THE ENERGY REEATIONS LY PHOTOSYXTHESIS 327 



are not the only products of photosynthesis. Apparently substances are 

 formed in pliotosynthesis with a higher heat of combustion than that of 

 glucose. 



Puriewitsch also determined the coefficient of absorption of leaves, 

 that is, the per cent of the total incident radiation which is absorbed by 

 leaves. By means of a bolometer he determined the total amount of radiant 

 energy falling on the leaf, and with another bolometer the energy which 

 passes through the leaf when this is in an atmosphere containing carbon 

 dioxide and when it is in a carbon dioxide-free atmosphere. He found 

 that a leaf which is photosynthetically active (in an atmosphere containing 

 carbon dioxide) absorbs more of the energy falling on it than a leaf in 

 which no photosynthesis is taking place (in an atmosphere free of carbon 

 dioxide). The amount of energy absorbed varied considerably with 

 ditTerent species of leaves, from about 18 to 27 per cent. The leaves which 

 were in an atmosphere containing carbon dioxide absorbed from 1 to 2.6 

 per cent more energy than those in an atmosphere without carbon dioxide. 

 This excess of absorption by leaves in which photosynthesis is taking place 

 is greater when the incident radiation is filtered through solutions which 

 absorb a portion of the solar rays. When filtered through an alum solution 

 leaves of Acer plantanoidcs, in an atmosphere containing COo. showed 

 an excess of 4.93 j^er cent over the leaves in a COo-free atmosphere ; when 

 the light was filtered through a solution of gentian violet which was trans- 

 parent for rays of 720-597 [i[i and from 490 [i\.i to the end of the visible 

 spectrum, there was an excess absorption of 4.25 per cent by the leaves in 

 an atmosphere containing carbon dioxide. When the light was filtered 

 through a ruby glass, transparent to rays of 590-690 \i[i the excess ab- 

 sorption was 11.7 per cent. There is no adequate explanation for this 

 excess absorption by leaves in which photosynthesis is taking place over 

 those which were in an atmosphere free of carbon dioxide. It is possible 

 that the products formed in photosynthesis may increase the absorption 

 of the leaves in a measure. Results similar to those of Puriewitsch had 

 previously been obtained by Detlefsen,^° though by less accurate means. 

 Ursprung ^^ reinvestigated the problem, using a filament electric lamp 

 as a source of light with a maximum intensity of 70.000 Lux. and air 

 containing as much as 8 per cent carbon dioxide. He could not detect the 

 effect reported by Detlefsen and by Puriewitsch so that we have here two 

 sets of directly contradictory results. 



This phenomenon, of an excess absorption of light due to photosyn- 

 thesis, if it actually exists, is not without some theoretical interest. In 

 photosynthesis the radiant energy of the sun is converted into potential 

 chemical energy. Draper's fundamental law states that only the light 

 which is absorbed is chemically active. Also it may be assumed that the 

 light which is transformed into chemical energy must disappear as such. 

 In two leaves, one of which is photosynthetically active and the other not, 



"Detlefsen, Arb. bot. hist. Wi'irsburg, 3, 534 (1888). 

 "Ursprung, Ber. bot. Ges., 36, 124 (1918). 



