Astragalus amblytropis Barneby
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Authors
Rupert C. Barneby
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Authority
Barneby, Rupert C. 1964. Atlas of North American Astragalus. Mem. New York Bot. Gard. 13(2): 597-1188.
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Family
Fabaceae
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Scientific Name
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Type
" ... ten miles west of Clayton, Custer Co., Idaho, 28 June 1946, fl. & fr., Hitchcock & Muhlick No. 14115."—Holotypus, WTU! isotypi, MO, NY, RSA, WS!
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Description
Species Description - Slender, caulescent, diffuse or trailing, with vertical or obliquely penetrating taproot and buried rootcrown, densely strigulose with appressed, filiform or somewhat flattened and scalelike hairs up to 0.25-0.35 mm. long, the herbage cinerous or silvery-canescent, the leaflets somewhat bicolored, the upper surface almost as densely pubescent as the lower but of a yellowish-green cast beneath the vesture; stems few or solitary, prostrate or nearly so, 1—4 dm. long, naked and subterranean for a space of 2-10 cm., thereafter divaricately branched from the first to the fifth emersed node, the more vigorous branches again branched or spurred, the branchlets (and, upward, the peduncles) sometimes paired with a spur, the whole forming a flat, fanshaped spray; stipules 1-2.5 mm. long, the subterranean ones early decaying or broken and their structure obscure, those at the early aerial nodes amplexicaul and connate into a low, scarious, brownish, usually several-nerved collar, the rest herbaceous, often purplish, ± semiamplexicaul, ovate or deltoid-acuminate, with spreading or deflexed blades; leaves mostly divaricate or deflexed, 1-3 (4.5) cm. long, shortly petioled or the uppermost subsessile, with 9-13 commonly crowded, obovate, cuneate-obovate, or oblanceolate, obtuse or retuse (and then sometimes obcordate), flat or loosely folded leaflets 3-9 (14) mm. long; peduncles spreading or incurved, (3) 5-22 mm. long, commonly shorter than the leaf; racemes loosely but shortly, sometimes subumbellately (4) 6—10 (13)-flowered, the flowers subhorizontal at full anthesis, declined thereafter, the axis 0.5—1.5 (2) cm. long in fruit; bracts scarious, ovate or lanceolate, 0.6—1.5 mm. long; pedicels at anthesis spreading-ascending, slender or filiform, 1—2 mm. long, in fruit variably spreading or recurved, somewhat thickened, 1.8-2.5 mm. long; bracteoles 0; calyx (2.8) 4.1-5 mm. long, densely white-strigulose, the symmetric disc 0.4—0.7 mm. deep, the tube 1.9-2.7 mm. long, 1.9—2.1 mm. in diameter, the linear-subulate to shortly subulate teeth (0.9) 1.5-2.3 mm. long, the whole becoming papery, ruptured, marcescent; petals dull straw-yellow often ± suffused with lilac, the banner often lilac-veined, the keel-tip maculate; banner recurved through 40—50°, ovate-cuneate or somewhat flabelliform, 6.4—8.3 mm. long, 5—6.6 mm. wide; wings slightly longer, 6.8-9 mm. long, the claws 2-3.1 mm., the oblong, obtuse or shallowly emarginate, nearly straight blades 5.6-6.8 mm. long, 2-2.4 mm. wide; keel nearly as long or a trifle longer than the banner, 6.8—8.3 mm. long, the claws 2.2—3.3 mm., the obliquely blunt-triangular blades 5—5.7 mm. long, 2.5—3.1 mm. wide, rather abruptly incurved through 50-95° to the very obtuse apex; anthers 0.45-0.55 mm. long; pod spreading or ascending (humistrate), ovoid or ellipsoid, bladdery-inflated, 2-3.5 cm. long, 1.2—2 (or when pressed seemingly up to 2.6) cm. in diameter, rounded at base, contracted distally into a short, deltoid or triangular, erect or slightly incurved, laterally flattened or subconical beak, otherwise a little obcompressed, shallowly sulcate along both sutures, the thin, pale green but purple cheeked, densely to thinly strigulose valves becoming papery, stramineous, subtransparent, delicately cross-reticulate, inflexed across the width of the cavity (below the beak) as a complete septum 3-7 mm. wide; dehiscence apical, basal, and ultimately through the ventral suture; ovules 25-32; seeds pale ocher or olive-brown, sometimes minutely purple-speckled, smooth but dull, (2.6) 3-3.7 mm. long.
Distribution and Ecology - Steep slopes of soft clay, mobile shale and clay detritus, or volcanic gravel slides, 4600-5600 feet, locally plentiful along the canyon of the Salmon River and its tributary creeks for a distance of ± 30 miles up- and downstream from Challis, in Custer and Lemhi Counties, Idaho.—Map No. 137.—Late May to early July.
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Discussion
The Challis milk-vetch, A. amblytropis, is a delicately delightful little astragalus of marked and singular individuality, recognizable at a glance by its repeatedly and divaricately branching stems, neat, silvery foliage, small flowers with petals of nearly equal length, and bladdery, bilocular pods. The plants fruit prolifically, and the subdiaphanous bladders, at first pale green suffused with purple on the side turned toward the sun, then straw-colored and somewhat lustrous when ripe, are enormous in proportion to the leaves and flowers. The partition within the pod, unlike that of A. platytropis described above, is formed almost wholly by the genuine septum, the funicular flange remaining quite narrow and the seed-funicles not greatly elongated. The Challis milk-vetch is remarkable in its section for the subterranean root-crown, which has no doubt developed in adaptation to the almost obligate habitat on steep gullied clay slopes and banks of mobile shale and gravel. However, occasional stray seedlings from the slopes above, which become established on sand bars on the canyon floor, retain the cryptophytic habit of growth, which must be an inherent and inherited character of the species.
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Objects
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Distribution
Idaho United States of America North America|