The Difference Between Open and Closed Meristems

Abstract

An open and a closed root meristem have been compared by investigating the cell kinetics of small regions of the apices of Helianthus and Zea. The cells of the stelar pole are quiescent in both and there is no exchange of cells between stele and cortex or stele and cap. The immediately distal cells in the closed meristem (Zea) are also quiescent and the few divisions that do occur can be transverse or longitudinal. In the open meristem (Helianthus) these cells are not quiescent, but they go out of cycle transiently, prolonging the potential cell-doubling time. Their divisions are transverse. It is a consequence of these differences that closed meristems form root caps discrete from the cortex whereas open meristems force instability in the boundary between the peripheral part of the cap and the cortex. Another consequence in roots with open meristems is a succession of columella complexes transversely displaced from each other by the state of flux in the meristem during the non-cycling phase of the proximal tier of cells, those immediately distal to the stelar pole. The results are discussed in relation to the ontogenetic onset of quiescence and the evidence for switches between open and closed operation of meristems.