Flora of North America species comparison

Species ca. 280 (239 in the flora).

Penstemon is nearly endemic to North America, with three species that range south into Guatemala; it is the third largest genus in number of species in the flora area after Carex (Cyperaceae) and Astragalus (Fabaceae). Some species, especially in the western United States, have exceedingly narrow ranges.

Orthography of the genus name varied from the 1700s into the early 1900s and is summarized by F. S. Crosswhite (1965d).

Monographic studies of most parts of the genus were carried out by F. W. Pennell (1920b, 1921, 1935, 1940), D. D. Keck (1932, 1936b, 1937, 1937b, 1938, 1940b, 1945), Keck and A. Cronquist (1957), and F. S. Crosswhite (1965, 1965b, 1965c, 1966, 1967, 1967b, 1967c). Many western species were studied for floristic treatments in the Intermountain region (N. H. Holmgren 1984, 2017) and California (Holmgren 1993; M. Wetherwax and Holmgren 2017, http://ucjeps.berkeley.edu/eflora/eflora_display.php?tid=11396).

Penstemon shares morphological similarities with Chionophila, most notably an epistaminal nectary of glandular hairs. Both genera have a base chromosome number of × = 8. R. M. Straw (1966) hypothesized the genera to be sister taxa. Relationships among genera in Cheloneae remain equivocal (A. D. Wolfe et al. 1997, 2002, 2006; S. L. Datwyler and Wolfe 2004). Penstemon is distinguished from Chionophila by shorter calyx tubes (the calyx lobes usually distinct nearly to their bases), thyrsoid inflorescences, bracteoles in the inflorescences, and unwinged or, rarely, narrowly winged seeds.

The only broad molecular survey of Penstemon (A. D. Wolfe et al. 2006) found limited support for morphology-based infrageneric groups, and it lacked sufficient resolution to test the monophyly of large parts of the genus. C. A. Wessinger et al. (2016) obtained better species-level resolution in a study that focused on several sections of the genus. Both studies demonstrated that some groups are monophyletic or could be made so with minor adjustments and suggested that the infrageneric classification that draws heavily on floral characters may need extensive revision. A major overhaul of the infrageneric classification of Penstemon must await studies to resolve topological incongruencies between gene trees and to provide greater resolution within and among clades, issues likely due to hybridization and the recent, rapid radiation of the genus. Except for shifts of some species among subgenera or sections, a morphology-based classification with two subgenera and 16 sections is followed here.

Published chromosome counts exist for about half of the species in the flora area. Most Penstemon species counted are diploid (2n = 16); published tetraploid, hexaploid, octoploid, and dodecaploid counts exist for 24 species in sect. Penstemon, sect. Saccanthera, and sect. Spectabiles in the flora area. Most polyploids are assumed to be allopolyploids owing to the high incidence of hybridization; that hypothesis has not been tested.

Although the taxonomy of Penstemon traditionally places heavy emphasis on flower morphology, not all authors have used descriptive terms consistently. An overview of important diagnostic terms as applied in this treatment follows.

Basic corolla parts are the tube, throat, and limb (with abaxial and adaxial lobes). Some authors use the term tube to refer to the entire corolla proximal to the limb. In most species, the basal part of the corolla, proximal to the point of adnation of the staminode, is cylindric and narrower than the rest of the corolla; that part of the corolla is herein called the tube. In most species, the corolla expands near the point of adnation of the staminode to the corolla, forming the throat; the throat extends from tube to limb. Relative to the tube, the throat can be unexpanded with essentially parallel surfaces (corolla salverform), slightly expanded with nearly parallel sides (corolla tubular), expanded with straight and slightly diverging surfaces (corolla funnelform), inflated abaxially (corolla ventricose), inflated adaxially (corolla ampliate), or inflated abaxially and adaxially (corolla ventricose-ampliate). Most species have distinctly bilaterally symmetric corollas; some (for example, Penstemon centranthifolius, P. cyathophorus, P. eatonii, P. goodrichii) have corollas nearly radially symmetric. Corollas with the abaxial surface of the throat strongly uparched and closing or nearly closing the orifice are described as personate.

Nectar guides are present in corollas of most species, especially those presumably pollinated by hymenopterans. Members of subg. Dasanthera basically lack nectar guides; the two prominent ridges in the abaxial surface of the throat usually are lighter in color compared to the rest of the corolla. This condition also occurs in some species in sect. Saccanthera.

The taxonomy of Penstemon relies heavily on stamen morphology, which varies among species in size, orientation, vestiture, dehiscence, and shape. Differences among species within sections are often subtle. Characters should be determined from mature anthers that have dehisced and shed their pollen. Pollen sac length is the distance from the distal tip of the sac to its point of confluence with the opposite pollen sac (the connective). Orientation refers to the relative position of paired pollen sacs at dehiscence: parallel (sacs parallel to each other and basically parallel to the filament); divergent (distal tips spreading from each other and the filament); and opposite (distal tips essentially opposite each other and in line with the point of attachment of the filament).

Sutures of dehisced pollen sacs vary from smooth to papillose (with papillae mostly less than 0.1 mm) or denticulate (teeth 0.1+ mm). Vestiture of the sides of the anthers varies from absent to densely lanate, the hairs reaching 4 mm and obscuring the external surface in most members of subg. Dasanthera.

Dehiscence patterns usually are consistent within a species with few exceptions. Complete dehiscence occurs when both pollen sacs open entirely from their apices proximally to, and across, the connective. Complete dehiscence can result in anthers that remain more or less boat-shaped (navicular) or become essentially flat (explanate). Two basic patterns of incomplete dehiscence exist: the proximal portion of each pollen sac can remain indehiscent (pollen sacs dehiscent distally), sometimes with only the connective not splitting, or the distal portion of each sac can remain indehiscent (pollen sacs dehiscent proximally). Anthers of the former type typically have been described simply as dehiscing incompletely proximally; the pollen sacs can remain straight, sometimes they twist sigmoidally. Anthers that dehisce incompletely distally usually are described as saccate because of their pouchlike appearance.

The diversity of floral forms in Penstemon has stimulated great interest in the evolution of floral morphology (R. M. Straw 1955, 1956, 1956b; J. Walker-Larsen and L. D. Harder 2001; M. C. Castellanos et al. 2004, 2006), pollination ecology and pollination syndromes (Straw 1963; F. S. Crosswhite and C. D. Crosswhite 1966; R. Schmid 1976; H. R. Lawson et al. 1989; R. R. Clinebell and P. Bernhardt 1998; R. S. Lange and P. E. Scott 1999; V. J. Tepedino et al. 1999, 2006, 2007; J. D. Thomson et al. 2000; Paul Wilson et al. 2004), and reproductive biology (R. L. Nielson 1998; J. Chari and Wilson 2001; G. Dieringer and L. Cabrera R. 2002; J. L. Hawk and Tepedino 2007). Studies show that flower color is a good predictor of pollen vector (red or pink, salverform or tubular flowers pollinated by hummingbirds; purple, blue, or white, tubular, funnelform, or ventricose flowers pollinated by hymenopterans). Hummingbird pollination has evolved independently nearly a dozen times in Penstemon.

Penstemons are popular as ornamentals. Readers interested in growing penstemons should consult R. Nold (1999) or D. Lindgren and E. Wilde (2003). Nearly two-dozen species of Penstemon throughout North America were used by Native American tribes for drugs, food, fiber, dyes, and ceremonies (D. E. Moerman 1998).

(Discussion copyrighted by Flora of North America; reprinted with permission.)