All plant tissue is ultimately derived from the meristems, and the molecular mechanisms that control growth of apical meristems have been widely studied (reviewed in). In contrast, much less attention has been paid to vascular meristems, such as the cambium and procambium, even though these meristems are the source of woody tissue and therefore generate the majority of plant biomass. Although biomass may represent a novel source of renewable energy, little is known about the molecular regulation of vascular-meristem activity. The vascular meristems participate in a highly ordered developmental process with a very prominent polarity. This polarity results in precisely orientated divisions of meristematic initials that generate files of cells, which differentiate into highly specialized and spatially separated xylem and phloem cells. The factors that are necessary to establish and maintain this polarity remain unknown. This manuscript describes the identification of the PXY mutant in which the spatial organization of vascular development is lost and the xylem and phloem are partially interspersed. The PXY gene encodes for a receptor-like kinase (RLK) that defines a novel role for RLKs in the meristem where it functions to maintain the cell polarity required for the orientation of cell division during vascular development.