Fig. 4. Shoot growth occurs from two apical initials. (A) Dorsal view of the flattened S. kraussiana shoot showing major and minor branch structures. White square indicates meristem position. (B) Scanning electron micrograph of a meristem, and schematic showing the four axes that are established during shoot growth. The oval indicates four cells at the tip. (C-F) Transverse sections through the shoot apex at 3 µm intervals showing: two central initials flanked by two lateral merophytes on the surface (C); the division of the two central initials to produce dorsal and ventral merophytes, and radial division of the two lateral merophytes (D); further radial divisions in the epidermal cell layer (E); and finally, at five cell layers below the surface, approximately 30 large epidermal cells, and a ventral region of five to six small epidermal cells in the predicted position of leaf initiation (p) (F). (G-J) Schematics of sections C-F showing epidermal cell division patterns deduced from both histology and sector analysis. Black lines indicate the most recent division, and insets indicate stages that are not apparent in the sections shown but that must have occurred between the layer shown and the next layer down. m, merophyte. (K-M) Schematic representation of shoot sectors occupying one-half (K), one-quarter (L) and one-tenth (M) of the shoot circumference. The sector in L is a twin spot, most likely induced by somatic recombination rather than by independent aneuploidy events in two neighbouring cells. Scale bars: A, 5 mm; B, 22 µm; C-F, 18 µm.