Astragalus amphioxys A.Gray
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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
"Southern Utah and New Mexico and Northern Arizona, Thurber, Parry, Palmer, etc...."—Lectotypus (Rydb., 1925, p. 148): Dona Ana, New Mexico, Thurber 295, GH!
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Description
Species Description - Low, tufted, or loosely matted, sub acaulescent or shortly caulescent, winter- annual or perennial of short duration, densely strigose-strigulose throughout with stiff, straight, appressed, usually subcontiguous (and on some peduncles and petioles a few narrowly ascending), sometimes mixed with shorter, sinuous hairs up to 0.65-1.25 (1.5) mm. long, the herbage silvery-white or rarely greenish, the leaflets equally pubescent on both sides, the inflorescence sometimes black- hairy; stems usually several, prostrate or weakly ascending, in seedlings sometimes solitary and erect, 1—7 (exceptionally, in shade, up to 15) cm. long, the internodes often concealed by stipules, but as commonly developed and up to 2 (4.5) cm. long but mostly less; stipules (1.5) 2.5—10 (13) mm. long, submembranous becoming papery in age, ovate, deltoid, triangular, triangular-acuminate or -caudate, semi- (the lowest sometimes fully) amplexicaul; leaves 2-10 (13) cm. long, all petioled, with (7) 11-21, or in some seedlings only (1) 3-7, elliptic, rhombic- elliptic, oblanceolate, or obovate, sharply acute to obtuse and mucronulate, exceptionally emarginate, distant or crowded, flat or loosely folded leaflets 3—13 (20) mm. long; peduncles ascending at anthesis, arcuate-recurved or prostrate in fruit, (1) 2.5-14 (20) cm. long, a little longer or shorter than the leaf; racemes loosely but often quite shortly (2) 4—10 (13)-flowered, the flowers ascending, the axis (0.5) 1-4.5 (6.5) cm. long in fruit; bracts submembranous becoming papery- scarious, narrowly ovate to lanceolate or lance-acuminate, 2.5—8 mm. long, pedicels ascending or arched outward in age, at anthesis slender, 0.6—1.8 mm., in fruit thickened, 1.2-2.5 mm. long; bracteoles 0-2; calyx 6.3-14.2 mm. long, pubescent like the herbage with white, mixed black and white, or all black hairs, the slightly oblique disc 1.2-2.7 (3) mm. deep, the cylindric or cylindro-campanulate, purplish tube 5.2-10.5 mm., the subulate teeth 1.1-3.7 (4.5) mm. long; petals bright pink-purple, the banner with a large striate lozenge in the fold, exceptionally pure white; banner recurved through ± 40°, oblanceolate, rhombic- oblanceolate, or spatulate, shallowly or deeply notched, 12.8—27 (28) mm. long; wings a little shorter, the blades linear- or narrowly lance-oblong, usually a trifle dilated above the claw, the distal half a little narrower and either straight or incurved, obtuse; keel 11-22.4 mm. long, the half-obovate or obliquely triangular- obovate blades rather abruptly incurved through 85-95° to the rounded apex; anthers 0.5-0.85 (0.9) mm. long; pod ascending (humistrate), variable in length and curvature, (1.5) 2-4 (5) cm. long, 5-10 mm. in diameter, most commonly 1) crescentic in profile, gently and evenly arched through ¼-1 circle, laterally compressed and acute to long-acuminate at both ends, obcompressed at the middle; or 2) straight, but otherwise as above; or 3) obcompressed throughout below the beak; or 4) when short, then obtuse and dorsiventrally compressed at base, acuminate and laterally compressed distally; in all cases both sutures prominent their whole length, the dorsal one often sinuous, commonly depressed at the middle of the pod and lying in a double groove, the green or sometimes mottled, ± fleshy, strigulose valves becoming stiffly leathery or subligneous, rarely a trifle spongy in age, reticulate and sometimes also wrinkled lengthwise, often rugulose on the obtuse, transversely dilated angles, either not inflexed or if so the septum present only near the middle of the pod and very narrow, not over 1.1 mm. wide; dehicence apical, through the gaping beak, and also basal, through the dorsal or both sutures; ovules (42) 44-70; seeds brown, variably pitted or wrinkled, sometimes lustrous, 2-2.9 mm. long.
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Discussion
The crescent milk-vetch, A. amphioxys, is a perplexingly polymorphic species. The pod is highly variable in length, degree of curvature, and compression, although the mode of compression is determined to large degree by the other two. The species is adapted to desert or near-desert climates and the populations fluctuate greatly in number and vigor in response to bountiful or meager spring rainfall. The plants are potentially perennial but relatively short-lived, often bearing flowers and fruits precociously, and sometimes perishing after the first or second summer. A favorable season after a period of several years drought, during which the older plants have died out, may bring out a swarm of floriferous seedlings easily mistaken for annuals. In the Rio Grande Valley, where summer rainfall prevails, the crescent milk-vetch can often be found in flower from late June or July onward, although elsewhere it is essentially a member of the early spring flora.
The flower of A. amphioxys is variable in length, both absolutely and in relative proportion of the calyx and petals. This type of variation is correlated with dispersal, permitting the segregation of three poorly differentiated and partly overlapping varieties.