14. Penstemon Schmidel (beard-tongue)

Plants perennial or rarely biennial herbs, often taprooted, terrestrial. Stems erect or loosely to strongly ascending, usually unbranched below the inflorescence, variously glabrous or hairy, sometimes glandular-hairy or glaucous. Leaves opposite or rarely in whorls of 3 (in P. digitalis) (overwintering basal rosettes usually produced but often absent at flowering), sessile (except sometimes in basal leaves), sometimes clasping the stem. Leaf blades simple, unlobed, variously shaped, the margins entire or toothed, the surfaces variously glabrous or hairy, sometimes glandular-hairy or glaucous, the venation pinnate or occasionally only the midvein apparent. Inflorescences terminal panicles, with mostly whorls of branches along an elongate central axis, these terminating in clusters of flowers, the branches sometimes relatively short and ascending with relatively dense flower clusters (then appearing more or less racemose), each node with a pair of inconspicuous or conspicuous, leaflike bracts, the individual flower stalks sometimes short but always noticeable at flowering, not becoming elongated at fruiting; bractlets absent. Flowers perfect. Calyces sometimes becoming slightly enlarged at fruiting, deeply 5-lobed, the lobes equal or slightly unequal in length, lanceolate to ovate, usually sharply pointed at the tip, glabrous or glandular-hairy. Corollas 15–50 mm long, bilabiate or nearly actinomorphic, 5-lobed, glabrous or glandular-hairy externally, the tube longer than the lobes, white or lavender to pale bluish purple or purple, the throat often lined with pink to purple and yellow nectar guides, the tube variously shaped, lacking a spur, the throat more or less open (the lower lip sometimes thickened, convex, and/or bearded toward the base), the lobes variously angled outward, spreading, or somewhat reflexed (those of the lower lip in P. pallidus often not or only slightly spreading). Fertile stamens 4, the filaments of 2 lengths, not exserted (exserted elsewhere), somewhat curved inward toward their tips, the anther sacs spreading in our species; staminode 1, about as long as the stamens, linear to narrowly strap-shaped, sometimes very slightly thickened toward the tip, positioned along the lower side of the corolla tube, usually with a dense beard of yellow hairs, at least toward the tip. Style 1, not exserted, the stigma capitate, unlobed. Fruits capsules, ovoid, tapered to a beaked tip, glabrous (sometimes with minute papillae along the sutures), the 2 locules equal in size, dehiscent longitudinally along the 2 sutures. Seeds numerous, minute, ovate to rectangular, more or less trapezoidal, or triangular in profile, rounded or 3–6-angled in cross-section, the surface dark brown to black, often with lighter ridges, also usually with a fine network of minute ridges. About 270 species, North America, Central America.

Penstemon is a relatively large genus that is almost entirely distributed in North America (Straw, 1966). The greatest species diversity is in the Intermountain region of the western United States and many of the species are narrow endemics. The genus is relatively closely related to Chelone and both are characterized by the presence of a staminode in the flower. One morphological difference between the two genera is that in Penstemon nectar is produced by glandular hairs that occur on the staminal filaments, whereas in Chelone the nectar is produced by a glandular disc (nectary) at the base of the ovary. Chelone also differs in its closed (vs. usually open) corolla throat; unbranched, spicate inflorescences; increased production of bracts, flattened, winged seeds, and base chromosome number.

The eastern and midwestern species of Penstemon are in need of detailed taxonomic study. Species limits within the polyploid complex of white-flowered penstemons in the region are not well understood and the role of past hybridization in the formation of species also requires more research. Several segregate taxa, some of these relatively rare and of potential conservation concern, have been accepted by some authors but not by others. The present treatment should be viewed as preliminary.

Species of Penstemon exhibit a variety of floral morphologies that correspond to specialized pollination syndromes involving bees, moths, butterflies, wasps, beetles, and hummingbirds. The species native to the eastern and midwestern United States are mainly bee-pollinated (Clinebell and Bernhardt, 1998; Dieringer and Cabrera R., 2002), which is considered the primitive condition in the genus (Wolfe et al., 2006). However, exceptions occur. In P. tubaeflorus, with its slender corolla tube, the main pollinators appear to be certain butterflies, such as the spicebush swallowtail, Pterourus troilus (L.), although some bees also contribute to pollination (Clinebell and Bernhardt, 1998). In the remaining Missouri species (except P. arkansanus, which has not been studied yet), Clinebell and Bernhardt (1998) reported that bumblebee queens are major pollinators, but other pollinators include smaller bees, bee-flies, the Penstemon wasp (Pseudomasaris occidentalis (Cresson)), and the bumblebee flower beetle (Euphoria sepulcralis (Fab.)). These authors also showed that self-pollination can be successful in P. digitalis, but not in four other Missouri species that they studied. Dieringer and Cabrera R. (2002), who studied pollination in P. digitalis, documented a diversity of bees of different body sizes as pollinators and also showed the importance of the staminode in facilitating pollination by most bees.

Penstemon species are popular native wildflowers in gardens, particularly in the western states. In the Midwest, the main species that are grown include P. cobaea, P. digitalis, P. grandiflorus, and P. tubaeflorus. Several cultivars involving primarily variations in flower color and/or overall plant color have been developed in P. digitalis and P. cobaea. In general, beard-tongues are easily-grown, sun-loving plants whose main drawback is that as perennials they tend to be relatively short-lived and usually must reseed themselves to remain present in a garden for more than a few years.

Some species of Penstemon were used medicinally by Native Americans, mainly in the treatment of sexually transmitted diseases (Moerman, 1998; Nold, 1999).