TROPISMS, forced movements of an organism (or of a large part of an organism) in response to external stimuli which bring about obligatory physiological reactions which are related to the direction in which the stimulus reaches the organism. A good illustration is the familiar phenomenon of plants turning to the sun, which De Candolle called heliotropism (1835). It is now known to have its counterpart in some animals. The tropistic mode of behaviour includes positive and negative thermotropism, towards or away from a source of heat ; positive and negative geotropism, towards or away from the direction of a gravitational stimulus; galvanotropism, towards or away from electrical radiations, chemotropism, in relation to chemical reagents; rheo tropism, in relation to currents, and so forth. The essential char acteristics of tropisms are their relation to the direction of the stimulus, and their obligatoriness. The young eel or elver is not trying to swim up-stream; it is physiologically forced to keep dead against the current. The physiological constitution of its body is such that it automatically adjusts itself towards securing equal pressure-stimulation on its two sides. The clarifying of the con cept of tropisms is mainly due to Jacques Loeb, and part of his definition may be quoted. "These tropisms are identical for animals and plants. The explanation of them depends first upon the specific irritability of certain elements of the body surface, and second, upon the relations of symmetry of the body. Sym
metrical elements at the surface of the body have the same irritability; unsymmetrical elements have a different irritability. Those nearer the oral pole possess an irritability greater than that of those near the aboral pole. These circumstances force an animal to orient itself towards a source of stimulation in such a way that symmetrical points on the surface of the body are stimulated equally. In this way the animals are led without will of their own either toward the source of stimulus or away from it." While engrained tropisms are eventually quite involuntary or forced reactions to external stimuli, it does not follow that there was no psychical factor in their racial evolution. Care must be taken not to exaggerate the role of "pure tropisms"; and one may even ask if there are any. A noteworthy fact is the reversibility of many tropisms, for the same animal may show a positive or a negative reaction to temperature, light, gravity or an electric current, the direction varying according to (a) the strength of the stimulation, (b) the internal physiological state, and (c) the coincident in fluence of the other factors. (See PSYCHOLOGY, COMPARATIVE.) See S. J. Holmes, Studies in Animal Behaviour (Boston, 1916) ; J. Loeb, Forced Movements, Tropisms, and Animal Conduct (Phila delphia and London, 1918). ( J. A. TEE.)