Astragalus molybdenus

  • Title

    Astragalus molybdenus

  • Authors

    Rupert C. Barneby

  • Scientific Name

    Astragalus molybdenus Barneby

  • Description

    5. Astragalus molybdenus

    Low, loosely tufted or matted, with a slender taproot and loosely, sometimes extensively branching subterranean stems persisting as rhizome-like caudex-branches (sometimes adventitiously rooting and thus vegetatively reproductive), the stems above ground thinly strigulose, the herbage thinly to densely strigulose-pilosulous with fine, nearly straight, appressed and ascending hairs up to 0.4-0.5 mm. long, greenish-cinereous to silvery-gray, the leaflets sometimes medially glabrescent above, the inflorescence black-hairy; stems of the season largely subterranean, the aerial tips prostrate or weakly ascending, 0.5-6 (14) cm. long, the internodes all short, or a few of them developed and up to 1.5 (4) cm. long, mostly shorter; stipules 2—5 mm. long, submembranous, purplish, glabrous dorsally, the lowest becoming pallid and scarious, all amplexicaul and connate through ± half their length, the free blades deltoid or lanceolate, obtuse or subacute; leaves 1.5—7 cm. long, petioled, with (9) 17—25 ovate, ovate-oblong, or elliptic, obtuse, mostly crowded, folded or involute leaflets 2—10 mm. long; peduncles at anthesis incurved- ascending, procumbent in fruit, 1—3 (6.5) cm. long; racemes loosely 3—6-flowered, the flowers loosely ascending, the axis scarcely elongating, 3-10 (15) mm. long in fruit, bracts membranous, lanceolate or ovate-acuminate, 2.5—5 mm. long; pedicels at anthesis 0.5—1.5 mm. long, in fruit either straight or arched outward, 1—2 mm. long; bracteoles 0-2, minute when present; calyx 5.2-7 mm. long, densely black- strigulose, the oblique disc 0.5—0.8 mm. deep, the campanulate tube 3—4.2 mm. long, 2.7—3.2 mm. in diameter, the broadly subulate teeth 2—3 mm. long; petals pink-purple, dull lilac, or whitish with banner veined and distally suffused with lilac, the keel-tip always maculate; banner recurved through ±45°, ovate-cuneate, notched, 10.7-12.5 mm. long, 5.2-7.2 mm. wide; wings 9.3-10.9 mm. long, the claws 3.3-4.4 mm., the oblong-obovate, obtuse, slightly incurved blades 6.6-8 mm. long, 2.5—2.7 mm. wide; keel 8.8—9.7 mm. long, the claws 3.8—4.7 mm., the broadly lunate blades 5.3-5.7 mm. long, 2.3-2.8 mm. wide, incurved through 90-100 to the bluntly triangular apex; anthers 0.45-0.6 mm. long; pod ascending (humistrate), sessile or nearly so, obliquely ovoid or ovoid-ellipsoid, a trifle in curved, 7-11 mm. long, ± 3.5 mm. in diameter, rounded at base, abruptly deltoid- beaked and cuspidate at apex, subtriquetrously compressed with acute ventral and rounded lateral angles, flattened or somewhat depressed dorsally but hardly sulcate, the prominent ventral suture either straight or a trifle concave in profile, the thin, submembranous, densely black- or more rarely partly white-strigulose valves not inflexed; dehiscence not seen; seeds (not seen quite ripe) ± 4.2-4.5 mm. long.— Collections: 11 (iv); representative: Weber, Rollins & Livingston 6514 (CAS, RM); Weber & Barclay 9182 (NY); Langenheim 4029, 4071 (RM, UC); Ripley & Barneby 10,402 (CAS, NY, RSA).

    Turfy hillsides and bare gravelly banks above timber line, sometimes creeping in scree or coarse debris at the foot of rock slides, in moist or summer-dry, well- drained soils, on granitic or shaley bedrock, 11,400-13,000 feet, locally plentiful in the highest Rocky Mountains of central Colorado, about the sources of the South Platte, Arkansas, Gunnison, and Grand Rivers.—Map No. 3.—July to August.

    Astragalus molybdenus (of lead, from Leadville, Colorado) Barneby in Leafl. West. Bot. 6: 70. 1950, a substitute for A. plumbeus (leaden) Barneby in op. cit. 5: 195. 1949 (non A. plumbeus Gontsch., 1946).—"Colorado: ... about 4 miles east of Leadville, Lake County ... Ripley & Barneby No. 9994 ... west slope of Mosquito Pass, east of Leadville, No. 10045." —Cotypi, CAS! isotypi, GH, K, NY, POM, RM, RSA, US, WTU!

    The Leadville milk-vetch, A. molybdenus, is one of very few astragali found in the tundra zone of the southern Rocky Mountains. On the South Platte-Arkansas divide its altitudinal range overlaps that of A. alpinus, a species very common in the timber belt, by a hundred feet or so; it becomes abundant only well above the tree line, especially in stony turf and at the edge of rock slides which it sometimes shares with the arctic-alpine Oxytropis podocarpa Gray. In such places A. molybdenus forms dense patches of silvery-gray or ashen-green foliage, spreading by slender sucker-like stolons which may give rise at intervals to an independent root system. The Leadville milk-vetch closely resembles the more densely pubescent Cordilleran phases of A. alpinus in superficial aspect, but is easily distinguished by its regularly graduated petals and by the sessile, unilocular pod. The habitally similar A. leptaleus, a denizen of moist meadows at much lower elevations, differs in its tender green herbage, usually white flowers with a shorter purple-tipped keel, and pendulous, slightly decurved pod.

    The first known collection of A. molybdenus is one taken by John Merle Coulter on Mt. Lincoln in the Park Range, at 13,000 feet, in the summer of 1873 (NY).