J. F. Smith, M. E. Kresge, M. Möller, and Q. C. B. Cronk, “A cladistic analysis of ndhF sequences from representative species of Saintpaulia and Streptocarpus subgenera Streptocarpus and Streptocarpella (Gesneriaceae)”, Edinburgh J. Bot., vol. 55, no. 1, pp. 1–11, Mar. 1998, doi: 10.1017/S0960428600004315.