Astragalus Purshii

Title

Astragalus Purshii

Authors

Rupert C. Barneby

Scientific Name

Astragalus purshii Douglas ex Hook.

Description

212.  Astragalus Purshii

Low, acaulescent, subacaulescent, loosely to densely tufted, sometimes pulvinate, or less commonly caulescent and diffuse, or loosely to densely matted, with a taproot and sometimes a well-developed, slenderly forking caudex, densely villous, villous-tomentose, or pannose, the vesture composed of fine, mostly sinuous or twisted (and often some straighter, ascending or spreading) hairs up to (0.8) 1-2.3 (2.5) mm. long, the stems, when developed, white-tomentose, the herbage gray or white, the pubescence often somewhat silky in youth, becoming duller and cottony in age; stems 0—1 dm. long, some internodes occasionally up to 2 cm. long, but usually all shorter than the stipules; stipules membranous, pallid with green midrib, becoming scarious in age, ovate, lanceolate, or lance-acuminate, or rarely -caudate, (2.5) 3-15 mm. long, decurrent around ± ½ of the stem’s circumference; leaves 1—12 (15) cm. long, all slender-petioled, with (3) 5—17 (21) crowded or moderately distant, flat or loosely folded leaflets varying from suborbicular and broadly obovate-cuneate and obtuse to narrowly elliptic and acute, 2-14 (20) mm. long; peduncles at anthesis ascending or erect, commonly shorter (rarely longer) than the leaves, sometimes almost 0, arcuate-procumbent or prostrate and radiating in fruit; racemes loosely but shortly, often subumbel- lately (1) 2-10 (11)-flowered, the flowers ascending at a narrow (rarely a wide) angle, the axis not or little elongating, up to 2 (4) cm. long in fruit, usually less; bracts membranous, lanceolate, lance- or ovate-acuminate, or -caudate, 2-9 mm. long; pedicels ascending, straight or a trifle arched outward, at anthesis 1-3.6 (4.5) mm., a little thickened and longer in fruit; bracteoles almost always 0, minute when present; calyx (5.5) 6-16 (18.7) mm. long, villous with white, mixed black and white, or all black hairs, the disc campanulate or turbinate, slightly to strongly oblique, the tube membranous, pallid or purplish, deeply campanulate to cylindric, the teeth subulate, lanceolate, or more rarely lance- caudate, or triangular, the orifice often oblique; petals whitish, ochroleucous, pale pink, or brilliant pink-purple, when pale the keel-tip maculate; banner gently recurved through dr 40°, broadly to narrowly oblanceolate, spatulate, rhombic- oblanceolate, or rarely obovate-flabellate, 9-25 (26.5) mm. long, shallowly to deeply notched; wings a little shorter, the blades lance-oblong, linear-oblong, or narrowly lance-elliptic, obtuse or rarely obliquely emarginate, straight or commonly a trifle incurved in the distal ?-½; keel again shorter, 8-21 (22.5) mm. long, the blades lunately elliptic to half-obovate, gently or quite abruptly incurved through 85-100° to the bluntly rounded apex; anthers 0.4-0.8 (1.05) mm. long; pod ascending (humistrate), sessile on a conical boss or well-defined gynophore up to 1.6 mm. long (its length correlated with the depth of the calycine disc), obliquely ovoid or broadly lance-ellipsoid, 7-27 (30) mm. long, (3) 3.5-11 (13) mm. in diameter, straight or nearly so and strongly or at least perceptibly dorsiventrally compressed in the lower ½-?, thence passing upward gradually or abruptly into the slightly to strongly incurved, shortly triangular to lance-triangular, laterally flattened, rigid beak, scarcely to quite deeply sulcate along either or both sutures, the fleshy valves becoming leathery, very densely villous- or hirsute- tomentose with sinuous, or largely almost straight, white or more rarely cream- colored hairs up to (1.2) 1.5-5 mm. long, the vesture nearly always concealing the brownish surface of the valves, these not at all inflexed, or in one var. inflexed as a nearly complete, rarely complete septum up to 1.4 mm. wide; ovules 14-38 (46); seeds ochraceous, greenish- or purplish-brown, sometimes purple-dotted, or almost black, smooth or variably pitted, dull, rarely wrinkled, 1.3-2.8 (3.4) mm. long.

The Pursh milk-vetch is the prototype of its group, a low, tufted or matted plant with softly gray-hairy foliage and proportionately long and narrow flowers giving rise to pods which resemble balls of cotton. The species is variable and polymorphic, composed of many geographic races. The following account follows the preliminary revision (Barneby, 1947, pp. 494—512) in most respects. Study of ever-increasing accumulations of new material and a more extensive field experience have modified my former concept of var. tinctus, especially as this occurs in the California Coast Range (var. longilobus) and Mohave Desert (var. flocculatus), and the limits of the species have been very slightly widened to admit the too closely related A. ophiogenes. The reader interested in the history or supposed differential characters of segregates mentioned below only in the synonymy is referred to the cited revision.

Attempts to devise a practical key to the varieties of A. Purshii are never wholly successful. This situation is due partly to the inherent variability of the plants, partly to the circumstance that flowers, preferably fresh (or with annotation of their color when fresh) are generally required for identification. Flower-size of fruiting material can, however, often be inferred from measurement of the marcescent calyx, always allowing for a shrinkage of ± 10% unless restored by soaking.