152 



Mr. F. Kidd. 



Section VI. — Further Light upon the Chemical Mechanism of Respiration in 



Plants. 



The results of these researches bear directly upon the problem of the 

 nature of the respiratory processes in plants ; and especially upon the question 

 as to whether the processes involved in the anaerobic C0 2 production which 

 is induced in plants in the absence of oxygen, are not also normally the first 

 processes in ordinary aerobic respiration. 



Two very definite effects of carbon dioxide have been demonstrated : — 



(1) An effect of increased concentrations of C0 2 in tissues upon anaerobic 

 respiration, decreasing the rate of CO2 production in direct relation to the 

 amount of CO2 in the tissues. 



(2) An effect of increased concentrations of CO2 in tissues upon normal 

 respiration, decreasing the rate of this process also in direct relation to the 

 amount of C0 2 in the tissues. 



The similarity of these effects of carbon dioxide upon two processes which 

 are different in their nature, the one producing C0 2 by molecular splitting, 

 the other by direct oxidation, naturally suggests at once two possibilities. 

 Either (1) the two processes are genetically connected, so that by limiting the 

 precursor the second process is naturally and automatically limited also ; or 

 (2) while the two processes are not genetically connected, the similar effect 

 produced on both under the influence of carbon dioxide is to be explained by 

 an action of carbon dioxide upon the medium in which they both occur. 

 Thus it is possible that a change in the permeability of protoplasm under the 

 influence of carbon dioxide might decrease equally the rate of two reactions 

 occurring in the cell but otherwise in no way connected with each other. 



The fact that carbon dioxide appears to act in its retarding effect mainly 

 upon the processes of floating respiration and not upon those of protoplasmic 

 or starvation respiration inclines us to the first of these possibilities. 

 Experiments were devised to test this question. 



It is necessary, however, clearly to set out in the first place the hypothesis 

 which we desire to test. It may be stated in sequence as follows : — (1) An 

 anaerobic splitting of carbohydrate into some easily oxidisable substance and 

 carbon dioxide, occurring in the initial stages of anaerobic respiration, is 

 always the first process in normal respiration. (2) The rate of this first 

 process producing easily oxidisable substances acts as a limiting factor upon 

 the amount of the succeeding process, in which these substances are oxidised 

 somehow by the oxygen of the atmosphere, oxygen being always in excess in 

 normal respiration and the oxidisable substances being thus always completely 

 removed. (3) Carbon dioxide retards the rate of the first process, that is the 



