Monographs Details: Astragalus sericoleucus A.Gray
Authors:Rupert C. Barneby
Authority: Barneby, Rupert C. 1964. Atlas of North American Astragalus. Mem. New York Bot. Gard. 13(2): 597-1188.
Family:Fabaceae
Discussion:

362.  Astragalus sericoleucus

Distinctly caulescent, prostrate, the freely branching stems radiating from the crown of a thick taproot and forming mats 1.5-9 dm. in diameter, the caudex-branches beset with a thatch of flaccid, persistent petioles, the herbage silvery-pilose with fine, lustrous, appressed and narrowly ascending hairs up to 3 mm. long or more; stems of the year very short or (especially the outermost of a mat) up to 5 cm. long or more; stipules 2-8 mm. long, hyaline, densely pilose dorsally, united behind the petiole and connate-amplexicaul; leaves 1-4 cm. long, the 3 narrowly to broadly oblanceolate, rarely obovate-cuneate, subacute leaflets 3-13 mm. long; peduncles 0.5—2.5 cm. long; racemes loosely (2) 3—5-flowered, the flowers ascending, the axis up to 1 cm. long in fruit; bracts membranous, lanceolate 1-2.5 mm. long; pedicels slender, at anthesis 1—1.5 mm., in fruit 1.5—3.5 mm. long; petals pink-purple with a pale lozenge in the fold of the banner, often drying yellowish, rarely pure white; banner recurved through nearly 90°, obovate-cuneate, 5.2-6.2 mm. long, 3.8-4.2 mm. wide; wings 5-5.8 mm. long, the claws 2-2.4 mm., the broadly oblanceolate, lunately incurved blades 3.2-4 mm. long, 1.3-1.7 mm. wide; keel 4-4.5 mm. long, the claws 1.9-2.3 mm., the half-obovate blades 2.2-27 mm. long, 1.4—1.7 mm. wide, incurved through 90—100° to the bluntly deltoid apex; anthers 0.3—0.4 (0.45) mm. long; pod ascending or recurved from arched pedicels, sessile, ± 5.5-7 mm. long, the ovoid-ellipsoid, slightly compressed body 3-4.5 mm. long, ± 2 mm. in diameter, commonly included in the unbroken calyx, tapering distally into a narrowly tubular, incurved beak nearly as long, the thin, densely silky-strigose or -pilose valves becoming papery, not inflexed; ovules 6-10; seeds (seldom seen) olive-brown, smooth, about 1.3-1.5 mm. long.—Collections: 20 (v); representative: Brenkle 43,101 (NY, SMU); A. & E. Nelson 6828 (NY); W. A. Weber 6040 (CAS, SMU); C. L. Porter 7262 (NY); Ripley & Barneby 9096 (CAS, RSA), 9126 (CAS, NY, RSA), 10,571 (RSA).

Barren ridges, knolls and hilltops, in gravelly clays and among distintegrating rocks, especially on shale outcrops, locally plentiful on the higher prairies between 3800 and 5300 feet from northeastern and eastcentral Colorado to western Nebraska and adjoining Wyoming, and extending west in scattered stations up the Platte and Little Laramie Rivers to the Seminoe Mountains in Carbon County and the base of the Snowy Mountains in Albany County, Wyoming—Map No. 162.— Late May to July.

Astragalus sericoleucus (white-silky) Gray in Amer. Jour. Sci. II, 33: 410. 1862, based on Phaca sericea (silky) Nutt. ex T. & G., Fl. N. Amer. 1: 343. 1838 (non A. sericeus DC., 1802).—"On the high hills of the Platte near the Rocky Mountains ... Nuttall"—Holotypus, labeled in Nuttall’s hand: "Phaca *sericea. R. Mts.-Platte," BM! isotypi, K, NY!—Tragacantha sericea (Nutt.) O. Kze., Rev. Gen. 942. 1891. Orophaca sericea (Nutt.) Britt. in Britt. & Br., I11. Fl. 2: 307, fig. 2155. 1897.

The silky orophaca, A. sericoleucus, is an attractive little plant at all stages of growth, as much for its closely woven mats of silvery three-fingered foliage as for the lavender- or rosy- purple flowers which although very small are sometimes produced in sufficient numbers to make patches of color visible from a distance. The flowers, however, are usually of pale or muted tones and never contrive the display afforded by A. tridactylicus on the red beds of the Laramie Hills or by A. aretioides on the Wind River. The species is commonest and most abundant locally on the short-grass prairies of western Nebraska and northeastern Colorado, where it is the only known member of its section; but it follows the North Platte westward to the mouth of the Sweetwater and has been traced up the Little Laramie River nearly to Centennial. There its range overlaps that of the mat-forming phase of the much larger-flowered A. tridactylicus. In southeastern Wyoming flowers, or at least the marcescent calyces, are required for positive identification. According to Rydberg (Fl. Neb.) A. sericoleucus extends north into South Dakota, but I have seen no specimens validating the claim. The record was dropped from Rydberg’s summary revision (1929, p. 311); and Claude Barr, long resident near Smithwick in southeastern Fall River County, tells me (in correspondence) that it is unknown from that corner of the state, where its place is taken by the very different, calciphile rather than shale-dwelling A. Barrii.

Besides the typus already cited, three Nuttall collections at PH deserve passing notice. The sheet bearing the same label as at NY consists of a mixture of A. sericoleucus and A. aretioides, which Nuttall evidently confounded; for the purpose of typification the element of the second species should be ignored. A second sheet of material belonging to A. sericoleucus, but possibly representing a separate collection, is ticketed by Nuttall as: "Phaca *trifoliata. Platte (R. Mts.)." The name was never published.