Astragalus desperatus var. desperatus

Flowers small, the banner 7.4-10.5 (11) mm. long, 5-7.5 mm. wide; wings 6.7-9.5 mm. long, the claws 2.8-4 mm., the blades 5-6.6 mm. long, 2.2-2.6 mm. wide; keel-claws 2.8-4 mm., the blades 3.5-4.5 (5) mm. long, 2.1-2.5 mm. wide; anthers 0.4-0.5 mm. long; pod 6-15 mm. long, 3-6 mm. in diameter.—Collections: 36 (xii); representative: Rydberg & Garrett 8428 (NY); Holmgren & Tillett 9445 (NY, RSA); McVaugh 14,572 (CAS, NY); Wm. A. Weber 3757 (TEX, WS); Barneby 12,668, 12,759 (CAS, RSA), 13,098, 13,145 (CAS, NY, RSA).

Rock ledges, crevices of rimrock pavement, and sandy detritus on boulder- strewn slopes under cliffs or at the foot of buttes, 4100-5600 feet, with piñon and juniper, on red or white sandstone, locally plentiful in the Colorado Basin south of Tavaputs Escarpment from near Grand Junction, Colorado, west to the San Rafael Swell, Utah, south, but becoming less common, to the foothills of the Kaibab and Moenkopi Wash in Coconino County, Arizona.—Map No. 96.—April to June.

Astragalus desperatus (despairing, in reference to Jones’s state of mind in searching for an epithet unoccupied in Astragalus: an exaggerated dilemma) Jones in Zoë 2: 243. 1891. —"This grows in sand along or near Grand River, in eastern Utah. Collected May 2, 1890."— Holotypus, collected by Jones at Cisco, Grand County, Utah, POM! isotypi, GH, RM, TEX, UC, US, WS!—Tium desperatum (Jones) Rydb. in Bull. Torr. Club 32: 660. 1905. Batidophaca desperata (Jones) Rydb. in N. Amer. Fl. 24 : 319. 1929. Astragalus desperatus var. typicus Barneby in Leafl. West. Bot. 5: 87. 1948.

Astragalus desperatus var. petrophilus (rock-loving) Jones, Rev. Astrag. 204, Pl. 45. 1923. —" ... in the San Rafael Swell region, Utah ... "—Holotypus, collected by Jones in the San Rafael Swell, May 17, 1914, POM! isotypus, NY!—Batidophaca petrophila (Jones) Rydb. in N. Amer. Fl. 24 : 320. 1929.

The common form of the rimrock milk-vetch, var. desperatus, is a neat little plant, notable for its tufted growth-habit, slender, wiry petioles from which the leaflets disjoint readily when dry, subscapose peduncles, and especially the small, deflexed, commonly much incurved, nearly always gaily mottled pod of papery texture beset with long, spreading, lustrous hairs minutely but always perceptibly enlarged at base. The small flowers are commonly bicolored, the white or pallid wing-tips contrasting with a purple banner and keel. The species is very characteristic of the sandstone butte and badland country of the Colorado Basin. It is usually found where its eventually woody taproot can strike down into a crevice of sandstone bedrock, thus often on rock pavement or on detrital slopes under cliffs with only a thin mantle of clay or wind-blown sand masking the rocky nature of the subsurface materials. The flowers vary somewhat in size; there is a series of variants connecting the small-flowered extreme exemplified by the typus of var. petrophilus with a normal or average state.

Over much of its range in Arizona and extreme southern Utah, var. desperatus is sympatric with A. sabulonum, a monocarpic species found on dunes and sandy valley floors where the rimrock milk-vetch cannot be expected to occur. Similar in many technical features, they have often been confused, but A. sabulonum is distinguished by its developed stems (short only in obvious seedlings), villosulous herbage, fewer (mostly 2-5) and slightly smaller flowers, and by the pod which, although similar in shape and structure, is more stiffly papery and merely hirsutulous (not lustrously hirsute) with much shorter hairs. Note must be made of A. pubentissimus, of which the papery-membranous, lustrously hirsute pod is similar to that of A. desperatus in orientation and structure; this species extends rarely south of Tavaputs Escarpment just into the range of A. desperatus. It is a coarse annual or short-lived perennial resembling A. sabulonum in its developed stems and type of vesture, and differs further in having a pod on the average longer (1.2-2 cm.) and fewer-ovulate, and beset with hairs not at all thickened at base.