THE ROLE OF CARBON DIOXIDE 



343 



There is also a constant exchange of carbon dioxide Between the oceans 

 and the atmosphere. In fact, on theoretical grounds, there is a good reason 

 to suppose that the carbon dioxide concentration of the atmosphere is more or 

 less effectively maintained in dynamic equilibrium with that of the oceans. 

 Carbon dioxide probably escapes from the oceans whenever its atmospheric con- 

 centration falls below the usual value, and dissolves in the oceans whenever a 



Fig. 84. The carbon cycle. 



contrary shift in atmosperic carbon dioxide concentration occurs. The main- 

 tenance of such a large-scale dynamic equilibrium between the oceans and 

 the atmosphere is probably the principal factor accounting for the constancy 

 of the carbon dioxide concentration of the atmosphere. 



The cycle of carbon in nature is shown diagrammatically in Fig. 84. 



2. The Entrance of Carbon Dioxide. — Some of the earlier plant physiolo- 

 gists believed that carbon dioxide gas was absorbed by leaves directly through 



