The ability of the plant to detect and respond to the length of daily period of light or more precisely, the relative length of day and night to which the plant is exposed, is called photoperiodism.
In responce to photoperiod the flowering responce of angiosperms falls into three basic catagories
[i) Short-day plants initiate flowering when the day become shorter than a certain critical length. If these plants are kept in day lengths in excess of this critical point, they will remain vegetative. The short day plants include Cosmos, Dahlia, Chrysanthemum, rice, etc., and are generally grown in the winter season.
[ii) Long-day plants begin flowering when the day length exceeds a critical length. The long-day plants are wheat, parley, sugar beet, larkspur, etc.
[iii) Neutral or intermediate day plants flower after a period of vegetative growth, regardless of
the photoperiod. Tobacco, cucumber, sunflower, tomato are some examples of the neutral or day intermediate day plants. The critical day length of both long-day and short-day plants tends to fall in the 12-14 hrs range.