102 HISTOLOGY AND PHYSIOLOGY. 



the amount of moisture on the epidermis. Another partial 

 hindrance to more rapid evaporation lies in the fact that 

 the water contains many substances in solution, which, as 

 is known, evaporates more slowly than pure water. 



129. The rate of evaporation is very slow. It was long 

 ago found that the amount of water evaporated from a vine 

 in twelve hours of daylight was equal to a film only .005 

 inch thick, with an extent equalling the evaporating sur- 

 face ; the amount from a Cabbage in the same time equalled 

 a film .012 inch thick; from an Apple-tree .01 inch. The 

 evaporation from most leaves is estimated to be about one- 

 third that from equal areas of water. A close-topped 

 Oak-tree, twenty feet high, having about 700,000 leaves, 

 evaporated (according to calculation) 246,630 pounds of 

 water during the growing season of five and a half 

 months. This amounted to a layer of water 1.31 inches 

 deep over the whole evaporating surface. Considered with 

 reference to the area of ground covered by the tree-top, it 

 was found that the evaporation from the tree was eight 

 times as great as the annual amount of water which fell 

 beneath it. The evaporation would be much less in dense 

 forests, yet even there it is sufiicient to deprive the ground 

 of its moisture in a short time in a dry, hot season. 



130. To supply the loss by evaporation there must be a 

 movement of water through the roots, stems, and 

 branches to the leaves. This is demonstrated by cutting 

 off a leafy shoot when evaporation is going on rapidly ; the 

 leaves wither quickly from loss of water, but if the cut end 

 be placed in water, the latter passes up through the stem 

 and supplies the loss from the leaves, which then retain 

 their normal condition. If in this experiment a colored 

 watery solution be used, it will be evident that some kinds 



