Development of the leafy shoot in Sphagnum (Bryophyta) involves the activity of both apical and subapical meristems

This light- and electron-microscope study of four species of Sphagnum revea ls that stem elongation involves meristematic activities unique to the grou p and hitherto unrecognized. The internal tissue of the mature stem arises by the concerted activity of an apical (primary) and a subapical (secondary ) meristem. The primary meristem comprises the immediate derivatives of the single apical cell. Following a small number of divisions, the primary der ivatives differentiate into highly vacuolate parenchymatous cells with a st oried arrangement. Subsequently, the large vacuoles are replaced by numerou s small vacuoles and the cells then divide repeatedly, by transverse septa, producing files of about nine short cells. Finally, ninefold elongation of these secondary cells is responsible for extension growth of the main stem below the mature capitulum. An early step in primary differentiation is th e confinement of pre-existing plasmodesmata to distinct pitted areas. Furth er enlargement of the cells during primary and secondary differentiation in volves the thickening of non-pitted wall areas, followed by expansion and t hinning out, while the pitted areas remain virtually unchanged. A cortical array of microtubules is regularly found in association with non-pitted wal l areas, while the unexpanded pitted areas are associated with smooth endop lasmic reticulum showing continuity with desmotubules. Though sharing much the same cytology as the conducting cells in bryoid mosses, in terms of the ir development the central stem cells in Sphagnum are not homologous with t hose of other mosses. The unique mode of stem development may be an importa nt factor in the ecological success of Sphagnum.