5. Astragalus aridus A. Gray, Proc. Amer. Acad. Arts 6: 223. 1864 Type: — USA, Interior Californian Desert, on the route between the mouth of the Gila River and San Diego, Thurber 3220 (Holotype: GH digital image 00058652 GH!). Tragacantha arida (A. Gray) Kuntze, Revis. Gen. 943. 1891. — Phaca arida (A. Gray) Rydb., N. Amer. Fl. 24: 354. 1929. Astragalus albatus E. Sheld., Minnesota Bot. Stud. 1: 128. 1894. Annual, short lived. Stems 1-few, up to 30 cm long, decumbent to ascendant distally or sometimes creeping, ashen, silver to white strigose to strigose-pilose, trichomes 0.5–1.1 mm long, of two sizes, some short and appressed, mixed with straight and longer ones. Stipules 1.5–6.3 mm long, semi-clasping, not connate, scarcely decurrent, triangular, acuminate, frequently purple. Leaves 2–9 cm long, leaflets 7–17, 4–16 mm long, elliptic, oblanceolate, obtuse, truncate or retuse, equally satiny or canescent on both surfaces. Peduncles 2–5.6 cm long, ascendant, or slightly curved, ashen strigose; racemes 1.5–5.5 cm long, flowers 3–9, loose. Flowers, white with rose or lilac tones, white tinged with levender, purple, becoming tan when dry; the calyx 3.2–4.4 × 1.5–2.4 mm, papery, campanulate, densely pilose, trichomes white and few black ones mixed, the tube 2–2.7 mm long, the teeth 1–1.6 mm long, triangular to subulate; the banner recurved, 3.3–6.5 × 2.5–3.9 mm, elliptic, retuse; the wings 3.5–4.6 × 1–1.6 mm, the claw 1.4–2.3 mm long, the blade, 2.1–3.7 mm long, oblanceolate; the keel 3–6.5 × 1.4–2 mm, the claw 1.4–2.5 mm long, the blade 2.1–3 mm long, obovate. Pod ascending, sessile, lunate to elliptic in profile view, 10–18 × 4.4–7 mm, somewhat inflated but laterally compressed at both ends, beak compressed, sutures narrow, the dorsal one convex, the ventral one strongly concave, the valves strigulose-pilose, similar color as leaflets, papery, opaque, scarcely evident reticles; seeds, 2.6–3.6 mm long, ochre to brown-reddish, opaque, frequently purple stained. Distribution:— Northeastern Baja California (Mexicali), northwestern Sonora (San Luis Río Colorado), near the border of both states (Fig. 5). Also in California and Arizona (USA) (Fig. 2). Habitat:— Sandy plateaus, sandy desert wash; granitic soils; microphyllous desert scrubland; 640 m. Rare. Comments:— Species morphologically similar to A. comonduensis, but this later species has longer erect, nondecumbent or trailing and longer stems, reaching up to 42 cm, shorter (0.2–0.5 mm long) and non silky or satinycanescent pubescence. The leaflets of A. comonduensis are mucronate and adaxially glabrate versus equally pubescent on both leaf surfaces and not apically mucronatefor A. aridus. The color and length of the flower component of both species is different also, A. comonduensis has shorter (2–2.8 mm long) and narrower calyx, with white and black intermixed trichomes unlike the uniformly white trichomes of A. aridus. Astragalus comonduensis has purple-lavender petals, and its keel has a very distinctive small curved backward beak, absent in A. aridus. Carpologically, the pods of A. comonduensis are completely inflated, bladdery, proportionally wider, shiny, and not laterally compressed (not even a little), opaque to sub-diaphanous, and minutely-strigulose. Specimens examined:—BAJA CALIFORNIA: 21 April 1915, Plain of San. [illegible], D. T. MacDougal 227 (NY); 16 March 1960, Sandy desert wash along highway about 14 miles west of Mexicali, I. Wiggins, D. B. Wiggins 15743 (CAS, MEXU, TEX-LL, US). SONORA: 26 February1958, Along Mexican Highway 2, 31 miles south-east of San Luis Rio Colorado, P. H. Raven 11642a (CAS, NY); 3 January 1904, Mesa near La Grulla, D. T. McDougal s.n. (NY).

Published as part of Castillón, Eduardo Estrada, Quintanilla, José Ángel Villarreal, Delgado-Salinas, Alfonso & Rebman, Jon P., 2023, The genus Astragalus (Leguminosae: Papilionoideae: Galegeae) in Mexico, pp. 1921-1935 in Phytotaxa 586 (1) on pages 1921-1935, DOI: 10.11646/phytotaxa.586.1.1, http://zenodo.org/record/7703999