The Intake of Carbon 211 



concordant, but sufficiently so to indicate that the curve 

 representing photosynthetic activity rises rapidly as the 

 content of CO 2 is increased to an air content of from .1 

 to 1 per cent, and subsequently the rise, if continuous, 

 is slow to about 10 per cent, after which it may decline. 



The amount of CO 2 in the present atmosphere of the 

 earth is sufficient for all the needs of plants throughout 

 imaginable -time. It must be assumed, indeed, that this 

 amount will never be much more or much less than at 

 present, and that, practically speaking, the forces governing 

 supply and demand are ultimately somewhat regulatory ; 

 although there is geological evidence that atmospheric 

 CO 2 has not been constant. The result of all animal and 

 plant respiration (see Respiration, p. 280) is to return to 

 the air daily an enormous quantity, - - an amount esti- 

 mated for mankind alone to be not less than 50,000,000 

 tons. The great present consumption of fuel- - coal, wood, 

 oil, etc. - - returns to the air several billion tons every year. 

 A moment's consideration of the production of coal in 

 the United States alone during 1907 (400,000,000 tons 

 yielding about 2 times this amount of carbon dioxid) is 

 alone sufficient to indicate the immensity of the quantities 

 which are involved in these exchanges. This coal repre- 

 sents in part, of course, the photosynthetic activity of 

 plants of the carboniferous age. In addition to these 

 sources of CO 2 there is also the disintegration of rock 

 carbonates. 



With rapid circulation of air the CO 2 of the atmosphere 

 is evenly distributed throughout, and plants, tall and low, 

 are in situations equally favorable. When, however, the 

 atmosphere is quiet, there is, especially in rich ground, 



