Sleep

(slēp.) [From A. S. sleep an, to slumber.] One of the most mysterious phenomena in the animal world, a state in which the body appears perfectly at rest and external objects act on the organs of sense without exciting the usual sensations. The voluntary exertions of our mental and corporeal powers being suspended, we are unconscious of what pa es around us, and are not affected by the ordinary impre ions of external objects. S. is generally attended with a relaxation of the muscles, but the involuntary motions, as respiration and the circulation of the blood, are continued. When S. is only partial, that is, when the brain does not fully participate in it-which is known by distinctly-remembered and painful and troublesome dreams -the refreshment it produces is greatly le ened. In childhood and in perfect health we dream but little; or at least so imperfectly and so slightly that the impre ions experienced are scarcely recollected.-S. of Plants. Light acts on plants as a powerful stimulus, e ential to their active and healthy vegetation. When it is withdrawn, the flowers of many plants close, and the greater number show a tendency to it, while leaves more or le decidedly incline to fold themselves up. The leaf-stalk also generally hangs down more or le , although in some plants it is more erect during sleep. The sleep of plants, however, is not always nocturnal. The flowers of some open and close at particular hours of the day. Thus, the crocus is a morning flower, and closes soon after mid-day, while some flowers expand only in the evening or during the night. Their hours of vegetative rest are probably as e ential to the health of plants as those of sleep are to animals. [s. 832]