The phenomenon of photoperiodism was first discovered by Garner and Allard (1920, 1922). They observed that maryland mammoth variety of tobacco could be made to flower only by reducing the light hours with artificial darkening. On the basis of photoperiodic response to flowering, plants have been divided into short day plants (tobacco), long day plant (eg, wheat, hanbane), short-long day plants (eg, Campanula), long-short day plants (eg, Bryophyllum), intermediate plants (eg, wild kidney bean) and day neutral plants (eg, cotton).