Popular Science Monthly/Volume 37/July 1890/Observations Upon Doubling of Flowers
|OBSERVATIONS UPON DOUBLING OF FLOWERS.|
By BYRON D. HALSTED, Sc. D.,
PROFESSOR OF BOTANY IN RUTGERS COLLEGE.
FUNCTIONALLY a flower is for the production of offspring, and in structure it may be considered as a transformed stem with its metamorphosed leaves. In a typical flower—that is, one having all the parts present and in an easily recognized form—there are four sets of organs. The calyx forms the outer whorl of leafy organs, and next within is the corolla, usually bright-colored and showy. Inside of these two cycles of floral envelopes are the essential organs: first the stamens, which bear the male element as pollen-grains; and the pistil or pistils, occupying the center of the flower, the lower portion of which bears the seeds. In the production of seed we find the aim and end of all floral structures. The pistil remains after the flowering period is past and becomes the fruit, which may or may not be accompanied by other portions of the flower. The stamens serve their purpose as they shed their pollen, and usually quickly wither away, and the latter is generally true of the petals.
As above stated, all the several parts of a flower are now considered as modified leaves. The calyx is often green, leafy, and indistinguishable from ordinary foliage. All gradations may be found between calyx and corolla. The bright color is no argument against petals being leaves, for leaves of the common sort often assume the most brilliant colorations. Other wild plants illustrate the transition from petals to stamens—as, for example, the flowers of the water-lilies; and pistils are frequently broad, green, and leaf-like, especially after the seeds are ripe, and the two infolded halves open out and take on the form and function of foliage.
We do not need to extend our examination upon this point in search of proof for the morphological significance of the floral structures, but plants under cultivation sometimes throw off some of their disguises and give additional evidence of no mean importance. Just as the man under the influence of intoxicating liquor may reveal qualities of his nature that might have otherwise remained securely hidden, so the distortions seen in cultivated blossoms furnish a key which unlocks the secrets of normal structures.
The common garden lily (Lilium tigrinum) often has, instead of the six normal and similar parts to the perianth—the six stamens and a single tricarpellary ovary—first a multiplication of the petals and sepals, usually about twelve, followed (passing inward in the flower) by a number of petal-stamens or stamen-petals. The outer of these last have nearly lost their stamen characteristics, being broad, highly colored, spotted, and with only vestiges of anthers; while the inner ones are exceedingly irregular, and suggest that a severe struggle might have taken place between a hidden force that unimpeded would have made a petal, and another aiming to produce a stamen. In all such flowers there were no perfect stamens; however, some of the petal-stamens bore anther-lobes along their contorted edges, in which seemingly perfect pollen-grains were produced in quantity. The pistil in all these doubled flowers is an amalgamation of five or more carpels, but the tricarpellary type is not obliterated. In one instance a petaloid structure was observed, with ovules arranged along the mid-rib upon the upper side; while above the two widely separated edges were lines of chocolate color, characteristic of the anther-lobes. In another instance the perianth was reduced to a spathe-like structure, upon the inner veins of which were long double lines of ovules. Within this structure was a much misshapen pistil, compounded of at least six carpels, judging from the styles and sections made of the ovary.
In the ordinary case of doubling it is considered that a stamen is replaced by a petal, and the additional petals of the doubled flower are limited in number by that of the stamens. It is at once seen that this view does not hold with the lily; for, in place of the six normal stamens, there are at least twelve petals, only a few of the inner ones of which retain any marks of stamens. There is, therefore, an augmentation of the petals and transformation of the stamens. In the cultivated tulip the perianth is often increased to three or more times the normal number (six) of parts, and in one flower the modified stamens were found increased to nine. The pistil frequently shows signs of transforming into petals and becomes winged and bright-colored along one or more sutures, while the ovules are sometimes exposed to view between the separated valves.
