177. Astranthium Nuttall, Trans. Amer. Philos. Soc., n. s. 7: 312. 1840.
Western-daisy [Greek astron, star, and anthos, flower, alluding to head as seen from above]
Guy L. Nesom
Annuals, biennials, or perennials, 5–50 cm (with taproots or fibrous roots). Stems erect to decumbent, usually branched throughout, sparsely to densely strigose to hirsuto-pilose. Leaves basal and cauline; alternate; mostly petiolate; blades 1-nerved, spatulate-obovate (proximal) or narrower, 1–8(–12) cm (some clasping to subclasping), margins usually entire, sometimes toothed, faces glabrous or strigose. Heads radiate, borne singly (usually on long, leafless or bracteate peduncles). Involucres campanulate or hemispheric, (3.5–8 ×) 4–8 mm. Phyllaries 15–30 in 2(–3) series, appressed, midnerves inconspicuous, broadly oblanceolate to linear-lanceolate, subequal, thin-herbaceous, margins often hyaline, faces glabrous or strigose. Receptacles conic, pitted, epaleate. Ray florets 10–65(–85), pistillate, fertile; corollas abaxially drying white or with blue to purplish blue midstripe, adaxially white. Disc florets 25–200[–250], bisexual, fertile; corollas yellow, tubes much shorter than tubular-funnelform throats, lobes 5, spreading, deltate; style-branch appendages lanceolate-acute. Cypselae (brownish) obovoid to oblanceoloid-obovoid, compressed, 2-nerved (nerves marginal, usually thin, sometimes thick), faces smooth, striate, or papillate-pebbly, and glabrous or glochidiate-hairy; pappi 0 or coroniform (of setae or scales to 0.1 mm). x = 3, 4, 5.
Species 12 (3 in the flora): c United States, Mexico.
Astranthium is characterized by its herbaceous habit, heads borne singly on long peduncles, subequal phyllaries in 2–3 series, conic receptacles, white or blue rays, tubular disc corollas, and flattened, papillate cypselae with coroniform or no pappi. Apically glochidiate cypsela hairs have each apical cell abruptly reflexed in opposite directions. The species are separated by small but consistent morphologic differences, often accompanied by differences in chromosome number. All but three of the species are known only from Mexico (D. C. D. De Jong 1965; J. Rzedowski 1983).
Vegetatively and florally, Astranthium is similar to the monotypic Dichaetophora, which has the same chromosome number (2n = 6) as northern species of Astranthium. Epappose species of Erigeron may be superficially similar to Astranthium (D. C. D. De Jong and G. L. Nesom 1982); they are distantly related.
De Jong, D. C. D. 1965. A systematic study of the genus Astranthium (Compositae, Astereae). Publ. Mus. Michigan State Univ., Biol. Ser. 2: 429–528.