386 



The Action of Light on Chlorophyll. 

 By Harold Wager, F.R.S. 



(Received February 6, — Read March 5, 1914.) 



The chemical changes brought about by light in the green leaf leading to 

 the production of sugars and starch from carbon dioxide and water are still 

 far from being clearly understood. 



To what extent the chlorophyll takes part in this process, whether it simply 

 performs the function of bringing the rays of light into contact with the 

 carbon dioxide and water in such a way as to enable them to effect a 

 synthesis of these two compounds, or whether the chlorophyll itself initiates 

 these changes by its own chemical decomposition, are problems still unsolved. 



It is a well-known fact that solutions of chlorophyll in the presence of 

 oxygen become decolorised by light, and Pringsheim showed that the chloro- 

 phyll in a living leaf becomes rapidly blanched when submitted to the action 

 of an intense light focused through a lens.* 



The earliest observations on the destructive effect of light on chlorophyll 

 appear to be those of Sir John Herschel, who in a series of papersf published 

 more than 60 years ago described many interesting experiments on the action 

 of the rays of the solar spectrum on the vegetable colours expressed from the 

 petals and leaves of plants. From these experiments he concludes that (1) 

 the action of light destroys colour, either totally, or leaving a residual tint on 

 which it has no further or much slower action ; (2) the action of the spectrum 

 is confined, or nearly so, to the visible rays, as distinguishe.d from the ultra- 

 violet and ultra-red rays, which are ineffective ; and (3) the rays effective in 

 destroying a given tint are, in a great many cases, complementary to the tint 

 destroyed. He pointed out that the green colouring matter expressed from 

 leaves and spread on paper shows, as in the elder, a maximum of action, as 

 indicated by the destruction of colour, in the red rays, from which the action 

 falls off rapidly with a slight intermediate maximum in the region of the 

 yellow, then falls off again, and about the termination of the green again 

 increases, reaching another maximum in the blue violet, after which it falls 

 off again, gradually, and ceases to be traceable as the termination of the violet 

 is reached. He points out that " photographic pictures may be taken readily 

 on such papers, half an hour in good sun sufficing ; but the glairy nature of the 

 iuices prevents their being evenly tinted, and spoils their beauty." He did 



* ' Pringsheim's Jahrb.,' 1881 and 1882. 



t 'Phil. Trans.,' 1840, 1842 ; 'Phil. Mag.,' 1843. 



