METABOLISM 69 



miles. The surface of the state of Oregon is given 

 as 94,560 square miles, that is to say, 945,600 cubic 

 miles of air cover that densely wooded state. On 

 the basis of the calculation made in the last paragraph 

 this would allow but 338.1 trees of the sort mentioned 

 to a cubic mile, or 3381 to a square mile of surface, 

 or one tree to every 918 square yards, a number 

 which is probably exceeded many times in any one 

 generation. 



Of course, both plants and animals " breathe out " 

 quantities of CO 2 in respiration as described in the 

 previous section, yet such amounts must be far 

 inadequate to make good the loss to the atmosphere 

 through plant growth. 



Since we have seen that the carbon compounds 

 in the soil cannot be utilized by the plants to build 

 up carbohydrates, it is evident that the air must 

 be constantly supplied with quantities of CO2 from 

 some source. Before we follow this farther, let 

 us consider for a moment the nitrogen balance 

 sheet. 



The plant draws from the soil all the requisites 

 for its protein, and, hence, for the bulk of its proto- 

 plasm. The rapidity with which a crop, say 

 of wheat, can exhaust the available and necessary 

 mineral food in the soil has been frequently and 

 strikingly demonstrated in America, where the 

 originally rich virgin soils have been repeatedly 

 " robbed " and then abandoned for other un worked 

 fields. Only recently, when the supply of free 

 land is reaching its end, has pressure been brought 



