184 EXHALATIOISr OF VAPOR BY TEEES. 



it held in solution or suspension.* The hygrometrical equihb 

 rium is then restored, so far as this : the tree yields up again the 

 moisture it had drawn from the earth and the air, though it does 

 not return it each to each ; for the vapor carried off by transpira- 

 tion greatly exceeds the quantity of water absorbed by the foliage 

 from the atmosphere, and the amount, if any, carried back to the 

 ground by the roots. 



The present estimates of some eminent vegetable physiologists, 

 in regard to the quantity of aqueous vapor exhaled by trees and 

 taken up by the atmosphere, are much greater than those of for- 

 mer inquu'ers. Du*ect and satisfactory experiments on this point 

 are wanting, and it is not easy to imagine how they could be made 

 on a sufficiently extensive and com23rehensive scale. Our conclu- 

 sions must therefore be drawn from observations on small plants 

 or separate branches of trees, and, of course, are subject to much 

 uncertainty. ]S^evertheless, Schleiden, arguing from such analo- 

 gies, comes to the surprising result, that a wood evaporates ten 

 times as much water as it receives from atmospheric precipita- 

 tion.f In the IS'orthern and Eastern States of the Union, the 



* Ward's tight glazed cases for raising and especially for transporting plants, 

 go far to prove that water only circulates through vegetables, and is again and 

 again absorbed and transpired by organs appropriated to these functions. 

 Seeds, growing grasses, shrubs, or trees planted in proper earth, moderately 

 watered and covered with a glass bell or close frame of glass, live for months, 

 and even years, with only the original store of air and water. In one of 

 Ward's early experiments, a spire of grass and a fern, which sprang up in a 

 corked bottle containing a little moist earth introduced as a bed for a snail, 

 lived and flourished for eighteen years without a new supply of either fluid. 

 In these boxes the plants grow till the enclosed air is exhausted of the gaseous 

 constituents of vegetation, and till the water has yielded up the assimilable 

 matter it held in solution, and has dissolved and supplied to the roots the nu- 

 triment contained in the earth in which they are planted. After this, they 

 continue for a long time in a state of vegetable sleep, but if fresh air and water 

 be introduced into the cases, or the plants be transplanted into open ground, 

 they rouse themselves to renewed life, and grow vigorously, without appear- 

 ing to have suffered from their long imprisonment. The water transpired by 

 the leaves is partly absorbed by the earth directly from the air, partly con- 

 densed on the glass, along which it trickles down to the earth, enters the roots 

 again, and thus continually repeats the circuit. See Aus der Natur, 31, B. S. 

 537. 



f Fur Baum und Wald, pp. 46, 47, notes. Pfaff, too, experimenting on 

 branches of a living oak, weighed immediately after being cut from the tree, 



