Astragalus aridus

  • Title

    Astragalus aridus

  • Authors

    Rupert C. Barneby

  • Scientific Name

    Astragalus aridus A.Gray

  • Description

    284.  Astragalus aridus

    Annual, of 2-4 months duration, variable in stature, either low and very slender or becoming robust and coarse in favorable seasons, with a slender or subfiliform taproot, densely strigulose-pilosulose (-pilose) with fine, shorter, appressed and straight together with some or many longer, straight or subsinuous and narrowly ascending hairs up to 0.6-1.1 mm. long, the stems and herbage silky- or almost satiny-canescent, the leaflets equally pubescent on both sides; stems solitary and erect in small plants, more commonly 3 from the root-crown and these often branched or spurred at each node preceding the first peduncle, decumbent with ascending tips or eventually diffuse and trailing, (2.5) 5-30 cm. long; stipules thinly herbaceous becoming papery, commonly purplish, ovate, triangular, or triangular-acuminate, 1.5-5 (6.5) mm. long, ± semiamplexicaul- decurrent; leaves (2) 3-9 cm. long, petioled but the uppermost shortly so, with (7) 9-17 oblanceolate, oblong-oblanceolate, or elliptic, obtuse, truncate, or emarginate, flat or loosely folded leaflets 4-16 mm. long; peduncles incurved- ascending, 2-5.5 cm. long, shorter than the leaf; racemes loosely (3) 4-9-flowered, the flowers ascending, the axis elongating, 1.5-5.5 cm. long in fruit; bracts subherbaceous, ovate or lanceolate, 0.8-2.2 mm. long; pedicels ascending, straight or a trifle arched outward in age, at anthesis 0.2-0.5 mm., in fruit thickened, 0.5-1 mm. long, persistent; bracteoles 0; calyx 3.3-4.3 mm. long, densely silvery-pilose with white and rarely a few black hairs, the subsymmetric disc 0.6—0.9 mm. deep, the turbinate-campanulate tube 2.1-2.7 mm. long, 1.5-2.3 mm. in diameter, the broadly subulate, herbaceous teeth 1—1.6 mm. long, the whole becoming papery, ruptured, marcescent; petals whitish faintly tinged with pink-lilac or flesh-color, drying stramineous, only slightly or (when small) often irregularly graduated, the wings then either a trifle shorter than the keel or longer than the banner; banner recurved through ± 40°, obovate-cuneate or broadly elliptic, shallowly but openly notched, (3.3) 3.5-6.5 mm. long, 2.5-3.8 mm. wide; wings 3.6-5.4 mm. long, the claws 1.4-2.3 mm., the oblong or oblong-oblanceolate, obtuse, straight or slightly incurved blades 2.1-3.7 mm. long, 1-1.6 mm. wide; keel 3.6-5 mm. long, the claws 1.4—2.5 mm., the obliquely obovate or nearly half-circular blades 2.1-3 mm. long, 1.4—2 mm. wide; anthers 0.25-0.4 mm. long; pod ascending, sessile on the receptacle, half-elliptic or lunately elliptic in profile, 1—1.7 cm. long, 4.5-7 mm. in diameter, a little inflated but scarcely bladdery, cuneate and laterally compressed at both ends, but strongly compressed only in the triangular beak, at the middle terete or very slightly compressed, the sutures both filiform, the ventral one straight or gently concave, the dorsal one convex, the thin, canescently strigulose-pilosulous valves becoming papery but opaque, obscurely reticulate, not inflexed, the funicular flange subobsolete, not over 0.2 mm. wide; dehiscence apical, after falling, the beak narrowly gaping; ovules (3) 4-6 (7); seeds ochraceous or chestnut-brown, often purple-dotted, pitted or wrinkled, dull, 2.7-3.7 mm. long. —Collections: 17 (ii); representative: Jones (from Yuma) in 1906 (CAS, NY, POM); L. S. Rose 37,155 (CAS); Abrams 3220 (NY); Ripley & Barneby 10,085, 10,091 (CAS, RSA); MacDougal 227 (NY, p. p.).

    Dunes and open sandy plains and valley floors, 200 feet below to 1200 feet above sea level, locally abundant in the Larrea zone of the Colorado Desert from Salton Sea to Borrego Valley, Imperial Valley, and along the lower Colorado from Palo Verde Valley south to Yuma Desert, southeastern California and immediately adjoining Arizona, Sonora, and Baja California.—Map No. 124.—February to April (May).

    Astragalus aridus (dry) Gray in Proc. Amer. Acad. 6: 223. 1864.—"Interior Californian desert, on the route between the mouth of the Gila River and San Diego, Dr. Thurber."—Holotypus, GH!—Tragacantha arida (Gray) O. Kze., Rev. Gen. 943. 1891. Phaca arida (Gray) Rydb. in N. Amer. Fl. 24 : 354. 1929.

    Astragalus albatus (whitened) Sheld. in Minn. Bot. Stud. 1: 128. 1894.—"Collected in the Colorado desert, in southeastern California, Ap. 1899, by C. R. Orcutt."—Holotypus, so labeled, MINN!

    The parched milk-vetch, A. aridus, is a most distinct species, recognized at a glance by its obviously annual root, silvery or satiny pubescence extending up to the small, ascending, turgid but scarcely bladdery fruits, and its tiny, flesh-pink flowers which turn straw-color in drying. The detached pod of A. aridus is rather similar in form to that of some phases of A. Palmeri, a potential perennial with more numerous and substantially larger, purple flowers native to the west edge of the Colorado Desert. I suspect that the two species may have arisen from a remote common source, although A. aridus has no really close allies. The two other annual Inflati found on the floor of the Salton Sink are A. insularis var. Harwoodii, distinguished by its short, strigulose indumentum, magenta-purple (when dry violet) flowers, and papery-membranous, greatly swollen, purple-speckled pod, and the coarsely hirsutulous A. sabulonum with its declined, villosulous pod of firmly chartaceous texture. In both of these the ovules are less than eight only in very rare instances, in contrast to the common counts of four, five, and six in the parched milk-vetch. The smallest flowers of A. aridus have petals slightly shorter than the calyx-teeth and a poorly developed banner sometimes shorter than the wings. Such flowers are self-fertilized and are nearly if not truly cleistogamous.