Monographs Details: Astragalus brazoensis Buckley
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:

358.  Astragalus brazoensis

Annual or winter-annual, variable in stature according to site and season, dwarf and slender or quite coarse and robust, thinly strigulose with appressed or narrowly ascending, straight hairs up to (0.35) 0.4-0.75 mm. long, the herbage green, the leaflets glabrous above; stems 3-several from the crown of a slender taproot, ascending, or decumbent and radiating with ascending tips, (0.5) 1-3.5 (4.5) dm. long, simple when slender, when robust branched or spurred from the base upward; stipules ovate or deltoid, 1.5-4.5 mm. long, the lowest early becoming papery, the rest herbaceous, semi- or the lowest a little more than semiamplexicaul, glabrous dorsally, thinly ciliate; leaves (2) 2.5-7.5 cm. long, the lowest petioled, the rest subsessile, with (11) 13-19 narrowly to broadly obovate-cuneate or obcordate, retuse, flat leaflets 3-10 (13) mm. long; peduncles erect or incurved-ascending, (3) 4-9 (11) cm. long, mostly surpassing the leaf; racemes rather closely 7-20 (25)-flowered, the subcontiguous flowers early spreading, declined in age, the axis little elongating, 0.7-2.5 (3.5) cm. long in fruit; bracts membranous, ovate-acuminate or triangular, 0.8-2 mm. long, spreading or reflexed in fruit; pedicels slender, at anthesis ascending, 0.5-0.8 mm. long, in fruit arched outward, a trifle thickened, 0.7-1.5 mm. long, persistent; bracteoles 0; calyx (2.7) 3-4.1 mm. long, strigulose with either white or black hairs, the oblique disc 0.40.6 (0.7) mm. deep, the shallowly campanulate, often purplish tube 1.6-2.4 mm. long, 1.6-2 mm. in diameter, the subulate teeth 1.1-1.8 mm. long, the orifice oblique, the whole becoming papery, marcescent unruptured; petals whitish, distally suffused with lilac, or bicolored, the banner then broadly purple-margined, the wing-tips white, the keel-tip always purple; banner recurved through ± 45°, sometimes further in late anthesis, obovate-cuneate, deeply or shallowly notched, (5.4) 6.7-8 mm. long, (3.7) 4.5-5.6 mm. wide; wings (5) 5.5-7.2 mm. long, the claws 1.7—2.3 mm., the oblong-oblanceolate blades (3.8) 4.3-5.6 mm. long, 1.3-2.4 (2.7) mm. wide, gentiy incurved through the distal half to the truncate or shallowly emarginate apex; keel 4.5-5.9 mm. long, the claws 1.9-2.6 mm., the obliquely triangular blades 3—4 mm. long, 1.7-2.1 mm. wide, abruptly incurved through 95-105° to the triangular, acute or subacute, nearly always slightly porrect apex; anthers 0.25-0.4 mm. long; pod reflexed, sessile but elevated out of the calyx on a slender, often decurved, stipelike gynophore 1.5—2.3 mm. long, the body very strongly obcompressed, peltiform, subcircular in dorsiventral view or often a little wider than long, 3.5—7 mm. long, 4.5—7.5 (9) mm. in diameter, truncate or shallowly retuse at either or both ends, contracted at apex into a conic-subulate, cusplike, more or less incurved beak 0.7—1 mm. long, straight or the whole gendy incurved (the ventral face then depressed in the middle and shallowly bowlshaped), the rather firm, green or purple-tinged, glabrous valves becoming stiffly papery, brownish or stramineous, somewhat lustrous, prominently cross-reticulate and often rugulose on the narrow but obtuse lateral angles, inflexed as a narrow but complete septum 0.5-0.8 mm. wide; dehiscence tardy, after falling, the pod separating into two halves along both sutures and through the septum; ovules 4, seeds brown or olivaceous, smooth, somewhat lustrous, 2.3—3.2 mm. long. Collections. 34 (i); representative: Heller 1483 (NY, SMU, WS); Cory 54,149 (OKLA, SMU, WS); Small & Wherry 11,965, 12,108, 12.177 (NY); Tracy 9091 (NY, WIS); Jones 28,200 (CAS, OB, POM, TEX); Ripley & Barneby 4160 (CAS, RSA).

Stony clay flats and sandy prairies, sometimes abundant and of weedy occurrence in disturbed soil along highways, 10-800 feet, widespread and locally plentiful in favorable seasons on the Gulf Coastal Plain of southern Texas in the drainage of the San Antonio and Nueces Rivers and the lower Rio Grande, extending just south into adjoining Tamaulipas.—Map No. 159.—February to April.

Astragalus brazoensis (of the Brazos River) Buckl. in Proc. Philad. Acad. 1861, p. 454. 1861.—"Western Texas."—Holotypus, collected in June, 1861, by Samuel Botsford Buckley, PH! isotypus, GH (fragm.)!—Tragacantha brazoensis (Buckl.) O. Kze., Rev. Gen. 943. 1891. Hesperastragalus brazoensis (Buckl.) Rydb. in Bull. Torr. Club 53: 166. 1926.

The Brazos milk-vetch is chiefly remarkable for its discoid pod which, as seen from above, is either circular or often wider than long and then almost transversely oblong and is, moreover, elevated out of the calyx on a decurved, stipelike gynophore often described in the past as a genuine stipe. The two faces of the fruit may be nearly flat, or with a slight inward curvature the ventral face becomes depressed in the middle to form a shallow bowl or saucer. Other notable features of A. brazoensis are the precise, consistent number of the four ovules, of which only one or both of each locule may ripen into viable seeds; and the minute puberulence of the style just below the stigma. This character is shared, apparently fortuitously, with another Texan annual, A. Lindheimeri, a species very different in its truly stipitate, ascending, compressed-triquetrous pod and much fewer and larger flowers.

The name of the Brazos milk-vetch is apparently a misnomer, for I find no modern record from the valley of the Brazos River, or from the county of the same name, the known eastern limit of the species lying in northern Aransas County on the coast and inland along the San Antonio River in Bexar and Karnes Counties. A collection ostensibly from the Pease River near Matador in Motley County (i.e., near the southeast corner of the Panhandle; cf. Small & Wherry 12,177, NY) is probably mislabeled. Another (Small & Wherry 11,776, NY), supposedly from Liberty County in the southeast corner of the state, needs verification and has been omitted from the map.