For Gratiola
Estes, D., and R.L. Small. 2007. Two new species of Gratiola (Plantaginaceae) from eastern North America and an updated circumscription for Gratiola neglecta. J. Bot. Res. Inst. Texas 1: 149-170.
Estes, D., and R.L. Small. 2008. Phylogenetic relationships of the monotypic genus Amphianthus (Plantaginaceae tribe Gratioleae) inferred from chloroplast DNA sequences. Systematic Bot. 33: 176-182.
Freeman, C.C. 2019b. Gratiola. In Flora of North America Editorial Committee. 2019. Flora of North America north of Mexico. Volume 17, Magnoliophyta: Tetrachondraceae to Orobanchaceae. Oxford Univ. Press, New York, NY. 737 pp.
Pennell, F.W. 1935. The Scrophulariaceae of eastern temperate North America. Academy of Natural Sciences of Philadelphia Monograph No. 1.
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