nastic movement

Plant movement that is caused by an external stimulus, such as light or temperature, but is directionally independent of its source, unlike tropisms. Nastic movements occur as a result of changes in water pressure within specialized cells or differing rates of growth in parts of the plant.

Examples include the opening and closing of crocus flowers following an increase or decrease in temperature (thermonasty), and the opening and closing of evening primrose Oenothera flowers on exposure to dark and light (photonasty).

The leaf movements of the Venus flytrap Dionaea muscipula following a tactile stimulus, and the rapid collapse of the leaflets of the sensitive plant Mimosa pudica are examples of haptonasty. Sleep movements, where the leaves or flowers of some plants adopt a different position at night, are described as nyctinasty. Other movement types include hydronasty, in response to a change in the atmospheric humidity, and chemonasty, in response to a chemical stimulus.