THE DIFFERENTIATION OF THE PLANT-BODY 35 
ordinary atmospheric conditions this is being continually 
exhaled as long as an excess of water is passing through 
the plant. The regulation of the process of exhalation 
depends mainly upon the condition of the guard-cells of 
the stomata, which can permit it to go on freely or can 
check it by partially or entirely closing the apertures 
according to various internal and external conditions (fig. 
37). 
