THE WATER OF SOILS. 263 



field crops, to correspond most nearly so 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, flr 

 and pine ranged, under the same conditions, from 30 to 70 pounds 

 of water only. In another year, these figures were increased for decid- 

 uous trees to from 500 to 1000, the conifers, 75 to 200 pounds. This 

 great variability indifferent 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 ever- 

 green 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 in- 

 jury 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. 



The shading of the ground by leafy vegetation will, of 



