260 Oxygen and Carbon Dioxide 



terial as a modifiable property of the environment. Because of the 

 reciprocal relation between carbon dioxide and oxygen in the funda- 

 mental reactions of nature mentioned above, we generally find that 

 wherever the supply of one of these materials has been depleted the 

 concentration of the other has been increased. 



Carbon Dioxide in the Terrestrial Environment 



It is a striking fact that carbon dioxide— a material so essential in 

 the ecological relations of organisms— is present in the earth's atmos- 

 phere in an extremely small amount; it constitutes only 0.03 per cent 

 of the air medium. Carbon dioxide is thus only about 34 oo ^s 

 abundant in the air as oxygen. Because of the great mobility of the 

 atmosphere, however, carbon dioxide is well distributed, and its low 

 concentration is generally sufficient for at least a moderate amount 

 of photosynthesis the earth over. Possibly the reduction in carbon 

 dioxide at great altitudes would seriously curtail the photosynthetic 

 rate of plants, but low temperature or poor soil conditions are usually 

 far more important in limiting plant growth in the mountains. 



0.200 

 ^0.175 

 ^0.150 



Q. 



"0.125 

 ■o 



O) 



M 



"^0.100 

 <v 



X 



■^ 0.075 



c 

 o 



" 0.050 

 0.025 



200 



800 



1000 



400 600 



Light intensity, foot-candles 



Fig. 7.8. Relation between rate of photosynthesis of wheat plants and light in- 

 tensity at normal atmospheric, at decreased and at increased concentrations of car- 

 bon dioxide. (From Meyer and Anderson, 1939, after Hoover et al., Smithsonian 



Misc. Coll., 1933.) 



