Monographs Details: Astragalus nuttallianus var. micranthiformis Barneby
Authors:Rupert C. Barneby
Authority: Barneby, Rupert C. 1964. Atlas of North American Astragalus. Mem. New York Bot. Gard. 13(2): 597-1188.

333b. Astragalus Nuttallianus var. micranthiformis

Habit and general appearance of var. Nuttallianus, the herbage dark green, thinly pilosulous with narrowly ascending or subappressed hairs up to 0.45-0.7 (0.8) mm. long, the leaflets glabrous or nearly so above; stems erect, ascending, or prostrate, 3-18 (25) cm. long; leaves 1.5—4.5 (6.5) cm. long, with (7) 9-15 leaflets 2—10 mm. long, those of the lower leaves obovate or oblong, retuse or emarginate, or obcordate, those of the upper leaves mostly longer, oblanceolate, elliptic, or linear-oblong, all or at least the lateral ones obtuse to subacute, the terminal one sometimes emarginate; peduncles (1.5) 2.5—5.5 (6.5) cm. long; racemes loosely (1) 3-7-flowered, the axis early elongating, (0) 0.5-2 cm. long in fruit; calyx 3.4—4.7 mm. long, loosely strigulose with mixed black and white, all black, or all white hairs up to 0.4—0.7 mm. long, the tube 2—2.8 mm. long, (1.5) 1.7-2.2 mm. in diameter, the teeth (1.2) 1.5-2.2 (2.5) mm. long; petals bright purple, the banner with a large pale eye; banner 6.3-7.6 mm. long, 4-6 mm wide; wings 4.9-6.5 mm. long; keel 4.5-5.9 mm., the claws 1.8-2.3 mm., the half-obovate blades (2.6) 3-4.1 mm. long, 1.6-2 mm. wide, abruptly incurved through 90-95° to the blunt apex; pod spreading or declined, sessile or nearly so, the body (1.2) 1.4-2 cm. long, 2.1-3.3 mm. in diameter, evenly and gently incurved through about ¼-½ circle, the glabrous or minutely strigulose valves inflexed as a complete septum 1.3-1.8 mm. wide; ovules 13-17.—Collections: 30 (vii); representative: Holmgren 3186 (NY, RSA, TEX, WS, WTU); Crandall 784 (NY, WS); Eastwood 5962 (CAS); Ripley & Barneby 5293 (CAS, RSA), 7559 (RSA, UTC), 8485 (RSA).

Dry sandy or stony hillsides, sandy talus beneath cliffs, rocky canyon washes, sometimes on dunes, commonly with piñon and juniper, mostly on sandstone or limestone, widespread, mostly between 4200 and 6200 feet, in the south and southeast parts of the Colorado Basin, from Grand Canyon (where descending to 3500 feet), Arizona, to Green River, Utah, and southwestern Colorado, extending less frequently to the Rio Grande Valley in northcentral New Mexico, and (perhaps interruptedly) thence northeast to the Arkansas River in southeastern Colorado. —Map No. 150.—April to June.

Astragalus Nuttallianus var. micranthiformis (resembling A. micranthus Nutt. = var. Nuttallianus), var. nov. inter var. Nuttallianum et var. austrinum quasi intermedia, illi propior, a prima dentibus calycinis brevioribus, racemo fructifero laxo, foliolis dimorphis (saltem lateralibus foliorum superiorum acutiusculis nec retusis), et legumine valde arcuato, a secunda et affinibus racemo laxo carinaque abrupte incurva obtusissima dissimilis.—Colorado: sand in the piñon belt, 5900 ft. Towaoc, Montezuma County, May 29, 1947, Ripley & Barneby 8390.—Holotypus, CAS! isotypus, RSA!

The first collection of var. micranthiformis from the Colorado Basin, Crandall’s plant from Grand Junction, Colorado (1898, cited above), was referred by Rydberg (1927, p. 326) to Hamosa Nuttalliana, our var. Nuttallianus, which was not known otherwise to extend west of Oklahoma and central Texas. Possibly Rydberg interpreted the occurrence as a chance introduction, but we now possess many examples of a similar plant from the Four Corners country and upper Rio Grande in New Mexico, unquestionably native, which match Crandall’s plant in the deep green, thinly pubescent foliage, several or many (but never all) retuse or truncate-emarginate leaflets, and obtusely rounded keel-tip. The var. micranthiformis does resemble var. Nuttallianus in general mien and foliage, but its pod is ordinarily incurved its whole length (and not only at base) through ¼ to nearly ½ circle, the calyx-teeth at least relative to the tube are decidedly shorter, and the raceme elongates substantially as the flowers fade. The variety is most strongly marked in the drainage of the Colorado River upstream from Grand Canyon, in which area it is the only form of A. Nuttallianus known to occur. A collection from the Arkansas Valley in Crowley County, southeastern Colorado (Ripley & Barneby 8301, RSA) is geographically and morphologically intermediate between var. micranthiformis and var. Nuttallianus, having the calyx nearly of the latter, but the pod and leaflets of the present form. Immediately south of the Mogollon Escarpment in Gila County, Arizona, forms of A. Nuttallianus intermediate between var. micranthiformis and var. austrinus are apparently not uncommon. For example, Eastwood 16,876 (CAS) combines an obtuse keel-tip with leaflets all elliptic, and Barneby 12,647 (RSA) a subacute keel-tip with retuse leaflets. Since var. micranthiformis evidently intergrades with both var. Nuttallianus and var. austrinus, and at the same time has a blunt keel-tip in common with var. macilentus, it serves to link together the three principal morphological types in the specific complex and is thus crucial to the taxonomy of the group.

Although only recently recognized as a distinct form, var. micranthiformis was first collected as early as 1847 by Fendler (No. 165, MO) on the banks of the Rio Grande near Santa Fe.