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Vascular Plants of California
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Malacothamnus arcuatus


Higher Taxonomy
Family: MalvaceaeView DescriptionDichotomous Key
Habit: Annual to tree; generally with stellate hairs, often with bristles or peltate scales; juice generally mucilage-like; bark fibrous. Leaf: generally cauline, alternate, petioled, simple [palmate-compound], generally palmate-lobed and/or veined, generally toothed, evergreen or not; stipules persistent or not. Inflorescence: head, spike, raceme, or panicle, in panicle or not (a compound panicle), or flowers >= 1 in leaf axils, or flowers generally 1 opposite a leaf or on a spur; bracts leaf-like or not; bractlets 0 or on flowering stalks, often closely subtending calyx, generally in involucel. Flower: generally bisexual, radial; sepals 5, generally fused at base, abutting in bud, larger in fruit or not, nectaries as tufts of glandular hairs at base; petals (0)5, free from each other but generally fused at base to, falling with filament tube, clawed or not; stamens 5--many, filaments fused for most of length into tube around style, staminodes 5, alternate stamens, or generally 0; pistil 1, ovary superior, stalked or generally not, chambers generally >= 5, styles or style branches, stigmas generally 1 or 1--2 × chamber number. Fruit: loculicidal capsule, [berry], or 5--many, disk- or wedge-shaped segments (= mericarps).
Genera In Family: 266 genera, 4025 species: worldwide, especially warm regions; some cultivated (e.g., Abelmoschus okra; Alcea hollyhock; Gossypium cotton; Hibiscus hibiscus). Note: Recently treated to include Bombacaceae, Sterculiaceae, Tiliaceae. Mature fruit needed for identification; "outer edges" are surfaces between sides and back (abaxial surface) of segment. "Flower stalk" used instead of "pedicel," "peduncle," especially where both needed (i.e., when flowers both 1 in leaf axils and otherwise).
eFlora Treatment Author: Steven R. Hill, except as noted
Scientific Editor: Steven R. Hill, Thomas J. Rosatti.
Genus: MalacothamnusView DescriptionDichotomous Key

