Until recently the interest of plant physiologists in the structure and function of the root, as compared with the above-ground organs, was rather limited. In contrast to the data obtained from agricultural research, where the root is understood as an organ of water and nutrient uptake, recent studies on root systems have evolved as a consequence of an increase in our general knowledge of the transport of substances across biological cell membranes. This has led to increased interest in studies of ion uptake and transport from a novel viewpoint. Parallel to this, the complexity of the histological structure of the root has necessitated a study of this structure in relation to processes which take place in the root. The evaluation of such complex relationships is, in this book, concentrated on recent findings concerning the anatomical structure of the root in relation to its functional manifestations. Based on the principles of structure and function, the metabolic processes in the root, as well as the uptake and transport of water and ions are dealt with. The emphasis throughout is on the structural features. Finally, a short chapter has been included about the root under conditions of environmental stress.