THE FOEEST AS ORGANIC. 17^ 



The vegetable mould, resulting from the decomposition of 

 leaves and of wood, serves as a perpetual Tnulch to forest-soil by 

 carpeting the ground with a spongy covering which obstructs the 

 evaporation from the mineral earth below,* di'inks up the rains 

 and melting snows that would otherwise flow rapidly over the 

 surface and perhaps be conveyed to the distant sea, and then 

 slowly gives out, by evaporation, infiltration and percolation, the 

 moisture thus imbibed. The roots, too, penetrate far below the 

 superficial soil, conduct water along their surface to the lower 

 depths to which they reach, and thus by partially draining the 

 superior strata, remove a certain quantity of moisture out of the 

 reach of evaporation. 



The Forest as Organic. 



These are the principal modes in which the humidity of the 

 atmosphere is affected by the forest regarded as lifeless matter. 

 Let us inquire how its organic processes act upon this meteoro- 

 logical element. 



The commonest observation shows that the wood and bai'k of 

 living trees are always more or less pervaded with watery and 

 other fluids, one of which, the sap, is very abundant in trees of 

 deciduous fohage when the buds begin to swell and the leaves ta 

 develop themselves in the spring. This fluid is drawn principally, 

 if not entirely, from the ground by the absorbent action of the 

 roots; for though Schacht and some other eminent botanical 

 physiologists have maintained that water is absorbed by the leaves 

 and bark of trees, yet most experiments lead to the contrary re- 

 fall through another, where vapor would not be condensed. Two saturated 

 strata of different temperatures may be brought into contact in the higher 

 regions, and discharge large rain-drops, which, if not divided by some obstruc- 

 tion, wiU reach the ground, though passing through strata which would vapor- 

 ize them if they were in a state of more minute division. 



* The only direct experiments known to me on the evaporation from the 

 surface of the forest are those of Mathieu. — Surell, Mude sur les Torrents, 

 2ded., ii., p. 99. 



These experiments were continued from March to December, inclusive, of 

 the year 1868. It was foimd that during those months the evaporation from 

 a recipient placed on the ground in a plantation of deciduous trees sixty-two 

 years old, was less than one-fifth of that from a recipient of like form and di- 

 mensions placed in the open country. 



