Small-fruited varieties of Astragalus purshii (var. lagopinus, var ophiogenes) when first described by Barneby (1964) appeared to be geographically isolated—the former from southcentral Oregon and the latter from southwestern Idaho. However, collections made in the last two decades have shown the two varieties to be geographically overlapping. Our study asked whether populations are differentiated, as suggested by Barneby, or represent a case of clinal variation. We measured 13 morphological characters in 103 herbarium specimens. We analyzed the variation using Principal Components Analysis, which distinguished the two varieties. Ten characters including leaflet number, number of flowers per raceme, calyx tooth length and pod curvature differed (p < 0.05) between the two varieties but for all characters the range of values overlapped between the two varieties. New collections of var. lagopinus found in Malheur Co. and western Idaho inhabit gravelly rhyolitic and basaltic soils, whereas var. ophiogenes primarily occupies sandy, lacustrine, sedimentary soils. We conclude that varietal distinction should be retained based on differential preference of substrate in the region of geographical overlap.

Morphological analysis of small- fruited varieties of Astragalus purshii (Fabaceae) confirms varietal distinction