Angle Meristem Origin and Development in Selaginella martensii

The origin and early development of angle meristems (AMs) in Selaginella martensii were studied using a high-resolution mould and cast technique for scanning electron microscopy, in conjunction with light microscopy. Both dorsal and ventral AMs originate in the apical region shortly after the most recent bifurcation of the shoot apex. AMs appear to arise from a single superficial initial cell, which subsequently undergoes an oblique anticlinal division to establish the apical cell of the AM. The AM apical cell has a relatively short active life and divides laterally usually only six to twelve times; most cells of the AM originate from repeated divisions of apical cell derivatives. Dorsal and ventral AMs show similar developmental sequences, although they differ slightly in time of appearance. Ventral AMs originated first, dorsal AMs appearing shortly thereafter. At a given branch point, the ventral AM is typically larger than the corresponding dorsal AM. Both dorsal and ventral AMs grow out as rhizophores (aerial roots), although ventral AM development greatly precedes dorsal AM development. It is possible that the apparent earlier decrease in apical cell activity in dorsal AMs compared to ventral AMs could account for the lag in dorsal AM development as a rhizophore at a given branch point.