My own experiments, as already pointed out, have involved the use of wild species from Mexico, and the influence of these wild crosses has undoubtedly been felt in the rather striking results attained in working with a race of flowers that, despite its comparatively recent advent in the horticultural garden, is already highly specialized. That further improvements of striking character will be attained can scarcely be doubted by any one who takes into account the fact that the dahlia is a parvenu among the admitted aristocrats of the flower garden. It is impossible that the hereditary resources of any plant should have been exhausted within the comparatively brief period of time that has elapsed since this extra-ordinarily responsive and adaptable flower was first brought from the wilds.
-Brought from sub-tropical Mexico to the relatively cold climate of England, the dahlia soon showed the effects of the altered climatic condition. The tendency to vary was accentuated and presently there was a new race of dahlias, so utterly divergent from the parent form as to be almost unrecognizable.
This text is from: Luther Burbank: his methods and discoveries and their practical application. Volume 9 Chapter 7