N. K. B.  Robson, Studies in the genus Hypericum L. (Guttiferae): 3. Sections: 1. Campylosporus to: 6a. Umbraculoides., vol. 12. Bull. Brit. Mus. (Nat. Hist.) Bot., 1985, p. 325p. (1985) - illus., maps, chrom. nos., keys.