73. Prenanthella Rydberg, Bull. Torrey Bot. Club. 33: 160. 1906.
[Genus Prenanthes and Latin -ella, diminutive, alluding to original assignment of type species]
Kenton L. Chambers
Annuals, 5–30 cm; taprooted. Stems 1–5+, erect (leafy proximally, often flexuous distally), branched near base bases or distally, glandular-puberulent to glabrescent. Leaves basal or proximally cauline; sessile or petiolate; blades spatulate to oblanceolate, often runcinate, margins irregularly dentate to lobed, teeth often spinulose (faces glabrous). Heads (10–150+, on flexuous, bracteate, intricately-branched axes) in paniculiform arrays. Peduncles not inflated, minutely bracteate . Calyculi of 2–3, minute, deltate, unequal bractlets. Involucres ovoid, cylindric at flowering, 2–3 mm diam. Phyllaries 3. 5 in 1 series, linear-lanceolate, equal, herbaceous, margins hyaline, apices acute. Receptacles flat, smooth (with scars), glabrous, epaleate. Florets 3–4; corollas pink or white (slightly surpassing phyllaries at flowering). Cypselae pale brown, columnar, tapering slightly proximally, not beaked, 5-ribbed and grooved, minutely rugulose, glabrous; pappi tardily falling, of 80+, basally connate, white, unequal, stiff, smooth bristles. x = 7.
Species 1: sw United States, nw Mexico.
The single species of Prenanthella was usually assigned to Lygodesmia in treatments prior to A. S. Tomb’s (1972b) reassessment. Its base chromosome number of x = 7 was taken as evidence of a possible relationship with Malacothrix, Calycoseris, and Anisocoma, also x = 7 (Tomb 1974). Recent molecular phylogenetic studies (J. Lee et al. 2003) place Prenanthella in a clade with Pleiacanthus, Stephanomeria, and Rafinesquia, all with x = 8. This group, together with Munzothamnus, constitutes the subtribe Stephanomeriinae of Lee and B. G. Baldwin (2004).
Tomb, A. S. 1972b. Re-establishment of the genus Prenanthella Rydb. (Compositae: Cichorieae). Brittonia 24: 223–228.