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 
and carbon dioxide. The latter gas is present 
in very small quantities only, say about 2°5 
ee SY a —o 
oy Wi ule rbd 
SO oe 
oy alt 
| i .M. 
E 
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; 8.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 
