The anatomical structure and development of adventitious roots were analysed in the basal monocotyledon, Acorus calamus, to determine to what extent those features are related to phylogenetic position.Root specimens were harvested and sectioned, either with a hand microtome or freehand, at varying distances from the root tip and examined under the microscope using a variety of staining techniques.Roots of Acorus calamus possess a unique set of developmental characteristics that produce some traits similar to those of another basal angiosperm group, Nymphaeales. The root apical meristem organization seems to be intermediate between that of a closed and an open monocotyledonous root apical meristem organization. The open-type root apical meristem consists of a curved zone of cortical initials and epidermal initials overlying the vascular cylinder initials; the epidermal part of the meristem varies in its association with the cortical initials and columellar initials of the promeristem. The cortex develops an endodermis with only Casparian bands, a dimorphic exodermis with Casparian bands and suberin lamellae, and a polygonal aerenchyma by differential expansion, as also observed in the Nymphaeales and some dicotyledonous species. The stele has characteristics like those of members of the Nymphaeaceae.Specific anatomical and developmental attributes of Acorus roots seem to be related to the phylogenetic position of this genus.