182 THE FOREST AS ORGANIC. 
The vegetable mould, resulting from the decomposition of 
leaves and of wood, serves as a perpetual mzlch to forest-soil by 
carpeting the ground with a spongy covering which obstructs 
the evaporation from the mineral earth below,* drinks 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 per- 
colation, 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 mois- 
ture 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 bark 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 foliage when the buds begin to swell and the leaves 
to develop themselves in the spring. This fluid is drawn prin- 
cipally, if not entirely, from the ground by the absorbent action 
of the roots, for though Schacht and some other eminent botan- 
ical physiologists have maintained that water is absorbed by the 
leaves and bark of trees, yet most experiments lead to the con- 
tion, will 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.—SurRELL, Ltude sur les Torrents, 
2d ed., ii., p. 99. 
These experiments were continued from March to December, inclusive, of 
the year 1868. It was found 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 
dimensions placed in the open country. 
