For Houstonia
Church, S.A. 2003. Molecular phylogenetics of Houstonia (Rubiaceae): descending aneuploidy and breeding system evolution in the radiation of the lineage across North America. Molec. Phylogen. Evol. 27: 223-238.
Church, S.A., and D.R. Taylor. 2005. Speciation and hybridization among Houstonia (Rubiaceae) species: the influence of polyploidy on reticulate evolution. Am. J. Bot. 92: 1372-1380.
Glennon, K.L., and S.A. Church. 2015. Gene flow contributes to genetic diversity of tetraploid populations of the North American plant genus Houstonia (Rubiaceae). Rhodora 117: 41-66.
Guo, X., R.-J. Wang, M.P. Simmons, P. P.-H. But, and J. Yu. 2013. Phylogeny of the Asian Hedyotis-Oldenlandia complex (Spermacoceae, Rubiaceae): evidence for high levels of polyphyly and the parallel evolution of dilpophragmous capsules. Molecular Phylogenetics and Evolution 67: 110-122.
Pease, A.S., and A.H. Moore. 1907. An alpine variety of Houstonia caerulea. Rhodora 9: 209-210.
Rogers, G.K. 1987. The genera of Cinchonoideae (Rubiaceae) in the southeastern United States. J. Arnold Arb. 68: 137-183.
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.
Shanks, H.L. 2015. Molecular phylogeny of the genus Houstonia and allies in Rubiaceae. M.S. thesis, Biological Sciences, Old Dominion University. DOI: 10.25777/th0k-rv14 https://digitalcommons.odu.edu/biology_etds/3
Terrell, E.E. 1959. A revision of the Houstonia purpurea group (Rubiaceae). Rhodora 61: 157-181, 188-207.
Terrell, E.E. 1986. Taxonomic and nomenclatural notes on Houstonia nigricans (Rubiaceae). Sida 11: 471-481.
Terrell, E.E. 1991. Overview and annotated list of North American species of Hedyotis, Houstonia, Oldenlandia, and related genera. Phytologia 71: 212-243.
Terrell, E.E. 1996. Revision of Houstonia (Rubiaceae-Hedyotidae). Syst. Bot. Monographs 48: 1-118.
Terrell, E.E. 2001. Taxonomy of Stenaria (Rubiaceae: Hedyotitideae), a new genus including Hedyotis nigricans. Sida 19: 591-614.
Terrell, E.E. 2007. Relationships of Houstonia prostrata (Rubiaceae) of Mexico and Arizona and a review of Houstonia subgenera and sections. J. Bot. Res. Inst. Texas 1: 109-119.
Turner, B.L. 1995b. Taxonomic overview of Hedyotis nigricans (Rubiaceae) and closely allied taxa. Phytologia 79: 12-21. [with corrected map: Phytologia 80: 142-143]
Turner, B.L. 1997. Rebuttal to Terrell's taxonomic notes of Turner's treatment of Texan and Mexican Hedyotis. Phytologia 82: 82-85.
Ward, D.B. 2004c. New combinations in the Florida flora II. Novon 14: 365-371.
Wikström, N., S. Neupane, J. Kårehed, T.J. Motley, and B. Bremer. 2013. Phylogeny of Hedyotis L. (Rubiaceae: Spermacoceae): redefining a complex Asian-Pacific assemblage. Taxon 62: 357-374.
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