A Taxonomic and Cytological Study of the Genus Wyethia, Family Compositae, with Notes on the Related Genus Balsamorhiza

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Abstract
Wyethia, a heli-anthoid genus of western U. S-, is treated in this study as comprising 3 sections: Agnorhiza (transferred from Balsamorhiza) with 6 spp., Alarconia with 2 spp., and EUWYETHIA with 6 spp. W. scabra var. attenuata and W. s. var. canescens are new, and new combinations include W. bolanderi (Balsamorhiza b. A. Gray) and W. invenusta (Helianthus (?) invenustus Greene). Chromosome numbers n = 19, 2n = 38 are recorded for Balsamorhiza careyana, B. incana, B. rosea, B. sagittata, B. serrata, W. amplexicaulis, W. angustifolia, W. helenioides, and W. mollis, and are discussed relative to phylogeny. The spp. in this treatment are biological units, that is, populations which are geographically or ecologically discrete are morphologically distinguishable at all times, and are effectively isolated from all other such populations by one or more barriers. The most obvious barriers to genic contamination are the strong ecologic preferences and actual physical separation by distance or by geographic barriers. In addition, 2 spp. occurring in the same area are usually effectively separated by a difference in the time of flowering. In a number of cases, nevertheless, it appears that the development of genetic isolating mechanisms has not kept pace with other types of barrier formation. Natural hybridization involving 6 pairs of spp. in Wyethia is recorded and discussed.

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