Astragalus obscurus

  • Title

    Astragalus obscurus

  • Authors

    Rupert C. Barneby

  • Scientific Name

    Astragalus obscurus S.Watson

  • Description

    166. Astragalus obscurus

    Low slender, diffusely caulescent or tufted and subacaulescent, with a woody taproot and thickened root-crown or shortly forking, suffruticulose caudex, strigulose nearly throughout with fine, straight, appressed (exceptionally ascending) hairs up to 0.3-0.55 (0.9) mm. long, the herbage greenish or subcinerous, the leaflets commonly more thinly pubescent above than beneath, exceptionally glabrous above; stems several or numerous, simple, 5-15 cm. long, when short the leaves and subscapose peduncles tufted on the root-crown, when developed with at least l and up to 5 intemodes 1-5 cm. long, then radiating, prostrate or weakly ascending; stipules (1.5) 2-5.5 mm. long, the lowest papery, usually loosely imbricated, ovate or triangular, decurrent around 3/4 to the whole stem’s circumference, free (or rarely a trifle connate at very base), the upper ones narrower, deltoid-acuminate or lanceolate, herbaceous or with herbaceous, deflexed tips; leaves (2.5) 4-10 cm. long, all petioled but the uppermost sometimes shortly so, with slender but rather stiff rachis and (5) 7-13 (15), remote or less often moderately spaced, flat or folded, thick-textured leaflets 2-10 (15) mm. long, varying greatly in shape and size from broadly oblong-elliptic in all or in all but the uppermost leaves to lance-oblong or linear-elliptic in all but the lowermost leaves, mostly obtuse, sometimes truncate or shallowly retuse when broad, or subacute when narrow; peduncles wiry, usually straight and rather rigid, sometimes incurved-ascending, 3-15 cm. long, commonly surpassing the leaf; racemes (3) 6-14-flowered, rather compact in early anthesis, the axis soon elongating, (1) 2-8 cm. long in fruit; bracts membranous or membranous-margined, ovate or lanceolate, 0.8-2.5 mm. long; pedicels erect or ascending at a narrow angle, straight, at anthesis 0.5-1 mm. long, in fruit somewhat thickened, 1.2-2.2 mm. long; bracteoles 0-2, usually 0; calyx (3.1) 3.6-5.5 mm. long, strigulose with black and white hairs variably proportioned, the disc 0.6-1 mm. deep, the campanulate tube (2.3) 2.8-4 mm. long, (1.7) 2-2.6 mm. in diameter, the subulate or triangular-subulate teeth 0.4—1.6 mm. long, the ventral pair often a little shorter than the rest, the whole becoming papery, marcescent, ruptured; petals ochroleucous or sordidly whitish, often suffused with dirty lilac; banner recurved through ± 45°, obovate-cuneate, deeply emarginate, 7-10.5 mm. long, (4.5) 5.5-8 mm. wide; wings (6.2) 6.7-9 mm. long, the claws (2.8) 3-4 mm., the lunately oblong, linear-oblong, or oblanceolate, obtuse blades (3.5) 4.3-6 mm. long, (1) 1.3-2 mm. wide; keel equaling or slightly surpassing the wings, (6.3) 7-10 mm. long, the claws 2.9-3.7 mm., the lunately triangular blades 3.6-5.5 mm. long, 1.6-2.6 (3) mm. wide, incurved through 80-90° to the narrowly triangular, often porrect and beaklike but ultimately obtuse apex; anthers (0.4) 0.45-0.6 (0.65) mm. long; pod erect, sessile or nearly so, the stipe if perceptible not longer than thick, the body linear-oblong in profile, straight, (1) 1.3-2.5 cm. long, 2.4-3.3 mm. in diameter, cuneate at both ends, cuspidate at apex, obtusely triquetrous with convex lateral and openly grooved dorsal faces, keeled ventrally by the prominent thick suture, the thinly fleshy, pale green, finely strigulose valves becoming leathery, stramineous, reticulate, inflexed as a nearly complete septum 1-1.4 mm. wide; ovules 14—23; seeds brown or greenish-brown, smooth or rugulose-punctate, 2.2— 2.7 mm. long.—Collections: 59 (viii); representative: Maguire & Holmgren 26,426 (NY, RSA, WS); Cusick 2368 (ND, NY, POM); Ripley & Barneby 6040 (CAS, RSA); Macbride & Payson 2851 (CAS, NY); M. & G. Ownbey 2787 (CAS, NA, NY, RSA, SMU, WS); Holmgren 1984 (NA, NY); Austin & Bruce 2211,2215 (NY).

