Monographs Details: Astragalus troglodytus S.Watson
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:

307.  Astragalus troglodytus

Acaulescent or nearly so, with a tough, woody taproot and shortly forking caudex or knotty root-crown, the herbage green or gray-green beneath the pilose vesture, the leaflets glabrous or medially glabrescent above; stipules papery-membranous, several-nerved, 3—11 mm. long, semi- or fully amplexicaul but free, the lowest ovate and glabrous, the upper triangular-acuminate or broadly lanceolate, pilose or glabrescent dorsally, ciliate; leaves 3.5—11.5 cm. long, slender-petiolate, with (7) 11-19 obovate, obovate-cunate, rhombic, or broadly oblanceolate, subacute, obtuse and mucronulate, or rarely obtuse, flat or loosely folded leaflets 4—13 mm. long; peduncles scapiform, 4.5—13 cm. long, incurved-ascending at anthesis, straight, prostrate, radiating in fruit; racemes densely 10—30-flowered, the spreading and ascending flowers forming a globose or oval, headlike cluster 1.5-2.5 cm. in diameter; bracts submembranous, linear-lanceolate, 3—8.5 mm. long, at least the lowest ones reflexed; pedicels at anthesis 0.4—0.8 mm., in fruit not over 1 mm. long; bracteoles 0—2, setaceous when present; calyx 5.9—8.2 mm. long, thinly hirsute with lustrous, divaricate, and ascending hairs up to 1.1—1.8 mm long, the obconic disc 0.6—1.2 mm. deep, the tube at anthesis deeply campanulate, 3.4—5.2 mm. long, 2.3—2.9 mm. in diameter, the linear-lanceolate or setaceous teeth 1.9—3 mm. long, the tube becoming membranous, at first distended by the forming pod, ultimately ruptured, marcescent; petals dull reddish-purple, marcescent; banner recurved through about 45°, 9.5—11.4 mm. long, the cuneately tapering claw expanded into an oval-ovate, retuse or subentire blade 4.2-6.3 mm. wide; wings (a trifle shorter to 0.5 mm. longer than the banner) 9.2—11.2 mm. long, the claws 4.3-5.6 mm., the narrowly oblong, obtuse, gently and equally incurved blades 5.5-6.8 mm. long, 1.6-2.1 mm. wide; keel 7.9-9.1 mm. long, the claws 4.1-5.6 mm., the lunately half-oval or -obovate blades 3.3-4.1 mm. long, 1.9-2.5 mm. wide, incurved through ± 100° to the blunt apex; anthers (0.35) 0.4-0.55 mm. long; pods spreading and ascending in oblong or globose, headlike clusters, plumply ovoid or subglobose, 5-7 mm. long, 4-4.5 mm. in diameter, rounded at base, abruptly contracted distally into a triangular, cuspidate, declined beak 0.8-1.5 mm. long, somewhat turgid, terete or a trifle compressed laterally, carinate ventrally by the thick suture, the thinly fleshy, green or purple-tinged, sparsely long-hirsute valves becoming firmly papery, stramineous or brownish, rugulose-reticulate; ovules 8-12; seeds (not seen fully ripe) ± 1.5—1.8 mm. long.—Collections: 18 (iii); representative: A. & R. Nelson 2094 (NY, RM); Loomis 6927 (ARIZ, NY); Deaver 1147 (CAS, NY), 2824 (CAS); Ripley & Barneby 4897 (CAS, RSA).

Dry slopes and flats in yellow pine forest, in poor stony soil derived from scoriae and rotted pine needles, 6500-7000 feet, very local, Flagstaff Plateau and adjacent Mogollon Rim, Coconino County, Arizona.—Map No. 140.—May to August.

Astragalus troglodytus (cave dweller, from the type-locality) Wats. in Proc. Amer. Acad. 20: 362. 1885.—"In the San Francisco Mountains, Arizona, near the "Cliff-dwellers’ Ravine," Mr. and Mrs. J. G. Lemmon, August, 1884."—Holotypus, Lemmon 3225, GH!— Cnemidophacos troglodytus (Wats.) Rydb. in N. Amer. Fl. 24: 288. 1929.

In the sum of its peculiar technical characters the cliff-dwellers’ milk-vetch, A. troglodytus, stands apart as one of those detached, highly distinctive monotypes in which the genus is so rich. However its general habit of growth, its large, imbricated stipules, and especially the form and arrangement of the small pods in dense, humistrate heads express its near kinship with A. gilensis. These two species are found in almost identical environments in the Mogollon pine forests and are vicariant in distribution. Around Flagstaff A. troglodytus is sometimes associated with the subacaulescent A. castaneiformis, which is easily distinguished by its dolabriform vesture, longer and narrower flowers of a pallidly lilacine or luridly purplish hue, and much larger pods arranged in loose, short racemes of ten or less. No other North American Astragalus has flowers of quite the same reddish-purple tint that I have seen in A. troglodytus. Unfortunately the color fades rapidly in most pressed specimens.

The cliff-dwellers’ milk-vetch has been reported (Kearney & Peebles, 1951, p. 458) to extend north to the Kaibab Plateau, but this is probably a mistake. Specimens so labeled by Dr. Kearney have turned out to be the superficially similar Oxytropis oreophila Gray.