190 EXHALATION OF VAPOR BY TREES. 
Besides this, trees often grow luxuriantly on narrow ridges, 
on steep declivities, on partially decayed stumps many feet above 
the ground, on walls of high buildings, and on rocks, in situa- 
tions where the earth within reach of their roots could not possi- 
bly contain the tenth part of the water which, according to 
Schleiden and Pfaff, they evaporate ina day. There are, too, for- 
ests 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 consumption. It must be remembered, also, that in the 
wood the leaves of the trees shade each other, and only the 
highest stratum of foliage receives the full influence of heat 
and light ; and besides, the air in the forest is almost stagnant, 
while in the experiments of Unger, Marshal, Vaillant, Pfaff 
and others, the branches were freely exposed to light, sun, and 
atmospheric currents. Such observations can authorize no con- 
clusions 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, 
exhale a quantity of moisture equal to an inch and one-fifth of 
precipitation per day, and it is hardly conceivable that so large 
a volume of aqueous vapor, in addition to the supply from other 
sources, could be diffused through the ambient atmosphere 
without manifesting its presence by ordinary hygrometrical tests 
much more energetically than it has been proved to do, and in 
fact, the observations recorded by Ebermayer show that though 
the relative humidity of the atmosphere is considerably greater 
in the cooler temperature of the wood, its absolute humidity 
does not sensibly differ from that of the air in open ground.* 
* EBeRMAyYER, Die Physikalischen Hinwirkungen des Waldes, i., pp. 150 
et seg. It may be well here to guard my readers against the common error 
which supposes that a humid condition of the air is necessarily indicated by 
the presence of fog or visible vapor. The air is rendered humid by containing 
invisible vapor, and it becomes drier by the condensation of such vapor into 
fog, composed of solid globules or of holiow vesicles of water—for it is a dis- 
puted point whether the particles of fog are solid or*vesicular. Hence, 
