As they were first supposed to be proteins, the earlier representatives of the group were given characteristic names ending with the suffix _in_, similar to that of the proteins. Since this idea has been found to be incorrect, however, a system of nomenclature has been adopted which assigns to each enzyme the name of the material upon which it acts, followed by the suffix _ase_. Thus, cellulase is the enzyme which accelerates the hydrolysis of cellulose; glucase, that acting upon glucose; amylase, that acting upon starch (_amylum_), etc. The substance upon which the enzyme acts (or, strictly speaking, the substance whose hydrolysis, oxidation, or other chemical change, is catalytically affected by the enzyme) is called the _substrate_. Most enzymes are catalysts for hydrolysis reactions and are, hence, classed as _hydrolytic_ in their action, and may be spoken of as "hydrolases." Those which accelerate oxidation are called "oxidases"; while those that stimulate reduction reactions are "reductases"; those that aid in the splitting off of ammonia, or amino-acid groups, are "deaminases"; and those that aid in the splitting off of CO_{2} from COOH groups are "carboxylases," etc. The hydrolytic enzymes are further subdivided into the sucroclastic (sugar-splitting), or sucrases; the lipoclastic (fat-splitting), or lipases; the esterases (ester-splitting); proteoclastic (protein-splitting), or proteases; etc. OCCURRENCE AND PREPARATION FOR STUDY Enzymes are present in all living matter. In animal tissues, they occur in the largest amounts in those glands or organs where active vital processes take place, as in the brain, the digestive tract, blood, etc. In plants, they may be found in all living cells, and are especially abundant in the seeds, where they serve to render soluble and available to the young plant the stored food materials. The enzymes of moulds, and other parasitic plants, are usually extracellular in type, being secreted for the purpose of making the material of the host plant available to the parasite. Extracellular enzymes are also developed in seeds during germination, in order that the stored food material of the endosperm may be rendered soluble and translocated into the tissues of the growing seedling. But most other plant enzymes are intracellular in type. Hence, in all preparations of plant enzymes for study, or for commercial use, the first step in the process is, necessarily, a thorough rupturing of the cell-walls of the plant material.