The Mechanism of Carbon Assimilation. 



101 



The presence of an acting peripheral mechanism in cardiectomised animals 

 suggests the following possibilities : — 



1. That the peripheral mechanism is active to some small degree in all 

 parts of the normal body; it is, perhaps, this mechanism which favours 

 local action of substances. 2. That this mechanism may take an active 

 share in the process of distribution in organs which are normally deficient 

 in circulation. The brain, for instance, has no lymphatics, and the exchange 

 of fluid material with the blood capillaries is said to be there somewhat 

 deficient. 3. That the peripheral mechanism gets into prominence in 

 pathological conditions in which there is either a local or general deficiency 

 of the cardio-vascular circulation. 



The Mechanism of Carbon Assimilation : Part III. 

 By Francis L. Usher and J. H. Priestley. 



(Communicated by Dr. M. W. Travers, F.R.S. Received April 13, — Read 

 June], 1911.) 



Some experiments and conclusions recorded in two papers* published in 

 1906 have been subjected to criticism by several investigators, and the 

 present paper has been written with the object of presenting some new facts 

 bearing on the problem of carbon assimilation, which incidentally support 

 some of those conclusions. We also take this opportunity to restate the 

 theory originally advanced, with such modifications as may be necessary, 

 and to reply to a few of the more important objections to it which have been 

 raised. 



The observations recorded below are concerned only with the initial stages 

 of the photosynthetic process, that is to say, with the formation of the 

 primary photolytic products from carbon dioxide, and with the evolution of 

 oxygen. In the papers referred to some evidence was given in support of 

 the belief that aqueous carbon dioxide is decomposed by light under the 

 conditions obtaining in a green leaf, the immediate products of this decom- 

 position being hydrogen peroxide and formaldehyde; and it is easy to see 

 that the production of these two substances would satisfactorily account both 

 for the oxygen and the carbohydrate, which are the first visible results of the 

 natural process. As the evidence put forward was to some extent indirect, 



* ' Eoy. Soc. Proc.,' B, vol. 77, p. 369 ; and vol. 78, p. 318. 



