Astragalus zionis M.E.Jones
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Authors
Rupert C. Barneby
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Authority
Barneby, Rupert C. 1964. Atlas of North American Astragalus. Mem. New York Bot. Gard. 13(2): 597-1188.
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Family
Fabaceae
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Scientific Name
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Type
"No. 5261w. May 17, 1894, Springdale, Utah, 4000° alt., in red sand ..."—Jones also cited his numbers 5249h, 5224d, 5001b, 5249g, all collected the same year and in the same immediate region.—Holotypus, Jones 5261w, POM! isotypus, US!
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Synonyms
Xylophacos zionis (M.E.Jones) Rydb.
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Description
Species Description - Shortly caulescent or subacaulescent, loosely or densely tufted, with a long taproot and at length repeatedly forking, suffruticulose caudex beset with the scaly remains of stipules and sometimes also with a thatch of stiff, persistent petioles (the whole sometimes buried in drifting sand), densely villous-villosulous, or exceptionally strigulose, with straight or somewhat sinuous, loosely ascending or rarely subappressed, stiff, lustrous hairs up to 0.6-1.1 (1.3) mm. long, the stems commonly villous-tomentulose, the herbage silvery or less often greenish, the inflorescence black-villous; stems several or numerous, (0.5) 1.5-7 cm. long, when developed diffuse or prostrate, simple or few-branched near the base, the internodes either concealed by stipules or some of them commonly up to 1 (1.7) cm. long, mostly shorter; stipules submembranous, pallid or purplish, several-nerved, becoming papery-scarious and ultimately brownish and glabrate, (1.5) 2-5.5 mm. long, mostly broadly ovate, broader than the stem and often broader than long, the uppermost sometimes lanceolate, all or all but the uppermost amplexicaul-decurrent and shortly connate; leaves (2) 3-12 (25) cm. long, with slender, wiry petiole and (11) 13-21 (25) ovate, elliptic, or rhombic- elliptic and acute, rarely obovate and obtuse, flat or loosely folded leaflets 2-12 (16) mm. long, either remote or crowded on the rachis, usually diminishing in size upward; peduncles mostly stout, (1) 3-12 (15) cm. long, shorter or exceptionally longer than the leaves; racemes shortly but loosely (1) 3-11 (20)-flowered, the axis little elongating, 0.5-4.5 (9) cm. long in fruit; bracts membranous, ovate- acuminate or lanceolate, (2) 2.5-5 mm. long; pedicels ascending, straight or nearly so, at anthesis 1-2 mm., in fruit thickened, 1.3-3 mm. long; bracteoles commonly 2, sometimes rudimentary or 0; calyx (8.3) 10-13 (18) mm. long, villous-villosulous with all or predominantly black, more rarely with largely white hairs, the oblique disc 1.2-2 mm. deep, the membranous, purplish, deeply campanulate or cylindric tube (6.5) 7-10 (12.7) mm. long, 3.6—4.7 mm. in diameter, the subulate teeth (1.5) 2.2-3.7 (5.7) mm. long, the dorsal one often shortest, the ventral sinus cut back, the orifice oblique; petals purple, the claws pale; banner broadly oblanceolate or spatulate, 18.5-22.5 (26) mm. long, the rhombic-oval, shallowly notched blade 9.2—12.5 mm. wide; wings 17—20.5 (?) long, the claws 8.5-12.2 mm., the narrowly lance-oblong, obtuse blades 9.4—11.2 mm. long, 2.4-3 mm. wide, slightly incurved above the middle; keel 14.6-18.6 (19) mm. long, the claws 8.5—12.4 mm., the lunately elliptic blades 6.4—7.8 mm. long, 2.6-3.3 mm. wide, rather gently incurved through 85-95° to the blunt apex; anthers 0.65-0.85 mm. long; pod ascending (humistrate), obliquely ovoid-, oblong-, or lance-ellipsoid, (1.5) 2—2.7 (3) cm. long, (5.5) 6—9 mm. in diameter, rounded or broadly cuneate at base, gently incurved through up to ½-circle but mostly less into a triangular- or lance-acuminate, laterally compressed beak, dorsiventrally compressed (except sometimes at very base) through the proximal ¾, openly sulcate ventrally at and below the middle and dorsally either at and above the middle or its whole length, bicarinate by the prominent but ± depressed sutures, the fleshy, loosely strigulose or villosulous valves brightly purple- or red-mottled on a green, at length stramineous ground, becoming leathery or subligneous, ± 0.2-0.4 mm. thick and cross-reticulate, either not inflexed, or inflexed in the lower half to form a rudimentary septum less than 1 mm. wide, the walls of the septum sometimes separated, leaving a filiform-tubular chamber behind the dorsal suture; dehiscence apical and part-way downward through the ventral suture; ovules 24-30; seeds brown or yellowish-buff, smooth or sparsely pitted, 2-3 mm. long.
Distribution and Ecology - Rock ledges, sandy talus in canyons and at the foot of cliffs, sometimes washed down and temporarily persisting on the valley floor, in sandy soils derived from red and white sandstone, 4000—7100 feet, locally abundant along the Zion Escarpment in Washington and Kane Counties, Utah, and in scattered stations along the Colorado and San Juan Rivers and their tributary creeks from Grand Canyon, Coconino County, Arizona, northeast across northern Arizona into southern San Juan County, Utah.—Map No. 80.—Late April to June.
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Discussion
The Zion milk-vetch is a variable species, consisting of many isolated populations scattered through the deeply dissected sandstone country of the Colorado Plateau. The vesture of the whole plant is commonly villous, but in some small areas, as along Clear Creek in Zion Park, the leaves are strigose with appressed or subappressed hairs. The plants from the canyons of the San Juan and Colorado Rivers are generally, perhaps always larger-flowered than those from the Zion Escarpment, but this point needs verification from more abundant flowering material. Despite its connate stipules, anomalous in sect. Argophylli, the Zion milk-vetch is closely related to A. argophyllus, and even though the species has marked individuality of aspect (and is seldom misidentified), technical differential characters are hard to find. It is maintained as a species distinct from A. argophyllus because the connate stipules and brightly mottled pod are linked with characteristic ecological and distributional patterns. On Zion Escarpment A. zionis occupies the same ecological niche as the taxonomically isolated A. striatiflorus; the two species have twice been found in immediate association on the hummocked dunes close under the summit cliffs. These two astragali are similarly organized up to the flower and fruit, and it may be significant that A. zionis differs from A. argophyllus var. Martini precisely in certain characters which are highly developed in A. striatiflorus, notably in the united stipules and an incipient septum. It is perhaps too fanciful to assume that introgression between A. striatiflorus and A. argophyllus var. Martini (however conveniently near at hand on the plateau above the escarpment) has given rise to the Zion milk-vetch, for the features common to A. zionis and A. striatiflorus may have been acquired by convergent evolution acting within a narrow set of physiological pressures. In any case A. zionis is certainly a self-perpetuating species which now occupies a range much more extensive than A. striatiflorus and ecologically separated from that of A. argophyllus.
In the preliminary account of sect. Argophylli (Barneby, 1947) the then imperfectly known A. arietinus var. stipularis was listed as a synonym of A. zionis, thereby seeming to extend the range of the species into Colorado. The var. stipularis is now known to belong with A. naturitensis Pays. and the Zion milk-vetch reaches its known eastern limit at Bluff on the San Juan River.
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Objects
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Distribution
Utah United States of America North America| Arizona United States of America North America|