Most laboratory studies of proteolytic enzymes are carried on with preparations of the powerful members of this class of enzymes which are found in the digestive tract of animals, namely, the pepsin of the gastric juice, which acts in the acid medium, in the stomach, and the trypsin of the pancreatic juice, which acts in the alkaline medium of the intestinal tract. But even these powerful proteases require several hours for the transformation of an amount of soluble albumin into its amino-acid constituents which is equivalent to the amount of starch which is hydrolyzed to maltose by diastase in a very few minutes. Enzymes which govern oxidative changes, known respectively, as _catalases_ and _oxidases_, are almost universally present in plants. Catalase decomposes peroxides, with the liberation of free oxygen. It is, therefore, necessary to the final step in the process of photosynthesis, as elucidated by Usher and Priestley (see page 26), and serves to prevent the destructive action of hydrogen peroxide upon chlorophyll. The almost universal presence of oxidases in plant tissues has been repeatedly demonstrated. They are present in especially large amounts in tissues which are being acted upon by parasitic fungi or are combating unfavorable conditions of growth. The oxidases, in such cases, seem to be the agents by which the plant is able to stimulate its metabolic activities to overcome the unfavorable environment for its normal development. In vegetables and fruits, the common browning, or blackening, of the tissues when cut surfaces are exposed to the air has been demonstrated to be due to the catalytic oxidation of the tannins or of certain amino-acids, especially tyrosine, under the influence of the oxidases which are present in the tissues. In fact, most pigmentation phenomena are due to changes in the oxygen content of the chromogens of the cells of the plant, under the influence of the oxidases which are present in the protoplasm of the cells in question. Hence, the oxidases may be said to be the controlling agencies for both the energy-absorbing activities and for respiration in plants. THE NATURE OF ENZYME ACTION The mechanism by which an enzyme accomplishes its catalytic effects has been the object of extensive studies during recent years, especially since the discovery by Büchner that enzymes could be isolated in solutions entirely free from the disturbing influence of growing cells. Several theories concerning the mode of this catalytic action have been advanced. The earliest and simplest of these was that the enzyme simply creates an environment favorable for the particular chemical reaction to take place, as by exposing large surfaces of the substance in question to the action of the hydrolytic, or other effective, agent, by means of surface adsorption of the substrate material on the colloidal enzyme.