Slender perennial, sometimes flowering the first season and probably never of long duration, with a taproot and (in mature plants) an incipient caudex, rather loosely strigulose with incurved-ascending hairs up to 0.3-0.4 mm. long, the herbage cinereous or greenish, the leaflets bicolored, deeper green and glabrous or nearly so above; stems erect and ascending, 1.5—4 dm. long, branched or spurred at 1-several nodes below the middle, floriferous upward from near or above the middle; stipules subherbaceous becoming papery, ovate-deltoid or triangular- acuminate, ± semiamplexicaul, 1-4 mm. long, erect or deflexed; leaves (3) 4-11 cm. long, shortly petioled, with (13) 17-27 obovate, elliptic, or oblong, obtuse or retuse-emarginate, flat or marginally involute, more rarely folded leaflets (2) 3-16 mm. long; peduncles slender, ascending or incurved, 3.5-14 cm. long; racemes loosely or remotely (3) 5-15-flowered, the flowers ascending, the axis early elongating, (1) 3-10 cm. long in fruit; bracts submembranous, ovate or lanceolate, 1-2.5 mm. long; pedicels ascending, at anthesis slender, 0.8-1.4 mm., in fruit thickened, persistent, 1.5-3 mm. long; bracteoles 0-2, minute when present; calyx 4.8-6.4 mm. long, strigulose with white or partly black hairs, the slightly oblique disc 0.9-1.2 mm. deep, the tube 3-3.8 mm. long, 1.8-2.7 mm. in diameter, the subulate or lance-subulate teeth 1.8-2.7 (3) mm. long; petals pale purple, the wing-tips white; banner recurved through ± 40°, rhombic-ovate, -obovate, or -elliptic beyond the cuneate claw, shallowly notched, 10-13 (15) mm. long, 5-8 (9) mm. wide; wings 9.6-12 mm. long, the claws 3-4.2 mm., the linear-oblong to narrowly oblanceolate, obtuse, subtruncate, or erose-emarginate, straight or distally incurved blades 6.6-8.5 mm. long, 1.8-2.6 mm. wide; keel 6.7—9 mm. long, the claws 3-4 mm., the half-obovate blades 4.3-5.5 mm. long, 2.1-2.8 (3) mm. wide, incurved through 90-95° to the rounded apex; anthers 0.45—0.55 mm. long; pod erect or ascending at a narrow angle, sessile or nearly so, continuous with the receptacle, linear-oblong in profile, slightly incurved, 1-1.6 cm. long, 2.2—3.3 mm. in diameter, broadly cuneate at base, contracted distally into a short, triangular, cuspidate, unilocular beak, otherwise triquetrous, keeled ventrally by the salient suture, openly grooved dorsally, the lateral faces nearly flat, the thin, greenish, strigulose valves becoming stramineous, rather stiffly papery, obscurely reticulate, inflexed as a complete or almost complete septum ± 1—1.2 mm. wide; seeds brown, sometimes purple- speckled, smooth but scarcely lustrous, 2.1—3 mm. long.—Collections: 6 (i); representative: Palmer 234 (GH, K, NY, US), 305 (GH, US, in part); Purpus 6511 (UC); Ripley & Barneby 13,513 (CAS, NY, RSA, US).
Dry rocky hillsides and boulder-strewn washes, on limestone, ± 5600-6200 feet, apparently rare, known only from southern Coahuila (near Parras) and northern Zacatecas (Concepcion de Oro).—Map No. 150.—February to May; September to November.
Astragalus coahuilae (of Coahuila) Jones, Rev. Astrag. 256, Pl. 65. 1923 ("Coahuilae").—"Parras Coahuila, Nos. 1078 and 4672 Purpus."—Cotypi, collected in March, 1905, and October, 1910, respectively, POM! isotypi, GH (both), UC (both), MO (4672)!—Hamosa coahuilae (Jones) Rydb. in Bull. Torr. Club 54: 23. 1927.
The first collection of A. coahuilae dates back to 1880, when Edward Palmer discovered the species near Parras. The specimens were misidentified by Watson (in Proc. Amer. Acad. 17. 343. 1882) as an erect form of A. arizonicus, a species fundamentally different in its dolabriform pubescence and recognized at a glance by the fewer leaflets of narrower outline and the acutely triangular keel-tip. The record was repeated by Hemsley in Biologia Centrali-Americana, and the error was not fully rectified until much later (Rydberg, 1927, 1. c.). Jones (in herb.) had already changed Watson’s determination to A. nothoxys, but this species, although technically similar in most respects, is lower and more diffuse in habit of growth, has a (nearly always) appendaged keel-tip, and differs especially in the longer, readily deciduous pod. The Coahuila milk-vetch is the only perennial member of sect. Leptocarpi in which the fruit dehisces on the raceme through the gaping beak and not through both ends after falling to the ground. The fruiting raceme recalls that of A. (Reventi-arrecti) remotus, but the individual pods are essentially sessile.