A GREEN LEAF 



77 



coming polluted by having poured into it enormous 

 volumes of carbon dioxide, or more familiarly 



Fig. 50. Section through the base of a leaf-stalk prior 

 to its fall, showing how the bark cells separate 

 the tissues 



carbonic acid gas. Large manufacturing works 

 may turn thirty tons or more of this impurity into 

 the atmosphere in the course of a single day, while 



