Slender, diffuse perennial, with a taproot or obliquely ascending root-stock and subterranean root-crown or shortly forking caudex, the stems, leaf-rachises, and peduncles densely pilose-hispidulous with horizontal or retrorsely spreading hairs up to 0.5-0.8 mm. long, the leaves more thinly incurved-strigulose or -hirsutulous, greenish, the leaflets either glabrous or pubescent above; stems 1-5 dm. long, simple or at length freely and divaricately branching, abruptly flexuous or zigzag distally; stipules dimorphic, those at the lowest, leafless nodes 1.5-2 mm. long, evidently but shortly connate, the median and upper ones lanceolate to deltoid- or lance-acuminate or -caudate, free or nearly so, 2.5-7.5 mm. long, sharply reflexed; leaves mostly subsessile, (2) 3-10 cm. long, with (11) 13-19 (21) obovate to broadly oblong, retuse, flat, thin-textured leaflets 3—15 (17) mm. long; peduncles spreading or incurved, 2-7 cm. long; racemes shortly but loosely 3-8-flowered, the flowers at full anthesis subhorizontal, declined in age, the axis (0.5) 1-2.5 cm. long in fruit; horizontal, or declined, sessile, persistent, narrowly oblong or lance-oblong in profile, slightly incurved, 1—1.7 cm. long, 3—4.3 mm. in diameter, strongly compressed-triquetrous, narrowly grooved dorsally, the thin, green, glabrous valves becoming stramineous, papery, lustrous, finely reticulate, inflexed as a complete septum up to 2—2.2 mm. wide; ovules 9—18; dehiscence and seeds unknown.— Collections: 6 (o); representative: Wynd & Mueller 375 (GH, K, NY, US); Pennell 17,497 (NY); Barkley, Webster & Rowell 7201 (TEX).
Moist wooded canyons and along streams in desert mountains, or in light shade of desert shrubs, mostly (perhaps exclusively) on limestone, about 3000-5500 feet, known only from eastern Coahuila (municipios de Múzquiz, Monclova, y Saltillo), apparently not common.—Map No. 155.—May to July and probably intermittently later following summer rains.
Astragalus Greggii (Josiah Gregg, 1806—1850, frontier trader, amateur physician, author, traveled widely in n. Mexico) Wats. in Proc. Amer. Acad. 17: 343. 1882.—"In the mountains east of Saltillo (Palmer 238); also collected by Gregg (439), the locality not stated."—Holotypus (Palmer 238), collected 6 miles e. of Saltillo, July 17-20, 1880, GH! isotypi, K, NY, PH, US! paratypus, Gregg 439, MO!—Hamosa Greggii (Wats.) Rydb. in Bull. Torr. Club 54: 20. 1927.
The Gregg milk-vetch may be recognized at a glance by the dense, hispidulous pubescence of the stems and petioles in contrast to the thinly and shortly hairy, ordinarily greenish leaflets of broad outline and thin texture. The small, loosely racemose flowers of characteristically short and broad proportions appear on the older specimens to have been ochroleucous with a faint brown penciling on the banner, but traces of purplish-pink are found in all recently collected material. The pod of A. Greggii is of a common hamosoid type, unusual only to the extent that it is glabrous and continuous with the receptacle, although completely sessile. Presumably it dehisces apically on the raceme, but fruits mature enough to show the mode of opening and the ripe seeds are still lacking. Collectors have reported A. Greggii as occurring in shady or even moist places, and the membranous foliage is what might be expected of a mesophytic astragalus. Probably, however, it is essentially a xerophyte like A. scalaris or many Strigulosi, species of the Mexican plateau which are in active growth during the hot months coinciding with the rains, and which seem to require more moisture at the root than the vernally flowering and summer- dormant astragali adapted to existence in open desert.