Astragalus Tidestromii

Title

Astragalus Tidestromii

Authors

Rupert C. Barneby

Scientific Name

Astragalus tidestromii (Rydb.) Clokey

Description

202.  Astragalus Tidestromii

Low, sometimes rather coarse, acaulescent or subacaulescent, perennial but of short duration, sometimes flowering the first season, with a taproot and ultimately a shortly forking but scarcely woody caudex, the herbage densely villous- hirsute and commonly also tomentose with incumbent or curly and ± entangled together with some longer, straighter, ascending and spreading hairs up to (0.75) 1-1.5 mm. long, the stems (when visible) white-felted, the leaves varying from white to greenish-gray, the inflorescence black-hairy; stems several, mostly reduced to obconic crowns beset with a thatch of imbricated stipules, sometimes a little developed and up to 4 (7) cm. long, then prostrate, and the internodes not over 1 cm. long; stipules thinly herbaceous becoming papery-membranous, triangular or lance-acuminate, 3-8 mm. long, decurrent around ± half the stem’s circumference; leaves subradical (or appearing so), (3) 4-15 cm. long, with rather thick, stiff, recurving petiole and (7) 11-19 obovate-cuneate, broadly elliptic, or suborbicular, commonly obtuse or subtruncate, rarely subacute, flat, thick-textured leaflets 414 mm. long; peduncles stout, (2.5) 5-13 cm. long, shorter or slightly longer than the leaves, ascending at anthesis, arcuate-procumbent or prostrate in fruit; racemes loosely but shortly 5-16-flowered, the flowers loosely spreading-ascending, the axis somewhat elongating, (1) 2-7 cm. long in fruit; bracts membranous, lanceolate, 2.5—5.5 mm. long; pedicels ascending, straight, or in age a little arched outward, at anthesis 0.8-1.5 mm., in fruit thickened, 1.3-3.1 mm. long; bracteoles 2, sometimes conspicuous and up to 2 mm. long; calyx (5.5) 5.8-9.7 mm. long, villous-hirsute or villosulous with predominantly black hairs, the scarcely oblique disc 0.8-1.4 mm. deep, the membranous, deeply campanulate tube 5-7.3 mm. long, 2.7-4 mm. in diameter, the broadly subulate or triangular, obtuse teeth 0.8-2.7 mm. long, the ventral pair often longest and broadest; petals whitish tinged with sordid lavender, the keel and wings tipped with dull dark purple; banner recurved through ± 40°, broadly oblanceolate or rhombic-elliptic, shallowly notched, 12-17.7 mm. long, 6.5-9.5 mm. wide; wings (11.6) 12.3-17 mm. long, the claws 5.5-8.7 mm., the linear-oblong or narrowly lanceolate, obtuse, straight or nearly straight blades 7.2-9.5 mm. long, 2.3-2.9 mm. wide, rather gently incurved through (85) 95° to the blunt apex; anthers 0.5-0.65 mm. long; pod ascending or incurved-ascending (humistrate), variable in size and outline, the basic outline obliquely lance-acuminate, (1.5) 2-5.5 cm. long, 6-16 mm. in diameter, obtusely cuneate or shortly acuminate and laterally compressed at base, thence obcompressed-quadrangular and incurved or inwardly hooked through ¼ to nearly 1¼ circles, passing distally into the usually long, narrowly triangular- acuminate, laterally compressed, rigid beak, bicarinate by the prominent thickened sutures but the ventral suture depressed and lying in an open groove, the lateral angles formed by transverse dilation of the valves rather narrow but obtuse, the green, fleshy valves strigulose with appressed or incumbent, sometimes curly hairs up to 0.5—0.7 mm. long, becoming stiffly leathery or subligneous, brownish-stramineous, coarsely cross-reticulate and rugulose on the angles, not inflexed; dehiscence apical, through the beak and part way down through the ventral suture, after falling; ovules 26—49; seeds light or yellowish-brown, often purple-speckled, pitted, sometimes greatly distorted by crowding, 2.5-3.6 (4) mm. long.—Collections: 24 (v); representative: Clokey 8596 (NA, SMU, TEX, WIS, WS); C. B. Wolf 10,354, 10,581 (CAS); Ripley & Barneby 6389 (CAS, RSA).

