Early research on auxins led the American Society of Plant Physiologists to appoint a committee to propose some definitions. Plant regulators were defined as organic compounds, other than nutrients, which in small amounts promote, inhibit or otherwise modify any physiological process in plants (Tukey et al. 1954). The committee further recommended that plant hormones be defined as regulators produced by plants, which in low concentrations regulate physiological processes. Hormones usually move within the plant from a site of production to a site of action (Tukey et al. 1954). It was considered that growth regulation by auxin resulted from a physiologically effective auxin concentration acting on a responsive target tissue or organ (Thimann 1937, Leopold 1955). The early workers also viewed the regulation of physiological processes as an interaction between auxin and other growth-regulating compounds (van Overbeek 1959).