Popular Science Monthly/Volume 33/September 1888/Heliotropism: The Turning Motions of Plants

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HELIOTROPISM: THE TURNING MOTIONS OF PLANTS.
By CONWAY McMILLAN.

AS its derivation would indicate, heliotropism means "turning toward the sun," and is the technical name applied to all such phenomena in the vegetable kingdom. It was well known to the ancients that plants exhibited a remarkable sensitiveness to light, for Aristotle mentions it, and indeed, in its more apparent forms, it is conspicuous even to the naïve observer of to-day. The sunflower, or tournesol, as the French name it, follows the daily course of the sun with its disk-like inflorescence; plants, potted and placed in a window, bend toward the light, unless, perchance, the plant is an ivy, in which case it bends away from the source of illumination; trees and shrubs in the edge of a thicket or forest may be seen to slant toward the open, and in general it may be said that there is scarely a plant which does not respond more or less distinctly to the directive action of light. Exceptions, as shown by Darwin and by Edouard Morren in his treatise on insectivorous plants, are for the most part carnivorous species like Dionea, Drosera, and Nepenthes—the Venus's-flytrap, sundew, and pitcher-plant, respectively—and twining plants. The reason why these plants should not fall under the rule will be apparent when the uses of heliotropism are discussed. Parasitic and the so-called saprophytic plants of the lower orders—those which live upon once-living matter—are commonly insensible to heliotropic stimulus, and, in short, all plants devoid of the great light-product—chlorophyl—manifest in this direction either weak irritability or none at all.

Heliotropism, it must be remembered, is not confined to plants as individuals, but is manifested by the different organs in varying degrees. Tendrils, for example, are either distinctly heliotropic, or far more commonly apheliotropic, as Darwin calls it—that is, negatively heliotropic; leaves are transversely or diaheliotropic—in other words, they tend to place themselves perpendicularly to the incident rays; stems, flower-peduncles, even roots, each in its own way, reply to the stimulus of lateral light. In passing, it should be mentioned that a plant has but one way of responding to conditions without, and this is by curvature. Even the sleep of leaves, the spontaneous movements of the sensitive-plant, or of that singular pulse, the Hedysarum gyrans, in which the two lateral leaflets keep up an incessant jerking motion; the reaction to a cut, bruise, or wound of any kind—as may be seen in an injured tendril; the effect of ether or chloroform, and indeed of natural forces such as electricity, gravity, or heat-vibrations, is in every case a modified curvature. This is the one way which. a plant has of reacting to the external world. Motile organisms, as, for instance, the microscopic swarm-spores of Hæmatococcus or Botrydium—a couple of fresh-water algæ—may seem to stretch the strict interpretation; but their movements may be considered under the law if one remembers that they are free, solitary cells, and must act accordingly.

Twining plants are, perhaps, the most interesting examples of an-heliotropic irritability, for their habit of growth—by no means leaving the instinctive circumnutation of the tip out of account—is a manifestation of insensibility to light. The morning-glory is a perfect example. Regardless of the sun, it twines regularly along its support, never for a moment being deflected or turned aside through conditions of unequal illumination. Of the same thing the wistaria is an equally instructive illustration. It may be safely presumed that, originally, twining plants were not twining plants at all, but were creeping in their habits; and from this it seems probable that heliotropism was once present, but is now lost. In its first appearance the habit of twining must have been accidental, and just how heliotropic tendencies were overcome by the newly developed trait is difficult to explain. It is interesting to notice, however, that the shoot of the morning-glory, when it first peeps from the ground, is distinctly heliotropic, and this must be considered an embryonic feature significant precisely as the branchial development of the fœtal mammal is significant—that is, there is indicated by it a line of descent.

A distinction must first be made between the periodic movements of leaves and stems and the true heliotropic movements. As pointed out by Dr. Julius Sachs, the first—such as the well known phenomenon of sleep—are dependent upon the intensity of illumination, while the second are almost entirely due to the direction from which the rays chance to be falling upon the plant. It will be indispensable to a clear comprehension of what true heliotropism is to speak somewhat generally of light-action in vegetable physiology.

There is certainly no more important agent in the whole system of Nature than light. It is only in light that green plants can form their chlorophyl, and, since this chlorophyl is absolutely essential in the assimilating processes, its importance can be conceived. Even chlorophylless plants—the fungi and slime-molds—and all animals are indirectly dependent, as is well known, upon the possibility of chlorophyl-formation. The whole organized world, then, depends upon light as one of its essentials, for undulations of the ether are necessary to the well-being of all protoplasm. But not only does protoplasm react to these undulations, in very many instances, by the elaboration from itself of the obscure carbon compound we call chlorophyl—it also manifests what is named irritability. This irritability is a property of all living things, and is what distinguishes them from lifeless things. It is the one great difference between a monad and a crystal. Not only in the presence of light is irritability manifested by a living creature, but also when the influence of any other natural force is felt. It is, however, only with light that we have to do at present.

