Photoperiodic sensitivity of the cotyledon and first true leaf of Xanthium

Three days after germination of Xanthium seeds, the seedlingswith only the first pair of opposite leaves attained full photoperiodicsensitivity and flowered in response to a 16-hr dark periodtreatment. The cotyledons of three- to six-day-old seedlingswere completely insensitive to inductive dark treatment. Six-day-oldseedlings could not be induced to flower with a 9-hr dark period.But with a dark period of 10 hr or longer, floral inductionoccurred and the optimum dark period was 15 hr. Developmentof the flowering apices started within 2 days after the inductivedark treatment and was completed by 9 to 11 days. ¹This work was supported by the United States Atomic EnergyCommission Contract AT (11-1)-1338.²Present address: Plant Physiology Section, Crops Research Institute,P.O. Box 3785, Kumasi, Ghana, West Africa. (Received March 28, 1974; )