“…A number of situations are recorded in the literature regarding unusual patterns of vascular organization in dicotyledons, including the presence of bicollateral, medullary or cortical bundles, intraxylary and interxylary phloem and the development of arcs, strands or complete rings of xylem and phloem alternating concentrically with one another in a root or shoot (see Solereder, 1899;Metcalfe & Chalk, 1950;Metcalfe, 1983). The origin and functioning of the secondary tissues in question have been subject to much debate (Philipson & Ward, 1965;Balfour, 1965;Studholme & Philipson, 1966;Esau & Cheadle, 1969;Wheat, 1977;Mikesell, 1979) particularly as to whether cambial activity is unidirectional or conventionally bidirectional, and, if successive cambia are present, whether these form from a previous cambium or independently outside the existing vasculature. This paper reports on the occurrence in certain species of Daviesia (Fabaceae) of a highly distinctive type of secondary thickening in which concentrically arranged layers of interconnecting vascular strands are added progressively to the root.…”