Similar to A. Spaldingii but of more robust growth, with a thick, woody taproot and shortly forking, knotty caudex, densely villous-tomentose throughout with extremely fine, curly and entangled together with straighter, loosely ascending and spreading hairs up to 1—1.5 mm. long, the stems and herbage somewhat silky in youth, becoming gray-cottony in age, the vesture of the inflorescence turning cream-color when dry; stems several or numerous, (1) 1.5-5.5 dm. long, simple except for an occasional spur in some lower axils, prostrate or weakly ascending, forming loose mats or low, tufted clumps; stipules membranous or early becoming so, lanceolate or lance-acuminate, 3-6 mm. long, semiamplexicaul-decurrent; leaves 5-14 cm. long, the uppermost subsessile, with (7) 15-21 oval-obovate, obtuse or subacute, or (in some upper leaves) elliptic and acute, flat leaflets 6-17 mm. long, the blades thin-textured, but appearing thick from the matted vesture; peduncles subcorymbosely disposed in the upper axils, erect or incurved-ascending, 5—12 cm. long; racemes (10) 20—40-flowered, the flowers crowded at anthesis into a dense, ovoid or oblong head, this elongating into a narrowly cylindric spike, often somewhat interrupted at base, the axis (2.5) 4—12 cm. long in fruit; bracts linear or linear-lanceolate, 2-5 mm. long, scarious-margined, ultimately reflexed; pedicels at anthesis subobsolete, in fruit obconic, 0.5-1 mm. long; bracteoles 0; calyx densely silky-villous, at anthesis 6.6-7.8 mm. long, the slightly oblique, saucer-shaped disc 0.8-1 mm. deep, the broadly campanulate, membranous tube 3.9-4.5 mm. long, 3.5-4.1 mm. in diameter, the narrowly subulate, herbaceous teeth 2.7-3.5 mm. long, the whole somewhat accrescent, the tube becoming ovoid, tumid, papery, ruptured or not, but with the connivent teeth enclosing the small pod; petals pale yellow, all pubescent dorsally above the middle, marcescent; banner recurved through ± 50°, spatulate, 9-12 mm. long, the obscurely notched blade 5-8 mm. wide; wings 8.3-10.3 mm. long, the claws 4.2-5 mm., the obliquely elliptic, obtuse, ± incurved blades 5-6.4 mm. long, 2.1-2.6 mm. wide; keel 7.1-8.2 mm. long, the claws 4-5 mm., the half-circular blades 3.3-3.6 mm. long, 2-2.3 mm. wide, abruptly incurved through 100-120° to the obtusely deltoid apex; anthers 0.5-0.6 mm. long; pod horizontal or a trifle declined, abruptly contracted at base into an obliquely attached stipelike neck 0.4-0.6 mm. long (but this apparently disjointing in age), the body obliquely ovoid, 4.5-6 mm. long, 3 mm. in diameter, shortly cuspidate at apex, obscurely trigonous, bluntly keeled ventrally by the thick, prominent, nearly straight suture, the dorsal face impressed as a short, open groove or rounded depression below the middle, the lateral faces plumply convex, the stiffly papery, canescently tomentulose valves inflexed as a complete or subcomplete septum ± 2 mm. wide produced into the pod’s apex; dehiscence unknown; ovules (6) 8; seeds (fully formed but unripe) ± 2.1-2.5 mm. long.—Collections: 11 (ii); representative: Peck 26,171 (CAS, RSA), 26,246 (RSA, WILLU, WS); Ripley & Barneby 10,826 (CAS, NY, RSA, WILLU, WTU); J. W. Thompson 4956 (MO, WILLU, WTU); C. L. Hitchcock 20,741 (NY).
Dry hillsides and valley floors, in sandy clay soils overlying basalt, commonly among sagebrush, 1500-1800 feet, locally plentiful but known only from the east foothills of the Cascade Range on the lower White and adjoining Deschutes Rivers in Wasco County, Oregon.—Map No. 156.—May to July.
Astragalus tyghensis (of Tygh Valley) Peck in Proc. Biol. Soc. Wash. 49: 110. 1936.—"dry slope near Tygh Valley, Wasco County, Oregon, Peck 17367."—Holotypus, collected May 29, 1933, WILLU! isotypus, RSA!—A. Spaldingii var. tyghensis (Peck) C. L. Hitchc. in Univ. Wash. Pub. Biol. 17: 262. 1961.
The Tygh Valley milk-vetch was first collected as early as 1894 by Francis E. Lloyd (NY), but the specimens were passed over by Rydberg and others as representing a robust form of A. Spaldingii. The two species are close kindred and there is some reason to evaluate A. tyghensis as no more than a strongly marked variety or geographic subspecies of its common relative; however, the differences, closely observed, are several and striking, especially in the field. The individual plant of A. tyghensis is (at least on the average) stouter, with more numerously flowered and longer fruiting racemes, and the pannose vesture is thicker, softer, and more intricately tangled. The fresh petals of A. tyghensis are distinctly yellow, pale but clear; those of A. Spaldingii, although they turn stramineous when dry, are at first white or dirty white, often suffused with grayish-lavender. Furthermore, the minute but definite stipe in A. tyghensis is attached so obliquely as to form almost a right angle with the base of the pod-body, and not (as in A. Spaldingii where, if at all developed, it remains extremely short and obscure) in a plane perpendicular to the body. The pubescent petals of the Tygh Valley milk-vetch are immediately noteworthy and furnish the most easily observed diagnostic character.
The known range of A. tyghensis extends from a point a few miles south of the village of Maupin northward in a narrow strip across the valley of the White River as far as the south slope of Tygh Grade, a distance of some fifteen or possibly twenty miles. The species is locally abundant, forming large colonies, and shows little variation. The nearest established station for A. Spaldingii lies about one hundred miles distant to the northeast, in the Rattlesnake Hills of Benton County, Washington.