Heliotropism

HELIOTROPISM (from the Greek helios - sun and trepo - I turn), or phototropism, is the phenomenon of organisms reacting to the action of light rays. Many motile plants or animals, when illuminated, change their movement, heading toward the light source (positive heliotropism) or moving away from it (negative heliotropism). Positive heliotropism is exhibited by many green flagellates (Euglenina, Volvocales, and others). Colorless Protozoa, for example, ciliates, either do not react to light at all or only to the strongest light stimuli, and moreover in a negative sense (excluding amoebae). Frequently, heliotropism is relative in character, changing depending on the illumination intensity. Most organisms exhibit a certain optimum of illumination, on both sides of which the animal behaves differently: below the optimum it is positively heliotropic, while above it it is negatively heliotropic. Sometimes intensified illumination of animals has an inhibitory effect on movement (in amoebae). Ultraviolet rays exert a particularly strong effect. Thus, paramecia (Paramaecium), which are insensitive to ordinary daylight, become negatively heliotropic under the action of ultraviolet rays with a wavelength of 280 mµ. It is interesting that the addition of fluorescent substances (eosin, erythrosine) to water with protozoa makes animals that were insensitive to light negatively heliotropic. This is the so-called induced heliotropism, discovered by Metzner's experiments on paramecia. In attached animals, for example, on parts of hydroid polyp colonies, heliotropism is manifested by a change in the direction of their growth.

V. Dogel. In botanical literature, two terms are distinguished: heliotropism (phototropism) and heliotaxis (phototaxis). The former denotes the reaction to unilateral illumination of attached plants, expressed by the direction of their growth, and the latter denotes the reaction of movement of motile forms. Heliotropism is of the greatest importance precisely for plants. Most stems and flowers possess positive heliotropism; negative heliotropism is rarer, for example, in certain aerial roots and tendrils of climbing plants. In lateral organs (leaves and lateral branches), transversal heliotropism is also distinguished, i.e., the arrangement of the organ perpendicular to the action of light. In many cases, it has been established that the site of perception of the light stimulus and the site of reaction to it in the form of curvature do not coincide. For example, in young oat or millet seedlings, the stimulus is perceived by the very tip (plumule), and the curvature occurs significantly lower. Thus, the transmission of the stimulus takes place here; furthermore, it can occur not only through living cells, but also through a layer of, for example, gelatin, if the severed perceptive tip is glued back in its former place with its help. From this, one can think that the transmitters of the stimulus here are some water-soluble substances that are produced at the site of stimulus perception and then spread by diffusion. Some authors compare them with hormones.