Astragalus riparius
Amer. Midl. Naturalist 55: 490. 1956.
Plants clump-forming, slender, 25–50 cm, strigulose, hairs basifixed; from superficial caudex. Stems erect to ascending, with 2–5(or 6) developed internodes, strigulose. Leaves (5–)10–21 cm; stipules 2.5–7 mm, membranous becoming papery-scarious; leaflets 21–33, blades linear-oblong, linear-oblanceolate, or narrowly lanceolate-oblong, 2–21 mm, apex obtuse or truncate-emarginate to subacute, surfaces strigulose abaxially, glabrous adaxially. Peduncles erect, 14–30 cm, together with racemes much longer than stems. Racemes (8–)12–21-flowered, flowers spreading; axis 4–12(–18) cm in fruit; bracts 1.2–7.5 mm; bracteoles 2. Pedicels 0.7–5 mm. Flowers 11.2–14.1(–17) mm; calyx short-cylindric, (6.9–)7.6–10.6 mm, strigulose, tube 4.6–5.8 mm, lobes lanceolate-subulate, (1.9–)2.2–5.2 mm; corolla greenish white, drying yellowish, immaculate; banner recurved through 45°, apex crinkled, 10.5–14 mm; wings 12.8–17 mm, longer than banner; keel (9.4–)10.2–12.8 mm. Legumes erect, brown or stramineous, straight or slightly incurved, obliquely oblong-ellipsoid or narrowly ovoid-ellipsoid, obcompressed, 15–25 × (5.5–)6.5–10 mm, unilocular, fleshy becoming leathery, glabrous; septum to 0.6 mm wide; sessile. Seeds 23–28. 2n = 24.
Phenology: Flowering Apr–Jun.
Habitat: Dry bluffs, canyon banks.
Elevation: 200–600 m.
Distribution
Idaho, Wash.
Discussion
Astragalus riparius occurs along the lower Snake River tributaries from the mouth of the Clear Water to the Tucannon rivers, Nez Perce County, Idaho, and historically in Latah County as well as in the southeastern Washington counties of Asotin, Columbia, Garfield, and Whitman.
The combination of the sessile, unilocular, glabrous fruits of Astragalus reventus with the uniquely modified flowers of A. arrectus led R. C. Barneby (1964) to suggest hybrid origin for A. riparius. Despite the unique backward-folded margin of the banner, giving the illusion of a foreshortened flower, the plant closely simulates in all other aspects A. sheldonii and its close relatives, and may be more properly allied there.
Selected References
None.