PERSPIRATION OF LEAVES. 149

riment of gathering the leafy branch of a tree, andimmediately stopping the wound at its base withmastick, "wax, or any other lit substance, to preventthe effusion of moisture in that direction. In a veryshort time the leaves droop, wither and are dried up.If the same branch, partly faded, though not dead,be placed in a very damp cellar, or immersed inwater, the leaves revive, by which their power ofabsorption is also proved. Hence the use of a tinbox to travelling botanists, for the purpose of re-straining the evaporation of plants, and so preser-ving them fresh for some days till they can be ex-amined ; as well as of reviving faded plants, if theinside of the box be moistened before they are shutup in it.

Dr. Hales found that a plant of the Great An-nual Sun-flower, Helianthus annum , lost 1 lb. 14 oz.weight in the course of twelve hours in a hot dryday. In a dry night it lost about three ounces;in a moist night scarcely any alteration was obser-vable, but in a rainy night it gained two or threeounces. The surface of the plant compared withthat of its roots was, as nearly as could be calculated,in the proportion of five to two; therefore the rootsmust have imbibed moisture from the earth of thepot in which the plant grew, and which was all pre-viously weighed, in the same proportion of five totwo, otherwise the leaves would have faded. The