Sleep

Sleep, in plants, the assumption by leaves, especially the leaflets of compound leaves, of the nocturnal position, which is generally a folding in a vertical plane, under the influence of darkness or excessive illumination. The movements exhibited are identical with those induced in some of the cases, such as the sensitive plant, by contact, and with those which occur spontaneously, in spite of the inhibitory action of light, in others, such as the telegraph plant. "Sleep" movements only occur between 15° and 50° C. Prolonged darkness destroys the power; but for a time the leaves of the sensitive plant (q.v.), in the dark, exhibit spontaneous alternating movement, like those of the telegraph plant. Sudden variations in the intensity of light induce "sleep." The mechanism of the movements consists, as in the sensitive plant (q,v.), in the action of the pulvini.