1046 BIOLOGICAL EFFECTS OF RADIATION 



One of the most successful of the earher attempts to use this method of 

 determining efficiencies was made by Brown and Escombe (9). These 

 investigators gave a very clear analysis of the disposition of the energy 

 incident on the leaf. While their analysis lacks somewhat in complete- 

 ness and accuracy, it still stands as one of the most comprehensive inves- 

 tigations in this field. The partition of energy which these workers 

 found is given in the following table: 



Per Cent 



Energy used in photosynthesis . 66 



Energy used in transpiration 48 39 



Energy transmitted by leaf 31 . 40 



Energy lost by thermal radiation 1 9 . 55 



The researches both of Puriewitsch and of Brown and Escombe were 

 carried out at high light intensities and the very low efficiencies which 

 they obtained may be partially explained on this ground. 



More recent attempts have been made by Briggs and by Burns to 

 obtain estimates of the apparent energy efiiciencies in different regions 

 of the spectrum. Briggs (8) has measured the oxygen evolution of 

 different leaves in yellow-red, green, and blue light. These results are 

 shown in Table 3. 



Burns (12) has attempted to obtain the quantum yields of the photo- 

 synthetic process at various wave-lengths for Pinus Strohus, Picea excelsa, 

 and Picea Engelmannii. 



THE REAL EFFICIENCY OF THE PHOTOSYNTHETIC REACTION 



The theoretical aspects of the energetics of photosynthesis have been 

 investigated by Warburg and his associates (135) and by Wurmser (148). 

 Their researches have dealt not with the efficiency of the conversion of the 

 light incident on the leaf under natural conditions, but with the efficiency 

 of the conversion of the light absorbed by the photosynthetic organism 

 under as nearly optimal conditions as possible — the real efficiency of the 

 photosynthetic reaction. 



According to the photochemical equivalence law the primary process 

 in any photochemical reaction is the absorption of one quantum of energy, 

 Nhv (Table 5)\ 



In order for a quantum of light to cause a reaction to take place, it 

 must be large enough to supply the activation energy of the reaction. 

 In the case of an endothermic reaction the quantum of energy must be 



» A^ is Avogadro's number, 6.061 X 10"; h is Planck's constant, 1.566 X lO"'^ cal. 

 sec; and v is the frequency of the light used, i.e., 2.9986 X 10'" cm. sec."' divided by 

 the wave-length in Angstrom units X 10~* cm. Therefore Nhv = 2.846 X 10*/X, 

 where X is expressed in Angstrom units. A useful nomograph for calculations of 

 quantum energies has been published by Mecke and Childs (77). 



