For Richardia
Krings, A. 2002. Floral variation and disgnosis of Richardia (Rubiaceae) in the Carolinas. Castanea 67: 329-330.
Lewis, W.H., and R.L. Oliver. 1974. Revision of Richardia (Rubiaceae). Brittonia 26: 271-301.
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.
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