1905.] Mechanism of Carbon Assimilation in Green Plants. 371 



lasted over three weeks in very dull weather, gave uranium peroxide and 

 formic acid. The different results obtained here from those in the case of 

 similar experiments with the acetate, may be due to the " reduction potential " 

 falling below the limit required for the completion of the second stage of the 

 decomposition. 



In all the foregoing experiments, except in the case of the liquid carbon 

 dioxide tubes, blank experiments were simultaneously performed, (1) with 

 uranium solution and carbon dioxide in the dark, and (2) with carbon dioxide 

 free solution in the light. In neither case was any precipitate formed. 



Decomposition of Carbon Dioxide in the Plant. — If a similar reaction, 

 resulting in the formation of formaldehyde and a peroxide, takes place 

 in the first stage of the absorption of carbon dioxide by the plant, it 

 is obvious that both the initial products of decomposition must undergo 

 a rapid change. 



On account of its intensely poisonous nature, formaldehyde must be very 

 rapidly converted into some physiologically inert substance ; and the peroxide 

 must be decomposed with evolution of gaseous oxygen, a process which 

 follows exposure to light by an interval of one or two seconds. 



The problem, then, is to ascertain the process by which oxygen is dis- 

 engaged ; to show the actual presence of formaldehyde localised in the 

 neighbourhood of the chloroplasts ; and to trace the steps by which the 

 formaldehyde is polymerised. 



The Mechanism of the Evolution of Oxygen from the Green Plant. — In the 

 experiments relating to the decomposition of carbon dioxide outside the 

 plant, no evolution of oxygen gas is ever observed ; it remains in the system 

 as a peroxide. 



There have been conflicting statements with regard to the presence of 

 hydrogen peroxide in plants, but even if traces are to be found, there is no 

 evidence that it is a product of decomposition of carbon dioxide. It has 

 indeed been shown that several organic substances, notably the organic acids, 

 e.g., oxalic, give rise to hydrogen peroxide on exposure to light, and such 

 substances as these are of common occurrence in the leaves of plants. 

 If, however, hydrogen peroxide is one of the first products of the photolysis 

 of carbon dioxide, we are more directly concerned with the elimination of 

 oxygen in the gaseous form than with the detection of the peroxide. 



Hitherto those writers who have recognised the difficulty at all have 

 suggested some method of reduction, which, of course, leads back to the 

 starting point. It appeared much more probable that this step in the process 

 was brought about by a catalyst, probably an enzyme. To test this, some 

 Modea was immersed in a dilute solution of hydrogen peroxide. An 



