Geotropism

GEOTROPISM (from the Greek ge-earth and trepo-I turn), or geotaxis, is the phenomenon of certain organisms reacting to the action of the force of gravity. The reaction manifests itself in movement or growth either in the direction of the force of gravity (positive geotropism) or in the opposite direction (negative geotropism). Unicellular organisms provide many examples of geotropism. Thus, infusoria (Paramaecium) and certain flagellates gather at the upper end of a vertically standing test tube, while some bacteria gather at the lower end (positive geotropism). There are a number of theories attempting to explain geotropism. Verworn initially thought to explain the negative geotropism of slipper animalcules purely mechanically, by the fact that the posterior end of their body is heavier than the anterior, and therefore the animal positions itself with its anterior end upward, which is where it moves. According to Jensen's theory, geotropism depends on the varying degree of pressure of the water column at different depths, with the majority of Protozoa moving in the direction of the least hydrostatic pressure, i.e., upward. Finally, Lyon and O. Koehler believe that the direction of geotactic movements is influenced by heavy inclusions (crystalline excretory bodies) scattered in the body of the infusorian. They act like statocysts (auditory vesicles) in Metazoa. By the pressure of their weight on the plasma in a certain direction, the bodies transmit the irritation of the force of gravity to the organism and force negatively geotropic animals to move in a direction opposite to the action of this force. Koehler confirmed this view with experiments combining the action of gravity and magnetic attraction on paramecia fed with fine powder of iron particles.

V. Dogiel. The phenomena of geotropism are best known in botany, where they were established as early as 1770 (Dodart) and were studied later by Knight, Darwin, and many other authors up to the present time. In botanical literature, a distinction is made between geotropism proper, as a reaction of a fixed organism in the form of a change in the direction of growth (curvature), and geotaxis, as a reaction of a mobile organism in the form of a change in the direction of movement. For plants, geotropism proper is of primary importance, and it is mainly what determines the orientation of organs in space (cf. also Heliotropism). Any deviation from the normal position of an organ causes irritation and a reaction in the form of curvatures of its growing part, so that growth again continues in the normal direction (vertically for so-called orthotropic organs—main stems and roots, and horizontally or at a certain angle to the horizon for plagiotropic organs—mostly leaves, rhizomes, lateral branches). In many cases, the site of perception of the irritation and the site of the reaction to it do not coincide; for example, in the root, perception is concentrated predominantly in the very tip, while the reaction is carried out in the growing zone, removed from the tip by several millimeters, i.e., transmission of the irritation must take place here. It is highly probable that here, as in heliotropism, this transmission occurs through the mediation of special substances produced at the site of irritation perception (analogous to hormones). As for the mechanism of geotropic irritation, the so-called statolith theory of Haberlandt-Nemec is widely accepted. According to it, the direction of the force of gravity is perceived in certain special cells containing large starch grains. Upon a change in the position of the cell, the heavy starch grains move within it and press on the hyaline layer of the protoplasm in a new, unusual place, which causes the irritation. In some cases, geotropic phenomena take place even where no moving grains are contained in the cells, e.g., in fungi. Thus, the statolith theory of geotropism does not have universal significance.

L. Kursanov. Lit.: Loeb J., Forced Movements, Tropisms, and Animal Conduct, pp. 93-102, Moscow, 1924 (lit.).