Amphiachyris is a genus of annual flowering plants in the family Asteraceae (tribe Astereae, subtribe Gutiereziinae), described by Thomas Nuttall in 1840 and endemic to the United States. Commonly called broomweed, the genus contains two species native to the southern Great Plains and Texas. Plants are taprooted annuals reaching 20–100(–200) cm tall, glabrous, and emit a strong terpenoid odor when crushed. Stems are erect, usually branched in the upper portion, with bases becoming somewhat woody. Leaves are alternate, sessile, linear to lanceolate, with entire margins and gland-dotted surfaces. The inflorescences are radiate heads borne in open panicles or dense corymb-like clusters. Involucres are narrowly bell-shaped to top-shaped, 2–4 mm wide, with 12–15 phyllaries in 1–2(–3) series. Ray florets number 7–12, are pistillate and fertile, with yellow corollas. Disc florets number 10–21 and are functionally staminate (lacking ovaries), also yellow. The fruits (cypselae) are purplish black, obovoid-turbinate, with 4–9 ribs and dense white appressed hairs; the pappus is crown-like. Amphiachyris is closely related to the monotypic genus Thurovia and was formerly placed within Gutierrezia, from which it differs in its glandular hairs restricted to the lower leaf surfaces, functionally staminate disc florets, and connate disc pappus scales. The chromosome base numbers are x = 4 and 5.
Etymology
The genus name Amphiachyris comes from Greek amphi- ("around") and achyron ("chaff" or "husks"), alluding to the ring of pappus elements that surrounds the cypsela apex. The common name is broomweed. The genus was originally published as Brachyris sect. Amphiachyris by de Candolle in 1836, then elevated to genus rank by Thomas Nuttall in 1840.
Distribution
Amphiachyris is endemic to the United States, occurring in the central and southern portions of the country. Amphiachyris amoena is restricted to Texas, while Amphiachyris dracunculoides ranges across the southern Great Plains with scattered populations farther east.
Taxonomy
Amphiachyris belongs to subtribe Gutiereziinae within tribe Astereae (Asteraceae). It was recently treated within a more inclusive Gutierrezia (Diggs et al. 1999), but phylogenetic evidence places it closer to the monotypic genus Thurovia (Suh & Simpson 1990). Key distinguishing features from Gutierrezia include glandular hairs restricted to the abaxial leaf surfaces, functionally staminate disc florets, basally connate disc pappus scales that equal the corollas, and phyllary nerves lacking green borders.
