Tropisms

TROPISMS, taxes (from Greek tropos - turn, direction and taxis - order, arrangement), changes in the direction of movement or growth in plants, lower organisms, and some mobile cells of higher animals, caused by external irritations. In this context, tropisms are understood as orienting reactions in relation to the stimulus in attached organisms, while taxes are considered reactions of freely living organisms. In essence, there are no fundamental differences between these phenomena, and some authors do not even make such a distinction. Depending on the orientation or movement of the organism toward or away from the source of irritation, positive or negative tropisms are distinguished. The most well-known and studied irritants are light, gravitational force, chemical irritations, electric current, irritation from touch, temperature, and some others. When the irritant is light, we speak of photo- or heliotropism (taxis). In plants, besides positive and negative phototropism, there is also dia-phototropism - the ability to place their organs, for example, leaf blades, perpendicular to incident rays. It is interesting to note that the sign of phototropism for the same organism is by no means always constant, but can change depending on both the intensity of illumination and environmental conditions. The same can be said about a number of other tropic or tactic reactions. The significance of phototropism in plant life becomes clear from the example that it is through phototropic movements that plant leaves form the so-called leaf mosaic, i.e., they are arranged perpendicular to light rays, taking such a position that some of them do not block the light for others as much as possible. This phenomenon ensures the maximum utilization of light energy, which accounts for the main part of plant life processes. In the case of positive phototaxis, it is not always possible to speak of an active movement of the organism toward the light. For example, in some photophilic bacteria (purple bacteria), the random occurrence of bacteria on illuminated places is observed during their disordered movement. However, they cannot leave the illuminated field, because at its boundary, due to a change in the direction of flagellar movement, movement in the opposite direction occurs. Thus, gradually all bacteria collect on the illuminated spot (the phenomenon of physiological trap, or phototactic reaction, according to Hartmann). Finally, it is also interesting to note the coincidence of phototropic reactions with the photochemical law of Bunsen-Roscoe. This analogy follows from the fact that the magnitude of phototropic curvature in plants depends on the total amount of light, i.e., it is determined by the product of the amount of light by the duration of illumination. If the irritant is gravitational force, we speak of geotropism (taxis) (see Geotropism). Here, as with phototropism, besides positive and negative, we also encounter diag-eotropism, as a result of which individual plant organs, for example, leaves, assume a horizontal position. Similarly, the sign of geotropism can change with changes in external or internal conditions. Thus, under the influence of low temperature, the negative geotropism of stems can turn into diag-eotropism, and then the shoot begins to grow horizontally. This phenomenon is observed in many spring plants, which for the first time are pressed to the ground and only later begin to grow straight upward. With a chemical irritant, we deal with the phenomenon of chemotaxis (see). Individual cases where the irritant is food substances, some authors single out into a special group of so-called trophotaxes, but in essence there are no serious grounds for such a separation. Varieties of chemotaxis are also aerotaxis (see) and the phenomenon of hydrotropism. In the latter case, the irritant is water or moistened objects. If the irritation is produced by an electric current, then we speak of galvanotaxis (see). Finally, a whole series of movements is caused by touching or pressure from solid bodies. These include the phenomena of stereo-, or thigmotropism (see also Barotaxis), and to a certain extent, rheotaxis - movement against the current of liquid, since it is apparently caused by the friction of water. Stereotropism was first observed by Dewitz on cockroach spermatozoa, which adhered to the surface of a coverslip under which they were examined, or to some smooth body placed in their medium. A fine example of thigmotropism is also provided by climbing plants. The adaptation for climbing is the tendril, which morphologically represents metamorphosed lateral shoots. In all cases, they appear as a thin, initially spirally twisted stem, which, as it elongates, begins to describe peculiar circular movements in the air. These movements are autonomous and apparently caused by uneven acceleration of growth on different sides of the tendril. During its movements, if the tendril encounters a solid support, it tightly coils around it due to a thigmotropic reaction. At the same time, the sensitivity of the tendril is so great that a brief touch with the finest hair is sufficient to cause a noticeable bending. Both stereotropism and rheotaxis apparently play a significant role in the phenomenon of fertilization, all the more so because for most animals it has not been possible to establish the secretion of positively chemotactic substances by egg cells, at least to the extent that was established for fern