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- 



