“…50 a small loss persists, unaccompanied by consumption of free oxygen. In all experiments the loss of ascorbic acid was greater than the equivalent of the oxygen absorbed (cf, p, 38), The results so far described are consistent with the assumption that barley juice contains a direct ascorbic acid oxidase similar to that previously obtained from other plants (cf, summaries by Boswell &Whiting, 1940 andGreen, 1940), Ascorbic acid is Fig, 2, Effect of cyanide on oxygen absorption by barley juice + ascorbic acid. O,Q barley juice + ascorbic acid only, # barley juice + ascorbic acid-t-Af/500 cyanide, ^ barley juice + ascorbic acid + M/iooo cyanide, also known to be oxidizable by other systems present in plants; notably by o-quinones arising from the activity of peroxidase (Szent-Gyorgyi, 1928) or catechol (polyphenol) oxidase (Johnson & Zilva, 1937;Keilin & Mann, 1938); and by the cytochrome system (Keilin & Hartree, 1938), The following sections detail the results obtained with each of these systems: Peroxidase.…”