In any study of the "strength," or possible catalytic effects, of an enzyme preparation, it is necessary, first, to determine what particular reaction it affects, by qualitative tests with various substrate materials, such as starch, sugars, glucosides, proteins, etc., and then to determine quantitatively its accelerating effect upon the reaction in question. The latter may be done by measuring either the _time_ required to carry a unit quantity of the substrate material through any determined stage of chemical change, or the _quantity_ of the substrate which is changed in a unit period of time. It would not be profitable to go into a detailed discussion here of the methods of making these quantitative measurements of enzyme activity. Such discussions must necessarily be left to special treatises on methods of study of enzyme action. It may be said, however, that generally both the qualitative tests for, and the quantitative measurements of, the accelerating influence of enzymes depend upon the observation of some change in the physical properties of the substrate material, such as the optical activity, electrical conductivity, or viscosity, of its solution. In some cases, it is convenient to make an actual quantitative determination of the amount of end-products produced in a given time, as in the inversion of cane sugar, the hydrolysis of maltose, etc., but such determinations necessarily involve the removal of some of the reaction mixture for the purposes of the determinations, and are not, therefore, suitable for the study of the progressive development of the reaction which is being studied. Enzymes are found in all parts of the animal organism and those which are active in the digestion of food, the metabolism of digested material, the coagulation of blood, etc., have been extensively studied. A discussion of these animal enzymes would be out of place in such a text as this, however, and the following list includes only enzymes which are known to occur in plant tissues. These well-known enzymes will serve as examples of the several general types which have thus far been isolated and studied.