The shoot apical meristem (SAM),which produces all the aerial parts of the plant,plays a major role in the establishment of plant architecture.In many higher plants,the SAM has a stereotypic organization in zones with different functions.At the summit of the dome-shaped meristem is a population of stem cells (called the central zone),required for meristem maintenance.Part of the cells produced by the central zone will enter a differentiation pathway and their descendants will serve as founders for the stems and lateral organs at the meristem periphery (or peripheral zone).