68 



ANNUAL REPORT SMITHSONIAN INSTITUTION, 193 2 



being the limiting factor for this range. At a lower light intensity 

 of 400 foot-candles departure from linear proportionality to concen- 

 tration begins at a much lower value, of the order of 0.01 per cent, 

 maximum being reached at approximately 0.10 per cent, after which 

 no further change takes place in the range of experiment. 



Referring to Figure 2 we see that for a carbon-dioxide concentra- 

 tion of 0.140 per cent the assimilation is proportional to the light 

 intensity for the range from to 1,000 foot-candles. On the other 

 hand, for a concentration of 0.01 per cent a maximum is reached for 



Figure 2.- 



<}00 SCO SOO 700 eOO SOO 1000 



LtGMT IMTESSStTY 



ASSIiMlLATION CURVKS FOR WHEAT 



-Assimilation as a function of light intensity for carbon dioxide couccntra- 

 tion 0.010, 0.036, 0.140 per cent 



light intensit}^ of 400 foot-candles, after which further increase in 

 light intensity produces no change, carbon dioxide being the limit- 

 ing factor. It thus appears that carbon dioxide may be the limiting 

 factor for sufficiently high light intensities, assimilation varying 

 proportionally to the carbon-dioxide concentration over a consider- 

 able range. On the other hand, for sufficiently high carbon-dioxide 

 concentrations the light intensity may become the limiting factor, 

 assimilation being proportional to the light intensity. There exist, 

 however, well-defined regions over which the assimilation is de- 

 pendent upon both factors. In examining the significance of this 

 transition range, however, it must be borne in mind that ideal condi- 

 tions can not be secured. Not all the chloroplasts can be main- 

 tained in the same radiation density, nor can exactly the same con- 



