330 PHYSIOLOGY 
low barometer may coincide with low humidity and therefore intense 
light, the excessive evaporation often becomes a powerful factor in 
dwarfing plants and in controlling their distribution. 
Temperature. The temperature of the plant itself tends normally 
to equal that of the air, since its extended surface permits quick gain 
or loss of heat toward equilibrium. A rise of temperature in the air, 
therefore, is quickly followed by a rise of temperature in the plant, and 
(even with no change in the relative humidity of the air) by increased 
evaporation. But the temperature of the plant depends also upon the 
energy absorbed by the green pigment in diffuse light or direct sunlight. 
In diffuse light the greater part of this energy is used in food making, 
and only a small portion exerts a heating effect. But in sunlight two 
thirds to three fourths of that absorbed is free to heat the tissues, and as 
soon as that begins, evaporation is thereby much accelerated. This 
tends to dissipate the heat. 
It has been proposed to call the evaporation due to the excess of energy absorbed 
by the chlorophyll, chlorovaporization. The term has its only value in promoting 
recognition of the fact; but chlorovaporization cannot be distinguished practically 
from the rest. 
Were it not for this transfer of energy to the water vapor, the tempera- 
ture of the tissues would rise to the danger point, or at least to a degree 
which retards food making. When transpiration is greatly reduced by 
enclosing a shoot in a glass chamber whose air quickly becomes nearly 
saturated while the light is absorbed, death quickly ensues. The 
" scalding " of leaves by sunshine after a summer shower is an example 
of the same effect. If a plant derives no other advantage from tran- 
spiration, this prevention of injury by overheating in direct sunlight 
is certainly one. For even temporary interference with food making 
might be serious, and permanent stoppage of it by the killing of any 
considerable area of leaves might be fatal to the whole plant. How- 
ever possible it might be for plants to meet this difficulty by other 
methods, if transpiration could be eliminated for other reasons, under 
the present organization transpiration is of real advantage in this 
particular. 
Amount transpired. Because of the extreme variation, from zero 
to the maximum, a quantitative statement of the amount of evaporation 
is of little value, though a voluminous literature records an enormous 
number of observations and calculations. The following will serve as 
illustrative examples. 
