Leafl. Bot. Observ. Crit. 1: 208. 1906.
Subshrubs or shrubs, sometimes root-suckering, glabrate to densely hairy, hairs various, simple or bifurcate, simple-glandular, and stellate, ca. 6–30-armed. Stems erect or ascending, ultimate branches strict to diffuse. Leaves: stipules ± persistent, filiform to subulate; blade ovate to round, rhombic, or reniform, unlobed or palmately 3-lobed, 5-lobed, or 7-lobed, base cordate to truncate or cuneate, margins usually toothed. Inflorescences usually axillary, rarely terminal, flowers solitary or in clusters in dense to open heads, spikes, racemes, or panicles; involucellar bractlets persistent, 3, distinct (basally connate in M. aboriginum). Flowers: calyx not accrescent, not inflated, lobes triangular to ovate, not ribbed; corolla exceeding calyx, campanulate to subrotate, usually rose to pale-pink, pinkish mauve, or mauve, rarely white; staminal column ± included; filaments terminal and subterminal; ovary 7–14-carpellate; ovules 1 per cell; styles 7–14-branched, (branches equal in number to carpels); stigmas capitate. Fruits schizocarps, erect, not inflated, disclike, subglobose-obovate in lateral view, not indurate, fragile, apically minutely stellate-hairy; mericarps 7–14, drying tan, 1-celled, asymmetrically suborbicular to obovoid-reniform, smooth-walled, without dorsal spur, apex muticous, dehiscence loculicidal, walls falling away as 2 fragile valves. Seeds 1 per mericarp, ascending, brown or black, obovoid-reniform, usually papillate-stellate or minutely stellate-hairy or rarely glabrous. x = 17.
Calif., nw Mexico
Species 11 (10 in the flora).
Most species of Malacothamnus are locally common in early burn successions of chaparral and adjacent vegetation types in California and northern Baja California. Malacothamnus foliosus (S. Watson) Kearney is the only species restricted to Mexico; it is found in northern Baja California.
Major morphological variants in Malacothamnus have been recognized as species, as treated by T. H. Kearney (1951). Insofar as known, most taxa are interfertile and in some instances intergrade in areas of proximity. Interpopulational variation within most species in indument and inflorescence characters is high and of moderately complex patterning.
|1||Involucellar bractlets 1–9 mm wide||> 2|
|1||Involucellar bractlets 0.2–0.8(–1) mm wide||> 4|
|2||Inflorescences open-paniculate, flowers solitary or in loose clusters, not subtended by conspicuous bracts; indument white and dense (hairs simple, fine), sometimes sublepidote (hairs stellate, sessile).||Malacothamnus abbottii|
|2||Inflorescences spicate, racemose, or headlike, flower clusters glomerate, subtended by conspicuous bracts; indument grayish, tawny, or white, dense to sparse, hairs stellate, sessile or stalked, sometimes mixed with glandular and/or armed hairs||> 3|
|3||Flower clusters in elongate, spicate or racemose inflorescences, involucellar bractlets distinct or basally connate.||Malacothamnus aboriginum|
|3||Flower clusters in headlike or spicate inflorescences, involucellar bractlets distinct.||Malacothamnus palmeri|
|4||Calyx lobes: length 1–1.5 times width, 1(–2) times tube, triangular to ovate, apex acute or acuminate to short-mucronate; involucellar bractlets 1/2–2/3 times calyx||> 5|
|5||Calyces sparsely bristly- or stellate-hairy, stellate hairs overlain by 1–few-armed, arms to 3 mm; flower clusters sessile, glomerate, inflorescences interrupted, spicate.||Malacothamnus densiflorus|
|5||Calyces sparsely hairy to densely stellate-hairy, if sparsely, not bristly, hairs usually 10–30-armed, arms 0.2–1.5 mm; flower clusters sessile or pedunculate, glomerate or loose, inflorescences non-interrupted, cymose, spicate, racemose, or open-paniculate||> 6|
|6||Leaf blades 2–6(–11) cm, usually thin, surfaces: adaxial usually gray-green and sparsely hairy, basal lobes not overlapping; branches usually slender.||Malacothamnus fasciculatus|
|6||Leaf blades 5–11(–20) cm, thick, surfaces: adaxial tawny and densely stellate-hairy, basal lobes usually overlapping; branches usually stout.||Malacothamnus davidsonii|
|7||Calyces not angled and/or winged in bud, lobes usually triangular or deltate-lanceolate, apex long-acute to ± acuminate; indument densely lanate, white-lanate, or villous, or sublepidote to tomentose||> 9|
|8||Flower clusters usually glomerate; inflorescences not basally leafy, interrupted, spicate; calyx indument sparse, hairs usually 1–2–few-armed, bristly.||Malacothamnus densiflorus|
|8||Flower clusters glomerate to open; inflorescences usually basally leafy, interrupted or not, spicate or racemose; calyx indument dense, stellate-hairy, not bristly.||Malacothamnus marrubioides|
|9||Petals white, fading lavender; leaf blades 3- or 5-lobed, surfaces: adaxial glabrate, dark green.||Malacothamnus clementinus|
|9||Petals pale pink to deep mauve; leaf blades unlobed or 3- or 5-lobed, surfaces: adaxial soft tomentose-white, pale ash green and velvety white or sparsely hairy and green||> 10|
|10||Inflorescences interrupted and spicate or narrow-paniculate to open-paniculate, flower clusters dense or loose, 6–10-flowered; branches stout or slender, not flexuous; leaf blades ovate, broadly ovate, suborbiculate, or reniform, 4–6(–11) cm.||Malacothamnus fremontii|
|10||Inflorescences usually open-paniculate or subracemose, flower clusters loose, 3(–6)-flowered, sometimes flowers solitary; branches slender, flexuous; leaf blades ovate, ± rhombic, or suborbiculate, 2.5–4.5(–7) cm.||Malacothamnus jonesii|