Low, sometimes dwarf or even diminutive, tufted, matted, or rarely mounded, essentially acaulescent, perennial, with a slender or at length stout and woody taproot and multicipital root-crown or closely forking caudex, densely strigulose throughout with straight, appressed, contiguous and parallel hairs up to 0.5-0.75 (1) mm. long, the herbage canescent or silvery, the vesture of the upper leaf-surface often turning a greenish-golden color when dried; trate), sessile, oblong-elliptic, linear-oblong, or lance-acuminate in profile, straight or gently lunate-incurved, (0.8) 1-2.1 (2.5) cm. long, (2.5) 3-4.5 mm. in diameter, rounded at base, cuspidate at apex, compressed-triquetrous with acute ventral and narrow but obtuse lateral angles, the plane or low-convex lateral faces broader than the narrowly grooved dorsal one, the thin, cinereously strigulose valves becoming papery, inflexed as a complete or nearly complete septum; seeds light or dark brown, sometimes nearly black, lustrous but irregularly pitted, 1.9-2.4 (2.8) mm. long.
The Torrey milk-vetch, A. calycosus, is a polymorphic species widely dispersed in the arid interior of western United States and is especially variable in habit of growth. Individual plants may form a diminutive tuft, or a depressed cushion up to 1.5 dm. in diameter, with 2-4-flowered racemes immersed in or barely projected beyond the foliage, but these common dwarf forms pass by gradual transition into a relatively robust, loosely tufted phase, with erect scapes and loose racemes of as many as seventeen flowers, the whole well over 2 dm. tall. The leaflets vary from one to thirteen to the leaf, the calyx from less than 5 mm. to over 10 mm. (with teeth 0.5-4 mm.) long, and the banner from less than 9 mm. to over 16 (very rarely over 20) mm. long. Segregates proposed and maintained on number of leaflets, number of flowers, length of calyx-teeth, or general growth-habit have no just claim to specific rank. The importance attached to length of the calyx-teeth, whether absolute or relative to the tube, has been overestimated. The pod, of the same form throughout the species, sens. lat., varies approximately between one and two centimeters long in each variety, and to nearly the same degree in many colonies of otherwise like plants. With evidence of instability in all the available differential characters, it has become increasingly difficult to define in mutually exclusive or exact terms the geographic varieties, which probably correspond to climactic points on a sliding scale of variation rather than to discrete entities.