94 RESPIRATION 



the wheat the ratio day rate/night rate was found to be i -042 

 in normal air and 1*010 in deionized air. The reason for the 

 difference is not obvious : it is known that during daylight the 

 air becomes ionized to various degrees under conditions of low 

 relative humidity. Ionized oxygen is more potent than deion- 

 ized oxygen so that the ionized oxygen of the daytime air, 

 according to Spoehr, possibly accelerates the purely oxidative 

 process of respiration, not the initial disruption of the respir- 

 able material. But until it can be said with certainty at what 

 stage in respiration oxygen becomes operative, an adequate 

 explanation is not possible. 



THE MECHANISM OF RESPIRATION. 



The catabolic processes of plants may be directly referable 

 to specific enzymes, zymase for instance in alcoholic fermenta- 

 tion ; but in the respiratory activities of higher plants, their 

 role is not defined with that precision and degree of complete- 

 ness which is desirable, although it is generally agreed that 

 enzyme action plays an important part in the process. En- 

 zymes associated with the common end products of respiration 

 are subjects for first consideration and of them most attention 

 has been given to oxidase, peroxidase, catalase, zymase, and 

 carboxylase. 



Oxidase, to use the generic term, has remarkable properties 

 of effecting with rapidity the oxidation of various substances 

 in the presence of oxygen.* They are of wide occurrence in 

 the vegetable kingdom, as should be the case if they are prim- 

 arily concerned in aerobic respiration, but whether they are 

 present in all living cells is doubtful. According to Atkins f 

 they are absent or inactive in tissues markedly acid in reaction 

 or containing large amounts of reducing substances. Bunzel J 

 also has shown that the activity of these enzymes is inhibited 

 by acids, their greatest activity being at or near the point of 

 neutrality ; the limits of the P H value corresponding to com- 

 plete inhibition io^the various subjects of experiment are 

 narrow and tljejngure of acid sensitiveness is almost invariable 

 in a partic^fer 



Vol. I., p. 392. 



kins: " Proc. Roy. Dublin Soc.," 1913, 14, 144., 

 unzel : " Journ. Biol. Chem.," 1916, 28, 153. 



