This phenomenon has been studied in connection with the zymogens of the digestive proteases, pepsin and trypsin. Trypsinogen may be rendered active by contact with either calcium salts or with another substance (apparently itself an enzyme) known as enterokinase, which is secreted in the intestinal tract. Similarly, proenzymes have been reported as occurring in numerous plant tissues. These proenzymes are believed to be present in the plant cells in the form of definite characteristic granules, which may be observed under the microscope, and which disappear when the enzyme becomes active. Thus, "proinulase" has been reported as occurring in artichoke tubers: "prolipase," in castor beans; "proinvertase," in several species of fungi; and, probably, "prooxidase," in tobacco leaves. In the case of the last-named zymogen, it has been observed that after the zymogen has been once activated, as in response to the need for increased activity due to the entrance of the germs of certain leaf-diseases, it can once again produce a second supply of the enzyme, but the process cannot again be repeated. Calcium salts, or very dilute acids, are usually energetic activators of proenzymes. PHYSIOLOGICAL USES OF ENZYMES There can be no doubt that enzymes exert a tremendously important influence in vital phenomena, by determining the rate at which the chemical changes which are involved in these phenomena shall proceed. Since they do not initiate reactions, and since they may catalyze reversible reactions in either direction, it cannot be said that they determine the type of reactions which will take place in any given mass of protoplasm; but, undoubtedly, they do exert a determining influence upon the rate at which the reaction will proceed, after the protoplasmic activity has determined the direction in which it shall go. Without the intervention of these catalyzing agents, it would be impossible for reactions between these non-ionized organic components of the cell contents to come to completion with anything like the marvelous rapidity with which these changes must take place in order to permit the organism to grow, to perform its necessary vital functions, or to adjust itself to the changes in its environmental conditions. Since the number of different reactions which take place within a living cell is very great, and since these chemical changes are extremely variable in type, it follows that the number of different enzymes which must exist in either a plant or an animal organism is likewise very large. For example, fourteen different enzymes have been isolated from the digestive system, and at least sixteen from the liver, of animals. They are universally present in living protoplasm of every kind, from the most minute bacterium to the largest forest trees, in the plant kingdom; and from the am[oe]ba to the whale, in animals.