Perhaps, in the whole field of biological science there is not a more obscure subject than this very one of protoplasmic irritability. Dutrochet follows the older botanists when heliotropism is presented for his consideration, and attributes the whole phenomenon to the creative intelligence behind the organism. Now Hartmann, the great German pessimist, following in the footsteps of his master, Arthur Schopenhauer, attributes the twining of the wistaria and the bending away from light of the ivy-shoot to an unconscious will in nature; and teleologists like Paley or Martineau would make the whole field a basis for argument. To be compelled to call upon the first cause for what unquestionably lies within the domain of secondary causes is, of course, no less or more than a confession of ignorance, and one which the modern worker in science is always undesirous of making. Without forgetting that Newton showed himself both a great scientist and a great philosopher when he spoke of himself as but an explorer of the sea-shore while an ocean of undiscovered truth lay beyond, it seems certain that some scientific knowledge of irritability is possible. As Sachs defines it, "it is the mode of reaction to stimuli which is peculiar to living organisms." It is what Herbert Spencer had in mind when he defined life as a continual adjustment between internal and external relations; it is what Brooks has in mind when he calls life "education," and what Haeckel calls attention to when he describes life as "memory." Irritability is really, it would seem, little more than a tendency to abandon an unstable for a stable equilibrium, and may be compared to the tendency to fall which a complicated structure of blocks, for instance, will exhibit upon the slightest disarrangement of any of its components. While the manifestations of irritability are by no means conditioned upon protoplasm alone, they always have their origin in this compound. Mechanical structures of cell-wall and cell-contents act their part in modifying, transmitting, or translating the original impulse; but this impulse itself is a characteristic of protoplasm. Sachs compares the state of things in a plant-cell, before stimulus is applied, to the state of things in a locomotive upon the throttle-valve of which the engineer's hand is placed. A slight expenditure of force will set in motion a vast quantity of matter and may liberate a totally disproportionate amount of energy. This is evidently—in the plant-cell just as in the engine—because things are in the condition to react to slight stimulus. Thus, when a ray of light falls upon a motile swarm-spore, and it swims toward the illuminated side of the drop of water in which it is confined, the undulations of the ether may he held to have caused a more or less continuous change in the molecular structure of the protoplasm; energy is liberated, and ciliary motion in a certain direction is the final resultant. In the same way, when the sun rises in the morning, rays fall upon the stems of the sunflowers, intimate structural changes take place in the cell-protoplasm, and, through mechanical contrivances which will be mentioned later, a slow curving toward the light is effected.

This remarkable instability of protoplasm—and the writer craves the privilege of considering it only as a chemical compound of astonishing complexity—is of deep interest when considered in its relation to growth. Upon this something must be said. The growing part of a plant is, as we know, only the living part. Heart-wood is always dead wood, and is incapable of reacting to the external world except as unorganized matter might. Furthermore—and this need scarcely be mentioned, since the rudiments of botanical knowledge have become so wide-spread—the whole mass of living, growing tissue is made up of cells more or less crowded together, more or less individual in their forms and functions, but all of the same general plan of structure. If one could imagine the Capitol-dome at Washington completely filled with a densely crowded mass of toy balloons, each balloon distended with water, and containing within the water, usually surrounding most of it, a sac-like piece of sponge, it will be a fair idea of what a growing-point would be like if seen upon a sufficiently large scale. The phrase "growing-point" will be understood to have the technical significance, meaning the extreme tip or apical area of a bud or shoot. Each cell-wall contains its cell-sap, or cell-fluid, and its cell-protoplasm, which was compared to the sponges. The protoplasm is, of course, the only essential living part, and the others are but elaborations and mechanisms by which the complicated cell-life, as part of an organic whole, is possible. Or the appearance of a growing-point might be compared to the mound of small bubbles which may be blown in a bottle half filled with suds. Hundreds of bubbles, each full to bursting of air, press each other in every direction, and constitute a more or less conical and coherent mass of bubbles. It is here that the important point is to be noticed—

Growth is possible only when the cells are in a state of tension.

As one may easily discover, a flabby leaf will not increase in size, and a limp and flaccid stem is equally incapable of growth. In other words, growth of a plant-cell is like the growth of a balloon at the gas-works. No increase in size is possible without increase of the internal pressure. Turgescence, as the state of tension is called in plant physiology, depends upon the amount of liquid in the cells, and may be regulated by the protoplasm. Indeed, a leaf may be strangled as readily as an animal which is taken by the throat. If one ties a string tightly around the petiole, water evaporates from the blade, and can not be supplied from below in sufficient quantities to keep the cells tense and elastic. Consequently, the whole leaf relapses into a state of flabbiness, and growth is impossible. Remembering, then, that growth of cells, and consequently of cell-tissue, is an unheard-of thing without turgescence, hydrostatic pressure, or, in a word, stretching, let us see what effect light has upon the condition of things within the plant-cell.

