DEPARTMENT OF BOTANICAL RESEARCH. 51 



relatively small and, from a thermodynamic consideration, the greatest lift 

 in energy must be in the photochemical reaction. 



The chemical reactions involved in these two processes are, in the case of 

 respiration, a series of oxidations, and in photosynthesis a general reversal 

 thereof in the reduction of C0 2 to C. There is much evidence to support the 

 conclusion that reduction and oxidation reactions in living organisms are 

 intimately connected and dependent upon the same or very closely allied 

 agents. It is therefore as yet impossible to state whether the acceptor forma- 

 tion in photosynthesis is directly the result of respiratory activity or whether 

 these two actions are brought about by the same agents or conditions, and that 

 what affects one activity also affects the other. On the basis of these experi- 

 ments and theoretical deductions the investigations on photosynthesis are 

 being extended in several directions. 



Temperature Coefficients and Efficiency of Photosynthesis, by H. A. Spoehr 



and J. M. McGee. 



One of the methods which is now being utilized to differentiate between 

 the two reactions in photosynthesis and to establish their properties is the 

 determination of the temperature coefficients. It has been found that much 

 of the confusion which has arisen in the interpretation of former work on this 

 subject has been due to inadequately controlled experimental conditions and 

 to a misinterpretation of van 't Hoff's law. Moreover, as the photosynthetic 

 activity is so decidedly influenced by the internal factor, consistent results 

 can not be expected without taking this factor into consideration. This can 

 be done only on the basis of adequate and accurate knowledge of the respira- 

 tory activity gained from separate experiments and analyses. Conditions 

 have been worked out where either one of the two reactions of photosynthesis 

 determines the rate of the entire process. By thus carefully selecting pre- 

 cisely controlled conditions, it is hoped that the leaf will reveal some of the 

 important characteristics of these two reactions. In view of the complexity 

 of the reactions and the many factors involved, conclusions can be drawn only 

 from a very extensive series of experiments. Special apparatus has been con- 

 structed in the constant-temperature chambers at Carmel which permits 

 very accurate control of all the factors involved in these studies. The ulti- 

 mate object of these investigations is to gather experimental data which 

 will permit the application of the recent developments of photochemistry and 

 energetics to the problem of photosynthesis. 



A determination of the fundamental question of the efficiency of the process 

 also depends upon these results. By means of Nernst's third law the free 

 energy of the photosynthesis process has been calculated by Warburg: 

 6 C0 2 +6 H 2 = C 6 Hi 2 6 +60 2 - 690,000 cal. 

 (0.0003 atmos.) (liquid) (solid) (0.2 atmos.) 

 that is, to reduce 1 mol of C0 2 at least 150,000 calories are required. Photo- 

 synthetic efficiency must be taken as the ratio of chemical work gained to 

 absorbed radiant energy. Various attempts have been made to determine 

 this quantity. However, no consistent results have been obtained, there 

 being large and apparently irregular variations. This has been substantiated 

 by the careful experiments of Mueller and Warburg, who found wide varia- 

 tions in the photosynthetic efficiency for light of a given wave-length under 



