798 EXHALATION OF WATERY VAPOUR BY LEAVES. 
has been the subject of various experiments, The most complete 
observations upon this point were made by Hales so long ago as 
1724. He found that a common Sunflower 33 feet high, weighing 
3 pounds, and with a surface estimated at 5,616 square inches, 
exhaled, on an average, about twenty ounces of Auid in the 
course of the day ; a Cabbage plant, with a surface of 2,736 
square inches, about nineteen ounces per day; a Vine with a 
surface of 1,820 square inches, from five to six ounces ; and a 
Lemon tree, exposing a surface of 2,557 square inches, six ounces 
on an average ina day. Hence if such a large amount of fluid 
be thus given off by single plants, what an almost incalculable 
quantity must be exhaled by the whole vegetation of the globe! 
It can therefore be readily understood that the air of a thickly 
wooded district will be always in a damp condition, while that 
of one with scanty vegetation will be comparatively free from 
humidity : and hence it will be seen that a country, to be per- 
fectly healthy, should have the proportion of plants to a par- 
ticular area carefully considered ; for while, on the one hand, 
too many plants are generally prejudicial to health by the 
dampness they produce ; on the other, a deficiency, or want of 
them will produce an equally injurious dryness. The same 
circumstances have an important bearing upon the fertility or 
otherwise of the soil, and in this way have an indirect influence 
upon the health of the inhabitants. Thus, it is a well-known 
fact, that as vapour is constantly given off by plants, rain is 
more abundant in those regions which are well covered with 
forests, than in those which are comparatively free from them. 
It is found, accordingly, that a great change may be produced 
in the climate of a country by clearing it too much of plants ; 
for while an excessive amount of vegetation is injurious to their 
healthy growth, if there be a great deficiency, it will become 
entirely barren from extreme dryness. By inattention to these 
simple but most important facts, which clearly indicate that 
open land and that furnished with plants should be properly 
proportioned the one to the other, many regions of the globe 
which were formerly remarkable for their fertility are now 
barren wastes ; and, in like manner, many districts, formerly 
noted for their salubrity, have become almost, or quite, unin- 
habitable. 
The fluid which thus passes off by the leaves of plants is 
almost pure water. This transpiration of watery vapour must 
not be confounded with the excretion of water containing 
various saline and organic matters dissolved in it, which takes 
place in certain plants, either from the general surface of their 
leaves or from special glands. In the peculiarly formed leaves 
of Dischidia, Nepenthes (fig. 390), Sarracenia (fig. 391), and 
Heliamphora (fig. 392), watery excretions of this nature always 
exist. From the extremities or margins of the leaves of various 
Marantaceze, Musaceze, Aroidaceze, Graminacez, and other 
