RATE OF CARBON ASSIMILATION 15 



the formation of chlorophyll is dependent upon the presence 

 of oxygen, the prolonged maintenance of a low oxygen 

 pressure will inhibit the development of more chlorophyll 

 and this will in turn react on carbon assimilation. 



Carbon assimilation is conditioned by various factors ; 

 wherefore the rate of the process will vary considerably ac- 

 cording to the inter-relationships of these factors. For this 

 reason it is hardly remarkable to find that different values 

 of the rate of carbon assimilation have been reported by 

 different investigators : differences in value due not only to 

 inappreciation of the conditioning factors, but also to dif- 

 ferent avenues of attack and to experimental error. 



The accompanying table gives a selection of values of 

 the rate of carbon assimilation in the open air of detached 

 leaves of the sunflower, Helianthus annuus, expressed in terms 

 of grams of increase in dry weight per square decimeter per 

 hour, obtained by the authorities named : 



Sachs* . . . '01882 

 Brown and Morris f . -00985 

 Brown and Escombe '00361 - '00551 



Thoday . . . -0169 (the average for fully turgid leaves) - 

 0016 (the average for quite flaccid leaves). 



The figures of Sachs and Thoday were obtained by the 

 direct determination of the increase in dry weight, the well- 

 known method of Sachs, whilst the others were calculated 

 from the amount of carbon dioxide absorbed and assuming 

 that carbohydrate only was ultimately formed. Since the 

 ultimate fate of the carbon dioxide is not entirely known, 

 the extent to which it is directly used in the elaboration of 

 fat or protein for example, the dry weight method would 

 appear to give the most accurate results.il 



THE FACTORS. 



It is obvious that little or no profit will accrue from the 

 contemplation of the above figures unless they be correlated 

 with the factors which determine and control the process. 



* Sachs: "Arbeit. Bot. Inst.," Wurzburg, 1884, 3, 19. 

 + Brown and Morris: " Journ. Chem. Soc.," Lond., 1893, 63, 604. 

 % Brown and Escombe: " Proc. Roy. Soc.," Lond., B. 1905, 76, 29. 

 Thoday : Id., 1910, 82, 421. 



II For a critical review of Sachs's and Brown and Escombe's methods, 

 see Thoday: " Proc. Roy. Soc.," Lond., B. 1909, 82, i. 



