Astragalus atratus

Title

Astragalus atratus

Authors

Rupert C. Barneby

Scientific Name

Astragalus atratus S.Watson

Description

138. Astragalus atratus

Low, diffuse, often wiry and sparsely leafy, strigulose nearly throughout with straight, appressed and subappressed hairs up to 0.3-0.6 mm. long, the herbage green, cinereous, or canescent, the leaflets equally pubescent on both sides or (more commonly) glabrescent to glabrous above; stems numerous, slender, radiating and weakly ascending (often supported by sagebrush), 3-30 cm. long, simple or bearing spurs or branchlets at one or more nodes preceding the first peduncle, together forming loose tufts or mats; stipules (1) 1.5-8 mm. long, dimorphic, the lowest ones approximate and often loosely imbricated, mostly ovate or broadly lanceolate and obtuse, papery-scarious, conspicuously adnate to the petiole-base, decurrent around half to the whole stem’s circumference, the median and upper ones narrower and shorter, with triangular-lanceolate, subherbaceous blades; leaves (1.5) 3-14.5 cm. long, with slender petiole and 7-15 distant or remote, narrowly oblong, or in some lower leaves oval-oblong, or in all leaves of some plants linear-elliptic or setaceous, obtuse, subacute, or (when broad) emarginate, commonly folded or involute leaflets 1.5—17 mm. long; peduncles weakly incurved-ascending, slender or filiform, (1.5) 3.5-16.5 cm. long, a little shorter to much longer than the leaf; racemes loosely or remotely (4) 5—16-flowered, the flowers nodding, the axis elongating, (1.5) 3-17 cm. long in fruit; bracts membranous, ovate or lanceolate, (0.8) 1-2.5 mm. long; pedicels very slender or filiform, (1.5) 2-4.7 mm. long at anthesis, in fruit straight and ascending at a wide angle or spreading, or ± contorted, or abruptly refracted, 2.5-6.5 mm. long; bracteoles 0, if present minute; calyx 3—7.6 mm. long, strigulose with black, white, or mixed black and white hairs, the subsymmetric disc (0.5) 0.7 1.1 (1.5) mm. deep, the campanulate tube 2.3-4.6 mm. long, 1.7-3.6 (3.8) mm. in diameter, the subulate teeth 0.7—2.2 mm. long; petals whitish or pale lilac; banner 6.3-13.4 mm. long, the shortly cuneate claw expanded into an oblong or ovate-oblong, deeply retuse blade 4—8.6 mm. wide, this pinched in at the middle in the form of two pocket-like folds and thus often appearing fiddle-shaped; wings (0.6-2 mm. longer) 7.4-14.4 mm. long, the claws 2.8-5.7 mm., the obliquely oblong-oblanceolate, obliquely obovate, or half-obovate blades 5.2-9.8 mm. long, gradually but strongly incurved into the rounded and erose, emarginate, or ± deeply 2-dentate or -lobed apex; keel 5.9-10 mm. long, the claws 2.9-5.6 mm., the obliquely half-obovate or triangular blades 3.5-5.2 mm. long, 2-3.2 mm. wide, abruptly incurved through 95-120° to the rather sharply deltoid, sometimes obscurely porrect apex; anthers 0.4—0.65 (0.8) mm. long; pod loosely pendulous, stipitate or subsessile, the stipe when present concealed by the marcescent calyx but up to 2 mm. long, the body linear-oblong, linear-oblanceolate, or narrowly elliptic in profile, (1) 1.2-2.2 cm. long, (2.3) 2.5-4.3 mm. in diameter, tapering at base into the stipe, ± abruptly cuneate at apex, gently decurved, or straight but the thick, salient ventral suture even then more convex than the dorsal one and the tip declined, variably compressed, uni- or bilocular, the slightly or scarcely fleshy, green, purplish, or mottled valves becoming leathery or papery, faintly reticulate, strigulose; ovules 10-29; seeds (little known) black, pitted or rugulose, ± 2-2.5 mm. long.

The picture of racial modification in A. atratus is complex, but its main outlines are clear enough. The species has differentiated dichotomously into northern and southern branches divergent from the physical barrier of the Snake-Humboldt water parting in northeastern Nevada, the populations found on this line and northward having subentire wing-petals and unilocular pods; to the south the wings are deeply toothed or lobed at apex and the pod is bilocular or nearly so. Within each area the leaflets vary from linear-filiform to oblong, leaflets of a narrow type being usually associated with an elongate, very slender raceme-axis, filiform pedicels, and a generally attenuated habit of growth. Furthermore, the smallest flowers commonly coincide with linear leaflets, but the correlation of these features is imperfect. The leaflet-variants of var. atratus in Nevada have been described as varietally distinct, but the extreme states are rare and intermediates much commoner. The stenophyllus form becomes predominant on the headwaters of the Humboldt where it approaches the range of var. owyheensis. Populations in this area of transition provide examples of a bilocular pod combined with the growth-habit of var. owyheensis and of an unilocular pod combined with jointed terminal leaflets, thereby confirming Jones’s evaluation of A. owyheensis as a geographic variant of A. atratus. In the northern area the leaflet-forms are more sharply differentiated, and it seems proper to separate var. inseptus, a relatively coarse and leafy plant with jointed terminal leaflets and leathery pods, from the extremely delicate var. owyheensis with decurrent terminal leaflets and pods of papery texture.

The var. mensanus is isolated from the rest of its species in the northern Mohave Desert and is distinguished by a laterally compressed pod devoid of dorsal groove. It may possibly deserve the rank of species, but scarcely differs from var. atratus except in this one respect.