McNeill, J.; Barrie, F.R.; Burdet, H.M.; Demoulin, V.; Hawksworth, D.L.; Marhold, K.; Nicolson, D.H.; Prado, J.; Silva, P.C.; Skog, J.E.; Wiersema, J.; Turland, N.J. (eds). 2006. International Code of Botanical Nomenclature (Vienna Code). Adopted by the Seventeenth International Botanical Congress Vienna, Austria, July 2005. A. R. G. Gantner, Rugell, Liechtenstein. online here