Monographs Details: Astragalus acutirostris S.Watson
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:

325.  Astragalus acutirostris

Slender, sometimes diminutive, with a subfiliform taproot and 1 (2) erect, or several decumbent and ascending or prostrate and radiating stems, loosely strigulose or pilosulous with subappressed or loosely incurved-ascending hairs up to (0.35) 0.4-0.6 mm. long, the herbage greenish-cinereous, the leaflets somewhat bicolored, brighter green and medially glabrescent above; stems 2-25 (30) cm. long, simple or (when vigorous) spurred or branched near the base, commonly tinged with brownish-purple; stipules membranous or broadly membranous- margined, deltoid, triangular-ovate, or lanceolate, pallid or early becoming so, 0.8-2.5 (3) mm. long, semiamplexicaul, glabrous or nearly so dorsally, ciliate with black or white hairs and sometimes a few minute processes; leaves (1) 1.5 4.5 cm. long, the lower ones slender-petioled, the upper subsessile, with (7) 9-13 (15) oblong-oblanceolate, obovate, or broadly to narrowly cuneate, almost consistently retuse, flat or loosely folded leaflets 2-8 mm. long; peduncles slender, erect, or spreading and incurved, (1.5) 2.5-7 cm. long, all or at least those of the main stems surpassing the leaf; racemes loosely (1) 3—6-flowered, the flowers at first ascending, early spreading or declined, the axis early elongating, (0) 1-3.5 (4.5) cm. long in fruit; bracts membranous, pallid, ovate-triangular or lanceolate, 0.7-1.3 mm. long; pedicels at anthesis straight, ascending, 0.4-0.7 (0.9) mm. long, in fruit somewhat thickened, decurved, contorted, or coiled, 0.9—1.7 mm. long, persistent; bracteoles 0; calyx (2.6) 2.8-3.5 (4.1) mm. long, loosely strigulose with black, black and white, or exceptionally all white hairs, the subsymmetric disc 0.4—0.7 mm. deep, the membranous, pallid or purplish tube 1.6—2.1 mm. long, 1.4-1.9 mm. in diameter, the subulate teeth mostly (1) 1.2—1.5 mm., more rarely up to 2.3 mm. long, the whole becoming papery, marcescent unruptured; petals whitish tinged or margined with lilac, pale or bright purple, the banner striate; banner recurved through ± 45°, (4.7) 5-7 mm. long, the short, cuneate claw expanded into an ovate, obovate, or suborbicular, notched or emarginate blade 3.5-5.3 mm. wide; wings 4.3-6.2 mm. long, the claws (1.2) 1.5-2 mm., the oblanceolate, obtuse, gently incurved blades 3.3-4.6 mm. long, 1.1-1.6 mm. wide; keel (0.4 mm. longer to 0.4 mm. shorter than the wings) 4.3—5.8 mm. long, the claws (1.4) 1.6-2.2 mm., the lunately half-elliptic blades (3) 3.2-3.8 mm. long, 1.3-1.7 mm. wide, incurved through 45-65 (80)° to the narrowly triangular, acute or subacute, often slightly porrect apex; anthers (0.2) 0.25-0.4 mm. long, pod pendulous, spreading, or ascending and resupinate from the variably decurved or contorted pedicel, disjointing from an obscure gynophore 0.4—0.8 mm. long, the body lunately linear-ellipsoid, very gently and evenly incurved, 1.2-3 cm. long, (2.2) 2.5-3.1 mm. in diameter, cuneate at base, more abruptly contracted distally into a short terminal cusp, laterally compressed-triquetrous, with s a lowly concave lateral faces broader than the deeply and narrowly sulcate dorsal face, the thin, green or purple-tinged, sparsely strigulose or exceptionally glabrous valves becoming brownish-stramineous, somewhat lustrous, delicately cross-reticulate, inflexed as a complete septum 1—1.9 mm. wide; ovules 12—26; seeds olivaceous, pale or dark brown, sometimes purple-speckled, prominently wrinkled and pitted, dull, 1.5-2.3 (2.5) mm. long.—Collections: 35 (v); representative: C. B. Wolf 6595 (CAS, WS), 6652 (ARIZ, CAS, NY, WS); Ripley & Barneby 3452 (NY, RSA), 5868 (CAS, NY, RSA); Munz & Johnston 5230 (CAS, POM); J. T. Howell 33,240 (CAS); Raven, Mathias & Turner 12,515, 12,542 (CAS).

