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

Although Penstemon tenuiflorus and P. hirsutus of eastern North America generally are regarded as closely-related species, the taxonomic level of their distinctiveness has been questioned. Based on evidence from herbarium, common garden, SEM, and field studies, we conclude that these two taxa are distinct at the species level. They differ in leaf pubescence, flower color, geographic distribution, and habitat preference. A key to the 18 taxa of Penstemon occurring east of the Mississippi River is provided.