    Stony flats, hillsides, and barren ridges, in sandy or stiff clay soils derived from basalt, commonly among or sheltering under low sagebrush, (2650) 35006500 (8000) feet, widely dispersed and locally common across northern Nevada in the drainage of the Humboldt and Carson Rivers north to the lake country of southern transmontane Oregon and northeastern California (rarely west to the foothills of the Cascades in Siskiyou County; cf. minor variant mentioned below), northeast in Oregon to the foothills of the Blue Mountain system (one station on the upper John Day River in Grant County, and two on the upper Deschutes in Crook and Deschutes Counties) and in Idaho to the southwest corner of the state in Owyhee and Elmore Counties.—Map No. 66.—May to July, the fruit long persisting.

    Astragalus obscurus (dark or obscure) Wats., Bot. King 69. 1871.—" ... foothills near Truckee Pass, Nevada; 5500 feet altitude; May (266)."—Holotypus, US! isotypi, GH, NY!- Tragacantha obscura (Wats.) O. Kze., Rev. Gen. 946. 1891. Tium obscurum (Wats.) Rydb. in N. Amer. Fl. 24: 394. 1929.

    Tium miserum sensu Rydb. in N. Amer. Fl., l.c., & in Bull. Torr. Club 57: 403. 1931; non A. miser Dougl.

    The arcane milk-vetch, A. obscurus, is a lowly inconspicuous astragalus of weedy aspect, demanding close scrutiny before it reveals its particularity of structure, but then easily recognized. The few leaflets, the small, dingy flowers ascending in a rather close raceme, the scarcely graduated petals with beaklike keel-tip, and the erect, sessile, linear-oblong, trigonously compressed pod form a combination unique in sect. Reventi-arrecti. Jones (1923, p. 181) thought the species related (or "in some forms very close") to A. atratus, but this differs greatly in the form of the flower and in the pendulous pod. It is true that A. obscurus and A. atratus are sympatric and often directly associated in northern Nevada, but this of itself is less suggestive of relationship (in fact, the opposite) than of similar ecological adaptation.

    Variation in outline and amplitude of the leaflets is a feature of A. obscurus, noted in detail by Jepson (1936, p. 372); it was Rydberg’s excuse for maintaining two species, Tium obscurum and T. miserum. The latter (wrongly identified with A. miser Dougl., q.v.) corresponds with the more pronouncedly xerophytic state in which narrow leaflets are supposedly, but not, in fact, correlated with short, tufted, subacaulescent stems. The relatively uncommon phase with broad leaflets is, however, nearly always strongly caulescent, tends to have a slightly larger flower, and occurs in more protected sites, often in shelter of sagebrush. The number of black calyx-hairs, emphasized as an additional distinguishing character (Rydberg, 1929, p. 385), has no bearing on the case.

    The vesture of the arcane milk-vetch is usually strictly appressed and composed of short, straight hairs up to 0.3-0.55, rarely 0.7 mm. long. A remarkable plant from the northern foothills of Mount Shasta in Siskiyou County (north of Weed, J. T. Howell 1786), pubescent with extremely fine, loosely ascending hairs up to 0.7-0.9 nun. long, is interpreted for the present as a minor variant, but may prove, as more material becomes available, to represent a distinct variety. The station marks the westernmost outpost of A. obscurus in California and lies nearly one hundred miles distant from the nearest in Modoc County.