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



peroxide of hydrogen is able to partially suspend its inhibitory action. In 

 the presence of barium chloride, apple oxidase acts as a " peroxidase," in its 

 absence, as an oxidase. 



Strong peroxide of hydrogen destroys the oxidase, and hence pieces of 

 fresh pulp when immersed in a strong solution of pure peroxide remain 

 colourless or show a faint browning along some of the veins. The peroxide, 

 as it penetrates, destroys the oxidase in the protoplasm before it comes into 

 contact with tannic acid. If the pulp is pounded with strong peroxide, the 

 browning is somewhat retarded, but still takes place, since the oxidase, 

 tannic acid, and peroxide are in contact simultaneously and react before 

 the oxidase is destroyed. Dilute peroxide accelerates the browning of 

 pounded pulp. 



Lead acetate appears at first sight to be incapable of preventing the 

 browning of pounded apple pulp. The pulp darkens to yellowish or greenish 

 brown, and even after 24 hours retains a distinct but enfeebled power of 

 decomposing hydrogen peroxide and turning guaiacum blue. The darkening 

 is, however, partly due to the precipitation of yellowish-white lead tannate 

 and the retention of the power of turning blue is easily explained, for lead 

 acetate itself gives a strong blue with guaiacum in the presence of hydrogen 

 peroxide, and can therefore act as an oxidase. Any poison which destroys 

 the oxidase also removes the power of turning brown, and the pulp of apples 

 turns brown when soaked in bulk in metallic poisons, because the slowness of 

 penetration allows the cell to be killed and browning to occur before the 

 oxidase is destroyed. Barium chloride, however, inhibits oxidation without 

 destroying the oxidase ferment. 



The Ghromogen of the Apple. 



In the apple the chromogen is known to be a form of tannic acid. In 

 a previous paper it was shown that tannic acid vacuoles appeared in the 

 protoplasm of pulp cells immersed in methyl blue or ferric chloride, so that 

 the assumption that the whole of the tannic acid was present in the cell sap 

 did not appear to be correct. No such vacuoles could, however, be detected 

 in living pulp cells, or in the protoplasm of cells killed by heat prior to 

 staining. Further, although pulp from which most of the sap has been 

 expressed turns, if anything, darker with FeCl 3 than before, this may be 

 merely because the tissue is more compacted. In addition, if slices of pulp 

 are subjected to very strong pressure between wads of filter paper until the 

 pulp is quite dry, on moistening with water the pulp remains colourless or 

 the veins may turn brown, and no distinct tannic acid reactions are given 

 with FeCl 3 , KCN, or iodine and ammonia. Microscopic examination shows 



