THE VEGETATIVE ORGANS OF PLANTS. 287 
number upon, the upper surfaces of land plants, which are 
exposed to the heat of the sun, while they exist in great 
numbers on the lower sides of all green leaves. In number 
and size, they vary remarkably. Some leaves possess but 
800 to the square inch, while others have as many as 
170,000 to that amount of surface. About 100,000 may 
be counted on an average-sized apple-leaf. In general, 
they are largest and most numerous on plants which be- 
long in damp and shaded situations, and then exist on 
both sides of the leaf. 
The epidermis itself is most dense—consists of thick: 
walled cells and several layers of them—in case of leaves 
which belong to the vegetation of sandy soils in hot cli- 
mates. Often it is impregnated with wax on its upper 
surface, and is thereby made almost impenetrable to moist- 
ure. On the other hand, in rapidly growing plants adapt- 
ed to moist situations, the epidermis is thin and delicate. 
Exhalation of Water-Vapor.—A considerable loss of 
water goes on from the leaves of growing plants when 
they are freely exposed to the atmosphere. The water 
thus lost exhales in the form of invisible vapor. The 
quantity of water exhaled from any plant may be easily 
ascertained, provided it is growing in a pot of glazed 
earthen, or other impervious material. A metal or glass 
cover is cemented air-tight to the rim of the vessel, and 
around the stem of the plant. The cover has an opening 
with a cork, through which weighed quantities of water 
are added from time to time, as required. The amount 
of exhalation during any given interval of time is learned 
with a close approach to accuracy by simply noting the 
loss of weight which the plant and pot together suffer. 
Hales, who first experimented in this manner, found that 
a sunflower, whose foliage had an aggregate surface of 
39 square feet, gave off 3 lbs. of water in a space of 24 
Lours. Knop observed a maize-plant to exhale, between 
