Monographs Details: Astragalus wrightii A.Gray
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
Discussion:

361.  Astragalus Wrightii

Slender but ultimately stiff or rigid annual, with a filiform taproot, silky-pilose throughout with fine, mostly straight, narrowly or loosely ascending, basally thickened hairs up to 1-1.6 mm. long, the herbage silvery-cinereous or -canescent in youth becoming greenish, the leaflets pubescent on both sides but often more thinly so above; stems either solitary, simple, and erect, or branched at or near the base and then 1-5, the lateral ones either erect or incurved-ascending, at maturity (2) 0 (35) cm. long, composed of ± 4-7 developed internodes, those succeeding the first of the 1—4 peduncles often much shorter than the lower ones, the inflorescence then appearing subterminal or falsely corymbose; stipules herbaceous becoming papery, lanceolate or triangular-acuminate, 3—6 mm. long, semi- to nearly fully amplexicaul, the margins pilose-ciliate and sometimes beset with a few minute clavate processes; leaves 1.5—5.5 (7.5) cm. long, the lower and median petioled, the upper subsessile, with 7—13 broadly to narrowly elliptic or oblanceolate, acute, or (in some early leaves) obovate and obtuse, flat or loosely folded leaflets (2) 4—17 (20) mm. long; peduncles erect and stiffly ascending, 0.7—7 cm. long, the first usually, and sometimes all, longer than the leaf, the uppermost often, rarely all, shorter; racemes capitately 3-7-flowered, the flowers suberect, the fruits erect or, when relatively many, stellately ascending, the axis not over 4 mm. long in fruit; bracts herbaceous, lanceolate, 1.5-5.5 mm. long; pedicels at anthesis subobsolete, in fruit thickened and up to 0.6-1 mm. long; bracteoles 0; calyx 5.2-6.2 mm. long, pilose like the herbage with white hairs, the subsymmetric disc 0.7-1 mm. deep, the turbinate-campanulate, pallid tube 2-2.6 mm. long, 2.1-2.6 mm. in diameter, the linear-lanceolate, herbaceous teeth 3.2-3.8 mm. long, the whole a trifle accrescent in fruit and the teeth then up to 5.5 mm. long, in age becoming papery, ruptured, marcescent; petals reddish-violet, lilac, or whitish faintly lilac-tinged, the color evanescent in drying; banner recurved through ± 30°, broadly oblanceolate or obovate-cuneate, shallowly notched, 5.2-6.2 mm. long, 3.2-3.5 mm. wide; wings 4.7-5.5 mm. long, the claws 1.9-2.3 mm., the oblong, obtuse, nearly straight blades 3.1-3.6 mm. long, 1.1-1.4 mm. wide; keel (a trifle shorter or longer than the wings) 4.7-5.5 mm. long, the claws 2.1-2.6 mm., the half-obovate blades 2.8 3.2 mm. long, 1.5-1.7 mm. wide, gently incurved through 70-80° to the blunt triangular apex; anthers 0.2-0.3 mm. long; pod erect, sessile on and firmly attached to the receptacle, oblong or lance-oblong in profile, 7-13 mm. long, 2.5-3.5 mm. in diameter, straight or nearly so, truncate or broadly rounded at base, abruptly contracted at apex into a short, subulate, decurved beak, bluntly compressed-triquetrous, the ventral angle broad and obtuse, the lateral faces low-convex, the dorsal face narrowly sulcate, the green, scarcely fleshy valves becoming brown or ultimately blackish, stiffly papery, transversely reticulate, loosely and densely pilose-hirsute, inflexed as a complete septum 1.2—2.2 mm. wide; dehiscence apical and downward through the ventral suture, the beak gaping to release the seeds, the dorsal suture finally splitting to expose the separating walls of the septum; ovules 5-9, commonly 8; seeds brown or yellowish-brown, often purple-speckled, lustrous but closely and irregularly pitted and wrinkled, 1.8-2.4 mm. long.—Collections: 28 (v); representative: J. Reverchon 1518 (NY, SMU); Wright 139 (BM, G, K, OXF); Heller 1633 (NY, SMU); Lindheimer 749, 749a, 749b (NY, TEX); Ripley & Barneby 11,062 (CAS, NY, RSA), 11,070 (CAS, RSA).

Openings in live oak and juniper woodland, gravelly hilltops and stony outcrops on prairies, 500—1000 feet, apparently confined to calcareous soils, locally abundant in southcentral Texas, in a roughly circular area of about 150 miles diameter circumscribed to the south and southeast by the Balcones Escarpment, extending northwest into eastern Kimble County, and north into Brown and Bell Counties; lately introduced along the railroad near Palestine, Anderson County (not mapped).—Map No. 161.—March to May.

Astragalus Wrightii (Charles Wright, 1811—1885, pioneer botanical collector in the Southwest) Gray in Boston Jour. Nat. Hist. 6 (Pl. Lindh. 2): 176. 1850. Texas, near Austin, Mr. Charles Wright... "; also cultivated in Cambridge Bot. Gard.—Holotypus, collected in the spring of 1848, GH! isotypi, mounted with plants grown at Harvard, NY, PH! Tragacantha Wrightii (Gray) O. Kze., Rev. Gen. 949. 1891. Hamosa Wrightii (Gray) Rydb. in N. Amer. Fl. 24: 437. 1929.

The Wright milk-vetch is an inconspicuous, somewhat weedy plant, an object of botanical curiosity rather than one of beauty. Its extra-American relationships have been discussed above. In Texas it is easily recognized among the annual species by its headlike racemes of few, tiny flowers, with poorly graduated, reddish-violet to whitish-lilac petals scarcely longer than the elongate, plumose calyx-teeth, and by the erect, narrowly oblong, bluntly trigonous pods, pilose like the herbage with ascending hairs. At first weak and pliant, the stems early become rigid, a feature common to some European Sesamei, and are so durable that one may sometimes chance upon the skeletal remains of a plant of the preceding year, with weathered, empty pods still attached, among the green colonies of the new generation. Like many annuals, A. Wrightii shows a marked response to seasonal variations in rainfall; flowering, when starved, at a height of a few centimeters, becoming quite rank and up to three decimeters or more in favorable years.