Psorothamnus arborescens

Title

Psorothamnus arborescens

Author(s)

Rupert C. Barneby

Scientific Name

Psorothamnus arborescens (Torr. ex A.Gray) Barneby

Description

4.  Psorothamnus arborescens (Torrey) Barneby

Stiffly irregularly branching, often thorny shrubs up to 3-10 dm tall, variably pubescent to almost glabrous, the foliage gray-tomentulose, silky-canescent, thinly pilosulous, or green and glabrate, the branchlets at nodes immediately preceding the terminal peduncles usually persistent as rigid lignified spines, the young parts (especially the raceme-axis) charged with subulate or prickle-shaped glands, the leaflets gland- pustulate beneath (the glands sometimes concealed by vesture); leaf-spurs up to 1 mm long; stipules narrowly subulate to linear-attenuate, 0.7-2 mm long, soon papery, brownish, fragile; intrapetiolular glands 0; post-petiolular glands 0 or minute; leaves (1) 1.3-3.5 (5.5) cm long, petiolate, with ± margined rachis and (3) 5-7, rarely up to 17 leaflets variable in outline according to varietas from ovate or lanceolate to linear, the terminal one often continuous with the rachis, the last pair often confluent, sometimes all decurrent on rachis, sometimes all sessile but jointed, the leaf rarely reduced to a simple blade; raceme sessile or subsessile (the lowest bracts sometimes foliose, or a branchlet intervening between the lowest and succeeding flowers), loosely but sometimes shortly 8-40-flowered, the flowers spreading-ascending, the axis (1) 1.5-12 cm long, becoming stiff and persisting after fall of the pods; bracts papery, narrowly subulate or linear, 2.5-4 mm long, deciduous; bracteoles similar but shorter, 1-2 mm long; calyx 5-9 mm long, externally glabrous to variably pubescent, the hypanthium 1.5-3 mm deep, the tube (measured to a lateral sinus) 3-4.5 mm long, the ribs becoming bluntly prominent, the firm intervals charged with small brownish glands, the 2 broad ventral intervals with ± 2 rows, the rest with 1 row of ± 3-5 glands, the teeth dissimilar, the ventral pair broadly lanceolate to triangular short-acuminate, 2-4.5 mm long (1 mm shorter to 0.5 mm longer than tube), (1.2) 1.4-2 mm wide near base, the rest lanceolate, it half as wide, the dorsal one often shortest, the sinus behind the banner commonly much shallower than the rest; petals indigo-blue or violet-purple, glandless, deciduous; banner 6.3-10.5 mm long, its claw very short, its blade flabellate or flabellate-obovate, deeply retuse, at base either cuneately tapering or subauriculate; wings a trifle longer than banner, the claw 2.2-3.6 mm long, the broadly oblong or oblong- obovate, obtuse or subemarginate blade 5.4-8.2 mm long, 2.2-3.5 mm wide; keel usually a trifle longer than wings, rarely a little shorter, 8.2-11.2 mm long, the claws 2.6-3.7 mm, the obliquely obovate blades 5.9-8.2 mm long, 3.3-4.9 mm wide; androecium (5.5) 6.5-9.8 mm long, the filaments free through (3) 3.5-5.6 mm, the anthers (0.85) 0.9-1.3 mm long; pod obliquely ellipsoid or ovoid-ellipsoid, the subcompressed body 8-10 mm long, (3.6) 4-6 mm diam, at least beyond the middle 2-carinate by the sutures, the convex valves charged with large, round or broadly elliptic blister-glands, between the glands either villosulous or glabrous, the ventral suture ciliolate; seed (4.7) 5-6 mm long.

These are small shrubs, never arborescent as suggested by the inappropriate epithet, and diverse in aspect because so variable in pubescence, in configuration of the leaves, and in flower- size. The species differs as a whole from the closely related Ps. schottii in its smaller stature and usually pinnate (not simple or trifoliolate) leaves, always excepting the anomalous Ps. arborescens var. simplifolius, discussed in more detail below and in relation to Ps. schottii itself.

Over a great part of the Mohave Desert in California and west-central Nevada Ps. arborescens is the only member of sect. Xylodalea and with the exception of Ps. polydenius the only Psorothamnus, and like the latter is found chiefly, perhaps exclusively, on granitic and volcanic bedrock. On the calcareous ranges of far southeastern Mohave it is replaced by the vicariant and perhaps too closely related Ps. fremontii, distinguishable by no feature of moment up to the distinctive pod. The pods of all forms of Ps. arborescens are like those of Ps. schottii charged with relatively few, large, circular blister-glands scattered over an eglandular field in polka-dot patterns. In D. fremontii the pod valves are covered by small orange glands of indeterminate size and shape which run together into irregular lines or ridges, the surface appearance being that of caramelized sugar.

Beyond the limits of the Mohavean floristic province Ps. arborescens is represented by two distantly isolated populations, one relatively extensive in the valley of Colorado River upstream from Grand Canyon in northern Arizona, the other dramatically disjunct on the Sonoran shore of Gulf of California, where it has been collected in only one station. The latter, surprisingly, cannot be distinguished at present from typical Ps. arborescens of southwestern Mohave Desert. The Arizona plant appears below as Ps. arborescens var. pubescens.