The common garden pæonia is another large-flowered species, but quite unlike the lily or tulip, because the stamens are very numerous in each blossom. In this plant there is no need of speculating as to the origin of the petals. They arise in large numbers from the failure of the stamens to develop as such. The filaments broaden out upon opposite sides and a petal results. In nearly half such petals the remnant of an anther can be seen at the tip of the petal, which is somewhat notched, often deeply, and in the center is the abortive anther. Near the center of the flower the transition is more evident, for here the filament-wings are not much broadened, and the anthers more prominent. Still nearer the great center pistil the ordinary stamens may be found, with their anthers bearing pollen. Occasionally the poppy illustrates a modification of the stamen in the opposite direction to that given above—namely, the inner ones become small simple pistils, which are either closely applied to the surface of the large central compound pistil, or adherent to it and blending with the stigmas.
The rose family and the crowfoots both furnish a long list of plants which uniformly produce double flowers under cultivation, and for this reason these two orders are rich in ornamental garden species. Both the roses and the buttercups abound in stamens; and, from what we have seen in the poppy, it should be expected that doubling would be easy in such plants. The examples of doubled flowers in these two families are so familiar that no further comment need be made. Among the hardy cultivated roses, for example, it is rarely that a blossom can be found not exhibiting all gradations between perfect stamens and unmistakable petals. It may, however, be stated that in a member of the rose family grown for its fruit—namely, the apple—petal-stamens were frequently met with. In the Tallman sweeting variety, upon one tree, the doubling was found as frequently as one flower in ten. Usually one stamen was transformed, but rarely so much so as to be distinctly petaline.
The abnormities which we have been considering, both generically and specifically, are rarely met with in wild plants in a state of nature. They are, therefore, transformations in flowers concomitant with culture. It is a well-established fact that culture induces changes in those parts for which the plant is cultivated and it might be added that they are cultivated because of this response. Varieties of any cereal differ mostly in the grain; beets, carrots, and turnips in the roots; apples, plums, and peaches in the fruit; and so on. In accordance with the general rule, plants grown for their flowers should vary most in the blossoms. A plant when under cultivation has been removed from the conditions which obtain in the wild state and is relieved from that fierce struggle for life which is everywhere in progress among feral plants. In other words, the cultivated plant is living an unnatural existence; stimulated by man's careful attention and guided by his will, it yields to the demands of its guardian. Variations quickly arise under such fostering conditions, and such changes as are advantageous to man are, if possible, perpetuated by him. This perpetuity in many instances can only be accomplished by non-sexual methods, as by cuttings, graftings, etc., and it therefore follows that the seeding process is either ignored or prevailed against. With their energies all turned in some other channel, plants may in time cease to develop seed.
A flower of the showy sort we may consider as the product of two great forces or groups of forces—namely, that which is within the plant, and for the lack of a better term may be called the constructive ability of the plant; and those forces which act from without, and are included in the general term environment The chief factor in this last or external force is the modifying influence of insects, due primarily to irritation. For example, the lily-flower in its wild state has reached its present condition because the mother plants and their insect attendants have worked together to produce a structure that is admirably adapted to the needs of each. It is, it seems to me, not asking too much of any one who is a disciple of evolution, even in its mildest form, to conceive that the simplest wind-fertilized flowers were the first of all floral structures to appear in the far-away geologic times. In those early ages, provided that we base our reasoning upon what is seen to-day, it is easy to understand that out of the foliar structures there were evolved the primitive ovary and the ante-Adamite stamen. That ancient ovary might stand in striking contrast with the simple pistil of a pine-cone or leaf serration of a cycas, and the corresponding stamen was perhaps only a slight modification of a common leaf. But out of these primitive essential organs came, by slow but by an ever-advancing adaptation to the surrounding world, the wonderful combinations of color, odor, and form which we see in the more complex floral structures of the present day.
