104 MAN THE ANIMAL 



phere, carbon constitutes on the average only 0.08 per cent. 

 And of this, only about one two-hundred-and-fiftieth part is 

 in such form as to be directly usable by plants, animals, and 

 men for the purpose of their living. The great bulk of it is in 

 insoluble forms such as coal and oil, where it was laid down 

 by the living beings of past geological eras after they had 

 finished with it. For the carbon cycle is, in considerable part, a 

 degradative one from the standpoint of living things. The total 

 store of carbon that is directly usable by and absolutely neces- 

 sary to organisms for their growth and living tends constantly 

 to diminish by being transferred to insoluble forms. 



The position regarding carbon will be clarified by the follow- 

 ing quotation from Lipman: 



"Under the head of debits, we may note the assimilation 

 annually of more than 16 billion tons of carbon by vegetation. 

 There is the fixing of large amounts of carbon in the weathering 

 of rocks and in the formation of insoluble carbonates. An equiva- 

 lent of 5 billion tons of carbon is fixed annually through the life 

 processes of sea organisms. Altogether, it appears that the carbon 

 dioxide withdrawn from the atmosphere by plants is balanced 

 by that added to the air by microorganic and other processes in 

 the soil. By way of further clarification, we should note that the 

 carbon given off by the soil comes partly from the destruction of 

 organic matter and partly from root respiration. According to 

 the investigations of J. W. Shive, the root respiration of a corn 

 crop amounts to about 35 per cent of the quantity of carbon fixed 

 as organic matter by this same corn crop. If this ratio is extended 

 to the entire plant world, it would mean that about 7,000 of 

 the 19,000 million tons of the carbon addition diffusing from 

 soils can be credited to root respiration, leaving 12,000 million 

 tons due to the destruction of organic matter. If we add the 

 7,000 million tons of carbon transpired by roots to the 16,300 

 million tons of the carbon assimilated by plants, we obtain a 

 total of 23,300 million tons of carbon utilized annually by 

 vegetation. The release through soil respiration is but 19,000 

 million tons." 



