For Penstemon
Bennett, R.W. 1963. Notes on eastern American species of Penstemon. Phytologia 9: 57-58. Google Scholar
Clements, R.K., J.M. Baskin, and C.C. Baskin. 1998. The comparative biology of the two closely-related species Penstemon tenuiflorus Pennell and P. hirsutus (L.) Willd. (Scrophulariaceae, section Graciles): I. Taxonomy and geographical distribution. Castanea 63: 138-153. Google Scholar
Estes, D. 2012. Penstemon kralii (Plantaginaceae), a new species from Alabama and Tennessee, with an updated key to the Southeastern U.S. taxa. J. Bot Res. Inst. Texas 6: 1-8. Google Scholar
Freeman, C.C. 2019d. Penstemon. 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. Google Scholar
Nelson, A.D., and J. Stanford. 2023. Penstemon albidus (Plantaginaceae) in the Lampasas Cut Plain Ecoregion of Texas. Phytologia 105(2): 25-28. Google Scholar
Pennell, F.W. 1935. The Scrophulariaceae of eastern temperate North America. Academy of Natural Sciences of Philadelphia Monograph No. 1. Google Scholar
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