Almost twenty years ago DE DuvE discovered the existence in rat liver cells of a novel class of subcellular structures which he termed lysosomes. As the lysosomes are the seat of various hydrolytic enzymes this discovery greatly stimulated the interest in cellular digestive processes. It is now recognized that compartmentation of hydrolases is an outstanding example of the dependence of metabolic functions on cell structure. The important role of lytic processes in plant metabolism made it interesting to correlate the facts on the various phenomena and present them from the point of view of cellular compartmentation. It is not intended to survey in extenso the literature on the various aspects of localization and function of hydrolases in plant cells as this would be an impossible task. The aim of this monograph is to emphasize the significance of hydrolase compartmentation rather than to strive for complete coverage of work on hydrolases in which other viewpoints have been considered. I thank Miss SONIA TURLER for her great help with the manuscript. I also wish to acknowledge the assistance given by Dr. ELSA HAUSERMANN, Mrs. DORLI FURRER, and Mrs. SILVIA STUNZI. Moreover, I am indebted to my collaborators and particularly to my colleague Dr. A. WIEMKEN for frequent and stimulating discussions on the subject. Zurich (Switzerland), February 1975 PH. MATILE Contents Abbreviations . XIII Introduction: Lysis and the Lytic Compartment. 1 1. Hydrolases and their Localization . . . . . 3 1. 1. Classification and Properties of Hydrolases 3 1. 1. 1. Peptidases . . . . . . . . .