Malacothrix DC. is a genus of flowering plants in the family Asteraceae, commonly known as desert dandelions or desertdandelions. The genus was described by Augustin Pyramus de Candolle in 1838 in his Prodromus (vol. 7, p. 192), and currently encompasses approximately 19 accepted species according to Plants of the World Online, with broader circumscriptions recognising up to 30 or more.
Plants are annuals or perennials, typically 2–70 cm tall (occasionally to 200 cm), with taproots and stems that may be erect or prostrate. Leaves are generally basal and cauline, sessile, with oblong to obovate blades bearing entire or somewhat dentate margins. The flower heads are arranged in corymb-like or panicle-like clusters and contain 15–270 florets; corollas are typically yellow or white, sometimes tinged reddish or lavender on the underside. Achenes are stramineous to brownish, prismatic or cylindro-fusiform with ribbed surfaces, and pappi range from absent to crowns of teeth plus bristles — key diagnostic characters within the genus.
Most species are native to western North America — California, Nevada, Arizona, New Mexico, and neighbouring states — with the range extending into northwestern and northeastern Mexico. A small number of species occur in South America (Argentina and Chile), and several are endemic to Pacific offshore islands. The genus is characteristically associated with desert and arid-land habitats. Phylogenetic studies have found the genus to be non-monophyletic, with some species more closely related to Atrichoseris, and others to Anisocoma and Calycoseris.
Distribution
The genus is centred in western and south-central North America, with native occurrences in Arizona, California, Colorado, Idaho, Montana, Nevada, New Mexico, Oregon, Texas, Utah, and Wyoming, as well as northeastern and northwestern Mexico. The range extends disjunctly to southern South America, with records from Argentina and Chile. Several species are endemic to Pacific offshore islands. The overall distribution spans from desert lowlands to montane arid slopes.
Ecology
Malacothrix species are predominantly inhabitants of desert and arid-land ecosystems in western North America and Mexico. Plants grow in sandy washes, desert flats, coastal bluffs, and rocky slopes. The plants are taprooted, an adaptation to dry substrates, and the genus includes both annual and perennial life forms suited to seasonally harsh, moisture-limited environments. Flower corollas are yellow or white — typical of insect-pollinated composites in open desert habitats.
Taxonomy
Malacothrix was formally described by Augustin Pyramus de Candolle in 1838 (Prodromus 7: 192) and is placed in subfamily Cichorioideae, tribe Cichorieae, subtribe Microseridinae within Asteraceae. Four heterotypic synonyms are recognised: Leptoseris Nutt., Leucoseris Fourr., Malacolepis A. Heller, and Malacomeris Nutt.
Chromosome base numbers of x = 7 and 9 have been recorded, indicating polyploidy within the genus. Molecular phylogenetic analyses have demonstrated the genus is not monophyletic: a portion of species cluster with Atrichoseris, while others are more closely allied to Anisocoma and Calycoseris. Species counts differ across treatments — POWO recognises 19 accepted species, SEINet lists 31, and GBIF records approximately 44 descendant taxa — reflecting different circumscriptions and ongoing taxonomic revision.
