Astragalus tetrapterus

Relatively robust but often of low stature, with a buried root-crown and ultimately woody caudex, strigulose in variable degrees with filiform or somewhat flattened, straight, appressed or subappressed hairs up to 0.3-0.5 mm. long, the stems and herbage varying from silvery or cinereous to pallid or dark green and nearly glabrous, the leaflets usually more densely pubescent above than beneath, occasionally glabrous above; stems several, rather closely tufted, at first erect or ascending but becoming diffuse or decumbent when weighed down by copious fruit, 1-3 (3.5) dm. long, simple, leafless, and subterranean for a space of 2-7 cm., thereafter leafy and branched or spurred at 1-6 nodes preceding the first peduncle, the primary branches often again branched or spurred, the spurs sometimes paired with a peduncle; stipules 2-5.5 mm. long, those at the buried and first emersed nodes scarious, pallid or purplish, strongly adnate to a vestigial or suppressed petiole to form a bidentate, semi- or almost fully amplexicaul sheath, the upper ones cauline, with triangular or triangular-acuminate, mostly reflexed, herbaceous blades; leaves (1.5) 2.5-8 cm. long, shortly petioled, with 9-21 linear, linear-oblong, narrowly elliptic, or rarely oval, obtuse to very acute, flat or folded, rigid and dorsally keeled or submembranous and flaccid leaflets 1-20 mm. long, the leaflets sub- homomorphic throughout the plant, or dimorphic, those of the lower leaves then broader and more obtuse, the terminal leaflet, at least of some upper leaves, continuous with the rachis, the lateral ones either similar or all jointed; peduncles ascending, or in fruit divergent, 1-5 (6.5) cm. long, nearly always surpassed by the leaf; racemes shortly and rather closely 6—15-flowered, the flowers ascending at anthesis, the axis little elongating, 1-4 cm. long in fruit; bracts membranous becoming scarious, 1.5-3.5 mm. long; pedicels at anthesis slender, erect, 1.4-3 mm. mm. long, in fruit a trifle thickened, arcuate-recurved, 2-4.3 mm. long; bracteoles commonly 2, minute or up to 1.5 mm. long, rarely 0; calyx 5.5-8.5 mm. long, strigulose with black or with black and some white hairs, the disc (0.8) 1—1.4 mm. deep, the membranous, pallid, cylindric or cylindro-campanulate or narrowly vaseshaped tube 4.7-7 mm. long, 2.3-3.1 mm. in diameter, the subulate or triangular- subulate, mostly obtuse, erect or spreading teeth 0.8-2.4 mm. long, the whole usually becoming hyaline, fragile, irregularly circumscissile, but sometimes papery and marcescent; petals whitish tinged with lilac, yellowish tinged with pink, or bright pink-purple with paler wing-tips, the color fugitive in drying; banner oblanceolate or rhombic-elliptic, emarginate to deeply notched, (13.5) 15—19 mm. long, 4.2-8 mm. wide; wings 11.6-16.5 mm. long, the claws 5.5-7.5 mm., the narrowly lanceolate or oblong, obtuse, obliquely truncate, or rarely retuse, straight or slightly incurved blades 6.6—10.2 mm. long, 1.5—2.4 (2.9) mm. wide; keel 10.2—13 mm. long, the claws 5.5—7.2 mm., the broadly lunate or half-elliptic blades 4.4-5.8 (6.3) mm. long, 2.1-2.7 (3.2) mm. wide, gently incurved through 80-90 (95)° to the bluntly deltoid apex; anthers (0.5) 0.55—0.7 mm. long; pod pendulous, sessile, obliquely oblong or broadly clavate-oblong in profile, crescentically incurved or coiled into a ring, 2-4 cm. long, (4) 6-10 mm. in diameter, nearly always broadest above the middle and tapering toward the obtusely cuneate base, abruptly contracted distally into a short, deltoid, cuspidate beak, when first formed succulent and a trifle obcompressed, ultimately (by collapse of the fleshy walls lengthwise along the sides of both sutures) becoming sharply tetragonal, both sutures then salient and acute and the lateral angles winged lengthwise (the wings up to 2.5 mm. wide), the 4 subequally wide faces all concave, the glabrous or thinly strigulose valves at first green or purple-mottled, becoming stiffly papery, stramineous or brownish, smooth but cross-reticulate, lustrous, not inflexed; dehiscence both apical and basal, the valves gaping distally and tending to coil out- and backward; ovules 28-38; seeds olivaceous or khaki-brown, rugulose-punctate but somewhat lustrous, 1.8-2.7 mm. long.—Collections: 22 (vi); representative: Peck 20,613 (WILLU), 21,695 (CAS); Eastwood & Howell 277, 287 (CAS); Maguire & Holmgren 25,484 (NY, UTC, WS); Jones 5200 (NY), 5210 (POM); Ripley & Barneby 6090 (CAS, NY, RSA), 6163, 6269 (CAS, RSA).

