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



According to Bertrand* the sugar-beet contains an oxidase capable of 

 oxidising tyrosin which he terms "tyrosinase," and this, according to 

 Gonnermann,f oxidases tyrosin to homogentisinic acid, which darkens rapidly 

 by direct oxidation to red, brown, or black. In the red beet the amount of 

 tyrosin present appears to be too small to appreciably affect the neutral red 

 colour. It may undergo oxidation, while the plant is living, and hence be 

 unable to accumulate. 



The Oxidase of the Parsnip. 



The parsnip differs from all the other vegetables used, in that a cut surface 

 is neutral or faintly alkaline instead of acid, and it resembles the carrot in 

 containing no chromogen oxidising on death. Neither carrot nor parsnip 

 oxidase will directly brown boiled potato or apple pulp, but if a little sodium 

 phosphate and H2O2 is added, they will cause tannic acid, apple pulp and 

 apple juice to brown distinctly and with fair rapidity. 



In both carrot and parsnip the oxidase is mainly present in the phloem and 

 outer cortex, and that of the carrot appears to be a little more abundant. 

 Hence of similarly prepared watery or glycerine extracts the former is a little 

 more active than the latter. 





Beetroot oxidase. 



Carrot oxidase. 



Parsnip oxidase. 





+ + + 



+ + 



+ +■ 



Do. +H,0» 



+ + + 



+ + £ 



+ + + 













+ 



+ 



+ 





+ + 



+ 





'Do. +H 2 2 



+ 



+ + + 



+ + + 



















Do. +H 2 0, 



+ 



+ 



+ 











Do. + 2 H 2 



+ 



+ 



+ + +_ 











Do. +H 2 2 









Tannic acid and sodium phosphate ... 









Do. +H 2 Oo 





+ 



+ 





+ + 







Do. +H„0 2 



+ 







Both ursol tartrate and hydroquinone when applied to a cut surface of a 

 carrot or parsnip show signs of oxidation particularly over the phloem ring.. 

 This is probably because the oxygen is more concentrated at the surface and 

 also a greater mass action is exercised upon the inwardly diffusing oxidant. 

 No assumption of the production of peroxides in the tissue is necessary to 

 explain the action. The addition of magnesium sulphate or of potassium 



* ' Bertrand, 'Compt. Kend.,' vol. 122, p. 1215. 



t Gonnermann, 'Pfluger's Archiv,' vol. 82, p. 289 (1900). 



