A *nastic movement
in which plant parts, especially leaves and flowers, assume a characteristic position at night. These sleep movements most often result in a folding together of leaflets. In certain Acacia species the process seems to be a form of *photonasty
, as the assumption of the night position coincides with the onset of darkness. In other species the cycle of folding continues even if the plant is kept in the dark; the process is then clearly *autonomic
rather than nastic.