It was found that the surface-areas of the leaves do not give reliable results, but that these depend very largely upon the thickness (mass) of the leaves. The dry weight of the latter was found, as in the case of field crops, to correspond most nearly to the observations made directly. It was thus found that _e. g._ birch and linden transpired during their annual period of vegetation from 600 to 700 pounds of water per pound of dry leaves; oaks 200 to 300, while the figures for ash, beech and maple were in between. On the other hand the conifers—spruce, fir and pine—ranged, under the same conditions, from 30 to 70 pounds of water only. In another year, these figures were increased for deciduous trees to from 500 to 1000, the conifers, 75 to 200 pounds. This great variability in different seasons, together with other elements of uncertainty, render these figures only roughly approximate; but it will be noted that the figures for deciduous trees are in general of the same order as those given above for field crops. Assuming the evaporation for citrus trees to be approximately the same as for the European evergreen oak (_Q. cerris_) viz. 500 pounds per pound of dry matter, and taking the weighings made by Loughridge of the leaves of a 15-year-old orange tree at Riverside as a basis (40 pounds of dry leaves), the water evaporated by each such tree would be about 20,000 pounds per year, or about 1000 tons per acre of 100 trees. This is equivalent to about 9 acre-inches of rainfall, out of the 35 inches commonly given. Since different plants evaporate very different amounts of water during a given time, according to their leaf-surface and the number and size of their stomates, the maintenance of the equilibrium between the soil-supply and the evaporation of the leaf-surface requires correspondingly varying moisture-conditions in the soil. Therefore desert plants, with their elaborate structural provisions against leaf-evaporation, will develop normally, and without wilting, under conditions which in the case of most culture plants would result in severe injury or death. Since diminution of leaf-surface will in all cases diminish evaporation, the heroic measure of cutting back the twigs and branches of shrubs and trees in seasons of severe drought is sometimes resorted to in order to save their life. In Nature this diminution of leaf-surface may be observed in many cases of desert plants, whose “fugacious” leaves are developed during the rainy season, in winter and early spring; dropping off so soon as the dry season begins, and leaving only the green surface of twigs, stems or spines to perform the functions of the leaves.