THE SYNTHESIS OF CARBOHYDRATES 



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 J -00361 — -00551 



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



•0016 (the average for quite flaccid leaves). 



The figures of Sachs and of Thoday were obtained by the 

 direct determination of the increase in dry weight, 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. || 



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. 



These factors are both external and internal : the external 

 factors are amenable to experimental control whilst the 

 internal are much more evasive and thus are less understood. 

 Of the external factors, the supply of raw materials, the degree 

 of temperature and the intensity and quality of the illumina- 

 tion are the most conspicuous ; of the internal factors, the 

 chlorophyll apparatus and the products of carbon assimilation 

 are the least elusive. 



The doctrine of limiting factors, now well known, is due 



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

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

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

 § Thoday : id., 1910, 82, 421. 



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

 see Thoday : " Proc. Roy. Soc," B, 1909, 82, 1. 



