Phyiologia (April 1993) 74(4):305-316. THE GENUS OREOSTEMMA (ASTERACEAE: ASTEREAE) Guy L. Nesom Department of Botany, University of Texas, Austin, Texas 78713 U.S.A. ABSTRACT A group of taxa of western North America that have most recently been treated as a subgenus within Aster is here regarded as the sepa-rate genus OTeostemma: O. peirsonii (Sharsmith) Nesom, comh. nov., O. elatum (E. Greene) E. Greene, and O. alpigenum (Torr. & Gray) E. Greene (the generitype). Three varieties are recognized within O. alpigenum: var. alpigenum, var. andersonii (A. Gray) Nesom, comb, nov., and var. haydenii (Porter) Nesom, comb. nov. Oreostemm.a is anomalous within Aster, but a close morphological similarity, however, is noted between Oreostemma and the primarily South American genus Oritrophium. KEY WORDS: Oreostemma, Aster, Asteraceae, Astereae Recent taxonomic overviews of the genus Aster L. have recognized a group of three species from the western United States as Aster subg. Oreostem.ma (E. Greene) Peck (Jones 1980) or Aster "subg. Ortastrum^ (Semple k. Brouil-let 1980). In a revisionary treatment of the group, Cronquist (1948j also treated these species within Aster, while confirming their status as a natural group. The first described species among them (see citations below), how-ever, was placed in Haplopappus DC. (as H. alpigenus) by Torrey & Gray, who judged that its closest relatives were among those of Haplopappus sect. Pyrro-coma (Hook.) H.M. Hall. In the description of the second species {Erigeron andersonii). Gray offered the following comment (1865, p. 540): "This is a North American representative of the Andine group of species, sometimes re-ferred to Aster, which Schultz has recognized as identical with Celmisia [Cass.], and Weddell has referred to Erigeron [L.]." These primarily South American species are now identified as the genus Oritrophium, (Kunth) Cuatr. (Nesom 1992a). Gray soon became dissatisfied with the disparity of this latter species with North American Erigeron and transferred it to the highly heterogeneous 305
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