Sandy and gravelly flats, desert hillsides, and outwash fans, on granitic and sometimes volcanic rock-formations, apparently of bicentric distribution, widespread in the zones of Larrea and Joshua-tree, about 2000-5200 feet, over most of the Mohave Desert, from lower Owens Valley and Death Valley south to the foothills of the San Bernardino and New York Mountains, east to the Belted Range in southern Nye County, Nevada, and south, becoming rarer, along the west edge of the Colorado Desert to the Mexican border; apparently disjunctly, at 100-1800 feet, on the west coast and interior deserts of Baja California, in lat. 30°-30° 30' N.—Map No. 147.—Late March to May.

Astragalus acutirostris (with sharp keel-tip) Wats. in Proc. Amer. Acad. 20: 360. 1885 (21 Feb.).—"Near Brown’s Ranch, Mohave Desert, by Parish Brothers, May, 1882, and on dry rocks above the Calico Mines, near Fort Mohave, by J. G. Lemmon, May, 1884."—Holotypus (S. B. & W. F. Parish 1276), GH! paratypus (Lemmon 3115), GH! isotypus, DS!—Oxytropis acutirostris (Wats.) Jones in Proc. Calif. Acad. Sci. II, 5: 677. 1895. Spiesia acutirostris (Wats.) Jones, l.c., nom. provis. Aragallus acutirostris (Wats.) A. Hell., Cat. N. Amer. Pl. 4. 1898. Hamosa acutirostris (Wats.) Rydb. in Bull. Torr. Club 54: 331. 1927. Astragalus Nuttallianus var. acutirostris (Wats.) Jeps., Fl. Calif. 2: 379. 1936.

Astragalus streptopus (with twisted pedicels) Greene in Bull. Calif. Acad. Sci. 1: 155. 1885 (7 May).—"Mohave Desert, 1884, collected by Mrs. M. K. Curran."—Holotypus, CAS! isotypus, GH!

The near relationship of A. acutirostris, despite its desert habitat, seems to lie in the direction of the cismontane annuals, among which A. pauperculus is technically very similar in its loose raceme and often resupinate pod. The species is superficially similar to and over almost its whole range of dispersal is associated with some variety of A. Nuttallianus; the two often grow together, are easily confused, and have often been distributed under the same label. Since they are treated in these pages as members of separate subsections, it seems proper and may well be helpful to present the differential characters in key form:

1. Pods readily deciduous from an obscure gynophore, dehiscent at both ends on the ground, disposed (when more than 1) on an axis 1—3.5 (4.5) cm. long, variably oriented by means of simple outward curvature or by more elaborate contortion of the pedicels, the body very gently and evenly incurved, shallowly crescentic; leaflets of all leaves retuse or truncate-emarginate               A. acutirostris

1. Pods sessile but persistent on the receptacle on a shortened axis (this rarely elongating and up to 2 cm. long), spreading or incurved-ascending from simply arched pedicels, the body nearly always incurved just above the base and therefore straight or nearly so (rarely evenly and shallowly crescentic); leaflets either all elliptic (then acute or obtuse but not emarginate) or dimorphic, obovate-obcordate in the lower leaves, elliptic in the upper; vars. imperfectus and cedrosensis of                                                                                   A. Nuttallianus

The importance attached in the past to the acute keel-tip of A. acutirostris was greatly exaggerated and the transference of the species to Oxytropis most unnatural. The keel is no more pronouncedly narrowed upward and no sharper at apex than that of many homaloboid species, and there is no suggestion of the precisely terminal beaklike appendage characteristic of the sister genus (of which all species are moreover sound perennials with quite differently formed pods).

The sharp-keeled milk-vetch in California varies little. The populations discovered quite recently (1958) by Raven and associates in northern Baja California appear to differ in some minor respects. The calyx-teeth are longer (1.5-2.3 mm. as opposed to 1-1.5 mm.) and the pod is also slightly longer and encloses 9-13 rather than 6-8 pairs of ovules and seeds. So far as we know at present the Mexican form is geographically disjunct, the known stations lying at points 150 miles or more distant from the previously established southern limit of A. acutirostris at the west edge of the Colorado Desert; very likely it represents a distinct entity. One individual plant from Baja California (Raven & al. 12,372, CAS, pro parte) is further peculiar in having elliptic and obtuse (not retuse) leaflets and a glabrous, inwardly arched pod such as I have not seen elsewhere in the species. This particular specimen was distributed with material of A. Nuttallianus var. cedrosensis, with which it was associated in nature; it may be a hybrid. The shape of the leaflets is suggestive of such a possibility, and the genes for a glabrous pod are present in many populations of var. cedrosensis.