OXIDASES 397 



of peroxidase and peroxide. According to them, there are 

 three categories of oxidizing ferments. 



(a) Oxygenases which produce the peroxide. 



(b) Peroxidases which transfer oxygen from the peroxide 

 to the substance to be oxidized. 



(c) Catalases which destroy peroxides so that oxygen is 

 given off. 



In the colour reactions mentioned above two actions are 

 possible. Either the plant juice, e.g. of the potato, gives the 

 blue coloration with the guaiacum tincture alone, or, the blue 

 colour will not occur, as, for example, in the sap of the 

 cucumber, unless a peroxide, such as hydrogen peroxide, be 

 added. 



On Bach and Chodat's hypothesis, there are present in 

 the potato oxygenase, peroxidase and peroxide; the peroxi- 

 dase transfers oxygen from the peroxide to the guaiacum, and 

 the oxygenase re-oxidizes the reduced peroxide. This may 

 be termed the direct action.* On the other hand, in the 

 cucumber juice, only peroxidase is present, so that in order to 

 obtain the blue reaction with guaiacum, hydrogen peroxide, 

 or other peroxide, must be added. This is the indirect action. 



This idea has been accepted by Palladin,f who considers 

 that normal respiration depends upon the presence of an 

 oxidizable substance, oxygenase and peroxidase. 



Peroxidases can practically always be found in living plant 

 members, but the oxygenases are less stable and are quickly 

 decomposed, giving origin to some of the respiratory carbon 

 dioxide. The amount of these enzymes varies with the stage 

 of development of the plant ; thus in the embyro, oxygenase 

 is at its 1 minimum, but increases with the development of the 

 plant and then diminishes as the growth of the organ ceases. 



On the other hand, according to Porodko,| oxidases play 

 scarcely any part in respiration. 



The views of Bach and Chodat are not universally held ; 

 thus Moore and Whitley, as a result of a number of experi- 

 ments, have arrived at the conclusions that the sole difference 



*Wheldale: " Proc. Roy. Soc., Lond.," B., IQII, 84, 121. 

 f Palladia: " Ber. deut. hot. Gesells.," 1906, 24, 97. 

 JPorodko: " Beih. Bot. Centrbl,, 1904, 16, i. 

 Moore and Whitley : " Biochem. Journ.," 1909, 4, 136. 