Although experiments along these lines are difficult to make, and, when made, difficult to interpret, it is the opinion of most botanists that the effect of light upon growth is one of retardation rather than of acceleration. It is true that plants will not thrive in darkness, but that is due to cessation in the assimilating processes. This is a comparatively clear case, and may be tested by experiment. Let a potato-tuber be cut in halves, each half containing one or more buds, or "eyes," and then let one half be allowed to sprout in darkness, while the other is brightly illuminated. Conditions of temperature and moisture should be precisely the same in each case. What, then, will be the result? Simply this: After a certain period, the length of which may vary from a day or two to more than a week, each half of the tuber will sprout, put forth a shoot, and upon this shoot there will be developed leaves. The two shoots will, however, be unlike. The one grown in light, or under normal conditions, will be short, plump, firm, green in color, and will bear well-developed green leaves. The shoot grown in the dark—the etiolated shoot, as it is technically named—will be long and slender, the leaves will be smaller, and in neither leaves nor shoot will there be a healthful green color. In other words, light seems to contribute to the production of a shorter shoot, and darkness to a longer. As to the growth of leaves, which are the assimilating organs of the plant, it is natural to suppose that in darkness they would be smaller; and such, in fact, is shown to be the case by the experiment with the potato. Having less to do, and less to do with, than leaves grown under normal conditions, they are correspondingly smaller and weaker. Microscopic examination of the two shoots will show furthermore that the fundamental tissue-cells in the etiolated shoot are much thinner-walled than in the normal shoot. In brief, the cells are stretched more tightly by the contained protoplasm when free from light than they are when exposed to its influence.

Perhaps these phenomena of growth-retardation in light are partly the result of dispersion of energy. In light there is heliotropism with other forms of irritability, chlorophyl-making, assimilation, and growth; while in darkness there is hut response to the influence of gravity and growth. Buds which contain a definite amount of stored-up energy will, perhaps, bring to pass different results, as the number of uses made of this energy may vary. Nature is constantly performing experiments along these lines which indicate such a probability. The alternation of day and night presents a natural periodic etiolation of most normally situated plants. Examination will show that growth is more rapid in darkness than in light, and in many plants it is only at night that any considerable increase of size takes place. Some plants, like the hop—which chances to be apheliotropic—do not grow more rapidly in darkness, and this may be attributed to a difference in irritability, or perhaps an inhibition of the growth process ' by some other. At night, however, the temperature is lower than in the daytime, and this is inimical to growth. Again, some flowers open only at night, and since the opening of a flower is evidence that growth is retarded, another apparently abnormal case is offered. Such instances are unusual and poorly understood. In general, darkness seems to favor a maximum of turgescence in plant-cells.

Motion, now, in plants is a phenomenon of growth—not, very possibly, of growth, viewing the plant as a whole, but considering the cells separately. The sunflower turns to the sun because upon the side next the source of illumination, the cells possess a kind of irritability, in view of which, through molecular changes in the irritated protoplasm—making it more absorbent—growth is retarded and curvature ensues. The old theory of De Candolle differed from this in that light was supposed to be inimical to nutrition, or cell-formation, and the meaning of turgescence and of irritability was not clearly understood. The ivy, which turns away from the sun, may possibly be accredited with a different kind of irritability, or, what seems more reasonable, habit may act as an inhibitor.

With regard to simple cells, the terms of the law that heliotropism is a phenomenon of growth must be modified a little. The Bacterium photometricum of Engelmann, which moves only under the influence of light, does not at the same time increase in size. Neither do the filaments of the Oscillaria nor the zoöspores of algæ. The plasmodia of slime-molds, which, except during the spore-forming stage, are negatively heliotropic, do not grow while turning from the light. Irritability in these organisms is translated at once into ciliary or mass movement, and has to do with growth only in a secondary way. It is, however, clearly analogous in the molecular changes, and need not be considered apart. The mechanism is different, though the forces are the same; just as steam is the same, whether it runs a rolling-mill or a locomotive.

The chemical or molecular theory of light-action is greatly strengthened by two considerations: The first is, that a latent period, after stimulus and before reply, can be detected in almost every case. This, as evidence of chemical action, is conclusive. Second, the researches of Wiesner, and especially those of C. M. Guillemin, recorded in the "Annales des Sciences Naturelles," series iv, vol. vii, in which careful investigations are made into the kind of light which has the greatest effect in heliotropism, must be noted. With regard to assimilation—as we should indeed expect—the yellow rays are found to be the most favorable; but when light is studied with reference to irritability, the results are entirely different. The maxima of action are now found to be at the extreme ends of the spectrum; the ultra-red and the ultraviolet or so-called actinic rays seem to be the ones best capable of giving the necessary stimulus. The minimum is found to be in the blue, near the "F" line. That this "division of labor," as it were, among the light-rays, should be so evident and so constant, offers strong testimony in favor of the theory that irritability and assimilation are equally molecular in their nature (if we may use such an expression), and the whole hypothesis seems unusually clear and satisfactory.

