For Hexasepalum
Bacigalupo, N.M., and E.L. Cabral. 1999. Revisión de las especies americanas del género Diodia (Rubiaceae, Spermacoceae). Darwiniana 37: 153-165.
Cabaña Fader, A.A., E. Bezerra de Souza, E.K. de Sousa Brandão, and E.L. Cabral. 2019. Hexasepalum nordestinum (Rubiaceae): A new species from two disjoint and fragmented areas in northeast Brazil, with a key to the American species of the genus. Syst. Bot. 44: 203-209.
Kirkbride, J.H., Jr., and P.G. Delprete. 2015. New combinations in Hexasepalum (Rubiaceae: Spermacoceae). J. Bot Res. Inst. Texas 9: 103-106.
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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