Monographs Details: Astragalus cremnophylax Barneby
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:

310.  Astragalus cremnophylax

Diminutive, matted or somewhat mounded, essentially acaulescent but suf- fruticulose, with a gnarled, closely ramifying caudex, the ultimate divisions becoming columnar from a sand-impacted thatch of fibrous, decaying leaf-bases, the herbage silvery-strigulose with truly appressed, straight hairs up to 0.3—0.35 mm. long, mature plants forming low-convex cushions up to 2 dm. in diameter; stems of the year not over 5 mm. long, concealed by imbricated stipules; stipules deltoid- ovate, subobtuse, 1—3 mm. long, fully amplexicaul but free, the nearly glabrous or distally puberulent blades squarrose, becoming papery and brownish in age; leaves 3-9 (12) mm. long, with (3) 5-9 crowded, sometimes subpalmate, elliptic, obovate, or suborbicular, obtuse, thick-textured, folded leaflets 1—2.5 mm. long; peduncles subfiliform, up to 5 mm. long, reclinate in fruit; racemes 1-3 (commonly 2)-flowered, the flowers loosely ascending, the fruiting axis not over 2 mm. long; bracts membranous, ovate, 0.6-1 mm. long; pedicels ascending, nearly straight, at anthesis about 0.7 mm., in fruit about 1 mm. long; bracteoles 0; calyx 2.7-3.5 mm. long, strigulose with mixed black and white hairs, the subsymmetric disc 0.3-0.4 mm. deep, the campanulate tube 1.7-2.3 mm. long, 1.5-1.7 mm. in diameter, the subulate, obtuse teeth 0.8-1.4 mm. long; petals pale pinkish-lilac, the wings white-tipped, the banner purple-veined, the keel-tip maculate; banner recurved through 45°, broadly ovate- or suborbicular-cuneate, shallowly notched, 5.3-6.1 mm. long, 4.3-4.6 mm. wide; wings as long or a trifle shorter, the claws 1.9-2.1 mm., the ovate-elliptic, obtuse blades 3.5-4.4 mm. long, 1.6-2 mm. wide, both incurved but the left one more strongly so and its inner margin infolded; keel 3.7-4.3 mm. long, the claws 1.7-2 mm., the half-obovate or nearly half- orbicular blades 2-2.8 mm. long, 1.3-1.7 mm. wide, abruptly incurved through 100-120° to the bluntly deltoid, minutely porrect apex; anthers 0.3-0.45 mm. long; pod ascending (mostly humistrate), obliquely ovoid, 3-4 (4.5) mm. long, about 2.5 mm. in diameter, rounded at base, flattened or slightly depressed dorsally, carinate ventrally by the prominent, thick, convexly arched suture, contracted distally into a minute, declined, deltoid beak, the thinly fleshy, pale green or purple- dotted, densely strigulose valves becoming papery and stramineous, obscurely reticulate; ovules 4-6; seeds amber-yellow or pale orange, smooth or nearly so, dull, 1.4-1.8 mm. long.—Collections: 3 (i); representative: Jones (from El Tovar) in 1903 (CAS, POM, US); W. J. Dress 3270 (G).

Crevices of limestone pavement in the piñon belt, about 7050 feet, known only from one colony of plants on the South Rim of the Grand Canyon of the Colorado River, near El Tovar, Coconino County, Arizona.—Map No. 140.— May to early June.

Astragalus cremnophylax (from cremnos, gorge, and phylax, watchman, from the dramatic habitat) Barneby in Leafl. West. Bot. 5: 83. 1848.—"Arizona: ... on the south rim of the Grand Canyon ... west of El Tovar, Coconino Co. ... 3 June, 1947, Ripley & Barneby 8473."—Holotypus, CAS! isotypi, GH, NY, RSA, UTC!

Evolutionary convergence of distantly related groups of plants from a commonplace herbaceous or suffruticulose type into the mounded or densely matted growth-form of the polsterpflanz, so highly developed in the Andes, in Patagonia, and to less extent in North America, is well illustrated by the sentry milk-vetch, A. cremnophylax, and the preceding species, A. siliceus, when compared with the superficially similar cushion astragali here referred to sects. Sericoleuci, Drabellae, and Ervoidei. The pulvinate Humillimi appear to be derived from A. gilensis, or from a similar and recent common ancestor, and have retained nearly all basic features of flower, fruit, stipule, and hair-attachment; but by reduction of nearly all organs either in size or number or both, in response to different but similar sets of ecological pressures and favorable circumstances, they have come to resemble species of widely differing ancestry. In practice A. cremnophylax should be easily recognized, although one must be careful to distinguish the reduced small-leaved form of A. calycosus, readily identified by its larger flower with bidentate wings and oblong, compressed, bilocular pod, the one other (and much commoner) dwarf, silvery astragalus found along the Canyon rim.

The sentry milk-vetch is one of our rarest species. It was first discovered in 1903 by Marcus Jones, who mistook it for A. humillimus and reported it (1923, p. 82, Pl. 6, fig. 19) as "apparently common at the Grand Canyon ... on sandy ledges." Search in recent years has revealed, however, only a single group of perhaps a hundred plants confined to a strip of rock pavement not over fifty yards in length. They are scattered over the area, some in full sun and rooting in scarcely visible cracks of the limestone, others standing back among the piñons and rooting into sand-filled hollows of the rock. It is to be sought elsewhere in the region, perhaps on some of the buttes which stand out in isolation from the canyon wall.