Structure of the Shoot Apex in Zamia

1. The shoot meristem of Zamia integrifolia, Z. silvicola, and Z. umbrosa consists of four distinct zones, as follows: Zone I, a group of superficial initials occupying the broad apex of the shoot. The entire meristem has its origin in these cells. Zone II, a central core of mother cells derived by periclinal divisions from zone I. This is a region of increase in volume accomplished by rapid cell division in its upper layers and by cell enlargement in the lower. These later are rejuvenated and contribute to the surrounding meristematic zones. Ginkgo seems to be the only other vascular plant so far investigated which duplicates this region. Zone III, a series of peripheral layers flanking zone II. The outer layers originate from the superficial cells of the apex and the inner ones by oblique divisions of the central mother cells. Leaves, cortex, vascular tissue, and probably a portion of the pith originate in this zone. Zone IV, a shallow rib meristem of vertically aligned cells, produced entirely from the base of the central mother cell zone and separating it from the maturing pith. 2. The occurrence of starch throughout the shoot apices in plants collected during January and March suggests that the meristem undergoes alternating periods of dormancy and active growth.