24 Forests — their Influence upon Climate and Rainfall. [January, 



On the other hand actual observation has shown that transpi- 

 ration is carried on with almost incredible activity — the rate at 

 which aqueous vapor is given off by plants being- more than one 

 and a quarter ounces per square foot of leaf surface for twelve diur- 

 nal hours. 1 Let the reader reflect upon the vast expanse of leaf 

 surface of a single tree giving off vapor at this rate, and then let 

 him consider the number of trees in a forest of only a few acres, 

 the number being variously estimated at from 150 to 600, and 

 multiplying these two factors he will be able to form some ap- 

 proximate idea of the enormous amount of aqueous vapor sup- 

 plied to our atmosphere in the most acceptable form. 



During the past summer I have instituted a series of experi- 

 ments with the view of determining the amount of water vapor- 

 ized from known areas of leaf surface, land surface and water 

 under similar circumstances, in order that a more nearly correct 

 estimate of evaporation from these various sources might be 



A pot plant having one square foot of leaf surface was carefully 

 prepared — in the manner previously described — so as to pre- 

 vent any evaporation from the pot in which it was growing. 

 Another glazed pot was filled with soil (a light clay loam) so as 

 to expose a surface area of only twenty-four square inches, the 

 pot being about the same size as that containing the plant, and 

 the depths of the pot very nearly six inches. The plant was 

 sufficiently watered to keep it in a thrifty condition, while the 

 earth in the plantless pot was kept generally well saturated. 

 Both were equally exposed to the outer air. The evaporation 

 from earth and plant was now tested simultaneously by weighing 

 the two pots at stated intervals, and it was found that the mean 

 evaporation was, in fair weather, nearly equal for the two sources, 

 with a slight preponderance on the side of the soil. For fourteen 

 consecutive days of clear and partly cloudy weather, the mean 

 transpiration from the plant was a little over one and a quarter 

 ounces, and the evaporation from the soil one and a third ounces. 

 This would place the rates of evaporation of equal areas of leaf 

 and land surface, under like circumstances of exposure, at about 

 six to one in favor of the soil, that is to say, one square foot of 

 soil will evaporate six times as much as one square foot of leaf sur- 



!" Transpiration of Plants," American Naturalist for March, 1878, by the 

 author. , 



