288 HOW CROPS GROW. 
May 22d and September 4th, no less than 36 times its 
weight of water. 
Exhalation is not a regular or uniform process, but varies 
with a number of circumstances and conditions. It de- 
pends largely upon the dryness and temperature of the 
air. When the air is in the state most favorable to 
evaporation, the loss from the plant is rapid and large. 
When the air is saturated with moisture, as during dewy 
nights or rainy weather, then exhalation is nearly or 
totally checked. 
The temperature of the soil, and even its chemical com- 
position, the condition of the leaf as to its age, texture, 
and number of stomata, likewise affect the rate of exe 
halation. 
Exhalation is a process not necessary to the life of the 
plant, since it may be suppressed or be reduced to a 
minimum, as ina Wardian case or fernery, without evident 
influence on growth. Neither is it detrimental, unless the 
loss is greater than the supply. If water escapes from the 
leaves faster than it enters the roots, the plant wilts; and 
if this disturbance goes on too far, it dies. 
Exhalation ordinarily proceeds to a large extent from 
the surface of the epidermal cells. Although the cavities 
of these cells are chiefly occupied with air, their thickened 
walls transmit outward the water which is supplied to 
the interior of the leaf through the cambial ducts. Other- 
wise the escape of vapor occurs through the stomata. These 
pores appear to have the function of regulating the exhala 
tion, to a great extent, by their property of closing, when 
the air, from its dryness, favors rapid evaporation. They 
are, in fact, self-acting valves which protect the plant from 
teo sudden and rapid loss of water, 
Access of Air to the Interior of the Plant.—Not only 
does the leaf allow the escape of vapor of water, but it 
admits of the entrance and exit of gaseous bodies. 
