Leaf explants of Convolvulus arvensis produce shoots when cultured on Murashige and Skoog salts, sucrose, vitamins and 0.05 mg/liter IAA plus 7.0 mg/liter 2-isopentenyl adenine. Shoot-inducing, root-inducing, or callus-inducing medium (SIM, RIM, or CIM) will cause small amounts of callus to form at the cut edges of the explant. This first-formed callus is developmentally interchangeable: SIM induces shoots in callus formed on CIM or SIM with equal effect and efficiency. Once induction begins in competent callus, the callus is no longer interchangeable. Under the continued influence of SIM, cells, or groups of cells become determined for shoot formation. This determination is strongly canalized for shoot formation: subsequent transfer to root-inducing medium does not affect the formation of shoots by the explant. The control of organogenesis by the auxin/cytokinin balance must occur between the time the tissue becomes competent and the time it is determined for shoot (or root) development. It is not known whether this control is a single or multiple phenomenon.