340 Preside7it's Address. 



Although the difficulties of accurate photometric (Ictermination of 

 the absorption spectra do not permit of comploto proof, yet tolerably 

 conclusive evidence against such a view is afforded by the 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-effect 

 upon the plant ; that the maximum of decomposition of carbonic acid 

 does not correspond with the maximum of absorption in the chloro- 

 phyll spectrum ; that leaves which 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 decompositions. 



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. Meyer and Mulder's view that the 

 colouring matter, instead of promoting assimilation, is formed in the 

 process, " green tissues give out oxygen not because they are green, 

 but because they become green," has been repeatedly controverted, 

 for the transformation of the starch content of chlorophyll-cor- 

 piiscles 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 set free. In later times, too, 

 Gerland has shown, in a discussion as to the relative energy of 

 colours in assimilation, 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, and an accumulation of 

 carbon takes place in the plant, but the colouring matter takes no 

 direct and immediate part in the process. 



The question then arises, How does this theory of the function of 

 chlorophyll affect the result of researches on assimilation 1 As the 

 oxidation and reduction processes in green tissues do not rise and 

 fall similarly in changing light-intensities and colours, it is necessary 

 in all questions concerning assimilation to take into consideration 

 the amount of respiration, and the extent to which it is influenced 

 by light. 



And firstly, as regards an optimum intensity of light for the de- 

 composition of carbonic acid. This cannot be determined simply 

 by the amount of oxygen given off' in light, for this in all intensities 



