Carbon Dioxide and Oxygen 45 



Plants require C0 2 and animals require oxygen in order 

 to live and both live through the continual exchange of 

 these staple commodities. This is the best known 

 phase in the cycle of food materials. The oxygen is 

 freed at the beginning of the synthesis of organic mat- 

 ter, only to be recombined with the carbon at the end 

 of its dissolution. And the well-being of the teeming 

 population of inland waters is more dependent on the 

 free circulation and ready exchange of the dissolved 

 supply of these two gases than on the getting of a new 

 supply from the air. 



The stock of these gases held by the atmosphere is 

 inexhaustible, but that contained in the water often 

 runs low; for diffusion from the air is slow, w T hile 

 consumption is sometimes very rapid. We often have 

 visible evidence of this. In the globe in our win- 

 dow holding a water plant, we can see when the sun 

 shines streams of minute bubbles of oxygen, arising 

 from the green leaves. Or, in a pond we can see 

 great masses of algae floated to the surface on a foam 

 of oxygen bubbles. We cannot see the disappearance 

 of the carbon dioxide but if we test the water we find 

 its acidity diminishing as the carbon dioxide is con- 

 sumed. 



At times when there is abundant growth of algae near 

 the surface of a lake there occurs a most instructive 

 diurnal ebb and flow in the production of these two 

 gases. By day the well lighted layers of the water 

 become depleted of their supply of C0 2 through the 

 photosynthetic activities of the algae, and become 

 supersaturated with the liberated oxygen. By night 

 the microscopic crustaceans and other plancton animals 

 rise from the lower darker strata to disport themselves 

 nearer the surface. These consume the oxygen and 

 restore to the water an abundance of carbon dioxide. 

 And thus when conditions are right and the numbers of 



