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

309.  Astragalus siliceus

Subacaulescent, densely pulvinate, the ultimate divisions of the repeatedly forking, suffruticulose caudex becoming columnar from the thatch of scaly marcescent stipules and fibrous leaf-bases, the very numerous, tightly packed leaf-rosettes forming low-convex mounds 1-3 dm. in diameter, the herbage silvery-strigulose with appressed and subappressed, mostly straight hairs up to 0.6-0.9 mm. long; stems of the year almost 0, rarely up to 1 cm. long, wholly concealed by stipules; stipules submembranous becoming pallid and papery, ovate or lanceolate, 1.5-3 mm. long, embracing ½-? the stem’s circumference; leaves 0.5-3 cm. long, slender-petioled, with (3) 5—9 crowded, elliptic, oblanceolate, or (on some short, early leaves) obovate, obtuse or subacute, loosely folded leaflets 1.5-5 mm. long; peduncles very slender, ascending, 1.5-6 (11) mm. long, much shorter than the leaves; racemes loosely 1-3 (commonly 2)-flowered, the flowers ascending, the axis (produced as a subulate appendage beyond the last flower) 1-3 (4) mm. long; bracts membranous, ovate or broadly lanceolate, 0.7-2.2 mm. long; pedicels 2.2—3.5 mm. long, flexuously ascending or at length somewhat arched outward, scarcely elongating or thickened in fruit; bracteoles 0; calyx 4-5.9 mm. long, silky-strigulose with white hairs, the subsymmetric disc (0.6) 0.8-1.1 mm. deep, the campanulate tube 3.2—4.2 mm. long, 1.9—2.5 mm. in diameter, the subulate teeth 0.8—1.8 mm. long; petals pink-purple, drying bluish, the wing-tips whitish, the banner purple-penciled in the fold; banner spatulate-oblanceolate or obovate above the long-cuneate claw, deeply and openly notched, 9.5-11.5 mm. long, 5—7 mm. wide; wings 9.4—11 mm. long, the claws 4—4.7 mm., the narrowly elliptic or elliptic-oblanceolate, obtuse or obscurely emarginate blades 4.9-7 mm. long, 1.6—2.2 mm. wide, both nearly straight and erect; keel 7.5—8.7 mm. long, the claws 4.1—4.9 mm., the half-obovate blades 3.6—4.5 mm. long, 1.6—2.1 mm. wide, abruptly incurved through 90° to the blunt apex; anthers (0.4) 0.45-0.6 mm. long; pod ascending or spreading, obliquely ovoid, 5-7.5 mm. long, 2.8-3.8 mm. in diameter, rounded at base, abruptly contracted distally into a short, declined (rarely suberect) cusp, laterally compressed and bicarinate by the sutures, but the ventral suture thicker, more prominent, and more strongly convex than the dorsal one, the faces low-convex, the green, thinly fleshy, densely strigulose valves becoming stiffly papery; ovules 8—10; seeds unknown.

Rocky knolls on high rolling plains, in arid grassland, on granite, about 6250 feet, known only from the type-locality on the Pecos-Rio Grande divide in central Torrance County, New Mexico.—Map No. 140.—May to June.

Astragalus siliceus (flinty, of the Pedernal or Flint Mountains) Barneby in Leafl. West. Bot. 8: 14. 1956.—"New Mexico: ... southerly foothills of the Pedernal Mts., 3 miles w. of Negra, Torrance County, 7 May and 22 May, 1955 ... Barneby 12,600 & 12,816."—Cotypi, CAS! isotypi GH, K, NY, POM, RM, RSA, US, WS!

The high rolling country that forms the watershed dividing the Pecos from the Rio Grande in central New Mexico is relatively little known botanically. Its flora is of interest to the botanical geographer since it comprises elements of southern and western Texan origin mingling with others derived from the Rocky Mountain piedmont and the higher Great Plains, here at or near the limits of their dispersal; and it is further enriched by a few endemic species. The Flint Mountains milk-vetch is probably to be numbered among the latter, although one must anticipate some extension of its range as exploration of the region progresses.

When past flowering the plants of A. siliceus are inconspicuous features of the sparse and low, markedly xerophytic flora of rocky knolls which stand up in isolation out of the sandy clay plains. In the early days of May, however, when the majority of other herbaceous plants are only just beginning to stir from winter dormancy, the silvery-gray mounds of the astragalus are lavishly decked with small, vivid flowers, forming patches of solid color visible from afar. Except for some pulvinate forms of the distantly related, superficially similar A. spatulatus and some alpine populations of A. Kentrophyta var. implexus, there is no more brilliant cushion astragalus in North America. In the context of its section A. siliceus seems to form a link between A. gilensis, of which it has the comparatively large flower and numerous ovules, and the two species following, similar in their depressed habit of growth, few-flowered racemes, and few small leaflets. In the laterally compressed pod it is nearest to A. humillimus, although very different in other respects.