The fact that plant life depends a great deal on enzymes was recognized early, and enzymes are receiving the constant attention of workers in botanical sciences. Because one deals with enzyme action rather than with isolated enzymes, enzyme chemistry is chiefly a science of methods. The results obtained in any specific case may yield a basis for conflicting conclusions. The reason for this lies in the fact that most methods for the estimation of enzymes have been worked out by enzyme chemists rather than by those interested in enzyme chemistry as a tool for the botanical sciences. The methods to be used, and the facts observed will greatly depend on the point of view of the worker. In too many cases, results and conclusions obtained by the botanist by the use of strictly enzyme chemical methods fail to take into account the actual role and significance of the enzyme in the plant. The enzyme chemist is aware of the unstable character of the enzyme. The botanist has to go one step farther because he is dealing with the equally unstable and complex plants which produce and contain the enzyme. From the standpoint of the determination of plant enzymes, the conditions under which the substrates of enzyme action exist in the cell should be considered first. The protoplasm is a constantly changing system in which the different materials exert great influence on each other. It has also been shown that many important chemical constituents of the cell such as proteins, polysaccharides, and lipids may form homogeneous or heterogeneous complexes or aggregates which have different properties, hence behave entirely differently toward enzymes. The size and complexity of such aggregates, furthermore, are undergoing almost steady changes in the protoplasm. Naturally the substrate may also react with other chemical constituents of the cell, producing reversible or irreversible compounds of different affinity to the enzyme. Thus the condition of the substrate of enzyme action in the cell may be entirely different from that in a test-tube, and also may show great qualitative and quantitative variation within a short period of time. Similar to an enzyme, a substrate may also go through changes making it unavailable or ' ' inactive. ' ' The substrate may be separated from the enzyme by occlusion. Occlusion, however, should be regarded as a case of spatial i Head before the joint session of the American Society of Plant Physiologists, American Society for Horticultural Science, and the Physiological Section of the Botanical Society of America with the A. A. A. S., held at Rochester, New York, June 17, 1936. Also an introduction to the recommendations on the determination of plant enzymes contemplated by the Committee on Chemical Methods of the American Society of Plant Physiologists, Approved by the Director of the New York State Agricultural Experiment Station for publication as Journal Paper no. 181. 845