Astragalus Rusbyi

  • Title

    Astragalus Rusbyi

  • Authors

    Rupert C. Barneby

  • Scientific Name

    Astragalus rusbyi Greene

  • Description

    29. Astragalus Rusbyi

    Slender perennial, with a shortly forking caudex at or just below soil-level, thinly strigulose with straight, filiform, appressed or subappressed hairs up to 0.35-0.7 mm. long, the stems glabrous or nearly so toward the base, the herbage green, the leaflets glabrous above; stems several, erect and incurved-ascending, 1.5—3.5 (4) dm. long, leafless at base, commonly branched or spurred at 1-4 nodes preceding the first peduncle; stipules 1.5—5.5 mm. long, dimorphic, those at the lowest nodes commonly the smallest, papery-membranous, amplexicaul and connate into a cuplike, subentire or obtusely bidentate sheath, the median and upper ones decurrent around about half the stem’s circumference, or some of them united by a low collar or stipular line; leaves (2.5) 3-8 cm. long, all but the lowest subsessile, with (11) 17-25 oval-oblong, oblanceolate, narrowly obovate, ovate, or (in some upper leaves) narrowly oblong-elliptic, obtuse, truncate, or shallowly retuse (and then often apiculate), flat or loosely folded leaflets (3) 4-11 mm. long, peduncles erect or incurved-ascending, (2) 4—11 cm. long, the principal ones longer than the leaf; racemes very loosely or remotely (10) 15-25-flowered, the flowers early spreading and nodding at full anthesis, the axis elongating, (2.5) 6-18 cm. long in fruit; bracts membranous, ovate-acuminate or lanceolate* 1-2.5 mm. long, pedicels at anthesis ascending and abruptly arched outward or hooked at apex, 0.8-1.8 mm. long, in fruit decurved or deflexed, a trifle thickened, (1.5) 2—3.2 mm. long, tardily disjointing with the fruit; bracteoles 0, rarely a minute scale; calyx 3.2-4.1 mm. long, loosely strigulose with black, fuscous, or largely white hairs, the subsymmetric disc 0.5—0.8 (1) mm. deep, the campanulate tube 2.1—2.6 mm. long, 1.8—2.3 mm. in diameter, the subulate or triangular-subulate teeth 0.8-1.6 mm, long, the whole becoming papery, marcescent unruptured; petals whitish, sometimes tinged with lavender; banner abruptly recurved through ± 90°, ovate-cuneate, shallowly notched or subentire, 6.3-7.2 mm. long, 3.5-4.5 mm. wide; wings 0.2-0.9 mm. longer than the banner, 6.9-8 mm. long, the claws 2.22.7 mm., the linear- or lance-oblong, or linear-elliptic, obtuse or bidentate blades 5-5.8 mm. long, 1.2-2 mm. wide, both incurved but the left one more abruptly and further than the right; keel 4.6-5.5 mm. long, the claws 2.1-2.6 mm., the half- orbicular or obtusely triangular blades 2.9-3.3 mm. long, 1.5-2 mm. wide, abruptly incurved through 100° to the blunt apex; anthers 0.3-0.4 mm. long; pod pendulous, stipitate, the straight, slender stipe 2.2-5 mm. long, the body linear- oblong or -ellipsoid, 1.3-2.2 cm. long, 3.2-4.1 mm. in diameter, straight or obscurely decurved, cuneate or cuneately attenuate at base, shortly cuspidate at apex, obscurely trigonous, with low-convex lateral and flattened or shallowly and openly sulcate dorsal faces, the thin, pale green valves minutely strigulose with black or mixed black and white hairs, becoming papery, stramineous, inflexed as a nearly complete septum 0.8—1.5 mm. wide; dehiscence apical and downward through the ventral suture; ovules 7—13; seeds not seen ripe.—Collections: 12 (i); representative: Jones 3946 (NY, POM); Schallert (from Navajo Ordnance Depot) in 1943 (CAS, NY); MacDougal 233 (NY); Bacigalupi 551 (DS).

    Meadows in yellow pine forest, or at the edge of thickets and aspen groves, in dry or temporarily moist basaltic soils, mostly 7000-8000 feet, local but forming colonies, Flagstaff Plateau and foothills of the San Francisco Peaks, descending along Oak Creek to 5400 feet, Coconino and perhaps immediately adjoining Yavapai County, Arizona.—Map No. 9.—June to September.

    Astragalus Rusbyi (Henry Hurd Rusby, 1855-1940, botanist, physician, collector of medicinal plants, organizer of the Herbarium of N. Y. College of Pharmacy, now at NY) Greene in Bull. Calif. Acad. Sci. 1: 8. 1884.—"On Mt. Humphreys, in the northern part of Arizona, collected July 2, 1883, by Mr. H. H. Rusby."—Holotypus, Rusby 573, CAS! isotypi, ND, NY, ORE, PH, US, WS!—Atelophragma Rusbyi (Greene) Rydb. in Bull. Torr. Club 55: 162. 1928.

    The Rusby milk-vetch and the two species next in order, A. Egglestonii and A. longissimus, are closely related, sharing all important technical features and a common general facies. They are rather tall, slender plants of xeric pine forest, with small, loosely or remotely and somewhat virgately racemose flowers giving rise to pendulous, stipitate, almost fully bilocular pods of linear-oblong outline and more or less trigonous compression. Like most Strigulosi they flower from midsummer onward into the fall months and vary in vigor and abundance in proportion to the amount and timing of the summer rains. These three astragali differ from each other in relatively minor characters of vesture, length of stipe, and number of leaflets; they might almost as well be interpreted as varieties of a collective species. Each, however, is apparently confined to a small, well-defined, disjunct and eminently natural area of dispersal, and there is no hint of intermediate populations. The trio of species was treated by Jones (1923, p. 186-7, Pl. 42. 77) as comprising a large part of A. strigulosus sensu Jones (non H. B. K.), an ambiguously comprehensive and obviously misnamed concept which included A. altus and probably A. hidalgensis or some other central Mexican forms. It is clear from Jones’s synonymy and figures that his typical A. strigulosus is equivalent approximately to our A. longissimus; his (but not Hemsley’s) var. brevidentatus is essentially A. Rusbyi with a small, misplaced element of A. altus; while his (but not Hemsley’s) var. gracilis, supposedly "less pubescent" and "the common form northward," was probably our A. Egglestonii. The last mentioned was described by Rydberg as a species of Tium, implying a pod of thicker texture than in related species which he referred to Atelophragma; but there is no real difference in this respect.

    Although much further removed geographically from A. longissimus, in Chihuahua, than from A. Egglestonii which enters eastcentral Arizona from New Mexico, the Rusby milk-vetch resembles the Mexican species the more closely, differing in its greener, thinly pubescent herbage, fewer leaflets, and rather more slender pod. The pod of A. Egglestonii differs in being hairless and borne on a shorter stipe only 1.5-2.5 mm. long.