For Digitalis
Barringer, K.A., and N.A. Harriman. 2019b. Digitalis. 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
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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