half-obovate or lunately oblanceolate in profile, with straight or gently concave ventral and strongly convex dorsal sutures, 1.5—2.1 cm. long, 5—8 mm. in diameter, broadest above the middle and tapering downward into the cuneate base, abruptly contracted distally into a rigid cusp 1.5-2.5 mm. long, the whole a trifle compressed laterally and obscurely trigonous, with rounded lateral and flattened or shallowly grooved dorsal faces, the green, somewhat fleshy, glabrous valves becoming leathery, reticulate, stramineous or blackish-brown, inflexed (especially toward the base) as a very narrow and incomplete septum 0.4-1.3 mm. wide; dehiscence apical, basal, and ultimately through the ventral suture, the dorsal suture also splitting to reveal the septum’s outer walls; ovules 18-26; seeds brown or olivaceous, often so heavily mottled or dotted with black-purple as to appear black, smooth but dull, 2.2-2.7 mm. long.—Collections: 15 (i); representative: Cory 52,870 (SMU, WS), 52,875 (SMU), 57,049 (OKLA, SMU); Barneby 11,044 (CAS, RSA); J. W. Hardin 525 (GH, US).
Openings in sandy oak woods and along sandy roadsides or in abandoned fields and clearings, ± 250-550 ft., locally plentiful in scattered stations in northeastern Texas, from the Sabine to the Trinity River.—Map. No. 138.—March to early May.
Astragalus Soxmaniorum (Dr. and Mrs. G. M. Soxman, amateur pteridologists of Dallas) Lundell in Field & Lab. 13: 3. 1945.—‘Texas: Nacogdoches County, off U. S. Highway 59, about 8 miles south of Nacogdoches, along sandy roadside, March 22, 1941, C. L. Lundell and Amelia A. Lundell 9750 ... Anderson County, 17 miles southeast of Athens, April 13, 1942, Lundell & Lundell 11151... Van Zandt Country, off highway 64, near Edom, in sandy fallow field, March 24, 1944, Lundell & Lundell 12701 (type in the herbarium of Southern Methodist University)."—Holotypus (SMU), not examined, but the description decisive.
The Soxman milk-vetch is closely related to A. distortus. The two species are alike in habit and all details of organization, and the pod of A. Soxmaniorum cannot be distinguished in practice from that of the sympatric A. distortus var. Engelmanni, from which the plants differ appreciably only in the larger flower, proportionately longer calyx-teeth, and ochroleucous petals. The differences are of a sort and of a magnitude which one might expect in allopatric geographic varieties of one species, but A. Soxmaniorum and A. distortus var. Engelmanni are found in similar habitats within the same range. Since no evidence whatever of intermediate forms is available, it may be assumed that the two entities are kept apart by some isolating mechanism, presumably of a genetic nature. As observed in Van Zandt County and afterwards in cultivation (RSA), A. Soxmaniorum is clearly self-perpetuating and breeds true to seed. It is interpreted with some confidence as a fixed mutation, stemming presumably from A. distortus var. Engelmanni, which has achieved an independent area of dispersal within that of its progenitor. The species was collected first in 1901, near Mineola, Wood County, by J. Reverchon.