Slender, prostrate or loosely matted, leafy and floriferous, perennial but sometimes flowering the first season, when mature with a woody taproot and shortly forking caudex, rather densely strigulose with straight, appressed or subappressed hairs up to 0.2-0.5 mm. long, the stems and herbage more or less cinereous, the leaflets bicolored, bluish-green and pubescent beneath, darker green and glabrous or medially glabrescent above; stems numerous, radiating, the principal ones (2) 4-15 (22) cm. long, freely branching at all the approximate lower nodes; stipules herbaceous, the lowest becoming papery in age, ovate, deltoid, or triangular, 1.5-3 mm. long, ± semiamplexicaul; leaves (1) 1.5—4.5 cm. long, with slender petioles and 7-15 oblanceolate or narrowly obovate-cuneate, retuse or truncate, sometimes obtuse, mostly involute or loosely folded, thick-textured leaflets 1.5—6 mm. long; peduncles slender, weakly ascending at anthesis, decumbent in fruit, 0.5—5.5 cm. long, those of the more vigorous stems surpassing, those of the branchlets shorter than the leaf; racemes loosely 3-12-flowered, the flowers ascending, the axis 0.5-4 cm. long in fruit; bracts ovate, 1-1.5 mm. long, at first herbaceous with membranous margins, becoming papery; pedicels slender, ascending, straight or flexuous, at anthesis 1.2-1.7 mm., in fruit 1.2-2.4 mm. long, naturally persistent after fall of the fruit but easily disjointing under pressure; bracteoles 0-2; calyx 5.8-8.5 mm. long, strigulose with white hairs, the turbinate disc 0.8-1.1 mm. deep, the membranous, deeply campanulate or cylindro-vase-shaped tube 4.3—5.7 mm. long, 2.4-2.9 mm. in diameter (the dorsal side straight or a little concave, the ventral side convex), the subulate teeth 1.5-2.8 mm. long, the whole becoming hyaline, a trifle distended but finally ruptured, marcescent; petals pale purple or whitish tinged with lilac; banner gently recurved through ± 40°, 12.4—16 mm. long, the blade 3.4-4.3 mm. wide just above the cuneate claw and thence narrowly lance- oblong, shallowly emarginate at apex; wings 11.5-13.5 mm. long, the claws 4.7 5.6 mm., the linear-oblong or -oblanceolate, obtuse or erose-emarginate, straight or distally a little incurved blades 7.2-8.7 mm. long, 1.7-2.5 mm. wide; keel 8.6-9.8 mm. long, the claws 4.7-5.8 mm., the half-elliptic or obliquely triangular blades 4-4.6 mm. long, 2-2.3 mm. wide, incurved through about 85° to the obtuse, sharply triangular, or minutely cuspidate apex; anthers 0.55-0.75 mm. long; pod ascending (humistrate), sessile, oblong or lance-oblong in outline, straight or a trifle incurved, 7-11 mm. long, (2.1) 2.5-3.5 mm. in diameter, rounded at base, abruptly contracted distally into a very short, triangular-cuspidate beak, bluntly triquetrous, obtusely carinate by the rather thick but not prominent ventral suture, the lateral faces low-convex, the dorsal face deeply and openly sulcate, the thin, green or purple-tinged, finely strigulose valves becoming stramineous, delicately cross-reticulate, inflexed as a nearly or fully complete septum 0.5—0.7 mm. wide; ovules 11-15; seeds brown, purple-speckled, wrinkled and pitted but rather lustrous, 1.9-2.2 mm. long.—Collections: 5 (o); representative: Palmer 18, 89 (K, NY, US); J. N. Rose 11,641 (US).
Dry sandy and gravelly plains, hillsides, and outwash fans, 4300—7400 feet, in mesquite-grassland or juniper-piñon forest, apparently uncommon, within and near the east foothills of the Sierra Madre, from westcentral Chihuahua south to westcentral Durango.—Map No. 141.—February to June.
Astragalus Pringlei (Cyrus Guernsey Pringle, 1838-1911, famed for the quantity and quality of his extensive plant collections from Mexico and s.-w. United States) Wats. in Proc. Amer. Acad. 21: 449. 1886.—"Plains near Chihuahua, Pringle, April, 1885 (no. 79)."—Holotypus, GH! isotypi, BM, G, K, NY, P (3 sheets), PH, US, WS!—Hamosa Pringlei (Wats.) Rydb. in Bull. Torr. Club 54: 336. 1927.
Astragalus Mexiae (Ynez Enriquetta Julietta Reygadas, nee Mexia, 1870-1938, collected widely in Mexico, Alaska, S. America, etc.) Jones, Contrib. West. Bot. 18: 44. 1933.—"No. 2563, Mexia from the Rancho Colorado, Chihuahua, district of Guerrero, Mex., May 27 1929 2200 meters alt... "—Holotypus. POM! isotypi, CAS, G, MO, NY!
The Pringle milk-vetch is technically similar to A. nothoxys, differing from this close relative in its shorter leaves and peduncles, shorter racemes of fewer but larger flowers, and shorter, proportionately plumper pod. A modest, loosely matted plant, it is very free flowering and must be quite showy at full anthesis. The petals have been described as white (Rydberg, 1929, p. 437); this is an error for there are traces of purple pigment in even the oldest specimens, and Mexia has them as "white tinged with lilac," a phrase applicable also to the paler forms of A. nothoxys. The relationship of A. Pringlei to A. nothoxys was not appreciated by Rydberg, who placed it in Hamosa sect. Hartwegianae, although he remarked (1927, p. 336) that it was habitally unlike the rest of that section. Jones (1923, p. 272, in clave) compared it with A. parvus, a much more delicately fashioned plant with tiny flowers. The identity of A. Mexiae, also compared by Jones with A. parvus, was first recognized by C. V. Morton (in Contrib. U. S. Nat. Herb. 29: 100. 1945).