Unusual morphological and anatomical features of two woody Madagascan endemics, Streptocarpus papangae and S. suffruticosus (Gesneriaceae), and their potential taxonomic value

As members of a small group of caulescent Madagascan endemics, Streptocarpus papangae and Streptocarpus suffruticosus are distinctive in being branched woody shrubs in a genus largely of herbaceous habit. The present study is the first detailed comparative investigation of any woody Madagascan Streptocarpus species and draws attention to certain notable morphological and anatomical characteristics, particularly their non-coherent anthers, foliar and stem sclereids, nodal anatomy (split-lateral or semi-girdling traces), and ovule morphology, features previously inadequately examined in Streptocarpus and their potential taxanomic value overlooked. For example, in a family where anatropous ovules were previously thought to be typical, the presence of hemi-anatropous ovules in Streptocarpus is confirmed and here recorded for the first time in the two species. Another feature is the occurrence of split-lateral traces, a distinctive nodal feature generally uncommon among angiosperms. Macrosclereids, rare among mainland African Streptocarpus have been observed in the stem, petiole, leaf lamina and receptacle of S. papangae but only near the base of the petiole in S. suffruticosus. The possible significance of these and other characters to the taxonomy and phylogenetic systematics of Streptocarpus is discussed.

► This is the first comparative anatomical study on two woody endemic Madagascan Streptocarpus, S. papangae & S. suffruticosus.
► Histological and other techniques revealed a number of notable features uncommon among other Streptocarpus species.
► These shrubby species are characterised by the presence of macrosclereids and noncoherent anthers.
► Split-lateral leaf traces and a basic chromosome number of x = 16 are an unusual combination of features in the genus.
► Aspects of these features were considered in terms of the systematics of the genus.