Molecular phylogenetic studies have been informative regarding the relationships of members of Cynareae, Centaureinae. Centaurea as traditionally recognized is polyphyletic. Plectocephalus is part of the basal grade in Centaureinae and is not closely related to Centaurea in a narrow sense (A. Susanna et al. 1995). Morphologically Plectocephalus is difficult to separate from the more derived and highly diverse Centaurea. Plectocephalus cypselae have obliquely basal attachment scars and the hilum of the seeds is basal. Centaurea cypselae have lateral attachment scars and hilum of the seed is lateral. The ovaries of Plectocephalus initially have 2-celled hairs; these may be absent from mature cypselae. Centaurea produces only 1-celled hairs on ovaries.