THE DEVELOPMENTAL PATTERN OF SHOOT APICES IN SELAGINELLA KRAUSSIANA (KUNZE) A. BRAUN

The pattern of cell division at shoot apices of Selaginella kraussiana is investigated using laser scanning confocal microscopy. The vegetative shoot possesses a single apical cell with two cutting faces that produce merophytes (apical cell derivatives) in a distichous manner. When the shoot dichotomizes, the apical cell divides into two cells, each of which gives rise to the apical cell of a new axis. Bifurcations of the apical cell occur close together, and by the time the first external signs of a dichotomy are apparent, the next bifurcation of the main axis may have already occurred. New apical cells have the same orientation as their parent, which accounts for the planar and plagiotropic habit of the shoot. Fertile (strobilar) shoot apices have an apical cell with three cutting faces and produce merophytes in a tristichous manner. Both vegetative and fertile shoots have a similar phyllotaxy, which suggests that the position of microphyll initiation is not determined by cell-lineage factors directly related to either apical cell shape or merophytes.