Monographs Details: Astragalus albens Greene
Authors:Rupert C. Barneby
Authority: Barneby, Rupert C. 1964. Atlas of North American Astragalus. Mem. New York Bot. Gard. 13(2): 597-1188.
Family:Fabaceae
Scientific Name:Astragalus albens Greene
Discussion:

317.  Astragalus albens

Delicate, diffuse, winter-annual or short-lived perennial, with a slender taproot, densely strigulose nearly throughout with straight, flattened (and the shorter ones scalelike), truly appressed hairs up to 0.35-0.55 mm. long, the herbage silvery- canescent; stems several, prostrate or decumbent, radiating from the root-crown or (in older plants) from the divisions of a forking caudex, (2) 5—30 cm. long, branched or spurred at 1-several nodes preceding the first peduncle, floriferous upward from below the middle, zigzag distally, together forming loosely woven mats; stipules submembranous, greenish, less densely pubescent than the adjoining stem, deltoid or triangular, 1.8-3.5 mm. long, about semiamplexicaul; leaves (1) 1.5-4 (5.5) cm. long, all petioled, with (5) 7-9 obovate or oblong-oval and obtuse or emarginate, or some few rhombic-obovate and subacute, flat or loosely folded leaflets 2-11 mm. long; peduncles ascending at a wide angle, 1.5-4.5 (8) cm. long, either a little longer or shorter than the leaf; racemes loosely 5-14-flowered, the flowers widely spreading or in age declined, the axis 2.5-4.5 (8) cm. long in fruit; bracts membranous, ovate or lanceolate, 0.8-1.5 mm. long; pedicels at anthesis ascending and arched outward, 0.8-1.5 mm. long, in fruit either more strongly arched, or abruptly divaricate, or sometimes contorted or geniculate at base, 1.4-2.5 mm. long; bracteoles 0; calyx 3.9-4.5 mm. long, densely strigulose with white and often a few black hairs, the subsymmetric disc 0.5-0.7 mm. deep, the campanulate, purple-tinged tube 2.2-2.7 mm. long, 1.6-1.9 mm. in diameter, the subulate teeth 1.4-2.3 mm. long, the whole becoming papery, persistent unruptured; petals pink-purple, drying violet or bluish, the banner with a pale, striate eye in the fold, irregularly graduated, the banner and keel of nearly equal length, the wings shorter than either; banner recurved through ± 40°, 7.3-9.5 mm. long, the shortly cuneate claw abruptly expanded into an ovate or subquadrately circular, shallowly emarginate blade 4.4-6.8 mm. wide; wings 6.2-7.5 mm. long, the claws 2.2-2.9 mm., the linear-oblanceolate or narrowly obovate, obtuse, nearly straight blades 4.8-5.7 mm. long, 1.2-2.4 mm. wide; keel 7-8.4 mm. long, the claws 2.6-3.4 mm., the obliquely triangular-obovate blades 4.9-5.7 mm. long, 2.7-3.3 mm. wide, abruptly incurved through about 90° to the broad, bluntly deltoid apex; anthers 0.35-0.55 mm. long; pod horizontal or commonly declined, sessile on a slender, glabrous gynophore 0.5-0.9 mm. long, narrowly lunate- oblanceolate in profile, incurved through about ¼-½ circle, 1.3-1.8 cm. long, acuminately tapering toward the base, 2.8-3.5 mm. wide just below the laterally compressed, triangular, cuspidate beak, otherwise compressed-triquetrous, with acute ventral and narrow but obtuse lateral angles, and broad, flat or slightly concave lateral and much narrower, sulcate dorsal faces, the thin, green or purplish-tinged, densely strigulose valves becoming papery, stramineous or brownish, inflexed below the beak as a nearly complete septum 1.3—2 mm. wide; seeds brown, smooth or nearly so, 1.8-2.6 mm. long.—Collections: 4 (ii); representative: Jones (from Cactus Flat) in 1926 (CAS, DS, NY, POM, US); Barneby 11,353 (CAS, NY, RSA).

Sandy or stony flats, rocky hillsides, canyon washes, and outwash fans, on granite or on mixed granitic and calcareous debris, very local but locally plentiful in years of sufficient rainfall, known only from Cushenbury Canyon in the northeast foothills of the San Bernardino Mountains, there extending from the desert edge (at about 4000 feet) near Box S Springs upward into the sagebrush zone on Cactus Flat (about 5800 feet).—Map No. 144.—Late March to May.

Astragalus albens (turning white, of the silvery vesture) Greene in Bull. Calif. Acad. Sci. 1: 156. 1885—"Mohave Desert, May, 1882, S. B. & W. F. Parish, No. 1274."—Holotypus, CAS! isotypi, BM, GH, DS, ORE, POM, US, WS!—Hamosa albens (Greene) Rydb. in Bull. Torr. Club. 54: 22. 1927.

The Cushenbury milk-vetch, A. albens, is a plant of great delicacy. It might be visualized as a refined and diminished version of its common relative A. mohavensis, from which it is easily distinguished by its few-ovulate pod of more slender outline and papery texture. As does A. mohavensis, the plants flower precociously; and a good proportion of them are probably monocarpic, especially in years of low rainfall, after which the populations become decimated or even annihilated except for the dormant seeds. In the first spring after a drought of several seasons duration, whole colonies of young plants can be found in prolific flower and fruit, giving the impression of an obligately annual species, but this is certainly a misleading picture. The genuinely annual astragali with hamosoid pods native to the Mohave Desert differ from A. albens in their greenish foliage, or fewer flowers, or regularly graduated petals, or acute keel-tip, or in some combination of these characters.

Not at all rare in Cushenbury Canyon, where it is sometimes associated with another narrow endemic, Erigeron Parishii Gray, A. albens is one of the most narrowly localized species of its genus. The Mohave Desert is now so well explored botanically that we can hardly anticipate any substantial extension of its presently known range of about five miles in diameter. The report of A. albens from the Colorado Desert (Rydberg, 1927, 1. c.) was based on a misidentified specimen of A. lentiginosus var. borreganus.