188 EXHALATION OF VAPOR BY TREES. 
each to each; for the vapor carried off by transpiration 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 
former inquirers. Direct 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 comprehensive scale. 
Our conclusions must therefore be drawn from observations on 
small plants, or separate branches of trees, and of course are 
subject to much uncertainty. Nevertheless, Schleiden, arguing 
from such analogies, comes to the surprising result, that a wood 
evaporates ten times as much water as it receives from atmos- 
pherie precipitation.* In the Northern and Eastern States of 
the Union, the mean precipitation during the period of forest 
growth, that is from the swelling of the buds in the spring to 
the ripening of the fruit, the hardening of the young shoots, 
and the full perfection of the other annual products of the tree, 
exceeds on the average twenty-four inches. Taking this esti- 
mate, the evaporation from the forest would be equal to a pre- 
cipitation of two hundred and forty inches, or very nearly one 
hundred and fifty standard gallons to the square foot of sur- 
face. 
The first questions which suggest themselves upon this state- 
ment are: what becomes of this immense quantity of water and 
from what source does the tree derive it? We are told in reply 
that it is absorbed from the air by the humus and mineral soil of 
the wood, and supplied again to the tree through its roots, by a 
circulation analogous to that observed in Ward’s air-tight cases. 
* Fiir Baum und Wald, pp. 46, 47, notes. Pfaff, too, experimenting on 
branches of a living oak, weighed immediately after being cut from the tree, 
and again after an exposure to the air for three minutes, and computing the 
superficial measure of all the leaves of the tree, concludes that an oak-tree 
evaporates, during the season of growth, eight and a half times the mean 
amount of rain-fall on an area equal to that shaded by the tree. 
