Movement of a flat plant part, oriented relative to the plant body and produced by a variety of stimuli that cause disproportioinate growth or increased turgor pressure in the tissues of one surface.

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 nasticisms and tropisms is sometimes unclear.