]16 



source of energy for the reduction-process lies in the colour- 

 ing matter. Although the difficulties of accurate photometric 

 determination of the absorption-spectra do not permit of 

 complete proofs yet tolerably conclusive evidence against such 

 a view is afforded by tlie facts that^ the rays absorbed in the 

 colouring matter, as indicated in the absorption-bands of the 

 spectrum, which must be of some value to the plant, play no 

 preponderating part in producing the light-efiect upon the 

 plant ; that the maximum of decomposition of carbonic acid 

 does not correspond with the maximum of absorption in the 

 clilorophyll-spectrum ; that leaves wliich are not active, show 

 the same chlorophyll-spectrum as those which are active; and 

 that artificial chlorophyll-solutions decompose no carbonic 

 acid. The source of energy is to be sought for only in the 

 light-absorption in the other cell-contents themselves, in 

 which intense light brings about such marked decomposi- 

 tions.^ 



The sharing of the colouring matter in assimilation has 

 been ere now questioned, though the grounds upon which 

 this has been done have been in part incorrect. Thus, Meyer^ 

 and after him Mulder,^ considered that the colouring matter 

 instead of promoting assimilation is formed in the process. 

 " Gx'cen tissues give out oxygen not because they are green 

 but because they become green." This view has been 

 repeatedly controverted,^ for the transformation of the starch 

 content of chlorophyll-corpuscles into wax, by which Mulder 

 accounted for the liberation of oxygen, does not take place ; 

 and moreover, in the formation of chlorophyll in the plant 

 oxygen is not set free. In later times,^ Gerland has shown, 

 in a discussion as to the relative energy of colours in assimi- 

 lation, that the conformity of absorption-spectra of leaves 

 with those in chlorophyll-solutions, and the decolorisation of 

 the latter in oxygen, are not easily reconciled with the theory 

 that colouring matter directly shares in assimilation. 



It would appear, then, that the increase of respiration in 

 light is retarded by the colouring matter, and that in this 

 way the reduction of carbonic acid and water is favoured, 



* That differences, dependent on their colour but difiicult to define exist 

 in their light-absorptions between active and non-active green tissues, has 

 been proved by IS. J. C. Miiller, *Bot. Untersuchungen,' Heidelberg, vol. i, 

 and 'Handbnch der Allgemeinen Botanik,' 1880, vol. i, p. 511. 



^ ' Pflanzenphysiologie,' Bd. ii, p. 162. 



^ * Allgemeine physiologische Chemie.' Uebersetzt von Kolbe, Braun- 

 scliweig, iS14, p. 273. 



* Moid, ' Verniisciite Schriften,' p. 360 ; Sachs, ' Experimental physio- 

 iogie,' p. 320; JJe Bary, ' Bot. Zeit.,' 1871, p. 612. 



5 'Ann. d. Phys. u. Chemie von Poggendorf,' vol. 143 (1871). 



