Polycentric vascular rays in Suaeda monoica and the control of ray initiation and spacing

Summary The early-formed xylem of Suaeda monoica Forssk. ex J. F. Gmel (Chenopodiaceae) is temporarily rayless. Vascular rays differentiate during later stages of its xylem ontogeny. The rays in Suaeda are heterogeneous, and some of them are aggregated. The mature xylem of this species is characterized by two unique types of vascular rays: (1) rays with several inside initiation centres of small cells formed by local frequent cell divisions in the cambium, and (2) huge xylem rays with radial phloem strands that are connected to the axial phloem. The spacing of the xylem rays is not even, and possible mechanisms controlling ray spacing are discussed. Our observations indicate that rays do not have an inhibitory zone around them in which ray initiation is prevented. The initiation of radial patterns of small cells which appear like inside rays within a large vascular ray suggests that initiation and spacing of rays is controlled by radial signal flows in relationship with axial signal fluxes.