796 The Beneficial Influence of Plants. [ December, 
vapor in twelve hours. Even supposing this to be much over- 
estimated, it may very fairly be concluded from the facts given 
that the evaporation of watery vapor from plants isa powerful 
agent in maintaining the humidity of the surrounding air. 
Some scientific critic might claim, as an objection to the latter 
assertion, that the amount of vapor exhaled from plants, though 
in itself large, is inconsiderable when compared with the evapo- 
ration from the surface of the earth. This seeming objection 
may, perhaps, best be confronted by a few calculations, and we 
first inquire what is the relative vaporization from a given 
area of leaf surface, and an equal area of earth surface? 
Taking the average rate of transpiration for soft, thin-leaved 
plants in clear weather to be one and a quarter ounces per 
_ square foot, and multiplying this by the number of days in a 
year the product would be twenty-seven pounds, or an equal 
number of pints. Now by reducing twenty-seven pints to cubic 
inches and dividing the result by the number of square inches in 
a square foot (144) we must obtain the depth in inches of the 
water transpired in the course of a year, about five and two-fifths 
inches, To continue our reckoning, we will next attempt to 
show the mathematical relation between the extent of the leaf 
surface of a section of country and the surface of the earth of the 
same section. According to the census returns for 1875, the per- 
centage of woodland of the entire area of the United States, 
including Territories, water surfaces, cities, highways, etc., is 25; 
that is to say one-fourth of the total area is woodland. Now if 
we suppose every 900 square feet of forest land to contain one 
tree, and estimate each tree to be one-thirteenth the size (only) of 
the Washington elm, at Cambridge, we shall have about one- 
third of an acre of leaf surface for every tree; then it will not 
require much mathematical skill to understand that by multiply- 
ing the supposed leaf surface of each tree (one-third acre) by the 
number of trees per acre, and this in turn by the fraction one- 
fourth, or the proportion of woodland in the United States, the 
product will be four to one in favor of leaf surface over the tota 
area of land surface. We have seen that according to our calcu- 
lations the depth of transpiration per year for soft, thin- leaved 
plants is about five and two-fifths inches, but this must be an 
over-estimate, for the plants are actively giving off watery vapo! 
only about five months of the year, that is, out of doors. We _ 
shall, ‘therefore, suppose eee twel of five and two- fifths inches 
