322 The Physiology of Plants Book in 



and its subsequent fate were made by Brown and Escombe 

 in 1898 and 1899 in connexion with their work which has 

 been already alluded to. They measured simultaneously 

 the total energy falling on a leaf by means of a Callendar's 

 recording radiometer, the amount of carbon dioxide 

 absorbed during the time of experiment, and the amount of 

 water transpired From the data thus obtained they deter- 

 mined that the leaf absorbed and converted into internal 

 work about 28 per cent, of the total radiant energy incident 

 on it when exposed to the strong sunlight of a brilliant 

 day in August ; of this 28 per cent., 27-5 per cent, was 

 used up in the vaporization of the water of transpiration, 

 and only -5 per cent, was employed in the processes of 

 photosynthesis. In strong diffuse light, instead of bright 

 sunshine, the leaf absorbed about 95 per cent, of the 

 incident energy, and employed in the processes of photo- 

 synthesis 27 per cent. 



These results to some extent explain the great dis- 

 crepancies noticeable in the figures of the earlier observers, 

 showing that the percentage used varies according to the 

 absolute intensities of the light received in different cases. 

 The total amount of light absorbed by the leaf may be 

 fairly constant, but yet bear a very different proportion 

 to the total incident amount, as appears in the figures 

 given under the two conditions by Brown and Escombe. 



Earlier quantitative experiments on the absorption of 

 light were made by Timiriazeff in 1870. Using an instru- 

 ment known as a phyto-actinometer, he found that when 

 bright sunlight fell upon a single leaf of the maple, the 

 latter absorbed 27 per cent, of the incident amount ; when 

 three leaves superposed were exposed to it, 31 per cent, was 

 absorbed. With the lime instead of the maple the figures 

 were 29 and 35 respectively. A single thickness of maple 

 leaf absorbed 13-8 per cent, of yellow and 10 per cent, of 

 red light when incident upon it. 



