The term hormone (Gk: hermao = to excite) was first used by W.M. Bayliss and E.H.1 Starling of London university college, in 1904 to denote chemical substance (secretin)] from intestine inducing pancreatic secretion.
Went and Thiemann (1937) defined a hormone as “a substance produced in any one part of an organism (called effectors), is transferred to another part (called target) and there influences a specific physiological process”.
The process of growth and development in both plants and animals are influenced by hormones along with nutrients and environment.
Thimann (1948) coined the term ‘Phytohormones” for plant hormones and defined them as “Organically compounds produced naturally in higher plants, controlling growth or other physiological functions at a site remote from its place of production and active in minute amounts.” Later on it was found that Phytohormones or plant growth regulators or plant growth substances can be substances naturally produced by plants or synthetic substances not secreted by plants.
Hence ‘plant growth regulators” is the term used to describe all those natural or synthetic compounds influencing growth and physiological process in plants. Hence, plant growth regulators are “naturally occurring or synthetic compounds which in very minute amount promote inhibit or modify the plant growth and physiological processes.”
Major kinds of plant hormones arc auxins, gibberellins, cytokinins, abscisic acid and ethylene. Of these, auxins, gibberellins and cytokinnins are growht promoter while abscisic acid and cytokinins are growth retarders or inhibitors.
The presence of growth regulating hormones in plants was first suggested by Julius Von Sachs in 1980. Famous evolutionist charles Darwin in 1880 for the first time proposed the presence of a growth substance in the tip of the plant responsible for causing curvature of decapitated coleoptile tip due to the action of unilateral light.