To return for a moment, now, to the potato-shoot grown in darkness, one other peculiarity besides those mentioned might have been noticed. That was this: the angle between leaves and leaf-axis, or stem, was always more acute than in the normally grown plant. The whole etiolated shoot seemed to be straining toward the light. Kraus believed that this was due to imperfect anatomical development in the fibro-vascular bundles; but such a view is scarcely confirmed by the facts, for etiolation is concerned not with the fibrous system alone, but with the fundamental. Rauwenhoff supposed the vertical position of shoots grown in darkness attributable to absence of heliotropism, and the consequent unmodified action of another force in plant-physiology, namely, negative geotropism, or, more clearly, "negative gravity." This is a kind of irritability, in view of which plant-shoots tend to increase in length in a direction opposite to the terrestrial attraction. This, he thought, might be favored by the feeble thickening of the cellular tissues. In this Sachs is inclined to support Rauwenhoff; but it seems probable that the whole matter will have to be laid at the door of heredity. The plant which has always struggled upward to the light will continue to do so when placed in darkness, and all its efforts will be concentrated upon this one end. Leaf-formation will be scanty; assimilation will be suspended; and the whole organism will reach out for the sunlight, as thousands of generations had done before its own life. That fungi of the mushroom type—needing no light, for they make no chlorophyl—reach upward too—and it is undeniably true that they elongate more rapidly in darkness—is to be considered as evidence of descent from an alga stock, and this is rendered probable by morphological as well as by this interesting physiological consideration. Heredity may come into play here as well as it does in the case of the moss antherozoids, which are attracted by the archegonium, or in the case of the fish-mold zoöspores, which swim toward decaying fish or putrid extracts of meat.

From all this, the meaning of heliotropism in the natural order of things becomes apparent. The phenomena which have been studied fit into the evolution theory as if made for the theory and not the theory for them. Plants which must have light to live are impelled toward this light by their own conditions of structure. The reaching upward is sometimes almost instinctive—almost conscious, one might fancy. Knight observed a vine-leaf try first one way and then another to reach the position of best illumination—a transverse one, which is now considered to be a result of the palisade structure, and not of a peculiar kind of irritability. Dutrochet noticed the tendril of a pea trying to avoid the light, and it finally seemed to send an impulse down to the petiole, and this bent backward. The question of resistance is probably, however, the only one which needs to be considered as modifying plant-action in such instances.

Climbing and twining plants, as Darwin observed, have lost their heliotropism because they would be pulled away from their supports if they always followed the sun. For the same reason tendrils, aerial roots, the suckers of Parthenocissus quinquefolia—the Virginia creeper—are, considered as separate organs, apheliotropic rather than the reverse. Carnivorous plants, which at least partially depend for sustenance upon a peculiar position of leaves and stem, and which have less need of light for assimilation, have also lost their powers of* response to the heliotropic stimulus. It must not be supposed, however, for a moment, that heliotropic irritability is not present. It may be there, and well developed too; but inhibited by heredity, by growth, by environment. Just as the compass-plant when grown in darkness allows its leaves to adopt the horizontal position, so does the Venus's-flytrap when growing normally, and it is probable that, if generation after generation of compass-plants could be grown in the darkness, the erect position of the leaves would permanently disappear. Just so the Venus's-flytrap has lost the power of responding to its heliotropic tendency, through its carnivorous habits. Vines, indeed, thinks a different kind of irritability, or at least a conspicuous lack of the normal kind, is denoted by such, habits of growth; but gravity, heredity, and anatomical peculiarities may be entirely responsible. Nor need the meaning of heliotropism in a theory of descent be seriously affected by the observations of Sachs upon certain roots which, although never normally in the light, showed marked heliotropic irritability when grown in illuminated water. In such cases a change in protoplasmic structure might easily have ensued after the change in life-conditions and before the manifestation of unexpected irritability. It is this which renders conclusions drawn from such data as Sachs had doubtful and, perhaps, fallacious.

Heliotropism, then, must be considered as a well-marked physiological trait; developed through ages of natural selection, in accordance with the laws of use and disuse, and here and there modified or altogether absent, as the needs of the organism chanced to demand. It is a result of irritability, and is usually manifested in connection with growth. As acknowledged above, it is still rather poorly understood, in its more recondite expressions; but, in general, it may justly be held to be a very complicated reaction in the department of molecular physics, or chemistry.