Psorothamnus fremontii

  • Title

    Psorothamnus fremontii

  • Author(s)

    Rupert C. Barneby

  • Scientific Name

    Psorothamnus fremontii (Torr. ex A.Gray) Barneby

  • Description

    5.  Psorothamnus fremontii (Torrey) Barneby

    (Plate IV)

    Low straggling or erect, irregularly branching shrubs up to 3-10 dm tall, stiff but hardly thorny (spinose branchlets at nodes preceding the racemes rare or 0), the raceme-axes persistent but not spinose, the often tortuous old stems pallid or whitish, the young twigs and foliage eglandular or almost so, strigulose with appressed or narrowly ascending white hairs up to 0.2-0.35 (0.4) mm long, the foliage ashen or silvery, the leaflets often more densely pubescent above than beneath, their lower face punctate, the inflorescence-axis charged with prickle-shaped glands; leaf-spurs up to 1 mm long; stipules subulate or subulate-attenuate, subglandular, brown or livid, 0.7-1.2 mm long; intrapetiolular glands 0; post-petiolular glands small; leaves alternate along young flowering shoots and fascicled on inhibited short-shoots of old wood, all peti- oled, 1-4 cm long, with 3-7 (9) leaflets varying in outline according to varietas from narrowly obovate to linear, 3-25 mm long, all sessile or nearly so, either all jointed to the rachis, or (especially in leaves low on the branchlets or fascicles) either the terminal one alone or the last 3 (rarely all) decurrent and ± confluent, their thickened margins revolute; peduncles 0-1 (2) cm long; racemes loosely (5) 10-25 (30)-flowered, the axis becoming 2-9 (11) cm long; bracts papery, brownish, 1.2-2 mm long, early deciduous; bracteoles similar but smaller, 0.4-1.7 mm long; pedicels 0.6-1.4 mm long; calyx (5) 5.4-7.7 mm long, the hypanthium 1.2-1.9 mm deep, the glabrous or strigulose tube (2.8) 3-3.7 (3.9) mm long, the internally silky, lanceolate or triangular- lanceolate teeth unequal, the ventral pair 2.2-3.8 (4.1) mm long, varying from 1.1 mm shorter to 0.6 mm longer than tube, each at base (1.2) 1.4-2.1 mm wide, nearly twice wider than the divergent, ultimately reflexed dorsal tooth, the sinus behind the banner shallower than the lateral ones, the ribs prominent but not cordlike, the firm intervals charged with 1 row of 2-5 (the broader ventral pair with 4-7 scattered) small brown glands; petals vivid magenta-purple, glandless, in form like those of Ps. arborescens; banner 7.3-9.3 mm long, the claw 1.1-2 mm, the blade (6) 6.3-7.4 mm long, (5.2) 5.6-6.8 mm wide; wings 7.5-9.9 mm long, the claw 2-3.1 mm, the blade 6.1-7.3 mm long, (2.6) 2.8-3.8 mm wide; keel (7.8) 8.2-9.8 mm long, the claws 2.2-3 mm, the blades (6) 6.4-7.5 mm long, (3.6) 4-4.7 mm wide; androecium 6.6-9.1 mm long, the filaments free through 3.2-4.4 mm, the pallid anthers (0.7) 0.9-1.2 mm long; pod very obliquely ovoid-ellipsoid, subcompressed, the body 7-10 mm long, 4.2-5.3 mm wide, carinate by the nearly straight ventral and strongly convex dorsal sutures, the faces charged with very numerous small orange glands confluent lengthwise (especially distally) into ridges, glabrous except for thinly ciliolate keels; seed castaneous, smooth, 4.4-5.8 mm long.

    The status of Ps. fremontii as a species distinct from Ps. arborescens is precarious, contingent on the emphasis laid on gland-patterns of the pod’s valves. The neat coincidence of this feature with dispersal is certainly significant of racial differentiation, and it is the rank, not the reality or the definitions that are open to controversial interpretation. A line drawn southward in California from Pahrump Valley astride the Nevada line so as to pass west of the Kingston, Old Dad, Providence, and Whipple mountains, thence turned east to the Colorado River, marks the boundary between Ps. arborescens and Ps. fremontii on the Mohave Desert. East of that line, from the Needles upstream to lower Lake Mead and up the Virgin River into southwestern Utah, Ps. fremontii is the only known Xylodalea. Beyond Zion and the Kaibab a complication arises in the shape of outlying populations of Ps. fremontii on the Escalante and Colorado rivers in southeastern Utah and extreme northern Arizona, closely approaching but not actually sympatric with the disjunct Ps. arborescens var. pubescens in House Rock Valley. These taxa are, however, strongly differentiated in ways that do not involve the fruit. At low elevations along and near the Colorado River downstream from lower Lake Mead a poorly differentiated form of Ps. fremontii characterized by proportionately long and narrow leaflets has been recognized in recent floristic literature under the names D. fremontii var.johnsoni or D. johnsoni var. minutifolia. The epithet minutifolius, originally applied to the common Psorothamnus of Death Valley, that is to Ps. arborescens var. minutifolius of the present account, was usurped; the plant is described below, technically for the first time, as Ps. fremontii var. attenuatus.