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Experiments indicate that photoperiod, or daylength, is of great importance in the heading of rice plants and that maturity of rice varieties can be classified in the field by their sensitivity to above-optimum daylength. Early varieties such as Colusa are less sensitive and will head in the field under the longer days of midsummer, while Caloro, a midseason variety, is more sensitive and heads in the field later in the summer after the photoperiod has shortened considerably. A very late variety such as Texas Patna 49 is highly sensitive to photoperiod and does not head until the days have become much shorter in the fall.