Monographs Details: Astragalus tridactylicus A.Gray
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:

364.  Astragalus tridactylicus

Acaulescent or shortly caulescent, the stems of the year rarely more than 1.5 cm. long, often all reduced to thick crowns terminating the divisions of a cespitose or more loosely forking (and then prostrate) caudex beset with a thatch of persistent leaf-stalks and stipules, together forming close tufts, cushions, or mats of foliage 0.5—3 (4) dm. in diameter, the herbage silvery-pilose or -silky throughout with lustrous, ascending, and subappressed (sometimes spirally twisted) hairs up to (0.75) 1-2 mm. long; stipules 5-10 mm. long, densely or loosely imbricated, either all pubescent dorsally or the inner ones of the shorter crowns glabrescent or subglabrous dorsally, pilose-ciliate, the ciliae sometimes mixed with minute tackshaped processes; leaves 0.5—6 cm. long, either all alike or dimorphic, all or only the early ones shortly petioled and with obovate or broadly cuneate-oblanceolate leaflets 2-10 (13) mm. long, or the later ones (developing with or after the flowers) with slender, elongate petiole and narrowly oblanceolate leaflets 0.8—2 cm. long; peduncles varying from subobsolete to 1 cm. long, much shorter than the leaves, often concealed within the stipular sheaths; racemes loosely but very shortly 2-6 (acc. Rydberg up to 10)-flowered, the flowers ascending, the axis not over 5 mm. long but often produced as a subulate appendage beyond the last flower, bracts hyaline with firm midrib, broadly lance-elliptic to obovate, involute-boat-shaped, 1.5-5.5 mm. long; pedicels 1-3.5 mm. long; calyx 4.5-6.9 mm. long, densely silvery-pilose, the campanulate or turbinately campanulate tube 2.7-3.8 mm. long, 1.6-3.8 mm. in diameter, the lance-subulate teeth 1.8-3.5 mm. long; petals pink-purple; banner obovate-cuneate, 8.2-11.5 mm. long, 4.2-6 mm. wide; wings 7.5-11 mm. long, the claws 3-4.4 mm., the blades 4.8-7.2 mm. long, 1.6-2.2 mm. wide; keel 5.3-7.2 mm. long, the claws 2.8—4.4 mm., the blades 2.8-3.2 mm. long, 1.6-2.2 mm. wide, obtuse; anthers 0.35-0.55 mm. long; pod (seldom observed) apparently as in A. sericoleucus; ovules 6—12.—Collections: 30 (v); representative: Clokey 4174 (CAS, NY, WIS, WS, TEX); W. A. Weber 3268 (CAS, TEX); Ramaley & Richards 15,952 (CAS, TEX, WS); Osterhout 6225 (NY, RM), 6472 (RM, WIS); A. Nelson 59, 207, 4320 (NY, RM); C. L. Porter 3060 (NY, RM, WTU); Ripley & Barneby 7719 (CAS, RSA).

Bluffs, knolls, and depressions on rolling plains, in sand or gravelly clays, on shale, sandstone, and limestone, especially abundant on the red sands of the Chug- water formation, 4800-7400 feet, locally plentiful in the Rocky Mountains drained by the north and south forks of the Platte River, from near Denver, Colorado, northwest to the Laramie Plains, Wyoming, and north, becoming less common, to the North Platte near Casper.—Map No. 162.—Late May to July.

Astragalus tridactylicus (three-fingered, of the leaves) Gray in Proc. Amer. Acad. 6: 527. 1865.—"Near Boulder City, Colorado Territory, in dense tufts or mats, on gravelly knolls, Dr. C. C. Parry, 1864."—Holotypus, Parry 54, GH! isotypi, MO, NY, P, US!—Tragacantha tridactylica (Gray) O. Kze., Rev. Gen. 948. 1891. A. sericoleucus var. tridactylicus (Gray) Jones, Contrib. West. Bot. 10: 69. 1902. Orophaca tridactylica (Gray) Rydb. in Bull. Torr. Club 32: 668. 1905.

Astragalus tridactylicus fma. coloradoensis Gand. in Bull. Soc. Bot. France 48: xvi. 1902. —"Hab. Colorado, ad La Porte (C. Crandall)."—Holotypus, undated, LY! probable isotypus, collected by Crandall in 1895 at Fort Collins (a mile or two distant from Laporte), NY!

Astragalus tridactylicus fma. pallidiflorus (pale-flowered) Gand. in loc. cit. 1902 ("pallidiflora").—"A. Nelson exs. no. 4430 ... Hab. Wyoming, ad Laramie (A. Nelson)."—Holotypus, actually A. Nelson 4320, collected on the Laramie Hills, May 30, 1898, LY! isotypi, NY, RM!

The foothill orophaca, A. tridactylicus, is in some respects intermediate between the two preceding species in habit of growth, but is a coarser plant than either and has at least slightly and often substantially larger flowers. It embraces two rather marked but intergradient forms or incipient geographic varieties. The nomenclaturally typical phase, restricted to the foothills and piedmont plains between approximately five and six thousand feet from Denver north to the forks of the Cache-la-Poudre in Larimer County, Colorado, is a truly acaulescent, tufted plant with notably dimorphic leaves, those maturing with and after the inflorescence having longer leaf-stalks and longer leaflets of narrower outline than those preceding or following them. The flowers in this form tend to be large for the species and to be very shortly pedunculate, appearing radical. Only the outer ranks of imbricated stipules in the year’s cycle are pubescent on the back, the inner ones concealed by them being glabrous or nearly so except for a fringe of long hairs around the margin. On the Laramie Plains in Albany County, Wyoming, at seven thousand feet or a little over, the common orophaca is technically similar but often shortly caulescent, so that mature plants form matted growths many times wider than high; its leaves are uniform or nearly so, all shortly petioled and with comparatively short and broad leaflets; its flowers are all small, either subsessile or manifestly pedunculate, and the stipules are all pubescent dorsally, the outermost densely so. These, however, are ideally conceived contrasts which do not hold good for many individual specimens. The Laramie phase has been traced north in scattered stations to the North Platte Valley in Platte and Natrona Counties, where it descends to elevations of about 4800-5200 feet. It there combines a subacaulescent habit with small flowers either radical or pedunculate. It seems impossible to admit any dismemberment of A. tridactylicus into named varieties.

The foothill orophaca makes a lasting impression on anyone who has seen it in full flower near Laramie, when the mats of silvery foliage, enameled over with a profusion of small, vivid purple blossoms, blaze out in the clear mountain light against their background of cinnabar- colored sand. Seen thus, A. tridactylicus might be coveted by the rock gardener. The species has proved difficult, however, to establish in gardens and loses its brilliance in moister, darker climes.