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TEXTBOOK OF PLANT PHYSIOLOGY 



the dependence of decomposition of carbon dioxide on each of 

 these two factors separately may be expressed by a logarithmic 

 curve. Suppose, now, that the intensity of the light increases, 

 while the content of carbon dioxide remains the same, a condition 

 that may be observed, for instance, during the day in a green 

 house. From the moment when the intensity of light attains a 

 level at which the carbon dioxide absorbed by the chloroplasts 

 during a certain period is completely used up, further increase of 

 assimilation becomes impossible, and the curve showing the 



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Light Intensity in Mc. 



Fig. 21. — Interrelation of the intensity of light and the concentration of carbon 

 dioxide in their influence on assimilation in the aquatic moss, Fontinalis (after 



Harder, from, Lundegardh) . 



dependence of CO2 decomposition on the intensity of light pro- 

 ceeds in a horizontal direction. If the CO2 content of the atmos- 

 phere is increased, then the bending of the curve takes place at a 

 higher temperature than before, and only with a certain excess of 

 carbon dioxide in the air, the actual dependence of photosyn- 

 thesis on the intensity of light energy may be clearly ascertained. 

 Thus it is seen that a very low concentration of carbon dioxide 

 limits the influence of gradually increasing light. On the other 

 hand, there are conditions where too feeble light is the factor 

 limiting the influence of the other external conditions. Hence, 



