PHYSIOLOGICAL STUDIES 65 



oxygen, were in error as shown by Emerson and Lewis due 

 to the incorrect assumption that the assimilatory quotient 

 was unity during such short periods as 10 minutes. More 

 recent experiments by Warburg, which have avoided many 

 of the sources of criticism levelled at his earlier work, con- 

 firm his previous finding of 4 quanta/molecule oxygen. 

 Later Burk and Warburg (1951) claim to have observed 

 a partial process in photosynthesis requiring only one 

 quantum /molecule of oxygen. This was observed when cells 

 were subjected to alternate periods of light and darkness of 

 1 1 minutes duration. The reliability of these measurements 

 is still uncertain and their interpretation will be discussed 

 in the last chapter. It appears most likely that in a steady 

 state 8 to 10 quanta of red light are absorbed per molecule 

 of oxygen produced. If each molecule of oxygen corre- 

 sponds to the reduction of a molecule of carbon dioxide to 

 carbohydrate the thermodynamic efficiency must therefore 

 be of the order of 30%. Higher values are not excluded but 

 have been found in the past by only a few investigators. 



For plants growing in the field the efficiency is much 

 smaller, being of the order of some 3% of the absorbed 

 light or about 2% of the incident light energy. Brown and 

 Escombe (1905) estimated that for Helianthus anniius i-o% 

 of the total absorbed solar radiation (about half of which is 

 in the visible) was used for photosynthesis and about 70% 

 or more is concerned in the vaporization of water. 



Photosynthetic efficiency in light of different wavelengths. 

 The effect of the colour of light upon photosynthesis was 

 investigated during the second half of the nineteenth century 

 and the results of Englemann (1882 onwards), whilst hotly 

 disputed by his contemporaries, are now being vindicated 

 by present-day researches. Englemann was the first to show 

 the general similarity between the amount x)f photosynthetic 

 activity in a spectral region and the absorption of light in 

 that region by the plant pigments. The action spectrum 

 relates the amount of photosynthetic activity for an equal 

 number of incident quanta to the wavelength of the incident 

 light. Measurements should be made with intensities of 

 illumination so low that there is an increase in the rate 



