Voluntary and autonomous turgescence-movements of this kind are only observed in a few of the higher plants, but stimulated movements that are accomplished by the same mechanism are very common in the vegetal world. We have, especially, the well-known "sleep," or nyktitropic movements, of many plants. Many leaves and flowers hold themselves vertically to the streaming rays of the sun. When darkness comes on they contract, and the calices of the flowers close. Many flowers are open for only a few hours a day. The mechanism of turgescence, which effects these swelling movements, consists in the co-operation of the osmotic pressure of the internal cell-fluid and the elasticity of the strained cell-membrane enclosing the cytoplasm. The strain of the outer cellulose membrane on the plasmatic primordial sac within it grows so much on the accession of osmotically active matter that the internal pressure is equal to several atmospheres, and the elastic strained membrane stretches from ten to twenty percent. When water is withdrawn again from one of these swollen or turgescent cells, the membrane contracts; the cell becomes smaller, and the tissue looser. Other stimuli besides light (heat, pressure, electricity) may produce these expansional variations, and, as a consequence of it, certain reflex movements (or paratonic variational movements). The most striking and familiar examples are the flesh-eating fly-trap (_dionæa muscipula_) and the sensitive plant (_mimosa pudica_); their contraction is caused by mechanical stimuli, shaking, pressure, or the touching of the leaves.