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  Tropism (Greek, ‘faculty of turning [towards the Sun]’), in the life sciences, is the phenomenon of a plant\'s involuntary response to an environmental stimulus which has directional orientation. The response of plants to gravity was the first tropism to be scientifically investigated, but the subject was expanded to include responses to light and wounds by Charles Darwin and his son Francis, who demonstrated that the mechanism of tropism was a curvature produced by differences in the growth on each side. Plants respond to touch and to chemicals by the same mechanism, which is co-ordinated by certain auxins (plant hormones). The term tropism has also been used to refer to bacteria, which may be motile; it has been shown that bacteria can exhibit tropism in response to magnetic forces. The use of the term in the context of animal movements is more controversial and, while simple animals, particularly single-celled animals, may respond involuntarily to stimuli such as light, ethologists prefer to use the term taxis for such animal movements. RB

See also endocrinology; ethology.



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