Independence of Organogenesis and Cell Pattern in Developing Angle Shoots of Selaginella martensii

Abstract

Angle meristems are mounds of meristematic tissue located at dorsal and/or ventral branch points of the dichotomising stem axes of many species of Selaginella (Lycophyta). The present study examined the development of ventral angle shoots of S. martensii in response to removal of distal shoot apices (decapitation). Scanning electron microscopy of sequential replicas of developing angle meristems and angle shoots revealed that for the first two pseudowhorls of leaf primordia, particular leaves are not attributable to particular merophytes of the angle meristem apical cell. Individual leaf primordia of the first (outer) pseudowhorl often form from more than one merophyte. Neither the shape of the angle meristem apical cell nor the direction of segmentation has any effect on the development of the angle shoot. Additionally, the apical cell of the angle meristem does not necessarily contribute directly to either of the new shoot apices of the developing angle shoot. The first bifurcation of the angle shoot shows a remarkably consistent relationship to the branching pattern of the parent shoot. The strong branch of the first angle shoot bifurcation typically occurs toward the weak side branch of the parent shoot. Anatomical studies showed that bifurcation of the young angle shoot involved the formation of two new growth centres some distance away from the original angle meristem apical cell; new apical cells subsequently formed within these. These results provide additional support for the view that cell lineage has little or no effect on final form or structure in plants.