Low, diffuse, with a taproot and knotty root-crown or shortly forking caudex, sparsely strigulose with fine, straight, appressed or subappressed hairs up to 0.2-0.45 mm. long, the stems often glabrous below the middle, the herbage green, the leaflets glabrous above; stems several, slender or moderately robust, decumbent or prostrate, radiating, (0.25) 0.5-3 (3.5) dm. long, commonly bearing short branchlets or spurs near the base, simple distally, together forming loose mats; stipules (1.5) 2-8 mm. long, green or purplish, the lowest becoming papery, mostly ovate- or broadly triangular-acuminate or the uppermost (all) lanceolate, decurrent around Vi, or the lowest around the whole stem’s circumference, the blades commonly recurved, glabrous dorsally, thinly ciliate; leaves of the principal stems (2) 4-10 (13.5) cm. long (those of the subbasal branchlets often smaller and with smaller leaflets), with slender but sometimes short petiole and (9) 13-25 (27) oval, obovate, elliptic-oblanceolate, or suborbicular, truncate or retuse, flat leaflets (2) 3-11 mm. long; peduncles (2) 5-14 cm. long, decumbent or prostrate in fruit, a little longer or shorter than the leaf; racemes (5) 10-21-flowered, rather dense at early anthesis, becoming looser in age, the flowers ultimately spreading and declined, the axis 1.5-5 (7.5) cm. long in fruit; bracts submembranous, ovate-acuminate or lanceolate, 1-3 mm. long; pedicels straight, ascending, or a trifle arched outward, at anthesis 0.8-1.4 mm., in fruit somewhat thickened, 1.5-2.4 mm. long; bracteoles 0-2, minute when present; calyx 3.1-6.3 mm. long, strigulose with white or mixed black and white hairs, the oblique disc 0.50.9 mm. deep, the triangular-subulate or subulate teeth ?-½ as long as the campanulate tube, the ventral pair often broadest and shortest, the whole becoming papery, marcescent, ruptured or not; petals pink-purple with pale wing-tips and pale, striate eye in the banner, all lilac-tinged, or all whitish with lavender keel-tip, rarely all white; banner recurved through ±45°, ovate-cuneate or rhombic-ovate, deeply notched, 8.2-15.3 mm. long; wings shorter, the blades oblong- oblanceolate, linear-oblong, or obliquely oblong-elliptic, gently incurved, obtuse and entire, undulate-erose, or (when broad) obliquely emarginate; keel much shorter than the banner, 5.5-9.3 mm. long, the half-obovate blades rather abruptly incurved through 95-105° to the blunt apex; pod normally ascending and humistrate, rarely declined of its own weight from ascending peduncles, sessile on an incipient or short gynophore 0.4-1.4 mm. long, varying in profile from narrowly lunate-elliptic to obliquely oblong-, ovate-, or obovate-elliptic, 13—25 mm. long, 3.5-7 mm. in diameter, shallowly sulcate only dorsally, or more deeply sulcate along both sutures, the green, thinly fleshy, glabrous valves becoming leathery, brownish and ultimately almost black, not inflexed, or inflexed as an incipient septum up to 1 mm. wide; dehiscence apparently apical, very tardy, after falling and weathering on the ground; ovules 16—37; seeds brown, sometimes purple- speckled, smooth or sparsely pitted, dull, 1.6-2.6 mm. long.
The Ozark milk-vetch, A. distortus, is variable in color and length of the petals, and in length and outline of the pod. The shorter type of pod, which is often but not quite always correlated with a relatively small flower, tends to be widest above the middle and thus obliquely obovate in profile; it is only shallowly grooved, commonly on the dorsal side alone, more rarely along both sutures, and is strictly unilocular. A longer and proportionately narrower pod of lunately elliptic profile is nearly always more deeply grooved both dorsally and ventrally, thus yielding a didymous cross-section, and is often narrowly septiferous within. The depth of the grooving along either suture, which increases as the valves dry out with advancing maturity, is apparently a function of the pod’s length-width ratio and of a corresponding shift in the tensions set up in the shrinking tissues. Thus the sulcus, on which Rydberg placed so much emphasis, furnishes a differential character of secondary importance. A full series of intermediate forms links the extreme phases of the fruit, and there is no abrupt discontinuity either in flower-size or ovule-number. In consequence the key, which attempts to separate a small-flowered, prevailingly Texan var. Engelmanni from the more widely dispersed var. distortus, cannot always provide satisfactory contrasts.