Slender, sparsely leafy, annual or biennial, perhaps sometimes shortly perennant, thinly strigulose with fine, straight, appressed and subappressed hairs up to 0.4—0.6 mm. long, the herbage green, the leaflets glabrous above; stems few, or in the second season several, from the crown of a subfiliform or at length woody but still slender taproot, prostrate and radiating, (0.25) 1-3 dm. long, simple, floriferous upward from near or well below the middle; stipules submembranous, triangular or lanceolate, 2-5 mm. long, about semiamplexicaul, glabrous or nearly so dorsally, the margins ciliate and often beset with a few minute processes; leaves (1.5) 2.5-8 cm. long, slenderly but the upper ones only shortly petioled, with (11) 13-17 ovate, oval, or broadly elliptic, truncate-emarginate, or obtuse and minutely apiculate, flat, thin-textured leaflets 2-10 mm. long; through 45-60°, rhombic-obovate or obovate-spatulate, notched or subentire, 7.8-11.2 mm. long, 4.5-5.9 mm. wide; wings 6.8-8.8 mm. long, the claws 3.3-4.8 mm., the oblong-oblanceolate or -obovate blades 4-4.7 mm. long, gently incurved distally and 1.5-2.2 mm. wide just below the shallowly and obtusely bidentate apex; keel (0.2-0.8 mm. longer than the wings) 7.2-9.3 mm. long, the claws 3.5-4.9 mm., the half-ovate-acuminate blades (3.7) 4-4.6 mm. long, 1.6-2.4 mm. wide, rather abruptly incurved through 70-90° into the narrowly triangular, acute, slightly porrect, beaklike apex; anthers 0.4-0.6 mm. long; pod ascending at a narrow angle, sessile on the slightly elevated receptacle, linear-lanceolate in profile, a trifle incurved, 1.1-1.5 cm. long, about 2.5 mm. in diameter, rounded at base, cuneately tapering distally into a short, triangular-acuminate or -cuspidate, laterally compressed unilocular beak, otherwise triquetrous, carinate ventrally by the suture, the lateral angles obtuse, the lateral faces flat or nearly so, the dorsal face narrowly sulcate, the thin, green, thinly and minutely strigulose valves becoming papery, stramineous, finely cross-reticulate, inflexed as a complete or nearly complete septum 0.8-1 mm. wide;ovules 13-16 (18); seeds dark brown, angled, smooth, scarcely lustrous, about 1.4 mm. long.—Collections: 6 (o); representative: Gentry 3648 (ARIZ, GH, K, MO), 7212 (NY), 8024 (ARIZ, DS); Knobloch 5009, 7048 (MSC).
Sandy stream beds and rocky canyon floors, 3500-6500 feet, in oak forest, west slope of the Sierra Madre Occidental in Sonora and Sinaloa, from the Rio Mayo south to the Sierra Surotato, and adjoining Chihuahua.—Map No. 142.— March to May.
Astragalus Gentryi (Howard Scott Gentry, 1903- ) Standl. in Field Mus. Bot. Ser. 22: 22. 1940.—"Mexico: San Bernardo, Rio Mayo, Sonora, ... February, 1935, Howard Scott Gentry xo (type in Herb Field Mus.)."—Holotypus, not examined; isotypus (Gentry 1345), collected February 23, 1935, MICH!
The nearest affinity of the rather weedy, strongly characterized Gentry milk-vetch is not settled. It is perhaps misplaced in subsect. Pringleani, although its deeply campanulate calyx- tube is proper to the group. The individual flower most closely resembles that of A. arizonicus in the prominent keel with its narrowly triangular, porrect tip surpassing by a trifle the broad and short, notched blades of the wings; the habit of growth is also similar. However A. arizonicus differs greatly in appearance on account of the narrow, usually distant leaflets and the dense vesture of silvery dolabriform hairs. The life-span of the average plant of A. Gentryi is not known for certain. Gentry (Rio Mayo Pl. 145. 1942) describes the species as a winter- annual, and the majority of specimens I have examined are certainly flowering in the first year of growth. Others, however, have an indurated, even though always slender taproot which could hardly be expected of an obligate annual. The point requires observation in the field. Annual examples of A. Gentryi would naturally be sought among the Californici, and the species has been introduced into the key to that subsection. I can find there no closer ally than the Lower Californian A. francisquitensis, which differs greatly in fine detail of the flower.
A specimen of the Gentry milk-vetch (Gentry 3648, MO, cited by Gentry, Rio Mayo Pl. 145, as A. Nuttallianus) was annotated by me as an undescribed species before I had seen authentic material of A. Gentryi. The name proposed there was never published.