Habit: Shrubs, sometimes spreading by rhizomes; sparsely to densely stellate-hairy and simple-glandular-hairy, flowers sometimes also with simple or 2-branched nonglandular hairs; stellate hairs stalked or not, 3--40-branched; glandular hairs not branched, often << stellate hairs. Stem: erect to ascending. Leaf: petioled; blades ovate to round (rarely diamond-shaped or +- reniform), unlobed or 3--7-palmate-lobed, margins generally toothed, bases cordate to truncate or wedge-shaped; stipules awl-shaped or linear to lanceolate or sometimes curved; transitioning in inflorescence into bracts +- resembling stipules. Inflorescence: Head-like to spike-like to panicle-like; bracts subtending the often highly reduced inflorescence internodes awl-shaped to linear to triangular to +- round, sometimes curved, occasionally 2--5-lobed, if 2-lobed resembling fused pair of +- modified stipules, smaller bracts sometimes deciduous; bractlets in whorl of 3 subtending calyx, distinct (occasionally fused at base in M. aboriginum), generally awl-shaped to linear, occasionally oblong or narrowly elliptic to ovate, green or partially to all red. Flower: calyx 5-lobed, not enlarging after flowering, not inflated, lobes triangular to ovate, tips acute to acuminate; petals exceeding calyx, unevenly obovate with rounded tip entire to notched or somewhat ragged-margined, pink to occasionally white and often varying in populations, generally drying closed after pollination or in some taxa drying partially to fully open; stamen tube +- included, filaments terminal and subterminal; ovary of 7--14 carpels, ovules 1 per cell, styles 7--14-branched, branches equal in number to carpels, stigmas head-like. Fruit: +- disk-like, fragile when dry, tip minutely stellate-hairy; segments 7--14, drying tan, 1-celled, wide-elliptic to obovoid-reniform, often notched near base, smooth-walled, fully dehiscent with each fruit segment splitting into two separate halves, beak 0. Chromosomes: 2n=34.
Etymology: (Greek: malakos, soft, thamnos, shrub) Note: Measurements for dry specimens; measurements for fresh specimens also provided in key. All Malacothamnus taxa can presumably hybridize; planting Malacothamnus taxa outside their natural range could threaten resident populations, a special concern for rare taxa. Hybridization/intergradation common where geographic ranges of some taxa meet; outside these zones of morphologically intermediate or intergrading plants, identification relatively simple and taxa relatively distinct. Such transition zones mostly between two taxa making parent taxa of intermediates easy to deduce; ranges of 3+ species abut near Santa Clarita making parentage of intermediate plants there unclear. Seeds generally germinate after fires in areas where woody plants burned; plants often short-lived, +- 5 years, but some may persist 20+ years post-burn.
eFlora Treatment Author: Keir Morse
Unabridged Reference: Morse 2023 Malacothamnus Volume 3 -- A Revised Treatment of the Genus Malacothamnus Based on Morphological and Phylogenetic Evidence
Malacothamnus arcuatus (Greene) Greene
Habit: <= 3 m, occasionally spreading by rhizomes. Stem: densely stellate-hairy, surface generally not visible through hairs without magnification, stellate hairs with branches <= 1.3 mm, mostly unstalked, stalks <= 0.7 mm, glandular hairs < 0.1 mm. Leaf: blades +- round to widely ovate (rarely elliptic or diamond-shaped), length generally >= width, unlobed or obscurely (rarely moderately) 3--5-lobed, lobes generally rounded, occasionally +- acute, bases wedge-shaped to cordate, surfaces ashy- to bright-green adaxially, paler abaxially, stellate hairs with branches <= 1.3 mm, mostly unstalked or many stalked, stalks <= 0.3 mm, abaxial stellate hair density 1--2× adaxial, glandular hairs < 0.1 mm. Inflorescence: spike-like to panicle-like; bracts subtending inflorescence internodes narrowly to moderately triangular or ovate, occasionally deeply 2-lobed, 1--7 mm, 0.5--3 mm wide, length 1--9× width; bractlets subtending calyx ovate or awl-shaped to linear, 1--6.5 mm, 0.2--0.8 mm wide, length 2--17.5× width, 0.2--0.8× calyx, green to red. Flower: calyx 4--9.5 mm, lobes 2--5.5 mm × 1.5--3.5 mm, lobe widest at base or <= 1 mm above base, length 0.8--2× width, ovate to widely so or triangular, tip acute, abaxial calyx stellate hairs with branches 0.1--1.8 mm, mostly unstalked, stalks <= 0.3 mm, abaxial glandular hairs < 0.1(0.3) mm (mean per plant < 0.1--0.1 mm); corolla drying closed, petals to +- 2 cm.
Note: Except for a transition zone of apparent intermediates, the two varieties are distinct morphologically and geographically. Var. arcuatus is distinguished from var. elmeri by having longer bractlets subtending the calyx and longer stellate hair branches on stem. These varieties are closest relatives in phylogenetic analyses, with some support for two taxa; population-level analyses needed to determine if var. elmeri should be returned to species rank as M. hallii. Some hybrids between var. elmeri and M. fremontii var. fremontii may key to var. arcuatus; these hybrids occur near parent taxa and not within geographic range of var. arcuatus.
Jepson eFlora Author: Keir Morse
Index of California Plant Names (ICPN; linked via the Jepson Online Interchange)

Previous taxon: Malacothamnus aboriginum
Next taxon: Malacothamnus arcuatus var. arcuatus

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Citation for this treatment: Keir Morse 2023, Malacothamnus arcuatus, in Jepson Flora Project (eds.) Jepson eFlora, Revision 12,, accessed on July 23, 2024.

Citation for the whole project: Jepson Flora Project (eds.) 2024, Jepson eFlora,, accessed on July 23, 2024.

Malacothamnus arcuatus  
var. elmeri
click for image enlargement
©2017 Keir Morse
Malacothamnus arcuatus  
var. elmeri
click for image enlargement
©2017 Keir Morse
Malacothamnus arcuatus  
var. elmeri
click for image enlargement
©2017 Keir Morse
Malacothamnus arcuatus  
var. arcuatus
click for image enlargement
©2017 Keir Morse
Malacothamnus arcuatus  
var. elmeri
click for image enlargement
©2017 Keir Morse

More photos of Malacothamnus arcuatus
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Geographic subdivisions for Malacothamnus arcuatus:
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map of distribution 1

(Note: any qualifiers in the taxon distribution description, such as 'northern', 'southern', 'adjacent' etc., are not reflected in the map above, and in some cases indication of a taxon in a subdivision is based on a single collection or author-verified occurrence).


Data provided by the participants of the  Consortium of California Herbaria.

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Blue markers indicate specimens that map to one of the expected Jepson geographic subdivisions (see left map). Purple markers indicate specimens collected from a garden, greenhouse, or other non-wild location.
Yellow markers indicate records that may provide evidence for eFlora range revision or may have georeferencing or identification issues.

CCH collections by month Flowering-Fruiting Monthly Counts

Duplicates counted once; synonyms included.
Species do not include records of infraspecific taxa, if there are more than 1 infraspecific taxon in CA.
Blue line denotes eFlora flowering time (fruiting time in some monocot genera).