For Richardia
Krings, A. 2002. Floral variation and disgnosis of Richardia (Rubiaceae) in the Carolinas. Castanea 67: 329-330. Google Scholar
Lewis, W.H., and R.L. Oliver. 1974. Revision of Richardia (Rubiaceae). Brittonia 26: 271-301. Google Scholar
Rogers, G.K. 2005. The genera of Rubiaceae in the southeastern United States, part II. Subfamily Rubioideae, and subfamily Cinchonoideae revisited (Chiococca, Erithalis, and Guettarda). Harvard Papers in Botany 10: 1-45. Google Scholar
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