SLEEP of PLANTS, is a certain faculty, peculiar to many herbs, flowers, &c. of assuming, during the night, a position essentially different from that which they bear throughout the day.—This change takes place principally towards the approach of night, in leaves and flowers; the appearance of which often varies so considerably, that the same plants can scarcely be recognized. During the night, their leaves are observed to rise or curl up, and sometimes to be pendent, according to the nature and genus of the plant, in order to protect the flowers, buds, or young stems.—This period of rest is absolutely necessary to vegetables; their irritability being exhausted by the light and warmth of the day. The circulation of the sap, also, is less rapid in their dormant state; from which circumstance Dr. Darwin conjectures that, as there is less wasted during their inactivity, it is probable that young plants may thrive more rapidly, in the same manner as animals are believed, during their youth, to grow faster when in a state of rest, than in that of exercise.