nastic movement, in botany, the movement of plant parts in response either to certain external stimuli or to internal growth stimuli. Nastic movements, which are generally slow, can be observed by time-lapse photography. Such movements as those of developing buds, which swell, open up, and eventually fall off, are examples of internally directed, or autonomic, nastic movements. The opening and closing movements of many flowers, and the responses of leaves to changes of temperature and light, are externally directed, or paratonic, nastic movements. Specialized plants, such as the insectivorous sundew, move in response to the touch and chemical stimuli of captured insects. Nastic movements are responses to stimuli that uniformly affect the plant or else elicit a uniform response regardless of the direction they come from, whereas tropisms are movements in response to stimuli coming from one direction; geotropism, for example, is the response to gravity. The distinction between the two is sometimes unclear.
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