Some special chemical substances also contribute to the growth of plants in addition to the food prepared by plants and minerals absorbed by them. These chemicals are produced in very little amount. These chemicals are produced in one tissue or organ of the plant and sent to other parts for growth and development. Such chemicalsubstances are known as plant growth regulators. These are also called plant hormones or phytohormones. The word �hormone� is derived from a greak work �hormao� which literally means to �stimulate�. Many organic compound also work as phytohormones. A plant hormone may be defined as an organic substance which is produced in one tissue of the plant and transfer to another tissue where in small quantities it affects the physiological activities of the plant, especially growth and development (F.W. Went and Thiman; 1937). The word hormone was first of all used by Starling (1906). He used this term for some stimulating chemical substances. Plant hormones or phytohormones can be divided into following five groups: 1. Auxins 2. Gibberellins 3. Cytokinins 4. Abscisic Acid 5. Ethylene.