Hormone is a Greek word derived from ‘hormao’ which means to urge or to stimulate. Thimann suggested to use the term ‘phytohormone’ for hormones of plant origin. According to him phytohormones are organic substances which are naturally produced in plants that regulate the growth or other physiological functions at a site remote from its place of production and active in extremely minute quantities. In plants growth is regulated through interactions of several growth substances. This can be well explained taking the example of vegetative growth in plants which depends upon cell division, enlargement and differentiation. All these are influenced by auxins, gibberellins, cytokinins, abscisic acid, ethylene etc. These have variable effects on the processes e.g. auxins increase cell division in cambium but inhibit the growth of lateral buds. Gibberellins also influence cambial growth and promote cell division in subapical (internodal) regions of shoot. Cytokinins promote cell division in callus tissues and promote growth of lateral buds. Abscisic acid inhibits cell division but its effect can be over come by exogenous auxins, gibberellins, cytokinins etc. Ethylene has little effect on cell division but it has strong effect on cell enlargement.
