Astragalus gracilis

  • Title

    Astragalus gracilis

  • Author(s)

    Rupert C. Barneby

  • Scientific Name

    Astragalus gracilis Nutt.

  • Description

    44. Astragalus gracilis

    Slender, sparsely leafy, strigulose nearly throughout with fine, straight, appressed or a few narrowly ascending hairs up to 0.3-0.5 mm. long, the stems and herbage greenish or cinereous, the leaflets glabrous or nearly so and brighter green above; stems few or several, 1.5-4 dm. long, diffuse, ascending, or sometimes erect, simple and subterranean for a space of 1-7 cm., commonly branched at the first emersed nodes, the branches divaricate and ascending, passing upward into spurs or directly into peduncles, less often simple throughout; stipules (1) 1.5-4 mm. long, dimorphic, those at the buried and lower aerial nodes papery-membranous or early becoming so, amplexicaul and connate into a truncate or bidentate sheath or low collar, the median and upper ones firmer, often herbaceous, progressively less united upward, the uppermost free or united by a stipular line, with deltoid, triangular, or triangular-caudate, mostly erect blades; leaves 2.5-7 cm. long, all shortly petioled or the uppermost subsessile, with 9-17 linear, Unear-oblong, narrowly oblong, oblanceolate, or cuneate-oblong, commonly retuse but when narrow truncate to obtuse, flat or involute leaflets (3) 5-20 (25) mm. long; peduncles erect, ascending and incurved, or divaricate, 3-9 cm. long; racemes loose or early becoming so, (6) 12-40 (55)-flowered, the flowers ascending and then horizontal or at length declined, the axis elongating, (1.5) 3—13 cm. long in fruit; bracts submembranous, ovate or subulate, 0.7-1.5 mm. long; pedicels at anthesis ascending, 0.7—1.1 mm. long, in fruit arched out- and downward or horizontally spreading, 1-1.8 mm. long; bracteoles 0-2, minute when present; calyx 2.1-3.3 mm. long, strigulose with white or rarely a few black hairs, the subsymmetric disc 0.4-0.7 mm. deep, the campanulate or turbinate-campanulate tube 1.5-2.7 mm. long, 1.6—2.4 mm. in diameter, the deltoid or triangular-subulate teeth 0.4—0.9 mm. long, the ventral pair broadest, sometimes broader than long; petals pale lilac or purplish, the banner often purple-veined; banner recurved through ± 45°, ovate- cuneate, 5.3-8.4 mm. long, 4-5.8 mm. wide; wings as long or a trifle shorter, the claws 1.5—2.7 mm., the obliquely elliptic or obovate, obtuse blades 4—5.7 mm. long, 1.6—2.5 mm. wide, both incurved but the left one more abruptly so and its inner margin infolded; keel 3.7—6.1 mm. long, the claws 1.6—2.9 mm., the half- obovate or nearly half-circular blades 2.4-3.8 mm. long, 1.6-2.2 mm. wide, abruptly incurved through 95-130° to the rounded apex; anthers 0.3-0.45 mm. long; pod declined or deflexed, or from decumbent branches horizontally spreading, sessile, obliquely suborbicular, plumply ovate, or ovate-elliptic in dorsal view, (4) 4.3-8 (9) mm. long, 2.2—3.6 mm. in diameter, obtuse at base, abruptly cuspidate at apex, obcompressed, the dorsal face either flattened or openly and shallowly sulcate, the ventral face convex, carinate by the prominent, thickened suture, the somewhat fleshy, densely strigulose or minutely villosulous valves becoming brown or stramineous, stiffly leathery and rigid, transversely rugulose-reticulate; dehiscence tardy; ovules 5-9; seeds brown or greenish-brown, smooth or minutely pitted, 2.3-3.3 mm. long.—Collections: 104 (viii); representative: C. L. Hitchcock 15,868 (NY, RSA, SMU, WS); C. L. Porter 3698 (NY, SMU, TEX); E. Nelson 3388 (NY); Rydberg & Vreeland 5982 (NY); Ripley & Barneby 10,568 (RSA).

    Sandy bluffs, barren hilltops, dunes, and gullied washes on the prairies, sometimes on shale outcrops, on gypsum beds, or on dry gravelly hillsides among junipers, (1300, northward) 2800-6500 (7250) feet, widespread and locally common on the Great Plains and west into the foothill valleys and tablelands of the Rocky Mountain piedmont, from southwestern North Dakota to the upper Missouri and Yellowstone Valleys in southcentral Montana, south to western Oklahoma, to the headwaters of the Colorado River in western Texas, and to the upper Canadian River in northeastern New Mexico; apparently isolated on the Saskatchewan River between Prince Albert and Rosthern, Saskatchewan.—Map No. 17.—May to July.