All the conspicuous parts of the flower outside of the essential organs are for the purpose of securing a transfer of pollen from the stamen of one flower to the pistil of another. This process of cross-fertilization, as has been abundantly shown, is an advantage to the offspring, which are stronger and therefore better able to cope with surrounding rivals. Therefore, any change in floral structure, however slight, born of accident as some would say, or the result of an inward impulse to improve, is one step toward that ideal condition of perfect adaptation between a plant and its surroundings. So far as the sexual elements are concerned, this ideal adjustment seems to be that of wide separation, and according to this view we find an explanation for the actual separation upon different plants of stamens and pistils in diœcious species, in different flowers in monœcious plants, and their practical separation in all cases when the stamens and pistils in perfect flowers mature at different times (dicogamy). Again, there is a long list of plants in which wide fertilization is secured by one flower having long stamens and short pistils, and another of the same species with short stamens and long pistils (dimorphism). Aside from all these well-defined plans for crossing, there are hundreds of others none the less obscure and often vastly more ingenious—plans so well worked out that the plant will fail to produce seeds unless a particular kind of insect visits it. All such species are constantly striving to arrive at a perfect adaptation between the flower and the peculiarities of its insect attendant. In short, the plan for wide fertilization is so thoroughly apparent along the many lines, that Darwin expressed the condition in the following concise and striking terms: "Nature abhors continual close fertilization."
The structure and form of the essential organs, like those of the floral envelopes, have come to their present condition through the prolonged interaction of plant and insect. Now, at the outset plants cultivated for their flowers were those already showy—that is, those in which the floral envelopes were conspicuous, fantastic, or sweet-scented. Let us bear in mind that these showy wild flowers became so in competition with hundreds of other species, and underwent all the expense of floral display for purely selfish ends. Each species worked out the problem of reproduction in its own way; and it is safe to assert that it became as much a part of the life of a wild rose to develop bright petals as to form compound leaves with large stipules.
In the historic development of such flowers it may be assumed that the essential organs came first, and the surrounding parts appeared and were preserved as they were found of service to the plant. As time went on, additional stamens and pistils may have been added, until the most economical number of parts was reached—if it has been reached. The number varies in many of the wild species to-day, and especially in those prominent in the flower-garden.
It is only fair to hold the successful floriculturist responsible for much of the seemingly stable increase of display in cultivated plants over their wild forms. This is the same credit that is freely given to the horticulturist who increases the size, for example, of the strawberry, by crossing, selection, etc., possibly at the expense of stamens, as seen in many of the pistillate sorts. By granting this there is no intention of overlooking the long-established tendency in the wild plant to develop in the direction taken when placed under the favoring conditions of culture.
Garden plants illustrate with accelerated force the working of the universal law of compensation. Fruits not only enlarge, hut "become seedless. Therefore, the increase in the size and other modifications in the flowers of such plants as are grown for their blossoms is only in accordance with a general law. An augmentation of floral parts is only a step beyond the increase in size of parts already present, and may be largely a matter of convenience in the arrangement of the parts in the bud. When we remember that any augmentation in the petals, etc., would be seized upon by the gardener, and if possible reproduced, the wonder is that the increase is not greater than it is. It is not claimed that such an augmentation is a direct advantage to the plant, any more than is the exaggerated size of a cabbage-head or the thick, rich pulp of a grape, especially when the cabbage splits open and falls apart of its own weight, or the grape-pulp monopolizes the whole substance, and no seeds result. When the guiding hand of man is withdrawn, cultivated plants soon or late find their way back to a stable condition called the natural form, and are again able to cope with their neighbors, depending entirely upon the conditions attendant upon the wild state.
The point that now calls for our attention is the development of one floral organ out of another widely differing from it in appearance. Augmentation, we have seen, is to be expected, but metamorphosis usually brings surprise. The unnaturalness of this arises in part from the constancy of organs in wild plants, and the general impression that a manifest difference in structure and use must indicate dissimilar origin of the parts. All of the various organs of a flower are now, as before stated, generally considered as lateral outgrowths from the stem, and in a state of nature their number, size, shape, color, etc., depend upon the service of each in the economy of the plant. In origin and early growth, therefore, there is no microscopic difference between the sepals, petals, stamens, and pistils. As shown at the beginning of this paper, by taking the whole range of wild plants, it is not difficult to find all gradations, from the outermost sepal to the central pistil. If these various parts have a common origin—namely, in minute cellular outgrowths afterward connected with the primary axis by a vascular cord—the wonder is that each type is adhered to so closely in the wild forms, and the surprise should be that under the modifying conditions of culture more striking combinations are not found. The petals (that is, the inner whorl of floral envelopes) and the stamens (the outer circle of essential organs) form the boundaries between the two primary divisions of the complete flower. It is here that the line of separation is most frequently broken, and especially in those flowers having an indefinite number of stamens and petals. In such plants in the wild state there is usually no established uniform number for eit her of these parts, and it may be that they vary as circumstances determine. In such cases it seems more natural to suppose that one gives place to the other, than that there is an independent development of a new part. However, when we come to the cultivated plants, this seeming chasm between petals and stamens is bridged, and the difficulty now turns upon deciding whether a certain organ is more or less stamen than petal.
