318 PHOTOSYNTHESIS 



a leaf is filtered through another similar leaf, the rate of photosynthesis 

 of the first leaf is reduced to %o of the original rate and when two leaves 

 are interposed, photosynthesis ceases. 



Brown and Escombe found but a very small increase in the values 

 of the coefficient of absorption with increasing age of the leaves. Tlieir 

 values are considerably higher than those obtained by Puriewitsch.^ The 

 latter by the use of a bolometer found that leaves of different plants 

 absorbed 18 to 27 per cent of the incident solar radiation, while Brown 

 and Escombe's results show an absorption of about 64 to 77 per cent. 

 The bolometer of Puriewitsch was probably more accurate than the 

 method of measuring the solar radiation used by Brown and Escombe. 

 It is difficult to determine the cause for the discrepancy between values 

 obtained by these workers. 



In this connection the observations of Timiriazefif * are of some in- 

 terest. This worker endeavored to determine the absorption caused by 

 the chlorophyll of a single leaf. Thus, he found that the absorption of 

 the direct sunlight by the green pigment dissolved in a volume of alcohol 

 just sufficient to fill a glass cell of the same cross-section as the area 

 of the leaf, ranged from 20 to 29 per cent for different leaves. These 

 values are obviously nearer to those of Puriewitsch than to those of 

 Brown and Escombe. At the same time it should be noted that the 

 latter workers also compared the absorption of green and albino leaves. 

 They found, for example, that the coefficient of absorption of the white 

 portion of leaves of Negundo aceroides was 0.745 while that of the green 

 portion was 0.787. This would indicate that in the method employed by 

 Brown and Escombe only a relatively small portion of the radiant energy 

 absorbed by the leaf can be ascribed to the absorption by the chloro- 

 phyll. However, there are other differences between the green and color- 

 less portions of a leaf besides the chlorophyll-content which may influence 

 the difference in absorption. 



Brown and Escombe consider that the error due to reflection from 

 the leaf surface which would tend to increase the values of the coeffi- 

 cient of absorption, was, under the conditions of their experiments, very 

 small. Jorgensen and Stiles,^ on the other hand, state that a black cloth 

 may reflect as much as 1 per cent of the radiant energy falling on it. 



In order to make an estimation of the energy exchanges in an illumi- 

 nated leaf it is necessary to know, besides the coefficient of absorption 

 of the leaf, also something of the thermal relations of the leaf to its sur- 

 roundings. The radiant energy which is absorbed by a leaf tends to 

 raise its temperature. If, for example, the solar radiation is taken as 

 0.8 calories per square centimeter per minute (a low value), the coeffi- 

 cient of absorption as 0.78, the weight of a square centimeter of leaf 

 as 0.020 gram and its specific heat as 0.879, there would result a rise 



'Puriewitsch, Jahrb. zinss. Bot.. 53, 209 (1914). 

 * Timirazeff, Froc. Roy. Soc, 72 B, 449 (1903). 

 'Jorgensen and Stiles, "Carbon Assimilation," p. 141. 



