1878. ] On the Transpiration of Plants. 171 
periments outside on a cloudy day, strong atmospheric currents 
did not by any means raise the daily quantity to what it was on 
a calm but clear day. But it must be remembered that currents 
are much more effectual in hastening the process in clear than in 
cloudy weather, for the simple reason that the sun’s rays opening 
the pores of the plant, allow of the more ready escape of senn 
vapor. 
Of the influence of currents, then, it might be stated,from what 
has been observed, that in clear weather they are very effectual 
in favoring the process; in cloudy weather their influence is not 
so noticeable. On clear days strong currents increased the 
amount over that of calm days by about one-fifth or even one- : 
fourth. : 
It was found, in every instance tried, that, other things being 
considered, the complement of the dew point, or the dryness of 
the air, modified ina marked degree the rate of transpiration; 
and this appeared to be, in a measure at least, independent of the 
temperature, as the latter condition did not seem to affect per- 
ceptibly the amount evaporated, unless, as is usually the case, the 
relative humidity was correspondingly low. 
A few calculations may serve to impress the importance of 
the ratio of transpiration, deduced from these experiments. Ac- 
cording to the above rate the Washington Elm, at Cambridge, 
a tree, it is stated, of no very large size, with its 200,000 square 
feet of leaf surface, would transpire 734 tons of watery vapor in ` 
twelve hours (day) of clear weather. 
Carrying the calculation further, a grove consisting of five | 
hundred trees, each with a leaf surface equal to that of the elm 
Mentioned, would return to the atmosphere 3906 tons of aqueous 
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 is a powerful 
agent in maintaining the humidity of the surrounding air. And 
if the above data be correct, a strong argument is furnished in 
Support of the belief that vegetation influences, in a great degree, 
the rainfall of a region of country. 
The practical advantage of keeping plants in occupied rooms, 
in which the air is generally dryer than outside, has, also, from 
the results obtained, received further demonstration. 
; PERETE Doo 13. 
