Quantitative Measurement of Radiant Energy. 1 4 r 



of sunlight measured. The leaf was intei'poseit above the coils of 

 the instrument for a few minutes and the intensity of radiation 

 again measured. The leaf was then withdrawn when the value of 

 the full intensity of radiation was ajsain recorded on the drum of 

 the self recorder. The ratio of the middle reading to the mean of 

 the first and third readings gives the coefficient of ti-ansmission, 

 and the difference between unity and the coefficient of transmission 

 is the coefficient of absorption. The following table shows the 

 coefficients of absorption and transmission found by Brown and 

 Escombe for various species. 



Table XLII. 



Coefficients of Absorption and Transmission of Radiant Energy 

 of Sunlight. 



No considerable difference in the coefficient of absorption was 

 found between leaves of the same species of different ages. 



Of course, in these determinations the part of the energy 

 reflected from the surface of the leaf is neglected. Brown and 

 Escombe regard the reflected energy as forming a very small 

 fraction of the total incident energy, but having regard to the 

 information available from pure physics it is unlikely to be negligible, 

 as a black cloth, for instance, may reflect 1% of the radiant energy 

 incident upon it. 



The difference between the total incident energy absorbed on 

 the one hand, and that used in assimilation and transpiration on the 

 other hand, gives that part of the energy lost by re-radiation, 

 conduction and convection, i.e., that lost by emission. 



The following numbers show the results of a typical experiment. 



