N. K. B.  Robson, ?Studies in the genus Hypericum L. (Guttiferae) 4(1). Section 7. Roscyna to 9. Hypericum sensu lato (part 1).?, Bull. Nat. Hist. Mus. (London), Bot., vol. 31, pp. 37-88 (2001) - illus., 2001.