Cotyledonary senescence of Xanthium strumarium was delayed by removal of the supracotyledonary shoot, and this delay was accompanied by an increase in dry weight and surface area of cotyledons. Gibberellic acid (GA3) was found to enhance cotyledonary senescence and benzyladenine to retard it. Growth and senescence of cotyledons was completed in three successive stages-an initial phase when both dry weight and surface area increased, a second phase of dry weight decrease and surface area increase, and a third phase when dry weight decreased and cotyledons ceased to expand. Removal of the supracotyledonary shoot delayed the commencement of the second and third phases by at least 10 days. Relative amounts of chlorophyll, nucleic acid, and protein differed in cotyledons of normal seedlings and cotyledons of those seedlings in which supracotyledonary shoots had been removed.