[Chap. XIV 



THE RATE OF PHOTOSYNTHESIS 



127 



tive humidity is 70 per cent. The lowest curve indicates that with only 

 0.01 of 1 per cent of CO2 in the air, an increase in the light intensity 

 beyond 40 per cent does not increase the rate of photosynthesis. 



The percentage of CO2 in the atmosphere near the earth's surface is 

 usually given as about 0.03 ( 3 parts in 10,000 parts of air ) , though it may 

 vary considerably just above the surface of the soil, particularly if there 

 is much decaying organic matter in the soil. Several measurements made 

 in a field of sugar beets showed that the percentage of COl> in the air at 

 the tops of the plants varied from 0.04 to 0.06, and at the ground level 

 from 0.05 to 0.28. 



The curves in Fig. 57 show that the rate of photosynthesis at a given 

 light intensity depends upon the available COi-, and vice versa. At low 

 light intensities the maximum rate of photosvnthesis is attained when the 

 available COl' is low, and at low concentrations of CO2 it is attained when 

 the available light is low. 



ILI 



f 50 



z 

 to 



2 40 



o 



5 35 



^30 I- 



(f) 25 



liJ 



^ 20 



a. 



15 

 UJ 



> 10 

 b 



Hi 



CL 



ith 1.227oC02 



10° 15° 20° 25° 30° 35° 40' 



TEMPERATURE IN DE6R.EES CENTIGRADE 



Fig. 58. Rates of photosynthesis in potato leaves in relation to temperature and the 

 concentration of carbon dioxide. Calculated from data by H. G. Lundegardh. 



The curves in Fig. 58 indicate the relative rates of photosynthesis in 

 leaves of a potato plant in relation to both temperature and the concen- 

 tration of CO2 in the air. The leaves were in full sunlight during each 

 experiment, which lasted only ten minutes. Longer exposures would 

 result in some changes in the curves, but without annulling the prin- 



