Photoperiodism. The ability of plant to detect and respond to the length of daily period of light or (relative length of day and night) to which the plant is exposed is called photoperiodism. It is a physiological change occurring in plants in response to relative length of day and night. The term photoperiodism. It is a physiological change occurring in plants in response to relative length of day and night. The term photoperiodism was used by Garner and Allard (1920) for the responses of plant to photoperiods expressed in the form of flowering. On the basis of photoperiod, there are three classes of plants. 1. Short day plant (SDP) e.g. Nicotiana tobaccum, Xanthium, Rice, Dahlia, Cosmos, Soyabean. 2. Long day plant (LDP) e.g. Henbane (Hyoscyamus niger), wheat, oat, sugar, beat, Radish, Lettuce. 3. Neutral or intermediate day plants or Short daylong plant (S-LDP) or Day neutral plants e.g. Tomato, Sunflower etc. Vernalization. The term vernalization was coined by Lysenko (1928) for promotion of flowering by a previous cold treatment. The method of inducing early flowering by pre treatment of seeds with a certain low temperature is known as vernalization. For flowering in winter varieties, winter cold treatment is necessary. In nature, plant requiring cold treatment is necessary usually behave as biennials. They germinate and grow vegetatively in first season and produce flowers during second season after getting the cold treatment. Some plants respond to the vernalization at seed or seedling stage e.g. secage cereale. It is now definitely known by various grafting experiments that growing point is the sire, which receives the cold stimulus. It is responsible for the production of a hormone like substance called as vernalin. It is basically a type of gibberellin.