Light and temperature may affect flowering in plants in various ways. The effect of photoperiods or daily duration of light hours (and dark periods) on flowering is called photoperiodism. For example, in short day plants flowering occurs when day length is below critical period, e.g., dahlia, rice, etc. In long day plants, flowering occurs when day length is above critical period, e.g., spinach, lettuce, etc. In short-long day plants, short photoperiod is required for floral initiation and long photoperiod is required for blossoming and vice-versa for long-short day plants.