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For Penstemon

Bennett, R.W. 1963. Notes on eastern American species of Penstemon. Phytologia 9: 57-58.

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.

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.

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.

Pennell, F.W. 1935. The Scrophulariaceae of eastern temperate North America. Academy of Natural Sciences of Philadelphia Monograph No. 1.

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