46 NATURE'S FOOD-SUPPLY. THE CARBON CYCLE. 
of decomposition, wood is converted into simple chemical com- 
pounds which join nature's food-supply in the air. 
By the means thus indicated the large part of the carbon ex- 
tracted from the air by plants is restored again to the air in the form 
in which it first existed. This carbon cycle is represented graphi- 
cally in the accompanying diagram (Fig. 13). 
