Biol. [The second element of HELIOTROPISM, GEOTROPISM, etc., used as an inclusive or generic term.] The turning of an organism, or a part of one, in a particular direction (either in the way of growth, bending, or locomotion) in response to some special external stimulus, as that of light (phototropism, heliotropism), heat (thermotropism), gravity (geotropism), etc.
1899. C. B. Davenport, Morphology, II. 480. All cases of true tropism are cases of response to stimuli: such are chemotropism, hydrotropism, thigmotropism, traumatropism, rheotropism, geotropism, electrotropism, phototropism and thermotropism.
1909. J. W. Jenkinson, Experim. Embryol., 273. The outgrowth and anastomoses of nerves, glands, ducts, the concrescence of layers may be tropisms of various sorts.
