Christenhusz, M.J.M., Fay, M.F. & Chase, M.W. 2017. Plants of the World, an illustrated Encyclopedia of vascular plants. Kew Publishing, Royal Botanic Gardens, Kew.
http://pza.sanbi.org/references/christenhusz-mjm-fay-mf-chase-mw-2017-plants-world-illustrated-encyclopedia-vascular