antherozoids. A tactic reaction can also be obtained under the influence of temperature irritations. In this case, it will be called thermotaxis (tropism). A thermotactic reaction was discovered by Mendelsohn in Paramaecia, which moved in a flat vessel from parts of it having a temperature of 38° or 10° to the side where the temperature was 25-26°. The same phenomenon of thermotropism can also be observed in plants. For example, roots, when at a temperature below the optimal one, bend toward the warmer area; conversely, at temperatures above the optimum, they give negative bends. One of the first to attempt to explain and systematize the large empirical material accumulated on these questions was J. Loeb. According to his concepts, the space in which the life process of organisms takes place is permeated with force lines of various categories (light rays, diffusion currents in the case of chemotaxis, etc.). The second essential moment is the symmetrical structure of the body, both in the morphological sense and in the sense of identical chemical properties. Due to this, the animal is forced to orient its body in a certain way relative to these force lines. Otherwise, more force lines will fall on one side; this will cause stronger physicochemical changes, which in turn will cause more intense movement of this side. As a result of the series described, the animal must turn and subsequently move only along the straight line connecting the given organism with the source of irritation. Loeb's theory of tropisms refers mainly to animal organisms. As for plants, Loeb limits himself to indicating the presence of symmetry in them as well, for which reason the above reasoning is applicable to them. Not limiting himself to lower animals, he transfers the above considerations to higher ones, pointing to the forced nature of their movements. He attempts to explain the action of the irritator through the sense organs on the muscular system of one of the symmetrical sides. Finally, from here he proceeds to an attempt to explain even more complex processes such as instincts and conditioned reflexes. Of course, forced movements, and even more so instincts, can no longer be reduced solely to tropic reactions. In animals with a developed and highly differentiated reflex arc, the processes of reacting to external stimuli acquire not only a quantitative but also a qualitatively new character. However, the first part of Loeb's theory is flawed by mechanism, since the author tries to reduce the phenomenon of irritability exclusively to physicochemical processes. But it should be noted that the biological features of tropisms have not yet been studied in detail, and their identification awaits a series of works carried out on the basis of the methodology of dialectical materialism. The possibility of a more detailed explanation of many forms of tropisms was given by the theory of Blaauw, confirmed and expanded in recent times by a number of authors. Its essence is as follows: in immobile plants, under the influence of external irritations, growth is accelerated or slowed down on the side of the most intense irritation. While in animals growth is almost exclusively the result of cell multiplication, in plants it can occur both as a result of multiplication and as a result of elongation of cells. The enhancement of growth due to elongation is apparently caused by some substances that can increase in quantity under the influence of external irritations.

These substances, which have been named auxins, are now being intensively studied by the school of Professor Went, as well as by a number of other scientists. Their significance for the theory of tropisms will become clear from the description of the following experiment: if the tip of an oat seedling is cut off, growth ceases for several hours. If this tip is placed on a plate of agar or gelatin, the growth substances pass into them, and when the plate is placed on the stump of the seedling, growth of the latter immediately resumes. When a piece of such agar is placed only on one side of the plant stump, that side will receive more growth substances, its growth will be more intensive, and the plant will curve. For example, let us analyze the experiment with geotropism. In a geotropically curved seedling, the tip is cut off and symmetrically placed on two agar cubes. Between them protrudes the blade of a razor, allowing the upper and lower sides of the seedling tip to be tested separately. If we then place both pieces of agar on vertically standing stumps of seedlings, the one onto which the lower piece was placed halfway will show significantly greater curvature than the seedling with the upper piece of agar placed on it. From these same experiments it follows that the front and rear (in relation to the stimulus) halves of the plant react independently of each other and also independently conduct these substances. Auxin is apparently not specific either in species relation or in relation to the stimulus itself causing its formation, but it can have different effects on different parts of the plant, for example by stimulating stem growth and inhibiting root growth. It is not difficult to notice that the basic principles of Loeb's theory remain valid even in these considerations. The mechanism of taxic processes is not yet studied, but it is possible that here too, under the influence of irritation, certain substances are produced that determine the reaction of the organism.