118 pringsheim's researches on chlorophyll. 



accurate experimenters, from Daubeny^ and Draper^ to 

 Sachs^ and PfefFer/ agree in showing that the greatest 

 activity for evolution of oxygen by green tissues resides in 

 the rays of middle refraction in the spectrum. Objections 

 urged against this statement, and these come mainly from 

 physiologists, who adopt a purely physical theory of assimi- 

 lation, are essentially theoretical, based upon the idea that 

 the colouring matter is the seat of light activity. Thus, 

 Lommel considered that light action must be dependent 

 on the degree of completeness of absorption of the rays 

 and on their energy as measured 'by their heat-effect or 

 mechanical intensity, and concluded that the chief activity 

 must lie in the absorption-bands of the chlorophyll colouring 

 matter, that is, in the red, because the blue, on account of 

 their small mechanical intensity, could have no effect. 

 Experiment does not, however, confirm this idea; and it 

 were nearer the truth had the seat of activity been looked for 

 in the cell-contents outside the chlorophyll colouring matter. 

 Those physiologists who hold that yellow and green rays are 

 more effective in decomposing carbonic acid than blue and 

 red, rightly enough express the observed facts; but there 

 remains for explanation the function of the rays so markedly 

 absorbed in the colouring matter. 



The experiments here recorded are not conclusive regarding 

 the effects of colour in the reduction-process. Green and 

 yellow are naturally more active than blue, because the 

 latter is absorbed to such an extent by the chlorophyll 

 colouring matter that it is unable to produce an effect, just 

 as in photography the silver salts behind an interposed 

 green glass-screen are much less sensitive to blue than to 

 yellow and green light. 



At the present time, then, notwithstanding many accurate 

 researches already made, the dependence of the decomposition 

 of carbonic acid upon the wave lengths of the light rays needs 

 elucidation. There is no doubt that, for green plants, yellow 

 and green rays are far more active for the evolution of oxygen 

 than blue, yet this is no clue to the dependence of assimila- 

 tion on colour. This process might equally well be stronger 

 in the blue, for researches say nothing certain on this point. 

 Yet a priori it appears more probable that blue rays have no 

 effect, as the absorption in the chlorophyll colouring matter 

 would be a more significant adaptation for the accumulation 



' ' Philosophical Transactions,' 1836. 



2 'Ann. d. Chetn. et Phys.,' 1844. 



3 'Bot. Zeit.,'1864, p. 253. 



* ' Arb. d. Bot. Instit. in Wiirzburg.' Part i, 18?1. 



