THE DIFFEBENTIATION OF THE PLANT-BODY 35 



if not quite, to saturation by aqueous vapour, and under 

 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. 

 87). 



D 2 



