The problem of the retardation of the processes of growth and differentiation is certainly as important as the processes of growth and differentiation themselves. It is striking, therefore, that whereas the analysis of growth has been carried out for a considerable period of time already, the analysis of inhibition was only commenced a few decades ago. It has to be admitted that Wiesner (1894) succeeded in demonstrating the presence of a substance retarding germination in the slime of the mistletoe (Viscum album), but this remained a solitary observation for some time. About 1920 a series of important publications appeared which deal with inhibiting substances. Oppenheimer (1922) discovered a substance of this kind in the fruit pulp of ripe tomatoes, Reinhard (1933) found one in tomato juice, Köckemann (1934) some in other pulpy fruits such as apples, pears, quinces and tomatoes, Lehmann (1937) one in the exocarp of buckwheat, Ruge (1939) some in the fruits of Helianthus annuus and Avena sativa, Fröschel (1939, 1940) one in Beta, Stolk (1952, 1953a) some in the roots of Fuchsia hybrida and Pelargonium zonale and in the roots of Allium Cepa.