Low, vigorous, loosely tufted, with a woody taproot and at length suffruticulose caudex, strigulose nearly throughout with straight, appressed, filiform or largely flattened and sometimes scalelike hairs up to 0.4-0.5 (0.7) mm. long, the stems thinly so, the leaflets strikingly bicolored, sparsely pubescent and yellowish-green beneath, silvery-canescent above; stems several or numerous, erect and ascending in clumps, 5-20 (25) cm. long, stiff, becoming stramineous and striate, simple or few-spurred below, zigzag distally, floriferous from near or commonly from below the middle; stipules membranous becoming papery, deltoid or triangular-acuminate, 1-4 mm. long, about semiamplexicaul, thinly pubescent or glabrate dorsally; leaves widely spreading or ascending, 7-20 cm. long, shortly petioled, with (17) 19-25 (27) distant, often scattered, ovate, elliptic, or obovate, sometimes obcordate, retuse or obtuse leaflets 3-12 mm. long, readily deciduous from the stiff, ventrally flattened but not sulcate, long-persistent rachis; peduncles erect and ascending, 9-20 cm. long; racemes loosely (5) 7-12 (15)- flowered, the flowers widely spreading-ascending, the axis elongating, (4) 6-18 cm. long in fruit; bracts membranous, ovate or broadly lanceolate, 1-2.4 mm. long; pedicels ascending, straight or nearly so, at anthesis 1-2.6 mm., in fruit thickened, 1.8-4.8 mm. long; bracteoles commonly 2, minute, or 0; calyx 6.1-7.6 mm. long, strigulose with black, mixed black and white, rarely all white hairs, the subsymmetric disc 1.2-1.5 mm. deep, the broadly campanulate, pallid tube 4.1-5 mm. long, 3.5-4.4 mm. in diameter, the subulate teeth 2-2.8 mm. long, the whole becoming papery, marcescent unruptured; petals ochroleucous, immaculate; banner recurved through ± 45°, ovate- or suborbicular-cuneate, openly notched, 12.6-15.7 mm. long, 7-10 mm. wide; wings 11-14.1 mm. long, the claws 5-6 mm., the broadly oblanceolate or half-obovate, obtuse or commonly erose-emarginate, nearly straight blades 7.1-9.6 mm. long, 2.8-3.6 mm. wide; keel 9.7-11 mm. long, the claws 5-5.6 mm., the half-obovate or lunately elliptic blades 5.5-6.2 mm. long, 2.6-3.2 mm. wide, abruptly incurved through 95° to the bluntly deltoid apex; anthers 0.5—0.6 mm. long; pod ascending, sessile on a stout gynophore (0.8) 1.2-2.5 mm. long, the body linear-oblanceolate in profile, gently incurved, 2.4—4.2 cm. long, 3.5—5.5 mm. in diameter, triquetrously compressed, sharply carinate ventrally by the prominent, thick, narrowly winged suture, the lateral angles narrow but obtuse, the lateral faces low-concave or nearly plane, the scarcely narrower dorsal face shallowly and openly sulcate, the thin, pale green, glabrous valves becoming papery, stramineous, lustrous, finely reticulate, inflexed as a nearly complete septum 1.5-2.1 mm. wide; ovules 20-24; seeds brown or purplish-brown, minutely pitted, scarcely lustrous, 2.6-3.3 mm. long.—Collections: 10 (i); representative: Munz & Johnston 5196 (CAS); Ewan 3096 (POM); Ripley & Barneby 3197 (RSA).
Hot, rocky slopes in canyons and along the edge of boulder-strewn desert washes, with Larrea and Encelia farinosa, between 1500 and 1850 feet, extending rarely upward into the Joshua-tree forest to 4000 feet, forming colonies but local, known only from around the north end of Coachella Valley, from Whitewater and Morongo Wash across the south face of the Little San Bernardino Mountains to the Orocopia Range, in central Riverside and adjoining San Bernardino Counties, California.—Map No. 145.—February to May.
Astragalus tricarinatus (three-keeled, of the fruit) Gray in Proc. Amer. Acad. 12: 56. 1876.—"White Water, San Bernardino Co., California, Parry, 1876." Holotypus (Parry 2, dated "1873"), GH! probable isotypi, NY (Parry & Lemmon in 1876), K (No. 88)!—Hamosa tricarinata (Gray) Rydb. in Bull. Torr. Club 54: 20. 1927.
The three-keeled milk-vetch, A. tricarinatus, is a handsome and easily recognized astragalus, notable particularly for the many small leaflets of broad outline, silvery-pubescent above and greenish beneath, disposed in distant pairs or more commonly scattered along a stiff rachis, from which they disjoint readily on drying. The shortness of the stems in relation to the long, stout peduncles is another unusual feature. As mentioned in the foreword to the subsection, the flower and inflorescence of A. tricarinatus are very similar to those of A. pachypus; and by confusion of flowering specimens of the two species, the three-keeled milk-vetch has been reported (Jones, 1923, p. 259; Jepson, 1936, p. 370) as extending north to the head of San Joaquin Valley in Kern County. Even at anthesis A. pachypus may be distinguished by its narrower, linear or linear-oblong leaflets; and its truly stipitate, laterally flattened, two-keeled pod of almost woody texture is unmistakable. The whole range of A. tricarinatus, extending from the Morongo Wash region to the hills northeast of Mecca, has a diameter of about fifty miles.