FORESTS IN THEIR RELATION TO RAINFALL. 229 



ground of transpiration alone, the atmosphere of a forest 

 is moister than that immediately above the surrounding 

 land, and it is desirable to conserve the forest growth on 

 that account. 



Botanists have made many measurements of its amount 

 and their results are extremely varied, due partly to the 

 fact that this function varies much naturally, and still 

 more to the fact that experiments are generally made 

 under conditions which are not natural to the plant. Sachs 

 says that it is no rarity for a tolerably vigorous tobacco 

 plant at the time of flowering, or a sunflower of the height 

 of a man, or a gourd plant with from fifteen to twenty 

 large leaves, to transpire from one to two pints of water 

 on a warm summer day; and, so far as may be judged by 

 the use of branches with the cut end in water, it may be 

 believed that large fruit trees, oaks or poplars absorb, 

 transport through their stems, and transpire through their 

 leaves, ten to twenty or more gallons of water daily. It 

 is not generally practicable to compare the transpiration 

 with known meteorological phenomena, such as evaporation 

 from a water surface, or from the soil, or the precipitation, 

 but some such comparisons have been made. For instance, 

 comparing the leaf surface to an equivalent water surface, 

 Unger makes transpiration from the former 0*33 of the 

 evaporation from the latter ; Sachs for white poplar 0*36, 

 the sun-flower 0*42. Comparisons have also been made 

 between the transpiration from plants and from the 

 evaporation from the surface over which the plants stand. 

 Schleiden thought that the transpiration from the forest 

 was three times that of the water surface equal to the 

 territory covered by the forest, Schiibler thought it only a 

 quarter ; and Pfafl, who studied a solitary oak in a garden, 

 found that it varied from 0*87 to 1*58. Comparing the 

 transpiration of plants with the evaporation of the bare 



