ABSORPTION OF CARBON 



43 



of the work is performed not by the plant but by the radiant 

 energy. 



13. Dependence of Assimilation on the Amount of Carbon 

 Dioxide. The Carbon-dioxide Cycle in Nature. — The gradual 

 lag in increase of the assimilation process in proportion as the 

 intensity of light is augmented, depends not only on the fact that 

 the chloroplasts are not able to utilize the excess of light energy, 

 but also on the fact that the carbon dioxide absorbed by them per 

 unit time is not sufficient for the full utilization of this energy. 

 An increase of the carbon dioxide content in the atmosphere, there- 



0.03 0.06 



0.09 0.12 0.15 0.18 0.21 

 C0 2 Content of Air.per cent 



0.24 0.27 



Fig. 18. — Dependence of assimilation of pine needles on the carbon dioxide content 

 of the air (after Stal/elt, from Lundegardh) . 



fore, always leads to an increase of the energy of photosynthesis. 

 This may be seen from Fig. 18, showing the increase in the assimi- 

 lation process of pine needles, with the increase of the carbon- 

 dioxide contents from 0.03 per cent, the normal content of the air, 

 to 0.28 per cent. It must be noted that the curve expressing the 

 dependence of assimilation on the carbon-dioxide content is also 

 of a logarithmic type, as was the curve showing the dependence 

 of assimilation on light, and that this is the usual form of curves 

 expressing the relationship of different physiological processes 

 to the intensity of external factors. 



Numerous investigations which have been conducted by differ- 

 ent authorities seem to agree that the "normal" or "natural" 



