PHOTOSYNTHETIC EFFICIENCY OF LEAVES 223 



Detlefsen (1888) found that the leaf absorbed more energy 

 when placed in an atmosphere containing carbon dioxide 

 than in one without it. The intensity of radiation was 

 measured by placing a thermopile behind the leaf. When 

 there was no assimilation the temperature indicated by the 

 thermopile was slightly higher ; the results showed that the 

 leaf did not utilise more than 0-3 to i-i per cent, of the 

 incident radiation. The average efficiency may therefore 

 be taken as about 0-7 per cent. 



Puriewitsch (1914) measured the incident energy by a 

 Bolometer, and the increase in the dry weight of the leaf 

 after insolation by the half-leaf method. The leaf of Acer 

 platanoides was exposed to sunlight for six hours ; the 

 increase of heat of combustion per sq. cm. after insolation 

 was found to be 2-208 gm. cal. The total energy incident 

 on the leaf per sq. cm. was 361-03 gm. cal. The efficiency 

 in this case was found to be o -6 per cent. 



There are several sources of error in the above deter- 

 minations. In prolonged experiments extending over 

 many hours it is difficult to obtain an accurate estimate of 

 the total radiation. The incident radiation is, moreover, 

 subject to various losses, and finally the determination of 

 the increase of dry weight by the half-leaf method cannot 

 be regarded as highly accurate. 



Difficulties in Estimating the Photosynthetic 

 Efficiency of Leaves of Terrestrial Plants 



Brown and Escombe (1905) carried out the most 

 elaborate and painstaking investigation on the subject, 

 making allowances for the various losses of the incident 

 radiation. Though every care was taken, the inherent 

 difficulties of the problem stood in the way of obtaining 

 accurate results. This will be understood from the following 

 considerations. 



Measurement of the energy stored. — Brown and Escombe 

 assumed certain values of the carbohydrate-factor and of 



