Dahlstedt (1900) synonymized P. diffusa with P. loxensis Kunth and this concept was followed by Yuncker (1974), Steyermark (1984) and Callejas (2003). However, P. diffusa is a distinct species with a much wider South American distribution than P. loxensis. P. diffusa has a more decumbent, more profusely branched habit and shows oblong to obovate leaves whereas P. loxensis is an erect plant with rather elliptic leaves [Mathieu 2007].