Zonal Structure of the Shoot Apex in Encephalartos, Bowenia, and Macrozamia

1. The structure of the shoot apex is described for six species of Encephalartos and for one species each of Bowenia and Macrozamia. 2. The shoot apex consists of four zones. The summit is occupied by a zone of initiation from which growth converges to a zone of central mother cells where increase in volume predominates. Growth is divergent from the central mother cell zone. 3. A peripheral zone, which is derived from the central mother cells and superficial layer, flanks the central mother cell zone. Leaf primordia, cortex, and vascular tissue are derived from this zone. 4. The zone of central mother cells is underlaid by a rib meristem which builds up the massive pith.