Diffuse, usually slender, perennial but of short duration, sometimes flowering the first season, with a taproot and at length shortly forking caudex or knotty root-crown, strigulose throughout with filiform or largely flattened, straight, appressed (and sometimes a few narrowly ascending) hairs up to 0.4-0.8 (0.9) mm. long, the stems and herbage silvery-canescent or cinereous, rarely greenish- cinereous, the leaflets equally pubescent on both sides or more densely so above than beneath; stems several or numerous, decumbent or prostrate with ascending tips, (0.5) 1.5-4.5 (5) dm. long, simple, branched at base only, or spurred at most axils preceding the first peduncle, flexuous or zigzag distally, together forming loosely woven mats; stipules submembranous becoming papery and pallid, ovate- or deltoid-acuminate, or broadly triangular, about semiamplexicaul, pubescent dorsally; leaves 2—8 (10) cm. long, all petioled or the uppermost sometimes subsessile, with (5) 9—15 (17) linear, linear-oblong, -elliptic, narrowly lanceolate, or (when short) narrowly oval, obtuse, mucronulate, or acute, flat or folded leaflets (1.5) 3-17 (21) mm. long; peduncles erect or divaricate and incurved, (1.5) 2.5-10 (15) cm. long, the lower, more vigorous ones a little longer than the leaf, humistrate in fruit; racemes loosely or at first anthesis rather closely (7) 10— 30-flowered, the flowers ascending, the axis elongating, (1.5) 2.5-10 cm. long in fruit; bracts membranous, ovate, triangular, or lanceolate, 0.8-2 (3) mm. long; pedicels ascending, usually at a narrow angle and straight or nearly so, at anthesis 0.5-1.6 (2) mm., in fruit 1-2.2 mm. long; bracteoles 0-2; calyx (4.5) 5-7 mm. long, strigulose with white or mixed white and black hairs, the symmetric disc 1—1.4 mm. deep, the campanulate tube 3.3—4.1 mm. long, 2.1—3.1 mm. in diameter, the broadly or slenderly subulate teeth (1.2) 1.4-3 mm. long, the whole becoming scarious, ruptured, marcescent; petals dull pink-purple, the banner with a pale, striate eye in the fold, the wing-tips paler or white, or all yellowish tinged with lurid lilac; banner recurved through about 50°, sometimes further in age, obovate-cuneate to suborbicular-flabellate, shallowly notched, (8.7) 9-12 mm. long, (6.1) 6.7-9.1 mm. wide; wings (6.8) 7.1-10 mm. long, the claws (2.4) 2.84 (4.3) mm., the narrowly oblong or oblong-oblanceolate, obtuse, nearly straight blades (4.7) 5—6.5 mm. long, 1.7—2.3 (2.5) mm. wide; keel (0.2—1 mm. longer than the wings, 0.3-2 (2.5) mm. shorter than the banner) 8.2-10.4 mm. long, the claws (2.6) 2.8-4 (4.4) mm., the lunately triangular blades (4.7) 5.2-6.5 mm. long, (1.9) 2.2-2.7 mm. wide, nearly erect in the lower half, thence abruptly incurved through 70—90° and produced into a narrowly triangular, porrect, acute or subacute, beaklike apex; anthers (0.4) 0.45-0.65 (0.7) mm. long; pod erect straight or gently incurved, 1.5-3 cm. long, (2.2) 2.5-4 mm. in diameter, cuneate at base, contracted distally into a short, triangular-acuminate, stoutly cuspidate, laterally compressed, unilocular beak, otherwise obtusely triquetrous, bluntly carinate ventrally by the thick suture, the lateral angles rounded, the lateral faces more or less convex at maturity, the dorsal face flattened or very shallowly and openly sulcate (the dorsal suture slender but prominent externally), the thin, densely strigulose valves becoming papery, stramineous, finely elevate-reticulate, inflexed as a complete or nearly complete septum 1.4—3 mm. wide; seeds quadrate, brown or greenish-brown, commonly purple-dotted, pitted and wrinkled but somewhat shining, 1.8—2.6 (3) mm. long. —Collections: 43 (iii); representative: Jones (from Skull Valley) in 1903 (CAS, NY, POM, WS); Eastwood 17,021 (CAS, NY); Barneby 12,640, 12,645 (CAS, NY, RSA); Eggleston 19,898 (CAS, NY); Keck 3972 (DS, US); Ferris 8789 (DS, NY).
Open stony hillsides, alluvial fans, washes, and canyon beds, sometimes abundant on gravelly roadsides, in desert-grassland, thorn scrub, oak scrub, or juniper forest, 2800-4650 feet, on various soils but apparently most frequent and vigorous on calcareous formations, widespread and rather common in the foothills around and within the upper Gila River Basin, from the head of the Hassayampa to the Salt and San Pedro Rivers, westcentral to southeastern Arizona, and south to the head of the Magdalena River in northern Sonora.—Map No. 143.—March to June.
Astragalus arizonicus (of Arizona) Gray in Proc. Amer. Acad. 7: 398. 1868 ("Arizonicus").—"Arizona, near Camp Grant, April, Dr. Edward Palmer."—Holotypus (Palmer 53), collected April 22, 1867, GH! isotypus, MO!—Tragacantha arizonica (Gray) O. Kze., Rev. Gen. 943. 1891. Hamosa arizonica (Gray) Rydb. in Bull. Torr. Club 54: 22. 1927.
The Arizona milk-vetch is the one species of its region in which an acutely triangular keel-tip is combined with a trigonous pod of narrow outline and a vesture of hairs attached laterally above the base; it is thus seldom misidentified. The herbage is thin and sparse, nearly always silvery-canescent, and the leaflets are commonly more densely hairy above than beneath. In the south half of its range it is sympatric and sometimes associated with A. nothoxys, a species technically similar but differing at first sight in its broader, bicolored leaflets, glabrous or nearly so above and only thinly strigulose beneath with basifixed hairs. The range of A. arizonicus, as known from reliably labeled specimens, is eminently a natural one. It has been reported from northern Arizona (Mohave and Coconino Counties) by Kearney (in Kearney & Peebles, 1951, p. 468), possibly by confusion with A. Nuttallianus var. micranthiformis, the common astragalus with "hamosoid" pod native to the south edge of the Colorado Basin. An old record (Hemsley, 1886, p. 28) from Coahuila was based on material of A. coahuilae, but Jones (1923, p. 257) still traced the Arizona milk-vetch south to "central Mexico," a doubtful statement. It is difficult to disprove even the most probably erroneous records, when the source-material was not identified in the first place and one has only negative evidence with which to confute them. Rydberg’s range extension of Hamosa arizonica (1927, p. 22) into northwestern New Mexico is certainly wrong, being based on misidenified specimens of A. albulus (Matthews 12, from Fort Wingate, GH) and A. proximus (C. F. Baker 427, from Aztec, GH).
In the early spring months A. arizonicus makes an attractive picture, as it spreads its round mats of silvery foliage, decked with an outer ring of pink- or lavender-purple flowers, over the calcareous gravels between bushes of cat’s-claw acacias, junipers, or sahuaros. Later in the season as the stems elongate and the fruits mature, the plants take on an untidy, withered look and are easily passed by unnoticed. The species was first discovered in flower in March, 1852, by C. C. Parry, then working for the Boundary Commission, at Tubac, Sonora, and at about the same time by Capt. E. K. Smith at Nogales. The specimens were noticed by Gray (1864, p. 234, No. 3) as representing an undescribed species which had been confused in the Botany of the Survey with A. sonorae. The fruiting typus was collected by Palmer almost immediately afterward.