Quantitative Measitrement of Radiant Energy. 1 45 



We may quote the results of atypical experiment of Puriewitsch. 



Two leaves of Acer platanoides were used. The intensity of 



radiation incident on the leaf was measured every 10 minutes 



during the experiment which lasted 6 hours, and the total energy 



calculated. 



The increase in dry weight was obtained by the half-leaf 

 method. 



The following results were obtained : — 



Before Insolation. After Insolation. 



Area of half leaf 316'6 sq. cm. ... 3168 sq. cm. 



Dry weight of half leaf ... 1-2494 gm. ... 1-3952 gm. 



Dry weight per sq. cm. ... 0*0039 gm. ... 00044 gm. 



Heat of combustion of 1 gm. 



dry weight 4300-21 gm.-cal. ... 4313-46 gm.-cal. 



Heat of combustion per sq. 



cm 16-770 „ ... 18-978 „ 



Increase of heat of combustion after insolation per sq. cm. — 



2-208 gm.-cal. 

 Total energy incident on leaf per sq. cm. 361-03 gm.-cal. 



Quantity of radiant energy used in assimilation 0-6%. 

 As we have stated before, conclusions in regard to the relation 

 between the intensity of the radiant energy and that part of it used 

 in assimilation, cannot be drawn until our information in regard to 

 the various factors is considerably enlarged. It can, however, be 

 concluded that with high light intensities only a small part of the 

 incident radiant energy is utilised for assimilation. The lowest 

 value, for instance, obtained by Puriewitsch was 0-6% for an average 

 light intensity of 1-003 gm. -calories per minute. But it will clearly 

 be seen from the table below that other factors besides light intensity 

 are operating. 



Brown andEscombe's figures exhibit similar variatiotw- in the 

 percentage of sun energy utilised in assimilation, but on the whole 

 they are lower. This difference is easily accounted for by the 

 different method employed in measuring the assimilation. Brown 

 and Escombe, as we have shown, used the intake of carbon dioxide 

 in conjunction with a carbohydrate factor and an estimated value 

 of the heat of combustion in order to obtain a measure of the 

 energy used in assimilation. Their method is likely to yield more 

 uniform though perhaps not more accurate results than the half 

 leaf method employed by Puriewitsch, who did not attempt to 

 correct any of the sources of error of the method, which it might 



