Low, slender, loosely tufted, with a taproot and superficial root-crown or shortly forking caudex, strigulose nearly throughout with appressed hairs up to 0.25-0.5 mm. long, the herbage greenish or subcinereous, the leaflets glabrous above; stems diffuse and ascending, 3-20 cm. long, sparingly branched or spurred near the base, floriferous from near or well below the middle; stipules 1.5—4.5 mm. long, the lowest ones small, ovate-deltoid, early becoming papery and brownish, the upper ones broadly or narrowly lanceolate, herbaceous, mostly erect; leaves (1.5) 2-9 cm. long, slender-petioled, with (7) 9-17 (19) narrowly oblong-obovate to oblanceolate or linear-elliptic, obtuse to deeply emarginate, flat or loosely folded leaflets (1.5) 3-10 mm. long; peduncles slender, (2) 3-7 cm. long, reclinate in fruit; racemes shortly but loosely (3) 5-12-flowered, the flowers spreading- ascending at anthesis, the axis little elongating, 4-16 (24) mm. long in fruit; bracts submembranous, ovate-subulate to linear-lanceolate, 1-2 mm. long; pedicels very slender, at anthesis 0.4-0.7 mm., in fruit little thickened, 1-1.8 mm. long, persistent (or perhaps very tardily disjointing); bracteoles 0; calyx 4—5.1 mm. long, white-strigulose, the subsymmetric disc 0.6-0.8 mm. deep, the rather narrowly campanulate or turbinate-campanulate, pinkish tube 2.3-3.1 mm. long, 1.3-2.1 mm. in diameter, the subulate teeth 1.4-2 mm. long, the whole becoming papery-scarious, ruptured, marcescent; petals pale purple; banner recurved through ± 40°, rhombic-oblanceolate or obovate-cuneate, notched, 7.5-9 mm. long, 3.5-5.8 mm. wide; wings almost as long or a trifle longer, the claws 2.8-3.1 mm., the narrowly oblong or linear-oblanceolate, obtuse or obliquely truncate- emarginate blades 4.8-6.5 mm. long, 1.3-2 mm. wide, both gently incurved but the left one sometimes further than the right; keel 5.4-6.2 mm. long, the claws 2.8-3.1 mm., the half-obovate blades 2.7-3.3 mm. long 1.4—2 mm. wide, incurved through 95-100° to the rounded apex; anthers 0.3-0.5 mm. long; pod ascending, sessile on an obscure, incipient gynophore, linear-oblong in profile, straight or slightly incurved, (8) 9—19 mm. long, (2) 2.5—3.7 mm. in diameter, obtuse at base, contracted distally into a short, compressed-triangular, cuspidate beak, otherwise bluntly triquetrous, with rounded lateral and bluntly keeled ventral angles, the lateral faces at first plane becoming convexly distended in age, the dorsal face openly and shallowly sulcate, the thin, green or purplish, sparsely strigulose valves becoming papery, stramineous, inflexed as a partial or complete septum (0.3) 0.6-1.3 mm. wide; dehiscence apparently both apical and basal after falling; seeds (little known) brown, wrinkled and pitted, sublustrous, 1.6-2.3 mm. long;—Collections: 3 (o); representative: Gomez 276 (NY).
Arid grasslands, ± 7000 feet, known only from southwestern San Luis Potosi (near San Luis; Villa de Arriaga).—Map No. 142.—May—June, sometimes again in fall.
Astragalus parvus (small) Hemsl., Biol. Centr.-Amer., Bot. 1: 226. 1880. North Mexico, region of San Luis Potosi, 6000 to 8000 feet (Pary & Palmer 174) ... Holotypus, K! isotypi, BM, GH, K, MO, NY, P, US!—Tragacantha parva (Hemsl.) O. Kze., Rev. Gen. 947. 1891. Hamosa parva (Hemsl.) Rydb. in Bull. Torr. Club 54: 335. 1927.
Astragalus Schaffneri (J. G. [Wilhelm] Schaffner, -1882) Jones, Rev. Astrag. 276, Pl. 69, 70. 1923. "No. 815 Schaffner from San Luis Potosi Mexico."—Holotypus (No. 611/815, collected in 1879), US! isotypi, GH (in part, mixed with A. esperanzae), BM, CAS, K, NY, P!
The central and northern states of Mexico are rich in small-flowered astragali with threesided linear pods; comparatively few of them, however, are likely to be confused with A. parvus. Either their flowers or fruits, and commonly both, are early deflexed, or at least some stipules low on the main stems are united into a sheath. All the Mexican species with ascending hamosoid pod and free stipules are members of the present sect. Leptocarpi, the majority being obligate annuals, whereas A. parvus, at least when mature, has a decidedly woody root and indurated caudex; the other perennial Leptocarpi have much larger flowers. In the arid hill-country west of San Luis the Schaffner milk-vetch, A. parvus, is sympatric with the singular A. diphacus which it resembles closely in flower-size and habit of growth. The latter is quickly recognized, however, as soon as its didymously swollen fruits begin to form.
The type-collection of A. Schaffneri was distributed partly as Schaffner 611 and partly as Schaffner 815; some labels bear both numbers, and it is clear that all form part of a single gathering. Watson (in Proc. Amer. Acad. 17: 343. 1882) identified the isotypus at GH as a luxuriant form of A. parvus, but it may be more correct to envisage the holotypus of A. parvus as somewhat depauperate and Schaffner’s material as normally developed under favorable conditions of soil and rainfall. Jones distinguished A. Schaffneri from A. parvus by a broader pod, but I must agree with Rydberg that the slight difference in this respect is to be interpreted as an individual variation, such as occurs in the vast majority of astragali.