THE PLANT OXIDASES 237 



and peroxide. Benzidine alone renders the petal slightly 

 brown and darkens the veins; whilst ^-phenylenediamine 

 gives a brown, changing to dark green, the distribution 

 being the same as is that of the benzidine reaction. These 

 experiments illustrate the graded action of the inhibitor 

 tannin upon the different reagents, and also, as some 

 think, upon the production of red anthocyanin, if it be 

 assumed that its chromogen is present. The coloured 

 flowers contain a complete peroxidase system, but in them, 

 too, the actions on the reagents differ in intensity. 



From among these numerous reagents, Keeble and Arm- 

 strong (1912, 1) selected a-naphthol and benzidine as being 

 the most suitable for general use in microchemical work. 

 Other authors also have employed these, and, in addition, 

 guaiacum for use with sections and tissue extracts. 



The above-mentioned reagents are only for the detection 

 of phenolase, and for tyrosinase the best substrate is, 

 according to Bertrand and Bourquelot (1896), undoubtedly 

 tyrosin. For the detection of the latter enzyme, Chodat 

 and Staub (1907) advocate the use of ^p-cresol, with the 

 addition of glycin or another amino-acid. The action is 

 greatly increased in rapidity by this group of acids, and 

 results in the production of a violet colour, changing to 

 blue with a reddish fluorescence. [See note, p. 250.] 



While it is necessary to bear in mind the different degrees 

 of sensitiveness of the various oxidase reagents, and the 

 very striking influence of inhibitors, it is nevertheless 

 obvious that those cells which act strongly upon all the 

 reagents employed are certainly those in which oxidation 

 processes are most active in the plant, and so their dis- 

 tribution must be of physiological significance. 



Since histological studies on oxidases are so intimately 

 connected with questions as to their physiological functions, 

 an account of this branch will be deferred till a later section. 



