Monographs Details: Astragalus detritalis M.E.Jones
Authors:Rupert C. Barneby
Authority: Barneby, Rupert C. 1964. Atlas of North American Astragalus. Mem. New York Bot. Gard. 13(1): 1-596.
Family:Fabaceae
Synonyms:Homalobus detritalis (M.E.Jones) Rydb., Astragalus spectabilis Ced.Porter
Description:Species Description - Dwarf, caespitose, the shortly forking caudex beset with remains of stipules and leaf-bases, the herbage densely silvery-strigulose or -strigose with fine, straight, appressed, parallel and subcontiguous hairs up to (0.5) 0.6—1.2 mm. long; stems of the year mostly reduced to rosulate crowns, together forming depressed tufts up to 15 cm. in diameter, or some of them a little developed and up to 3 cm. long, but the internodes very short, not surpassing and usually concealed by stipules; stipules 3—10 mm. long, scarious with green or at length brownish midrib, amplexicaul and connate through half their length or more into a sheath, glabrous or early glabrescent dorsally, the ovate or lanceolate free blades ciliate; leaves dimorphic, the lowest in the year’s cycle 0.5—2 cm. long, mostly reduced to a narrowly oblanceolate or linear-oblanceolate phyllodium not differentiated into petiole and blade, rarely all or nearly all palmately trifoliolate, the upper ones (1) 2-8 cm. long, with wiry petioles and 1-3 pairs of linear-lanceolate or -oblanceolate, or subfiliform, very acute, rather stiff and dorsally carinate, ± involute leaflets (0.3) 0.5-3 cm. long, all jointed to the rachis, sometimes obscurely so; peduncles scapose, erect or incurved-ascending, 1-7 (9) cm. long, either ascending or decumbent in fruit; racemes shortly 2-6 (8)-flowered, the flowers strongly ascending, the axis 0.2-2.6 (3.8) cm. long in fruit; bracts scarious with green, finally pallid midrib, ovate-acuminate or broadly lanceolate, boat-shaped, (2.5) 3-7 mm. long, glabrous or nearly so; pedicels ascending or erect, at anthesis 0.5-1.4 mm., in fruit 1.2-2.5 mm. long; bracteoles usually 0, rarely present and up to 2.5 mm. long; calyx at anthesis (5.1) 5.8-9.2 (9.6) mm. long, strigulose with white and sometimes a few black hairs, the symmetric disc 0.7-1.3 mm. deep, the submembranous, purplish tube 3.1-5.2 (5.4) mm. long, 2.5-4.1 mm. in diameter, the narrowly subulate, rather stiff, erect or up-curved teeth (1.6) 2-4.5 (4.7) mm. long, the ventral pair often shortest, the orifice oblique, the whole becoming papery and a little distended or accrescent, persisting usually unruptured; petals pink-purple, the banner pale-eyed, striate; banner recurved through ± 45°, oval-elliptic to broadly rhombic-oblong, openly notched, (12) 13.5-18 (20) mm. long, 5.5-8.5 (9.8) mm. wide; wings a little shorter to a trifle longer, (10.8) 12.5-16.3 (19.7) mm. long, the claws (4.4) 4.6-6.5 mm., the lanceolate or lance-elliptic, obtuse and sometimes obscurely erose blades (7.7) 8-12 (14.2) mm. long, 2-3.4 mm. wide, the left one more strongly incurved than the right and its blade infolded; keel (8.4) 9.4-12.8 (13.4) mm. long, the claws (4.1) 4.5-6.3 (6.8) mm., the half-obovate blades 5.2-7.3 (8.1) mm. long, 2.5-3.2 (3.7) mm. wide, gently incurved through 85-95° to the rounded or bluntly deltoid apex; anthers (0.5) 0.55-0.9 mm. long; pod erect or narrowly ascending, sessile, tardily if at all disjointing from the receptacle, linear-oblong, straight or a little incurved, 1.5-3.5 (3.8) cm. long, 2-3.5 mm. in diameter, obtuse at base, cuspidate at apex, strongly compressed and 2sided, carinate ventrally by the slender but salient suture, narrowly rounded dorsally, the lateral faces shallowly concave along the ventral suture and low-convex toward the dorsal one, the thinly fleshy, greenish and usually red-mottled, finely strigulose valves becoming papery, stramineous, finely cross-reticulate; ovules (15) 16-24; seeds brown, rugulose, ± 2-2.4 long.

Distribution and Ecology - Cobblestone bluffs, barren knolls, hilltops, and gullied badlands, on shale, sandstone, or stony alluvial clays, 5200—7000 feet, local but forming colonies in scattered stations within the Uinta Basin in Duchesne and Uintah Counties, Utah, and Rio Blanco County, Colorado.—Map. No. 27.—May to June.

Discussion:

The debris milk-vetch, A. detritalis, is most abundant on the white shales exposed around the southern periphery of the Uinta Basin, where it is commonly associated with Lesquerella subumbellata Roll, in open stands of Utah juniper. The astragalus and the lesquerella flower simultaneously, and the bright golden cushions of the little crucifer, intermingled with low tufts of carmine vetch and an occasional patch of Penstemon dolius Jones, are set off to brilliant advantage by the pale chalky background of the barrens. Eastward, along the Rio Blanco in Colorado, A. detritalis has been found on sandstone outcrops and among rock debris under sandstone cliffs, but the plants are smaller, less floriferous, and occur in small, scattered stands. On the soft red clay of gullied badlands near Vernal, where the species is represented by the unusually vigorous, large-flowered form described as A. spectabilis, it is associated with four Basin endemics, A. Hamiltoni, Cymopterus duchesnensis Jones, Hermidium alipes var. pallidum C. L. Port., and Cordylanthus uintahensis Penn.

Reduction of at least the early leaves to phyllodia, a feature of sect. Drabellae, is less extreme in some populations of A. detritalis than in its kindred. In some plants from populations growing between 6000 and 7000 feet, the lowest leaves, although always shorter than the later ones, are mostly and in a few cases all composed of three leaflets, and these are jointed to the rachis. However some phyllodia have been found in every colony of the species. The presence in all plants of some 3-7 foliolate leaves and the occasional absence of leaves reduced to a simple blade might suggest that A. detritalis is primitive in relation to the remaining Drabellae; but its greatly elongated pod and large flowers seem, in their context, specialized.

The flowers of the debris milk-vetch vary in size from one population to the next, as well as from plant to plant in colonies of limited extent, but the whole range of variation has not been seen in any one place. The smallest flowers, with banner 12-13.5 mm. long, came from barren shale hilltops at 7000 feet near the head of Pariette Wash in Duchesne County; the largest, with banner 16-20 mm. long, from protected gullies in the badlands of Asphalt Ridge, at about 5300 feet in Uintah County (= A. spectabilis). At other stations the banner (and other petals proportionately) varies from 13.5-16 mm. (Duchesne), to 14.5-16 mm. (Meeker, Colorado), and 15.5-17 mm. (southwest of Myton, Utah). Isolation of colonies has apparently permitted or encouraged slightly different ranges of variation in each; but with no evidence of an overlapping sequence and the pod varying little or not at all, it is clear that A. spectabilis is no more than a minor variant.

Distribution:Utah United States of America North America| Colorado United States of America North America|