318 



The Mechanism of Carbon Assimilation in Green Plants : the 

 Photolytic Decomposition of Carbon Dioxide in vitro. 

 By Fkancis L. Usher and J. H. Pkiestley, B.Sc. 



(Communicated by Professor Morris W. Travers, F.R.S. Received April 30, — 



Read May 10, 1906.) 



(From the Chemical and Botanical Departments, University College, Bristol.) 



In a previous paper* it was shown that carbon dioxide is decomposed in 

 the green parts of plants independently of vital or enzyniic activity, 

 formaldehyde and hydrogen peroxide being produced. It follows from the 

 analysis of the process of carbon assimilation there set out that this first 

 step, the photolysis of carbon dioxide, should be capable of being artificially 

 induced under laboratory conditions. 



It is impossible here to give even a brief account of the work of previous 

 investigators on these lines ; it is sufficient to state that all experiments 

 with chlorophyll solutions have given negative results, and as regards those 

 with other forms of chlorophyll, such as dried powdered leaves or expressed 

 iuice, the balance of evidence favours the view that no decomposition takes 

 place. Reference may be made to papers by Friedelf and Macchiatif on 

 the positive side, and by Harroy,§ Herzog,|| and, quite recently, Bernard,1F 

 who obtained only negative results. Experiments with uranium compounds 

 will be considered later. 



I. Experiments on the Chlorophyll Catalysis of Carbon Dioxide. 



In view of the very perfect chemical and physical arrangements which 

 assist the decomposition of carbon dioxide in the plant, it is obvious that 

 attention must be paid to these points in any experiment performed outside 

 it. It appears likely that the failure of so many attempts on the part of 

 previous observers to carry this out has been due to a neglect of such 

 considerations — not so much with regard to the chemical as the physical 

 conditions. The chemical conditions are those which are necessary when 

 any other highly reversible action is being dealt with, viz., that one at least 



* ' Koy. Soc. Proc.,' B, vol. 77, p. 369. 



t 'Comptes Rendus,' 1901, vol. 132, p. 1138. 



\ 'Bull. Soc. Bot. Jtal.,' 1903. 



§ ' Comptes Rendus,' 1901, vol. 133, p. 890. 



II ' Zeit. physio). Chem.,' 1902, vol. 35, p. 459. 



1 ' Comptes Rendus,' 1905, vol. 140, p. 509. 



