Monographs Details: Astragalus didymocarpus var. dispermus (A.Gray) Jeps.
Authors:Rupert C. Barneby
Authority: Barneby, Rupert C. 1964. Atlas of North American Astragalus. Mem. New York Bot. Gard. 13(2): 597-1188.
Family:Fabaceae
Discussion:

360c. Astragalus didymocarpus var dispermus

Herbage greenish-cinereous or canescent, the vesture commonly denser than in var. a, but the leaflets usually glabrescent medially above; stems diffuse, weakly ascending, or prostrate, (1.5) 4—27 cm. long, usually numerous and forming loosely woven mats, rarely few or solitary; leaves 1—3.5 cm. long, with (7) 9—15 (17), mostly 11-13 leaflets 2-8 mm. long; racemes 7-20-flowered, 7-9 mm. in diameter at full anthesis, the axis 3—12 mm. long in fruit; calyx (3) 3.4—4.7 mm. long, densely villous-pilosulous with white and not uncommonly (the teeth especially) some black hairs up to (0.55) 0.6—1 (1.1) mm. long, the tube 1.7—2.3 mm. long, 1.4-1.9 mm. in diameter, the subulate or subulate-setaceous teeth (1.3) 1.6—2.4 mm. long; banner (3.4) 4.3—5.4 mm. long, 1.6—2.8 mm. wide; wings (3.3) 3.9—5 mm. long, the claws (1.3) 1.7—2.4 mm., the blades 2.3—3.3 mm. long, 0.7—1.1 mm. wide; keel (3.4) 3.8—4.5 mm. long, the claws (1.4) 1.7—2.4 mm., the half-obovate blades 2—2.5 (2.8) mm. long, 1—1.4 mm. wide, abruptly incurved through 90-95° to the blunt, deltoid, or sometimes sharply deltoid and obscurely porrect apex; anthers 0.15—0.25 (0.3) mm. long; pod 2.3—4 mm. long, 1.6—2.5 mm. in diameter, densely to quite thinly strigulose-hirsutulous.—Collections: 23 (v); representative: C. B. Wolf 6486 (TEX, WS), 8430 (NY, WS) 10,323 (CAS, NY, TEX, WS); C. L. Hitchcock 6140 (NY, WS); Eastwood & Howell 8817 (CAS, WS); Munz 13,791 (POM, WS).

Open sandy flats and gravelly washes in the foothills of desert mountains, commonly with Larrea, 100-3000 (3550) feet, widespread and locally plentiful in the southwestern Mohave, Colorado, and Gila Deserts, southeastern California, southwestern Arizona, and adjoining Sonora, extending south in interior Baja California to about 30° N.; apparently isolated, at 4000 feet, in the far eastern Mohave Desert (near Kingston Peak).—Map No. 160.—February to May.

Astragalus didymocarpus var. dispermus (Gray) Jeps., Fl. Calif. 2: 376. 1936, based on A. dispermus (2-seeded) Gray in Proc. Amer. Acad. 13: 365. 1878.—"Wickenburg, Arizona, Dr. Palmer, 1876."—Holotypus, GH! isotypus, K!—Hesperastragalus dispermus (Gray) A. Heller, Muhlenbergia 1: 137. 1906.

Astragalus dispermus var. albus (white, of the calyx-hairs) L. E. James in Contrib. Dudl. Herb. 4: 68. 1951.—"Type in the Dudley Herbarium, no. 280,492, collected on the Mojave Desert, north side of Mojave River, 1½ miles from Daggett on road to Yermo, May 4, 1932, Wolf 3370."—Holotypus, DS!

The var. dispermus is the form of the two-seeded milk-vetch most prevalent in the southern Mohave Desert, where its range overlaps that of var. didymocarpus, and it is the sole representative of its species in the torrid, low-lying Colorado and Gila Deserts. It differs from var. didymocarpus chiefly in its denser and whiter vesture of slightly longer hairs, and especially in its longer, narrowly subulate or subsetaceous calyx-teeth. In habit of growth the plants are commonly prostrate, varying, according to season, from diminutive examples composed of about three stems a few centimeters long to freely branched and loosely woven mats up to five decimeters in diameter. Even on the deserts, however, the vesture is somewhat variable in density, and James (l.c.) has sorted these plants into two varietal categories, accepting a typical A. dispermus with greenish leaves and some black hairs on the calyx-teeth, and a var. albus with white herbage and no black hairs. To the former she referred some specimens from cismontane California which I have identified (in duplicate) as var. didymocarpus, one collection of var. obispensis, and the synonym Hesperastragalus Milesianus, thus extending the range of A. dispermus far northwest into coastal San Luis Obispo County. Obviously our interpretations of the material are so different that no valid comparison of the present var. dispermus with either of James’s varieties can be made. I must look with skepticism born of long experience on any proposition in Astragalus defined in terms of color or density of pubescence.