THE GREEN LEAF 



61 



be convenient in the first place to cast a brief 

 glance at the functions of the leaf. 



The leaf absorbs from the air chiefly oxygen 

 ami carbon dioxide. The latter gas is present 

 in very small quantities only, say about 2-5 



Fig. 9. Section of a Leaf, showing the internal structure 

 and also the surface of the lower epidermis (E) ; C, cuticle ; 

 E, E epidermis ; P, palisade cells containing much chloro- 

 phyll ; S.M., the more spongy lower tissue of the leaf with 

 abundant air spaces; S, stomata; V, B, " vein " or vas- 

 cular bundle, consisting of wood (W), and bast (B). 



to 3 parts in 10,000. Yet this carbon dioxide 

 represents the sole source of the carbon which 

 forms so large a part of the dry weight of the 

 tree. Free oxygen is required, as it is by 

 almost all living beings, for purposes of 

 respiration ; that is to say for the purpose of 

 oxidising certain chemical substances within 



