Recognizing Gesneria and Rhytidophyllum is relatively easy because of many shared derived characters (Fig. 2). In contrast, the distinction between Gesneria and Rhytidophyllum involves more careful evaluation. Taxonomists have differed on whether they should be combined as one large genus or retained as separate genera. Wiehler (1983) preferred a system of one genus (i.e., lumping “Rhytidophyllum” as a subgroup of Gesneria). The primary reason for Wiehler’s support of a single genus classification was based on the absence of a unifying character that defined or differentiated Gesneria and Rhytidophyllum.
In contrast, Skog (1976) published a comprehensive treatment of Gesneria where he recognized Gesneria and Rhytidophyllum as distinct genera. Also, Skog (1976) described the new genus, Pheidonocarpa, to accommodate a single species that did not fit the currently accepted genera. Although not the focus of this article, the recognition of Pheidonocarpa is supported as a monophyletic lineage based on molecular phylogenetic studies (Joly et al., 2017; Martén-Rodriguez et al., 2010; Watson 2015).