186 EXHALATION OF VAPOE BY TEEES. 



periments witli carefullj heat-dried field soil, tlie absorption was 

 five per cent, in eiglity-four hours, and when the water was first 

 warmed to secure the complete saturation of the air, air-dried 

 garden earth absorbed five and one-tenth per cent, in seventy- 

 two hours. 



In nature the conditions are never so favorable to the absorp- 

 tion of vapor as in these experiments. The ground is more com- 

 pact and of course offers less surface to the air, and, especially in 

 the wood, it is already in a state approacliing saturation. Hence, 

 both these physicists conclude that the quantity of aqueous vapor 

 absorbed by the earth from the air is so inconsiderable " that we 

 can ascribe to it no important influence on vegetation." * 



Besides this, trees often grow luxuriantly on narrow ridges, on 

 steep dechvities, on partially decayed stumps many feet above the 

 ground, on walls of high buUdings, and on rocks, in situations 

 where the earth within reach of their roots could not possibly 

 contain the tenth part of the water which, according to Schleiden 

 and Pfaff, they evaporate in a day. There are, too, forests of 

 great extent on high bluffs and well-drained table-lands, where 

 there can exist, neither in the subsoil nor in infiltration from 

 neighboring regions, an adequate source of supply for such con- 

 sumption. It must be remembered, also, that in the wood the 

 leaves of the trees shade each other, and only the highest stratum 

 of f ohage receives the full infiuence of heat and hght ; and besides, 

 the air in the forest is almost stagnant, while in the experiments 

 of Unger, Marshal, Yaillant, Pfaff and others, the branches were 

 freely exposed to light, sun and atmospheric currents. Such ob- 

 servations can authorize no conclusions respecting the quantitative 

 action of leaves of forest trees in normal conditions. 



Further, allowing two hundred days for the period of forest 

 vital action, the wood must, according to Schleiden's position, ex- 

 hale a quantity of moisture equal to an inch and one-fifth of pre- 

 cipitation per day, and it is hardly conceivable that so large a vol- 

 ume of aqueous vapor, in addition to the supply from other 

 sources, could be diffused through the ambient atmosphere with- 

 out manifesting its presence by ordinary hygrometrical tests much 

 more energetically than it has been proved to do, and, in fact, the 



* WiLHELM, D&r Boden und das Wasser, pp. 14, 20. 