Open gravelly hillsides, outwash fans, gravelly and sandy playas in the foothills of calcareous desert mountains, commonly with Larrea, but ascending into the juniper belt, (2000) 2400-5000 feet, locally plentiful and rather frequent in the eastern Mohave Desert, in particular about the Spring, Charleston, and Desert Ranges in Clark County, Nevada, and the Clark, Kingston, and Nopah Ranges in eastern San Bernardino and extreme southeastern Inyo Counties, California; somewhat isolated in the western Mohave Desert at the mouth of Cushenbury Canyon, San Bernardino Mountains.—Map No. 85.—March to May.

Astragalus Tidestromii (Rydb.) Clokey in Madrono 6: 214. 1942. based on Xylophacos Tidestromii (Ivar Tidestrom, 1864-1956) Rydb. in Bull. Torr. Club 52: 155. 1925.—‘Type collected in Charleston Mountains, Nevada, May 27, 1919, Ivar Tidestrom 9661 ..—Holotypus, collected at "Wilson’s Ranch, on mesa at base of the Charleston Mountains," US! isotypus (fragm.), NY!

Astragalus aimphioxys X Layneae Jones, Rev. Astrag. 215. 1923, as to description, but the collections cited ("Chimihuevis Mts., Arizona [south of Franconia] and my No. 5010 from the copper mine west of St. George, Utah...") are forms of A. amphioxys. Specimens agreeing with the description were collected by Jones in Clark County, Nevada, in 1905 (Good Springs) and 1906 (Indian Springs); either would serve if needed as typus of the supposed hybrid.— A. Marcusjonesii (Marcus Eugene Jones, 1852-1934, preeminent student of North American Astragalus) Munz in Leafl. West. Bot 3: 50. 1941, in large part, excl. typus and syn. Xylophacos melanocalyx.

The Tidestrom milk-vetch has been introduced at this point because it possesses the technical attributes, in particular the basifixed vesture, of the genuine Argophylli; but its closest affinity in the section is probably with A. amphioxys. a species of subsect. Missourienses, characterized by dolabriform hairs. It resembles A. amphioxys in habit of growth, and the pods are almost identical in size, in form, in curvature, and in the dorsiventral compression of the middle part which passes into lateral compression toward either end. The rather coarse villous-tomentose vesture of A. Tidestromii and the proportions of its relatively small, pale or luridly bicolored flowers are the features in which it differs most obviously from A. amphioxys; these are at the same time astonishingly similar, even in fine detail, to the comparable features of A. Layneae. The resemblance in flowering material is so great that A. Tidestromii has repeatedly been confused with A. Layneae in herbaria, even though the latter's solitary stems arising from slender, horizontal rhizomes and (eventually) its bilocular pod are fundamentally different. It is not surprising, however, that the earliest discoveries of the Tidestrom milk-vetch were interpreted as representing a hybrid A. amphioxys X Layneae, a hybrid which seemed all the more plausible in that it was found close to the line where the ranges of the supposed parents were known to converge. As the dispersal of A. Tidestromii has become more fully known and its habits studied in the field, its status as a self-perpetuating species has become established beyond any chance of controversy. Nevertheless its neat combination of morphological features of two distantly interrelated and wholly dissimilar astragali, and this within a restricted region abutting on the ranges of both, is bound to excite speculation as to its origin in the past through hybridization and subsequent recombination of diverse heredities. The situation is similar to that described under A. riparius in sect. Reventi-arrecti.

The ecology of the three species involved in this provocative triangle is of considerable interest, for it goes far to explain the mutually exclusive ranges of, at least, A. Tidestromii and A. Layneae (see map 85). So far as known, A. Layneae avoids sedimentary bedrock and is certainly calcifuge. In contrast A. Tidestromii occurs nowhere except on limestone or on out- wash fans built up out of calcareous debris, a fact dramatically illustrated by its disjunct occurrence on the calcareous island in Cushenbury Canyon and its absence from the granitic deserts intervening between the San Bernardino Mountains and the Kingston massif. The widely dispersed A. amphioxys is indifferent to the nature of the bedrock over most of its range, but toward its western limit occurs most frequently on sedimentary formations, both sandstone and limestone. It impinges on the range of A. Tidestromii in the foothills of the Spring and Charleston Mountains, but barely comes into contact with A. Layneae in the neighborhood of Kingman, Arizona, the only known spot where a hybrid involving the two species could be expected to occur at the present time.

Although described only in recent years, the Tidestrom milk-vetch was first collected in 1844, by Fremont (NY) on his second expedition, very likely in the neighborhood of the Vegas wash.