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



oxidase, such as the phloem and cortex ot the carrot, the veins of the apple and 

 potato. A potato kept until the tuber was watery hut still acid, and in which 

 the oxidase had nearly disappeared, showed no power of liberating iodine 

 from potassium iodide. 



The cut surface of the parsnip is neutral or feebly alkaline and, although 

 rich in oxidase, a cut surface shows only a feeble power of liberating iodine 

 from potassium iodide over the phloem ring and outer cortex after many hours' 

 exposure to air. The acid pulp of the orange and lemon, which contains no 

 oxidases, is unable to produce any liberation of iodine, nor does the wood 

 cylinder of the carrot, which is usually more acid than the cortex but contains 

 hardly any oxidase. 



This action is evidently due to the oxidase and not to the free acid. 

 The extracted oxidase, however, like pounded pulp is unable to produce any 

 liberation of free iodine from potassium when tested in the usual way. 

 Possibly this is because any iodine liberated would at once attack and 

 destroy the plant oxidase where this was in immediate contact with 

 potassium iodide. Free iodine does actually destroy potato oxidase. Hence 

 to produce any progressive liberation of iodine sufficient to stain the starch 

 the oxidase and potassium iodide would need to be separated by a semi- 

 permeable or colloidal membrane, such as is formed by the cell walls on the 

 cut surface. 



If pounded potato pulp or filter paper pulp saturated with a glycerine 

 extract of oxidase is covered by a layer of gelatine containing starch or of 

 starch paste, and a little potassium iodide poured on top when the colloid 

 layer has set, after one day a more or less prominent violet line appears on 

 or close to the pulp. Apparently the oxidase is only able to liberate iodine 

 from potassium iodide when the latter diffuses slowly to it, and this is 

 possibly a question of relative mass action and osmotic separation. 



In any case the liberation of iodine from potassium iodide on the cut 

 surface of a living tissue can be used as a confirmatory test for the presence 

 of an oxidase. It does not indicate the presence of hydrogen peroxide or of 

 any special " iodoxidase." 



The Influence of Anaesthetics on Oxidase Action. 

 Ether.- — Small cubes of potato soaked in saturated ether water for a day 

 and then exposed to air darkened distinctly. Pulp triturated with ether 

 darkened slightly, and gave strong oxidase reactions and decomposed H 2 2 . 

 The clear ether extract had no oxidase properties. Apple pulp pounded 

 with excess of ether turns a deep brown, but a little more slowly than in 

 the absence of ether. The ether extract is yellow, not owing to tannin but 



