Densely cespitose when young, becoming pulvinate, with a closely and repeatedly forking, suffruticulose caudex beset with a columnar thatch of persistent petioles and stipules, at length forming low, hard cushions or domed mounds 1—4 dm. in diameter, the herbage villous-strigose nearly throughout with narrowly and loosely ascending, straight or partly wavy hairs up to 1—2 mm. long, silvery-silky; stems of the year mostly reduced to crowns of loosely rosulate leaves and closely imbricated stipules, rarely a little developed and up to 2 cm. long, but the inter nodes always concealed; stipules hyaline, ovate or obovate, mostly 6-10 mm. long, adnate behind the petiole and (at least in vernation) connate opposite it into a loose, transversely wrinkled sheath, glabrescent dorsally, thinly long-ciliate; leaves 0.7—3 cm. long, with rather stiff, at length recurving petiole and 3 oblanceolate or (when short) obovate, subacute or obtuse, loosely folded leaflets 2.5-10 mm. long; peduncles very short or obsolete, concealed by the stipules, not over 4 mm. long; racemes subcapitately 1—2 (3)-flowered, the flowers erect, the axis not elongating; bracts hyaline, ovate or broadly lance-acuminate, boat-shaped, 3-5 mm. long; pedicels very short or subobsolete; calyx 7.3—10.7 mm. long, densely silvery-villous, the turbinate disc 1.3-2 mm. deep, the narrowly cylindric tube 5.8-7 mm. long, 2.1—3 mm. in diameter, the subulate or linear-subulate teeth 1.5—3.7 mm. long, the whole becoming scarious, fragile, distended and apparently ruptured by the swelling pod; petals whitish, the keel- and wing-tips faintly lilac-tinged; banner nearly erect, 12.5-17.6 mm. long, the oblanceolate or cuneate-oblanceolate claw 6.2-8.3 mm., the linear or linear-oblong, obtuse or emarginate, straight blades 5.6-9.2 mm. long, 1-2 mm. wide; keel 10-13 mm. long, the claws 6.5-9 mm., the ovate-elliptic, scarcely oblique blades 4—4.6 mm. long, 1.6—2.2 mm. wide, gently incurved through 45° to the obtuse apex; anthers 0.55-0.7 mm. long; pod apparently as in A. gilviflorus, densely pubescent when young, not seen fully formed; dehiscence and seeds unknown.—Collections: 15 (viii); representative: Osterhout 4008 (NY, RM, WIS); Ripley & Barneby 8937, 8951, 10,556 (CAS, RSA); E. Nelson 705 (RM), 4949 (NY, RM); Goodding 176 (RM, US); Barneby 13,224 (NY, RSA).
Barren hilltops, gullied bluffs, and badlands, on sedimentary formations, mostly shales and limestones, sometimes on red beds, 3500-7500 feet, locally plentiful in scattered stations along the North Platte River and its tributaries in southeastern Wyoming and western Nebraska, north to the west foothills of the Black Hills, South Dakota, and in Wyoming to the Powder River and west slope of the Big Horn Mountains, south to the Arikaree River in northeastern Colorado; reported (C. L. Porter, 1951, p. 6) from North Dakota.—Map No. 163.—Flowering from late June to early August.
Astragalus hyalinus (glassy-transparent, of the stipules) Jones in Proc. Calif. Acad. Sci. II, 5: 648. 1895.—"Upper Lawrence Fork, Kimball County, Nebraska, No. 80; Cliffs, Banner County, Nebraska, August 1890; Hills, Kiowa Valley, Scott’s Bluff County, Nebraska. All collected by Rydberg. Type in University of Nebraska and duplicate types in National Herbarium."—Holotypus, not examined; isotypi, Rydberg 80 (from Scott’s Bluff County), US! Rydberg in August, 1890 (from Banner County), LIS!
Phaca caespitosa ß Hook, in Lond. Jour. Bot. 6: 212. 1847.—"Stony plains, on the north and south Forks of the Platte ... Geyer n. 166."—Holotypus, K! isotypi (some dated "1845," but probably collected in 1843), BM, G, OXF!
Orophaca argophylla sensu Rydb. in N. Amer. Fl. 24: 310. 1929; non Phaca argophylla Nutt.
The three striking features of the summer orophaca, A. hyalinus, are the platyonychious banner, the pubescent petals, and the late season of bloom. The banner, like that of the related but early spring-flowering A. proimanthus, takes the form of two oblanceolate leaves of about equal length and width superimposed one above the other, making a fiddle-shaped figure quite distinct from the simply oblanceolate banner of A. gilviflorus. The flower of A. hyalinus is ordinarily much smaller than that of A. gilviflorus, the petals of which are with rare exceptions quite glabrous, and at best only puberulent on the back of the banner. Jones (1923, p. 86) suggested that A. hyalinus might be no more than an abnormal form of A. gilviflorus, but this is certainly not so. Though the two species inhabit almost identical environments and are partly sympatric, they are sharply differentiated morphologically and isolated genetically by the gap between their seasons of bloom. The period of anthesis is governed to some degree by altitude and exposure as well as by seasonal variation, but at the same latitude and elevation flowering of the two species is separated by an interval of three to four weeks. In western Nebraska A. gilviflorus is in flower from the last week in April through May, but rarely persists into the last week of the latter month; whereas A. hyalinus has been collected in flower in the same area only between the middle of June and the third week of July. In mid-June, 1961, A. gilviflorus was in ripe fruit on the Powder River in northeastern Wyoming at a time when the buds of A. hyalinus were only just beginning to emerge from the stipular sheaths. As seen in the field, A. hyalinus is often recognizable even when sterile by its habit of forming extremely large, domed cushions of neat and short, silvery foliage; the average plant of A. gilviflorus is at once coarser in detail and forms smaller tufts.
It is remarkable that although A. hyalinus was first discovered almost 120 years ago, the ripe fruit of the species is still unknown.