Low, rather slender, perennial, with a woody taproot and ordinarily subterranean (more rarely, perhaps only accidentally, superficial) root-crown or shortly forking caudex, strigulose throughout with largely sinuous, but the leaflets with straighter, appressed mixed with a few longer and looser, spreading and ascending hairs up to 0.5-0.9 (1) mm. long, the herbage silvery or greenish- cinereous, the leaflets pubescent on both sides but sometimes more thinly so, exceptionally subglabrous above; stems several, diffuse and incurved-ascending, buried for a space of 0-15 cm., the aerial part 4—35 cm. long, simple or, when vigorous, few-branched near the base; stipules membranous early becoming papery- scarious, 1-5 (6.5) mm. long, the lowest smallest, all amplexicaul and connate through half their length or more into a sheath, this sometimes ruptured in age, densely pubescent dorsally; leaves 1-6 cm. long, all but the lowest subsessile, with (11) 13-23 crowded or moderately distant, elliptic, oblong-elliptic, or oblanceolate, acute, obtuse, or emarginate, flat or loosely folded, dorsally keeled leaflets 3-10 (12) mm. long; peduncles relatively stout, 2.5-9 cm. long, surpassing the leaf; racemes densely 10—30-flowered, sometimes becoming looser and interrupted in late anthesis, the axis more or less elongating, 1-5 (7) cm. long in fruit; bracts membranous, ovate-triangular or lanceolate, 0.8-2.5 mm. long; pedicels arched out- and downward, at anthesis 0.3-0.8 mm., in fruit thickened, 1—1.5 mm. long; bracteoles 0-2, but only rarely present and then minute; calyx 3.2-5.2 mm. long, strigulose with black or mixed black and white hairs, the subsymmetric disc 0.5—0.9 mm. deep, the campanulate or turbinate-campanulate tube 2.2-3 mm. long, 1.7-2.4 mm. in diameter, the subulate or triangular-subulate teeth 0.8—2.5 mm. long, the whole becoming papery, ruptured, marcescent; TEX); Waterfall 13,930 (RSA); Ripley & Barneby 13,343 (CAS, MICH, NY, RSA, US).
Savannahs, stony plains, gullied badlands, hot, rocky hillsides with Cactaceae and thorn scrub, ascending into open pine and oak forest, about 6000-8000 feet, commonly on limestone (or gypsum) but also on volcanic and metamorphic bedrock, widespread and locally plentiful, southwestern Tamaulipas and central Nuevo Leon to Jalisco and Oaxaca.—Map No. 155.—April to October, apparently flowering with equal vigor after spring and summer rains.
Astragalus hypoleucus (whitish, or white beneath) Schau. in Linnaea 20: 747. 1847.— "In montanis Mexici, loco non notato, Aschenborn no. 343."—No typus examined, but the full description decisive.—Tragacantha hypoleuca (Schau.) O. Kze., Rev. Gen. 945. 1891. Hamosa hypoleuca (Schau.) Rydb. in Bull. Torr. Club 54: 334. 1927.
Astragalus luisanus (of San Luis Tultitlanapa) Jones, Rev. Astrag. 275, Pl. 69. 1923.—"The types are Purpus plants Nos. 3208 from near Oaxaca and 2477 from Esperanza in the state of Puebla... "—Holotypus (collected at San Luis Tultitlanapa near Oaxaca, in August 1908), POM! isotypi, MO, NY, US! paratypi (from Esperanza), GH, MO, NY, POM, UC, US!
The Aschenborn milk-vetch, A. hypoleucus, is the only Mexican Astragalus other than A. humistratus, which extends only feebly south into Sonora and Chihuahua, characterized by dolabriform pubescence and connate stipules. While the two species are alike in their diffuse stems and commonly ashen foliage, they are dissimilar in detail, the flowers of A. humistratus being much larger and differently shaped, and its pod humistrate, ascending, and unilocular. In practice A. hypoleucus could be confused only with some slender Micranthi, particularly with A. oxyrrhynchus, but these have basifixed hairs and all stipules free.
Variation in A. hypoleucus is sometimes striking but of a superficial nature, not more pronounced than should be expected of a species dispersed along the Cordillera through nearly nine degrees of latitude. The leaflets are commonly gray on both sides, rarely glabrescent medially or truly glabrous above. In the raceme the hairs vary from black, the commonest color, to nearly all white, and on the pod from white to white mixed with black or fuscous in a pepper-and-salt combination. The pod is somewhat unstable as to length, diameter, and curvature, these minor deviations from an arbitrarily assumed standard corresponding with no discernible pattern of dispersal. The typus of A. luisanus which, according to Jones’s key, should differ from genuine A. hypoleucus and A. Hartwegi in its few-flowered spikes, is in no respect exceptional.