Water is essential for life and without water no life exists. The liquid sur- rounding of an aqueous solution is the conditio sine qua non for most of the physiological responses and as such, water is as decisive for the occurrence of a single enzymatic reaction as it is for the global zonation of world vegetation. It is no wonder that scientists since early times have made every effort to describe and understand the functional interrelationships between water and the phe- nomenon of life. During the past half of this century, these endeavours in the field of botany have been marked by steps which might be symbolized by a series of books such as "The plant in relation to water" (1. Maximov, 1929), and "Die Hydratur der Pflanze in ihrer physiologisch-6kologischen Bedeutung" (H. Walter, 1931), then "Pflanze und Wasser" (Vol. III of the Encyclopedia of Plant Physiology, edited by O. Stocker, 1956), and "Plant-water relations" (R. O. Slatyer, 1967), or the treatment of "Displacement of water and its control of biochemical reactions" (S. Levin. 1974).