• Several kinds of radiation like gamma rays, X-rays, UV-rays cause mutation. • These are physical mutagens. • Such induced mutation in plants is done to develop improved varieties. The first natural cytokinin was isolated from unripe maize grain known as zeatin. The cytokinin that was obtained from degraded product of autoclaved herring sperm DNA was kinetin ( N6− furfuryl aminopurine). Infrared rays cause heating effect.