THE ENERGY RELATIONS IN PHOTOSYNTHESIS 329 



extinction has become a much debated subject, and. owing to the complex 

 nature of photochemical reactions, has become quite involved. There is 

 some evidence that Bunsen and Roscoe overestimated the magnitude of 

 light absorption which can be ascribed to chemical extinction, and Prings- 

 heim would ascribe this absorption to intermediate products in the photo- 

 chemical reaction. Some distinction must, however, probably be made 

 between the types of photochemical reactions, i.e., 1, those in which light 

 performs work against chemical forces and 2. those in which the action 

 of light is catalytic. In the former, light corresponding to the chemical 

 work is used, there is a diminution of the entropy and the light must vanish ; 

 photochemical extinction is in such a case at least highly probable. As 

 far as we know, photosynthesis is this type of reaction. To the second 

 type of reaction can be counted those in which light accelerates a reaction 

 proceeding slowly in the dark. There are as a matter of fact few endo- 

 thermal photochemical reactions known and none in which the increase 

 in potential approaches that of the photosynthetic reaction. It is obvious 

 that a discussion of photochemical extinction in relation to photosynthesis 

 raises many more questions than are capable of being answered, consider- 

 ing the meagre state of the experimental data available. The role of the 

 chlorophyll pigments in photosynthesis, the fluorescence of these, the influ- 

 ence of surface layers are problems which have hardly been approached 

 and the importance of which cannot be determined until a great deal more 

 experimental work has been carried through. 



Warburg and Negelein ^'^ approached the problem of the efficiency of 

 the photosynthetic activity in a different manner. They studied the prob- 

 lem by using an aquatic plant, the unicellular alga Chlorella vulgaris, and 

 thus avoided the difiiculties attendant on determining transpiration and 

 the indirect efifect of changes in the size of stomatal openings. 



If the radiant energy absorbed by the plant is designated by E and 

 the energy utilized by U, the quotient U/E can be regarded as an expression 

 of the efficiency. The ratio U/E changes with alterations in the intensity 

 of the absorbed radiation. The ratio is less with high intensity; with 

 decreasing intensity the value of the ratio increases and approaches a limit. 

 It is this limit or point of maximum efficiency which Warburg and Nege- 

 lein endeavored to determine by using low light intensities. The manner 

 of measuring the rate of photosynthesis was in principle the same as that 

 employed in their other investigations on photosynthesis, in which they 

 made use of the Barcroft mannometer, which has been described in the 

 chapter on Methods of Measuring Photosynthesis. Light of a limited 

 spectral region was used, 570-645 \i\i and the intensity was measured with 

 a bolometer. The vessel in which the photosynthesis was carried out was 

 fiUed with the alga so that all the light was absorbed and the absorbed 

 radiation was thus equal to the incident radiation. In order to obtain a 

 definite value for the ratio U/E, values for two dififerent incident radia- 

 tions were determined and then by extrapolation from these two values 



"Warburg and Negelein, Zeit. physik. Ghent., 102, 235 (1922) 



