Abstract
Astragalus asotinensis Bjork & Fishbein (Fabaceae) is newly described from a single population on limestone of the Limekiln Formation at the mouth of Hells Canyon in Washington and Idaho in the United States. This population of several thousand plants has yet to be found on any of the non-calcareous substrates in the vicinity. Its affinities appear closest to Astragalus sect. Podosclerocarpi A. Gray, which hitherto encompassed three species of the Columbia Basin of British Columbia, Oregon, and Washington, in northwestern North America. Astragalus asotinensis represents a disjunct element of section Podosclerocarpi, the nearest population of which, in the form of A. sclerocarpus A. Gray, occurs 150 km northwest. With species of section Podosclerocarpi, A. asotinensis shares a strongly cauline habit, creamy white petals, non-gibbous calyx base, and stipitate, curved fruits. It is unique within section Podosclerocarpi in having sparse pubescence, a much longer ratio of peduncle-to-raceme length (4:1), and an intermediate leaflet length-width ratio (5:1).
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