Monographs Details: Astragalus arthuri A.Gray
Authors:Rupert C. Barneby
Authority: Barneby, Rupert C. 1964. Atlas of North American Astragalus. Mem. New York Bot. Gard. 13(1): 1-596.
Family:Fabaceae
Synonyms:Atelophragma arthuri (M.E.Jones) Rydb., Tium arthuri (M.E.Jones) Rydb.
Description:Species Description - Relatively slender, loosely strigulose nearly throughout with narrowly ascending and incumbent, sometimes sinuous hairs up to 0.4-0.6 (0.75) mm. long, the herbage greenish or cinereous, the leaflets glabrous above; stems several, simple above the immediate base, 0.2—1.6 (2.6) dm. long, either all short and forming a close leafy tuft on the shortly forking caudex or all better-developed and then diffuse or ascending, composed of 1-5 (6) developed internodes up to 1.5-9 cm. long, stipules 2—6 (7) mm. long, thinly herbaceous becoming papery, often purple- tinged, the lowest decurrent around ± 3/4 the stem’s circumference, usually broader than the triangular-lanceolate or lance-acuminate upper ones; leaves slender- petioled or the uppermost subsessile, with (15) 19-25 (27) linear-elliptic, oval, or oblanceolate, obtuse, subacute, truncate-mucronulate, or shallowly retuse, flat or loosely folded leaflets (2) 4-12 (14) mm. long; peduncles ascending, often incurved, (7) 10-18 (20) cm. long, well surpassing the foliage; racemes loosely (5) 8-20-flowered, the flowers early horizontal, later declined, the axis becoming (2.5) 5-15 cm. long, the fruits secund; bracts membranous or early becoming so, lanceolate, 1.5—3.5 mm. long; pedicels slender, at anthesis ascending, (1.5) 2-3 mm. long, in fruit arched outward, or twisted and recurved, little thickened, (2) 2.5-4 mm. long; bracteoles usually 0, minute when present; calyx 6.6-8.8 mm. long, loosely strigulose with mixed black and white hairs, the disc 0.8—1.3 mm. deep, the deeply campanulate tube 4.5-6 mm. long, (2.5) 2.8-3.8 mm. in diameter, the lance-subulate teeth 1.8-3.5 mm. long, the whole becoming papery, marcescent unruptured; petals ochroleucous, immaculate; banner recurved through 50-85°, oblong-ovate beyond the cuneate claw, 11.7-15.2 mm. long, 4.8-8 mm. wide; wings (nearly as long or a trifle longer) 11.7-15.2 mm. long, the claws 5.3-6.7 mm., the lunately oblanceolate, obtuse blades 7.3-10 mm. long, 2.2-3.8 mm. wide; keel (9.5) 10-11.7 mm. long, the claws 5.3-6.4 mm., the half-obovate blades (4.6) 5-6.3 mm. long, 2.5-3.5 mm. wide, abruptly incurved through 90° to the bluntly deltoid apex; anthers 0.55-0.8 mm. long; pod pendulous, stipitate, the slender stipe 6-15 mm. long, the linear-ellipsoid body (2.5) 3-4.5 cm. long, 2.5-3.3 mm. in diameter, nearly straight or gently incurved, tapering at base into the stipe and distally into a slender, acuminate beak, laterally compressed-triquetrous, with rounded lateral angles and convex lateral faces, carinate ventrally by the suture, narrowly sulcate dorsally, the thin, greenish, finely and densely strigulose valves becoming papery, stramineous, the complete septum 1.6-2.3 mm. wide; ovules (16) 18-30; seeds narrowly oblong, dark brown or nearly black, smooth or minutely pitted, sublustrous, 2.5-3.1 mm. long.

Distribution and Ecology - Dry grassy hills and stony meadows, on basalt, 900-3100 feet, locally plentiful in the valley of the Snake River and its tributary creeks and rivers about the common boundary-point of Washington (Asotin County), Oregon (northeastern Wallowa County), and Idaho (Nez Perce and Idaho Counties).—Map No. 52. —Mid-April to June.

Discussion:

The stems of the Waha milk-vetch, A. Arthuri, are ordinarily quite short, composed of few or often only one developed internode, so that the leaves are gathered into a loose basal tuft and the earlier of the long, incurved-ascending peduncles appear subscapose. In protected sites and generally late in the season, the plants become more strongly caulescent, but even then the stems rarely equal in length the longest of the peduncles and racemes together. The extremely long, slender, linear-fusiform pods, which are the particular ornament and most distinctive feature of the species, are formed after the basic fashion of the Miselli; but they are less sharply trigonous than those of its close allies, the convex curvature of the lateral faces passing imperceptibly into that of the rounded lateral angles. The fruit-cavity is so narrow that the seeds are compressed into a narrowly oblong figure. Despite the striking modification of the pod A. Arthuri is closely related to A. Howelli, differing principally in its weaker, more diffuse stems, more nearly appressed vesture, and glabrous upper surface of the leaflets.

About the mouth of the Grande Ronde River and on the opposite bank of the Snake River in Idaho, A. Arthuri is common locally in the arid grasslands characteristic of the region. It has been traced up the Snake Canyon to Imnaha Creek and ascends the Clearwater a little way, its known limit in this direction being Myrtle Beach. Its main area of dispersal does not exceed fifty miles in diameter.

Distribution:Oregon United States of America North America| Washington United States of America North America| Idaho United States of America North America|