The structure of the shoot apex of Z. integrifolia, Z. silvicola, and Z. umbrosa is described. Within the massive meristem the cells are arranged in 4 distinct zones each of which plays a definite role in the development of the mature shoot. A superficial group of cells (zone I) occupies the apex of the shoot and gives rise to the entire meristem. A conspicuous core of mother-cells (zone II) underlies zone I from which it has been derived by periclinal divisions. This region is characterized by increase in volume accomplished by cell division in the upper layers and cell enlargement in the lower. A mantle of peripheral cells (zone III) flanks zone II. Its outer layers originate from the superficial cells of the apex and the inner ones from oblique divisions in the central mother-cells. Leaves, cortex, vascular tissues, and probably a portion of the pith are derived from this zone. The structure of the apex is completed by a shallow rib meristem (zone IV) of vertically aligned cells produced entirely from the base of zone II and which separates it from the maturing pith.