96 VEGETABLE PHYSIOLOGY 
some significance when it is remembered that the guard- 
cells of the stomata contain this pigment. The nature of 
the action of chlorophyll in this direction is not, however, 
fully understood. 
Apart from direct radiation, the temperature of the 
air, and its hygrometric condition, are important factors 
in causing an increase or a diminution of the watery 
vapour exhaled. They act principally by exerting an 
influence directly upon the evaporation from the cells, 
but several indirect effects can also be noticed. The 
general movements of water in the plant, as well as its 
absorption, are influenced particularly by variations of 
temperature, and the latter has also an effect upon the 
width of the stomatal orifices. A rise of the external 
temperature causes the saturated air in the intercellular 
passages to expand, as the air acquires the new temperature 
more rapidly than do the tissues of the plant. The escape 
of vapour is consequently accelerated as the temperature 
rises, even though the rate of evaporation from the cells 
into the intercellular spaces is not at first affected. 
The influence of the hygrometric condition of the air, 
apart from changes of temperature, can be seen when a 
plant which has been exposed to a dry atmosphere till its 
leaves have become flaccid is transferred to one saturated 
with moisture. After a short time the drooping leaves 
again become turgid. This is not due to an absorption of 
water in the form of vapour by the leaves, but to a 
diminished loss by the checking of transpiration. The 
return of turgidity is caused by the accumulation of the 
store drawn from the earth by the roots. This can be 
shown by comparing the behaviour of two plants treated in 
the way described, one of which is allowed to remain 
rooted in soil, while the other is taken up from the earth 
and exposed in that condition to the saturated air. There 
is in the latter case no recovery of turgescence. 
The temperature of the soil in which the roots of a 
plant are embedded has also an influence upon the exhala- 
