THE POLLINATION ECOLOGY Richard R. Clinebell 112 and PeterOF FIVE SPECIES OF Bernhardt2PENSTEMON(SCROPHULARIACEAE) INTHE TALLGRASS PRAIRIE'ABSTRACT The floral ecology of Penstemon cobaea Nutt. var. cobaea, P. cobaea var. purpureus Pennell, P. digitalis Nuit. ex Sims.P. grandiflorus Nutt., P. pallidus Small, and P. tubaeflorus Nutt. was studied by sampling populations at nine prairiesites in lllinois, Kansas, and Missouri. Ail five species show protandry, but the receptive stigma lies only 2 mm awayfrom the two pairs of fertile stamens. Bagging experiments on three Penstemon spp. show that only P. digitalis setsseed when insect visitors are excluded. Flowers of ail five species exhibit a horizontal presentation of the corolla andemit a slightly discernible scent. However, the presumed pollination systems of Penstemon spp. correlate with corollaform and the size of the floral sinus. The tubular, white flowers of P. tubaeflorus appear to be pollinated by a combinationof diurnal Lepidoptera and some native bees favoring a dorsal deposition of pollen on mouthparts and upper thoraces.The four remaining species have gullet-or bell-shaped corollas ornamented with violet-purple blotches or lines. Thesespecies appear to be pollinated primarily by polylectic/polyphagic bees (including six Bombus spp.), with P. digitalisand P. cobaea visited infrequently by the rare Penstemon wasp, Pseudomasaris occidentalis. Queens of Bombus penn-sylvanicus subsp. pennsylvanicus forage primarily on the large, gullet flowers of P. grandiflorus and both varielies of P.cobaea. Queens of Bombus nevadensis subsp. auricomus prefer those P. digitalis and P. pallidus showing a reducedgullet or bell form. Bee pollination in four Penstemon spp. may operate within a two-tiered system. Large-bodied Bombusspp., Megachile brevis, and anthophorids (Synhalonia hamata and Anthophora terminalis) carry dorsal depositions ofPenstemon pollen because they contact anthers and stigmas while they forage exclusively for nectar. In contrast, small-bodied members of the Anthophoridae (Ceratina), Colletidae (Hylaeus spp.). Halictidae (Augochlorella, Halictus, La-sioglossum), and Megachilidae (Hoplitis and Osmia) forage actively for Penstemon pollen encouraging repeated, ventralcontact with the sexual organs of the flowers. Bombus queens were more prevalent at large Penstemon populations(especially P. cobaea). Bombus workers were collected primarily on restored sites. The importance of small bees aspollinators appeared to vary indirectly with Penstemon population size. Penstemon (Scrophulariaceae: Cheloneae) is aNorth American genus of about 270 species (Wolfeet al., 1997) distributed from Alaska to Guatemala.Within the Great Plains, Freeman (1981) recog-nized 22 species of Penstemon in two subgeneraand five sections. The Upper Mississippi Valleysupports two additional species, Penstemon arkan-sanus Pennell and P hirsutus (L.) Willd., suggestingthat 24 Penstemon species are native to midwestemAmerican prairies. Despite the species richness of Penstemon inNorth America, analyses comparing life-historieswithin this genus lag far behind classical (Pennell,1935; Keck, 1938) and molecular (Wolfe & Elisens,1993) taxonomies. We lack significant literature onbreeding systems in Penstemon compared to otherscrophulariaceous genera (e.g., Pedicularis; Macior,1982) distributed through the Northern Hemisphere(Kampny, 1995). References to the pollination bi-ology of Penstemon by Pennell (1935) were derivedprimarily from predictions that were based on floralmorphology. In contrast, what literature does existon Penstemon pollination often shows a lack of con-sensus regarding the efficiency of different bee taxaas true pollen vectors. Field studies of Penstemon pollination beganwith Robertson (1892, 1929), who noted protandryin "P laevigatus" (= P. digitalis) and "P. pubes-cens" (= P pallidus) in Illinois and collected a totalof 20 different bee species in their flowers. Rob-' This work is part of the first author's doctoral dissertation being prepared in the Dept. of Biology, Saint LouisUniversity, under the direction of P. Bernhardt. Financial support for fieldwork has been provided by the ConservationFederation of Missouri (Bell Conservation Scholarship), the Kansas City Garden Club, the Missouri Botanical Garden(Litzsinger Road Ecology Center), the Missouri Department of Conservation, the Missouri Prairie Foundation, and theSt. Louis office of the Nature Conservancy. Donald Hardin provided assistance in the field and in the preparation oftables and graphies. Stanley Sawyer provided statistical advice. Charles Michener and colleagues identified the Hv-menoptera and referred other insect specimens to appropriate authorities. We thank Paul Wilson and an anonymousreviewer for helpful comments and criticisms. 2 Department of Biology, Saint Louis University, 3507 laclede Ave., St. Louis, Missouri 63103. U.S.A.ANN. MISSOURI BOT. GARD. 85: 126-136. 1998.