Psorothamnus thompsonae

  • Title

    Psorothamnus thompsonae

  • Authors

    Rupert C. Barneby

  • Scientific Name

    Psorothamnus thompsonae (Vail) S.L.Welsh & N.D.Atwood

  • Description

    9.  Psorothamnus thompsonae (Vail) Welsh & Atwood

    (Plate VII)

    Small freely branching, lavishly floriferous shrubs of rounded or depressed-rounded outline, 4-9 (15) dm tall and often as much in diameter, rather softly woody but the short sterile branchlets immediately below the inflorescences tapering into vulnerant, often divaricately bifurcate, persistent spines, the stems densely silky-pilosulous with narrowly descending (retrorse) hairs up to 0.2-0.4 mm long and pustulate with prominent orange blister- or prickle-shaped glands, the foliage more thinly strigulose-pilosulous than the canescent stems, commonly greenish, the leaflets usually more densely pubescent above than beneath, the lower face paucipunctate; leaf-spurs up to 0.6 mm long; stipules triangular, 0.2-0.9 mm long, deciduous, charged at apex and often on margins with 1-3 glands; leaves short-petioled, (1) 1.4-3 cm long, with narrowly margined, punctate rachis and 3-8 (9) pairs of obovate to linear-oblanceolate, obtuse to emarginate, flat or loosely folded, thick-textured leaflets 2-7 mm long; peduncles terminal to branchlets, 0.5-1.5 cm long; racemes loose and open, the flowers spreading at full anthesis, the glabrous or puberulent axis becoming (1) 2-9 cm long; bracts early deciduous, ovate-elliptic, submembranous, 0.6-1.5 mm long, gland-tipped; pedicels spreading-ascending, 0.7-1.4 mm long, charged near apex with a pair of orange glands; calyx 3.7-5 mm long, either pilosulous throughout with spreading hairs up to 0.3-0.5 mm long or glabrous up to the margins of the ciliolate, internally silky-pubescent calyx-teeth, the tube 2.4-3.3 mm long (measured to lateral sinus), the dorsal sinus shallower than the rest and the orifice hence oblique, the ribs prominent but not cordlike, the firm, pleated intervals charged above middle with 1-4 large red-orange glands, the teeth ovate to oblong-ovate, obtuse to subacute, gland-mucronulate, herbaceous, obscurely 3-nerved below apex, very unequal, the ventral pair nearly twice as broad and longer than the rest, 1.1-2.3 mm long; petals bright pink- or violet-purple, marcescent, glabrous, all glandless or the banner sometimes charged on back with a subapical gland; banner (5.5) 6-8.4 mm long, the claw (1.8) 2.2-2,5 mm, the ovate, basally broadly cuneate or cordate, sometimes suborbicular-cordate, notched blade (4.4)4.8-6.2 mm long, (4.2) 4.8-6 mm wide; wings 7.6-8.6 mm long, the claw 2-2.7 mm, the oblong obtuse blade 5.8-6.6 mm long, 2.6-3.2 mm wide; keel (only a trifle longer than wings) 7.4-9 mm long, the claws (1.9) 2.1-3.2 mm, the broadly obovate blades 5.6-6.7 mm long, 3.4-3.8 mm wide; androecium 10-merous, 6-7.7 mm long, the longer filaments free for 2.5-3.5 mm, the connective gland-tipped, the anthers 0.8-0.9 mm long; pod 4-4.5 mm long, compressed, membranous and glabrous at base, thence firm, rugulose, pilosulous, gross-glandular; ovules 2 (3); seed 1, 2.1-2.6 mm long.

    A highly decorative small shrub endemic to the canyonlands of southeastern Utah and adjoining Arizona, notable for the retrorse pubescence of the stems, the transformation of branchlets related to the inflorescence into stout vulnerant thorns, the open racemes of vivid pink-purple flowers, and the marcescence of the petals around the forming pod. Its open raceme and glabrous petals recall members of sec. Xylodalea, but the orange-pustular, retrorsely canescent-hairy stems and the spathaceous calyx are more nearly matched by Ps. scoparius and Ps. polydenius collectively. The species seems most nearly related to the latter, differing in the laxer raceme of larger flowers and in the broad, ovate or oblong- ovate calyx-lobes.

    Since Kearney & Peebles described Dalea whitingi, its sister species D. thompsonae has become much better known, and the variation within it nearly spans the discontinuity between the two. Typical Ps. thompsonae may have a calyx externally either glabrous or pubescent, and the differences between it and Dalea whitingi are reduced to shape of leaflets, these relatively broad and small in var. thompsonae, narrower and longer (but not necessarily fewer) in what must be called var. whitingi. The two varieties occur in the same environment and are clearly no more than vicariant races of one strongly characterized and isolated species.