    Astragalus gracilis (slender) Nutt., Gen. 2: 100. 1818.—"From White River to the Mountains, on the plains of the Missouri."—Nuttall cited his own A. gracilis of Fraser’s Catalogue, a nomen nudum, and Dalea parviflora Pursh, of which he had seen a spm. in herb. Lambert. The name A. gracilis is therefore best treated as a legitimate substitute for Dalea parviflora (small-flowered) Pursh, Fl. Amer. Sept. 474. 1814.—"On the banks of the Missouri."—Holotypus, labeled in Pursh’s hand "Dalea parviflora. Mr. Nuttall," PH! isotypus, labeled "D. parviflora. Herb. Lambert.," NY!—The Nuttall spms. from the Platte or the Rocky Mts. (BM, K, NY) probably date from 1834.—Tragacantha parviflora (Pursh) O. Kze., Rev. Gen. 941. 1891. Astragalus parviflorus (Pursh) MacMill., Metasp. Minn. Vail. 325. 1892; non Lamk., 1783. Microphacos parviflorus (Pursh) Rydb. in Bull. Torr. 40: 51. 1913. Astragalus Microphacos (the generic name) Cory in Rhodora 38: 405. 1936. A. gracilis var. parviflorus (Pursh) Gates in Trans. Kans. Acad. Sci. 42: 137. 1940.

    Astragalus gracilis (5 erectus (erect) Hook. in Lond. Jour. Bot. 6: 210. 1847.—Upper Platte ... [Geyer] n. 223."—Holotypus, labeled "Missouri & Oregon, 1845.," K! isotypi, BM, G!

    Astragalus parvifolius Nutt, ex Gray in Proc. Amer. Acad. 6: 202. 1894, in syn., nomen. —This name was taken from Nuttall’s specimen labeled ''''Astragalus *parvifolius. Missouri.," PH! (Non Phaca parvifolia Nutt.).

    Astragalus microlobus (with small pods) Gray in Proc. Amer. Acad. 6: 203. 1864.— "Plains of Nebraska, &c., to the Rocky Mountains."—Under this proposition Gray listed Nuttall specimens of A. gracilis at PH and in herb. Torrey.; Parry 189; and Hall & Harbour 119. The last mentioned, which provided the critical fruiting character, is chosen as lectotypus, GH! isotypus (fragm.), NY!—Tragacantha microloba (Gray) O. Kze., Rev. Gen. 946. 1891. Microphacos microlobus (Gray) Rydb. in Bull. Torr. Club 32: 663. 1905. A. parviflorus var. microlobus (Gray) Jones, Rev. Astrag. 193. 1923.

    The slender milk-vetch, A. gracilis, is quite variable in length and compression of the pod, though scarcely more so than related Scytocarpi, and for many years two specific or varietal taxa, under changing names and combinations, have been recognized within it. At the small extreme the fruit is about 5 mm. long, shallowly excavated dorsally and thus inversely boat-shaped, and the transverse reticulations are so closely crowded together that the surface of the ripe valves becomes sharply rugulose. Of equally common occurrence is a pod up to 8 or even 9 mm. in length, flattened but not or only obscurely excavated dorsally, and here the nerves, although as many and equally prominent, are separated by wider intervals, and the valves are not more than strongly reticulate. The depth of the dorsal sulcus and the prominence of the cross-ribs seem to be functions of the pod’s size rather than inherent qualities, and transition from one extreme condition to the other is gradual and complete. Those who have professed to see two entities within A. gracilis have emphasized several supporting characters. Gray associated the boat-shaped pod, under the epithet gracilis, with tiny flowers (± 6 mm. long) disposed in dense, many-flowered racemes and linear leaflets 1.5-2 cm. long; and the dorsally flattened pod, under A. microlobus, with larger flowers (± 8 mm. long) disposed in shorter and looser racemes and shorter leaflets of linear-oblong outline. Jones added to these differential characters the claim that A. parviflorus ( = A. gracilis sensu Gray) had only 5-9 leaflets as opposed to 9-17 in "var. microlobus." Rydberg repeated these statements in slightly modified form, but treated the plant with larger flowers and fruits as Pisophaca gracilis. Unfortunately the form of the fruit is not consistently correlated with any other character mentioned. Frequently a small, boat-shaped pod is combined with broad leaflets variable in length and number. Relatively large flowers may give rise to small, rugulose- reticulate pods. The racemes vary greatly in length, but independently of flower-size, although there is a strong tendency for the longer racemes to be smaller-flowered. It is a relevant fact that modern authors dealing with the complex have assigned approximately the same range to both species, a range over poorly differentiated prairie country where genetical separation of two admittedly close relatives is unlikely, unless there exists an effective mechanism to prevent cross-breeding. Yet if two species exist, they must hybridize freely to produce so many individuals of doubtful identity. Welsh (1960, p. 154) suggested that the form with short raceme and larger pod arose through introgressive hybridization between an original tall and many- flowered A. gracilis and the related A. flexuosus. But some of these forms are found so far outside the known range of A. flexuosus that the hypothesis is not convincing. It is probably correct to interpret the various states as individual variations of no taxonomic importance and to accept A. gracilis as genetically unstable and inherently variable. If this view is accepted, it will be academic to trace back through the literature the shifting estimates of Nuttall’s several collections on which the epithets gracilis, parviflorus, and parvifolius ultimately rest.