Gullied bluffs, barren knolls, and open valley floors, in loose sandy alkaline soils, mostly in exposed places but sometimes taking shelter under or entangled in sagebrush, commonly between 3500 and 6500 feet, but probably lower at its southern limit along the Colorado River, widely dispersed but uncommon, southeastern Oregon (about Malheur and Harney Lakes and along the Owyhee River) south through eastern Nevada to the Colorado Valley in northwestern Arizona and the Zion region in southern Iron, Washington, and western Kane Counties, Utah, west in Nevada to the headwaters of the Reese River in Nye County.—Map No. 108.— Late April to early July.

Astragalus tetrapterus (four-winged, of the pod) Gray in Proc. Amer. Acad. 13: 369. 1878.—"Southern Utah, Mrs. Thompson and Capt. Bishop, in flower (1871—73), and now (1877) found by Dr. Palmer in fruit, 25 miles north of St. George."—Lectotypus, Palmer 111, GH! isotypi, K, NY! paratypus (Thompson & Bishop), GH!—Pterophacos tetrapterus (Gray) Rydb., Fl. Rocky Mts. 1063. 1917. Astragalus tetrapterus var. typicus Barneby in Leafl. West. Bot. 4: 234. 1946.

Astragalus tetrapterus var. capricornus (ram’s-horn, the pod’s valves coiled in dehiscence) Jones, Rev. Astrag. 149, Pl. 31. 1923.—"...at Cobre, Nevada."—Lectotypus, collected June 16, 1906, by M. E. Jones, POM! isotypi, CAS, NY!

Pterophacos cinerascens (turning ashen, of the pubescent foliage) Rydb. in N. Amer. Fl. 24: 309. 1929.—"Type collected... southwest of Narrows, Harney County, Oregon, July 4, 1912, Morton E. Peck 3024... "—Holotypus, GH! (apparently no isotypus retained in herb. Peck., WILLU).—Astragalus cinerascens (Rydb.) Tidest. in Proc. Biol. Soc. Wash. 50: 21. 1937. A. tetrapterus var. cinerascens (Rydb.) Barneby in Leafl. West. Bot. 4: 235. 1946.

The four-winged milk-vetch, A. tetrapterus, is somewhat isolated taxonomically in relation to the two foregoing Pterocarpi. It might be more consistent with standards adopted elsewhere in this volume to provide for it a separate subsection, possibly a monotypic section. However the fruit, as first fully developed, is not different in any essential feature from that of A. Casei or of several Argophylli at the same stage of growth, the unique tetragonal compression being assumed only as the pulpy tissue of the mesocarp collapses inward in drying. Jones (1923, p. 148) pointed out that the cavity of the pod is transversely linear, indicating a primitively dorsiventral compression and the quadrangular, ripe section is apparently the result of stresses set up within the valve-walls during the process of maturing. The lustrous, reticulate exocarp is like that found in A. pterocarpus.

The species is superficially polymorphic. The vesture varies greatly in density and somewhat in distribution. The leaflets vary from flat, membranous, and mostly jointed to the rachis to involute, stiff, and decurrent. The flowers vary in size and coloring, sometimes in the same colony. In fact, hardly two populations are similar in all respects. The differential characters of likely use in the definition of geographic varieties within A. tetrapterus have been shown (Barneby, 1946, p. 234—236) to occur in a number of intergradient and interlocking combinations, and the var. cinerascens, arbitrarily defined by a glabrous pod, seems little better than one of a number of minor variants of equal stature. A hairless pod was formerly known only from southeastern Oregon, but even there it does not wholly replace the more ordinary pubescent form. Moreover, part of the type-collection of the certainly inconsiderable var. capricornus (CAS) has a glabrous pod, even though it came from a region where a pubescent ovary is more commonly encountered. In general, the more xerophytic type (= var. capricornus) becomes prevalent in the northern half of the species-range and the type with expanded, jointed, lateral leaflets is commoner southward. But intergradation between the extremes is continuous, and the extreme types do not seem to be firmly correlated with any edaphic or distributional factor.

In southwestern Utah A. tetrapterus has caused loco-disease (near Newcastle, acc. Marsh & Clawson in U. S. Dept. Agr., Dept. Circ. 81, 2 figs. 1920); it is very possible that all the Pterocarpi are poisonous. Fortunately none is really abundant, except very locally, and I believe that they are seldom browsed.