THE ENERGY RELATIONS IN PHOTOSYNTHESIS 317 



was made with direct solar radiation, then one with the interposition of 

 the leaf and finally a second observation of the direct radiation. The 

 difference between the mean of the first and third observations and the 

 second gave the coefficient of absorption when the direct radiation was 

 taken as unity. A considerable variation was found in the values for 

 the coefficient of absorption in leaves of different species, although fairly 

 constant values were obtained for mature leaves of the same species. 

 There is, however, a dearth of accurate experimental determinations of 

 the coefficient of absorption of leaves especially using different frequencies 

 and intensities of illumination. In Table 33 are given the values of the 

 absorption coefficient of various leaves determined by Brown and Escombe. 



TABLE ZZ 



Coefficient of Absorption of the Radiant Energy of Sunlight for Leaves of 

 Different Plants. (Brown and Escombe.) 



Coefficient of 

 Plant Absorption 



Helianthus annuus 0.686 



Polygonum Weyrichii 0.647 



" Sachalinense 0.691 



Petasites officinalis 0.728 



Silphium terebrinthaceum 0.699 



Arctium majus 0.728 



Verbascum olympicum 0.758 



Senecio grandifolius 0.774 



The green leaf is very selective in its absorbing power. This fact 

 is brought out by the results of the absorption of light by a number of 

 similar leaves superimposed. If there were no selective absorption by 

 the leaf the quantity of light transmitted would be decreased in geometric 

 proportion as the superimposed leaves were increased in arithmetic pro- 

 portion. Thus, if one leaf has a coefficient of transmission of 0.3, two 

 superimposed leaves should have a coefficient of transmission of 0.09 

 and three leaves 0.027, etc. That this is not the case is shown by the 

 experiments of Brown and Escombe who found the following quantities 

 transmitted by Polygonum Weyrichii: one leaf 35.3 of the total radia- 

 tion; two leaves 19.2 per cent and three leaves 15.5 per cent; similarly 

 for Helianthus annuus: one leaf 31.3 per cent, two leaves 17.4, three 

 leaves 11.6. 



Similar results were obtained by Timiriazeff who found that the 

 chlorophyll which had been dissolved from a unit area of one maple leaf 

 absorbed 27 per cent of the direct radiation while the absorption of a 

 solution three times this concentration was only 31 per cent. 



That a single leaf is capable of absorbing most of the rays which 

 induce photosynthesis, although the total radiation may be many times 

 that which the leaf is capable of utilizing, has also been demonstrated 

 bv Willstatter and Stoll.^ They found that when the Hght incident on 



" WillstJittcr and Stoll, "I'ntcrsuchungcn ii. Kohlensaureassim.," p. 125. 



