CO2 in the Terrestrial Environment 261 



The normal amount of carbon dioxide available in the atmosphere, 

 although allowing adequate plant growth under usual circumstances 

 at ordinary altitudes, is insufficient for photosynthesis to attain rates 

 that would be possible under favorable natural conditions if the con- 

 centration of carbon dioxide were higher. During conditions of low 

 illumination the light factor ordinarily controls the rate of photo- 

 synthesis, but under high illumination the small amount of carbon 

 dioxide in the air acts as the limiting factor. This situation is illus- 

 trated in Fig. 7.8 in which rate of photosynthesis of wheat plants in 

 relation to illumination is plotted under conditions of normal, in- 

 creased, and decreased tensions of carbon dioxide. In this experiment 

 under a normal carbon dioxide tension of 0.037 per cent the rate of 

 photosynthesis (as measured by utilization of carbon dioxide) in- 

 creases with increasing illumination up to 1000 ft-c. At higher il- 

 luminations, beyond the limits of the graph, little further increase in 

 photosynthetic rate took place, as the availability of carbon dioxide 

 became limiting. With carbon dioxide at 0.010 per cent the limiting 

 action of carbon dioxide availability became complete at 400 ft-c, 

 whereas an increase of the carbon dioxide concentration to 0.111 per 

 cent resulted in an augmentation of photosynthesis at all illuminations, 

 with the curve still steeply rising at 1000 ft-c. Practical advantage of 

 this situation can sometimes be taken by artifically increasing the car- 

 bon dioxide in sealed greenhouses. Improved growth of certain 

 plants is found for tensions of carbon dioxide of three to twenty times 

 the normal amount, but higher concentrations are harmful. 



In nature some instances of local significant increases in atmospheric 

 carbon dioxide have been reported. In certain volcanic areas carbon 

 dioxide escapes from fissures in the rocks and, being heavier than 

 air, forms a layer near the ground. From these "death valleys," such 

 as the one in Java mentioned earlier, all animal life is excluded. The 

 carbon dioxide in soil air generally is increased as a result of decom- 

 position and respiration beneath the ground surface under conditions 

 of poor aeration. The air under dense vegetation may exhibit values 

 considerably above normal, and the air just above the soil of certain 

 cultivated fields has been found to show a tenfold increase in carbon 

 dioxide content, but no evidence is available as to any ecological ef- 

 fects that may have been produced. In an experiment conducted in 

 Georgia, however, the carbon dioxide in the atmosphere was found 

 to be 25 per cent higher on certain foggy days, and during these pe- 

 riods the photosynthesis of plants under observation was increased 

 to as much as seven times the normal value (Wilson, 1948). 



Our atmosphere receives carbon dioxide from geological and in- 



