Low, diffuse or loosely tufted, annual, winter-annual, or perennial of short duration, with a taproot, densely strigulose throughout with straight, appressed and narrowly ascending (rarely a few shorter, curly) hairs up to 0.45-0.8 mm. long, the stems and herbage silvery-white or rarely greenish-canescent; stems 1-several from the root-crown, decumbent and weakly ascending, mostly 0.5-3.5 dm., rarely 4-5 dm. long, or in some seedling but already fruiting plants as little as 1-2 cm. long, when vigorous then branched or spurred below the middle, flexuous distally; stipules membranous or thinly herbaceous, deltoid or lanceolate, 1-3.5 (5) mm. long, not more than semiamplexicaul; leaves 2-10 (12.5) cm. long, all petioled, with (3) 5-11 oval, rhombic-elliptic, obovate-cuneate, or suborbicular, obtuse or (in some lower, rarely in some upper leaves) shallowly retuse, flat leaflets 3 18 mm. long; peduncles erect or divaricate and ascending, 1.5—7 (10) cm. long, often humistrate when weighed down by the fruits; racemes loosely 3-16-flowered, the flowers ascending at early anthesis, declined in age, the axis becoming 0.8—7 cm. long in fruit; bracts submembranous, ovate or lanceolate, 1—2.2 mm. long, pedicels at first slender, ascending, 0.6—1.2 mm. long, in fruit thickened, arched out- and downward, 1.5—3 mm. long; bracteoles commonly 2, minute, sometimes 0, calyx 4.4-7.2 mm. long, strigulose with white or mixed black and white hairs, the subsymmetric disc 0.5—1.4 mm. deep, the campanulate tube 2.5—4.4 mm. long, 1.9—3.3 mm. in diameter, the subulate teeth 1.6-2.8 mm. long, the whole becoming papery, nearly always ruptured, marcescent; petals pale purple or pink-purple, the claws paler than the blades; banner gently recurved through about 45°, obovate-cuneate or broadly rhombic-oval, 7-12.5 mm. long, the midnerve sometimes running out as a minute tooth in the apical notch; wings 6.4—10 mm. long, the blades narrowly oblong or oblong-oblanceolate, truncate-erose or shallowly notched, straight or gently incurved distally; keel 0.4 mm. shorter to 0.7 mm. longer than the wings, the broadly half-obovate or obliquely and obtusely triangular blades abruptly incurved through 85-95° to the rounded apex; anthers 0.45-0.55 (0.6) mm. long; pod declined or pendulous, sessile on an obscure or subobsolete gynophore up to 0.8 mm. long, varying greatly in size and curvature from broadly and plumply oblong and straight to linear-oblong or -oblanceolate and crescentically or even hamately incurved, (1.3) 1.5-3.2 cm. long, 3.5-8.5 mm. in diameter, fleshy, solid, terete or nearly so when freshly formed, becoming (when straight) laterally or (when greatly incurved) triquetrously compressed in ripening, the thick, green, densely strigulose valves becoming stramineous or brownish, leathery or subligneous, prominently cross-reticulate and often wrinkled lengthwise, inflexed as a partial or nearly complete septum 1-2.5 mm. wide, the apex unilocular; seeds brown, nearly smooth or sparsely pitted, sublustrous, 2-3.5 mm. long.
The Mohave milk-vetch, a rather pretty desert species easily recognized by its few, densely silvery leaflets, and small, purple flowers giving rise to leathery or almost ligneous fruits, is of particular interest on account of the great variation in length, curvature, and compression of the ripe legume. The three types of variation are, however, causally related. Thus a short pod, or one unusually broad and therefore relatively short, is commonly straight or nearly so, and in ripening its lateral faces collapse inward, leaving the sutures equally prominent and equally convex. In a long and relatively narrow, or even a quite short, narrow pod a strong tendency toward inward curvature develops; and since the dorsal suture is no longer or scarcely longer than the ventral one, but is now obliged to describe the outer and therefore longer arc, it is drawn inward or depressed and produces a flattened or depressed dorsal face to the fruit. Even though almost terete in the fresh state, the incurved pod commonly becomes bluntly triquetrous in the course of ripening. Finally a narrow, strongly incurved or hooked pod is decidedly sulcate dorsally, even when first formed, and presents, when ripe, a decidedly obcordate section. There is evidence of transition from one extreme type of pod to the other, and A. mohavensis thus serves as a valuable example of the ease with which the astragalus pod may undergo apparently profound changes by simple means and independently of structural modification. The logic of a classification based solely on fruiting characters would have compelled Rydberg, had he known A. hemigyrus, to refer it to Hamosa rather than to Brachyphragma, in which A. mohavensis had come uneasily to rest. Nevertheless I have no apology for treating them as varieties of a single species.