As seen from both a physiological and morphological point of view, the pistil is considered the most highly differentiated part of the flower, the stamen next, petal next, and sepal least. Under the conditions obtaining with the cultivated rose, stamens are less important than petals, and probably less easily produced. Instead of the slender filament surmounted by the two lobes of the anther, bearing thousands of expensive pollen-grains, there is a broad, loose-celled, showy petal. When a stamen is replaced by a petal, it is naturally termed retrograde metamorphosis. In the rose, as in many other cultivated plants, all gradations may be found, from a normal stamen, with a slight color-line along one side of the slender filament, to the perfect petal, which may have a small notch at the tip, marking where the anther might have been. So long as the demand for self-propagation is met in other ways, the tendency to produce seed may be overcome, and the plant spends its energies in the formation of showy blossoms, possibly losing, for the time at least, the power to ripen seed. If the selective power of the rosarian is now withdrawn, while at the same time the stimulation of high culture remains, the inference is not unwarranted that the retrogression would continue so far that no flowers develop. It may be that the so-called green roses sometimes met with furnish solid ground for such a view. At any rate, they are forcible examples of the throwing off of floral disguises, and the true nature of the parts becomes evident to the most skeptical.
Rosaceous flowers furnish examples of the simplest form of doubling. In many others the struggle between the two forces seems to have been more violent, and the results are far from uniform, even in the same flower. In some species the broadening of the stamen takes place above the anther, as if the filament had become prolonged and petaloid. Frequently with such structures the rudimentary anther is at the base of the petal, or one half is midway upon one side, and the other opposite it, the connective having broadened out into the body of the petal. It is not unusual to find one half of the organ petaloid, while the other is contracted, contorted, and bears an anther-lobe containing fully developed pollen. In the petunia the doubling of the flower is usually accompanied by a remarkable modification of the pistil—in short, a secondary flower is formed within the ovary. Botanists have long recognized an exceptional development of the floral axis which has been termed prolification. In this there may be a prolongation of the axis beyond the blossom, and the development upon it of ordinary foliage. The European larch furnishes a good illustration of this. Sometimes an ordinary leafy stem extends upward from the center of the cone for nearly a foot. In rare cases leafy branches have grown out from the i free or blossom end of pears, and buds and long branches have arisen from the center of a rose. In the petunia this prolification, if we may call it such, assumes the form of a small and much-contorted flower. Repeated examinations of normal flowers fail to show any unusual structure to the pistil. It is, therefore, associated with the doubling process in the petunia. Instead of the end of the floral axis, which terminates at the base of the single centrally situated pistil, remaining as such, it develops into another flower, and this within the ovary of the primary blossom. Just why we should have this peculiar form of prolification, or any, in fact, is not for us to decide. The ordinary forces which would construct a normal flower have been thrown into confusion, and retrograde metamorphoses and floral prolification have resulted. In fact, it seems evident that out of the substance ordinarily producing a capsule of petunia-seed has been formed in the same ovary an amalgamation of stamens, petals, and a rudimentary pistil. In short, the tendency to petaline display does not stop with the stamens, but invades the pistil, and transforms it as already described.
After doubling has once become established, and the tendency is an hereditary trait, it still remains true that surrounding conditions may favor or modify it. It is well known that among wild plants the absence of favoring surroundings will hasten the period of reproduction, and even augment the yield of fruit. With doubled flowering plants it may be that they strive toward the same end, but fall short because of non-reproductive tendencies developed in them by long-continued culture for their showy flowers only.