328 PHOTOSYNTHESIS 



only in the former will there be a transformation of radiant into chemical 

 energy, and in this leaf it might be expected that a greater absorption of 

 radiant energy takes place. In the leaf which is not photosynthetically 

 active there are a priori two possibilities of the disposition of the energy. 

 It is possible that the amount of energy used in photosynthesis, in the 

 case of the first leaf, is not absorbed by this leaf, but passed through the 

 leaf which thus shows lower absorption. This would correspond to the 

 findings of Detlefsen and of Puriewitsch. Or it is possible that in both 

 leaves the same quantity of radiant energy is absorbed. In the leaf which 

 is photosynthetically active a portion of the absorbed energy is converted 

 into potential chemical energy ; the rest is used in transpiration or is 

 reradiated. In the leaf which is not photosynthetically active all of the 

 absorbed energy is dissipated in transpiration or by reradiation. It might 

 be expected that in the latter case there would occur a higher heating effect 

 made evident by higher transpiration or thermal emission than in the 

 photosynthetically active leaf. Photosynthesis would thus decrease the 

 rate of transpiration. That this is the case has, in fact, been claimed by 

 Deherain ^- and Jumelle.^^ The point is, however, one of dispute,^* 

 though it ought not to be very difficult to establish which of the two possi- 

 bilities of the disposition of energy, just referred to, occurs in the plant. 

 In either case the possibility suggests itself, of using either the difference 

 in absorption or in heating effect between a leaf which is photosynthetically 

 active and one which is not, as a means of measuring photosynthesis. It 

 must be borne in mind,, however, that as the results of Brown and Escombe 

 have shown, a great deal more of the radiant energy absorbed by the leaf 

 is used in transpiration than in photosynthesis, under most conditions, so 

 that the lower rate of transpiration, should such be caused by photosyn- 

 thesis, in most cases would be very small. Also the indirect effects due 

 to the absence of carbon dioxide in the one case may be such as to obliterate 

 the slight increase in heating effect produced when photosynthesis is not 

 taking place. 



The results of Detlefsen and of Puriewitsch have a further theoretical 

 implication, that is in connection with the chemistry of the photosynthetic 

 process. They recall the early observations of Bunsen and Roscoe who 

 found that the absorption of light by chlorine and hydrogen separately was 

 less than when these two gases reacted in light to form hydrogen chloride. 

 Bunsen and Roscoe concluded that the absorption of the reacting mixture 

 was greater because energy is used to bring about the chemical reaction. 

 The absorption of light by the separate components is termed optical or 

 photo-thermal absorption and that absorbed to bring about the chemical 

 reaction is "photochemical extinction." ^^ The question of photochemical 



"Deherain, Ann. Sci. Nat. (6), 4, 177 (1876). 



" Jumelle, Rev. gen. Bot., 1, 37 (1890) ; 2, 417 (1891) ; 3, 241 (1892). 

 " Burgerstein, "Die Transpiration der Pflanzen.," Jena, 1904. 

 "Sheppard, "Photochemistry," 1914, p. 187. Weigert, "Die Chemischen Wirkun- 

 gen des Lichtes.," 1911, p. 49. 



