310 Prof. A. J. Ewart. Comparative Study of Oxidation by 



or light. The colour given is brown or brownish red. It picks out the 

 wood vessels in a slice of boiled potato or carrot in brownish red without 

 affecting the other tissues. The bundles or vascular network on the inner 

 surface of orange or lemon peel are coloured bright red on a white ground, 

 looking like blood vessels injected witli carmine. Conifer wood or a match 

 also colours bright red. As a direct test, it is simpler to apply than any 

 other lignin test, and the colour is confined to the walls of the vessels or 

 tracheides. In testing tissues for oxidase this reaction must be borne in 

 mind. 



The Action of Paraphenylenediamine on the Oxidases of Apple Sap and Potato. 



Apple pulp turns violet, in a few minutes rapidly darkening to blue, with 

 alcoholic paraphenylenediamine in the absence as well as in the presence of 

 peroxide of hydrogen. In the former case the blue colour remains permanent 

 for an indefinite length of time, whereas in the presence of peroxide of 

 hydrogen the oxidation is ultimately completed to a dark brown or black. 



Potato pulp remains colourless with alcoholic paraphenylenediamine for one 

 or more hours, but on long standing the liquid acquires a brown colour, 

 tinged with violet, and the pulp a weak but distinct violet tinge. In the 

 presence of peroxide of hydrogen a violet colour is rapidly produced, but 

 changes to brown in the presence of an excess of peroxide of hydrogen. 



Using a watery solution of ursol tartrate, apple pulp develops a violet or 

 blue colour in the absence and presence of peroxide of hydrogen, appearing 

 first in the veins and persisting for a long time. With potato pulp and in 

 the presence of peroxide of hydrogen a green colour is shown passing 

 through blue rapidly to a slate colour. In the absence of peroxide of 

 hydrogen potato pulp remains colourless, gradually acquiring a slight brown 

 colour in two days, but with no signs of any colour sequence, and the 

 brown is hardly stronger than that produced in pieces of boiled egg albumin 

 used as a control. In all cases no colour sequences were produced by boiled 

 apple or potato pulp. In needing hydrogen peroxide to produce a colour 

 sequence with ursol tartrate, potato oxidase therefore resembles copper 

 sulphate and salt, and, similarly, both the vegetable oxidase and the metallic 

 oxidase give a blue colour with guaiacum in the absence of hydrogen peroxide. 

 That is they are oxidases to guaiacum, " peroxidases " to ursol tartrate. 



On the other hand, apple oxidase resembles ferric chloride in its ability to> 

 produce oxidase colour changes with both ursol tartrate and guaiacum in the 

 absence of hydrogen peroxide. 



