Rhizophore and root development in Selaginella martensii: meristem transitions and identity

Rhizophores arise from angle meristems located at shoot branch points in many species of Selaginella and produce roots at their distal ends. We reinvestigated the origin and development of rhizophores and roots in Selaginella martensii using a nondestructive sequential replica technique for scanning electron microscopy and anatomical studies of sectioned material. We followed development in excised branch systems containing undetermined ventral angle meristems Analysis of patterns of division and expansion of cells on the surface of apical meristems and in transverse and longitudinal sections showed that the outgrowth from a ventral angle meristem had three distinct developmental phases: rhizophore, aerial root, and subterranean root, each with a distinctive apical meristem structure and activity. Rhizophore formation involved division of all cells in the angle meristem and establishment of multiple surface initial cells from which rhizophore growth ensued. The aerial root phase began with formation of two internal root meristems, each with a single prominent apical cell, replacing the multiple surface initials. From these meristems, dichotomously branched aerial roots formed, which continued to grow and branch internally. Eventually, subterranean roots formed, bearing root hairs and root caps. Formation of subterranean roots was induced at any time after aerial roots reached 0.5-1 cm in length when the tip of the organ touched a substrate. Our data provided support for the original concept of the rhizophore as a unique root-bearing organ in S. martensii, as well as for a developmental succession of apical meristem structure and activity, specific forms of which characterized the rhizophore and true root stages of angle meristem outgrowth.