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taxis(tăk`sĭs), movement of animals either toward or away from a stimulus, such as light (phototaxis), heat (thermotaxis), chemicals (chemotaxis), gravity (geotaxis), and touch (thigmotaxis). The turning movements of plants in response to stimuli are called tropismstropism
, involuntary response of an organism, or part of an organism, involving orientation toward (positive tropism) or away from (negative tropism) one or more external stimuli.
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A mechanism of orientation by means of which an animal moves in a direction related to a source of stimulation. There exists a widely accepted terminology in which the nature of the stimulus is indicated by a prefix such as phototaxis (light), chemotaxis (chemical compounds), geotaxis (gravity), thigmotaxis (contact), rheotaxis (water current), and anemotaxis (air current). The directions toward or away from the stimulus are expressed as positive or negative, respectively. Finally, the sensory and locomotory mechanisms by means of which the orientation is achieved are denoted by a second type of prefix forming a compound noun with taxis. Positive phototropotaxis thus describes a mechanism by means of which an animal carries out a directed movement toward a source of light along a path which permits the animal's paired eyes to receive equal intensities of light throughout the movement.
a locomotory reaction of motile microorganisms and very simple plants, individual cells of multicellular organisms (zoospores, spermatozoids, leucocytes), and organelles (nuclei, plastids). Taxes may occur in response to unilateral stimulation by light (phototaxis), temperature (thermotaxis), moisture (hydrotaxis), liquid current (rheotaxis), electric current (galvanotaxis), injury (traumatotaxis), chemical stimulus (Chemotaxis), or pressure (barotaxis).
Taxes are distinguished according to the direction of the reaction: positive taxis is movement in the direction of the stimulus, negative taxis is movement away from the stimulus, and phobo-taxis is a movement of “fright,” in which the orientation is not related to the source of the stimulus. The nature of a taxis may change depending on the intensity of the stimulus and the condition of the organism. Taxes should be distinguished from tropisms, which are reactions to unilateral stimulation of individual organs of plants or whole plants attached to a substrate.
V. I. KEFELI