174 
EXHALATION OF VAPOR BY TREES. 
tion in spring, some of this fluid certainly escapes through the 
buds, the nascent foliage, and the pores of the bark, and vege¬ 
table physiology tells us that there is a current of sap toward 
the roots as well as from them.* I do not know that the 
exudation of water into the earth, through the bark or at the 
extremities of these latter organs, has been directly proved, 
but the other known modes of carrying off the surplus do not 
seem adequate to dispose of it at the almost leafless period 
when it is most abundantly received, and it is therefore diffi¬ 
cult to believe that the roots do not, to some extent, drain as 
well as flood the watercourses of their stem. Later in the season 
the roots absorb less, and the now developed leaves exhale a 
vastly increased quantity of moisture into the air. In any 
event, all the water derived by the growing tree from the 
atmosphere and the ground is returned again by transpiration 
total volume of 311 cubic metres per hectare [166 cubic yards per acre]. 
After this age the volume increases, but the mean rate of growth dimin¬ 
ishes. At eighty years, for instance, the volume is 335 cubic metres, the 
annual production 4 only. The beech reaches its maximum of annual 
growth at one hundred and twenty years. It then has a total volume of 
633 cubic metres to the hectare [335 cubic yards to the acre], and pro¬ 
duces 5 cubic metres per year.”— Clave:, Etudes , p. 151. 
These measures, I believe, include the entire ligneous product of the 
tree, exclusive of the roots, and express the actual solid contents. The 
specific gravity of maple wood is stated to be 75. Maple sap yields sugar 
at the rate of about one pound wet sugar to three gallons of sap, and wet 
sugar is to dry sugar in about the proportion of nineteen to sixteen. Be¬ 
sides the sugar, there is a small residuum of “ sand,” composed of phosphate 
of lime, with a little silex, and it is certain that by the ordinary hasty 
process of manufacture, a good deal of sugar is lost; for the drops, con¬ 
densed from the vapor of the boilers on the rafters of the rude sheds 
where the sap is boiled, have a decidedly sweet taste. 
* “ The elaborated sap, passing out of the leaves, is received into the 
inner bark, * * * and a part of what descends finds its way even to 
the ends of the roots, and is all along diffused laterally into the stem, 
where it meets and mingles with the ascending crude sap or raw material. 
So there is no separate circulation of the two kinds of sap ; and no crude 
sap exists separately in any part of the plant. Even in the root, where it 
enters, this mingles at once with some elaborated sap already there.”— 
Gray, How Plants Grow, § 273. 
