Herbs, perennial, densely tufted to ± matted, ± aromatic, 1.5–5(–9) dm, sparsely to densely villous and glandular-pubescent; short-rhizomatous. Stems 1–20+, ascending to erect, green, stramineous, or reddish. Leaves winter-marcescent, primarily basal, cauline 2–7, reduced, alternate, odd-pinnate; stipules persistent, basally adnate to petiole, linear to lanceolate or elliptic, margins entire, cauline sometimes lobed; petiole present; blade narrowly linear in outline, ± cylindric, 4–20(–35) cm, foliaceous, leaflets 30–70, separate proximally, otherwise overlapping, sometimes imbricate, terminal confluent with distalmost lateral ones, broadly obovate to reniform in outline, margins flat, palmately divided nearly to base into linear to oblanceolate lobes, venation palmate. Inflorescences terminal, 5–20-flowered, narrow cymes, ± open or ± congested; bracts present, reduced; bracteoles absent. Pedicels present, straight. Flowers 10–15 mm diam.; epicalyx bractlets 5; hypanthium bluntly campanulate, flat-bottomed, 2–3 × 4–6 mm; sepals 5, reflexed, adaxially purple-streaked, lanceolate to narrowly deltate; petals 5, white, sometimes pink-tinged, narrowly oblong to obovate or spatulate; stamens 20, shorter than petals, filaments erect, ± flattened, forming tube, anther thecae dehiscing introrsely; torus elongate; carpels 25–50, glabrous, styles subterminal, slightly rough-thickened basally, otherwise mostly filiform; ovule 1. Fruits aggregated achenes, individually deciduous, 25–50 or less, obliquely ovoid to reniform, 1–1.5 mm, glabrous; hypanthium persistent; sepals persistent, erect; styles tardily deciduous, jointed.
Species 2 (2 in the flora).
Rydberg established Horkeliella as a subgenus of Horkelia, at the same time treating Ivesia as another subgenus. The species resemble Horkelia in having flat-bottomed, campanulate hypanthia, reflexed, purple-streaked sepals, flattened, erect filaments that form a column around the styles, and inwardly dehiscing anthers. This floral morphology is evidently an adaptation to bee pollination. Unlike Horkelia, Horkeliella has 20 stamens, and the cylindric leaves with finely divided leaflets are most similar to those of Ivesia species, especially sect. Comarella. D. D. Keck (1938) accordingly decided that the taxa in question were better accommodated in Ivesia and also reduced the three species of Rydberg to two subspecies of a single species. Subsequent regional floras (for example, P. A. Munz 1959) followed this basic arrangement, although L. Abrams (1944) placed the bitypic species in Horkelia rather than Ivesia. In contrast, worldwide generic compilations (for example, J. Hutchinson 1964–1967) considered the treatment by Rydberg 1908 of Horkeliella as a genus with three species, even while accepting the numerical circumscription of Ivesia by Keck.
Horkeliella was resurrected for regional floras by B. Ertter (1993b), comprising two species endemic to the southern Sierra Nevada of California. The genus may represent the transitional stage in the evolution of Horkelia from the ivesioid lineage.
|1||Filaments opposite center of sepals longer than those on either side; herbage with non-glandular hairs sparse, not obscuring abundant stipitate-glandular ones; leaflets divided into 5–10 ultimate segments; e of Sierra Nevada crest, Inyo and Mono counties.||Horkeliella congdonis|
|1||Filaments opposite center of sepals shorter than those on either side; herbage with non-glandular hairs sparse to dense, often obscuring stipitate-glandular ones; leaflets divided into 3–6 ultimate segments; w of Sierra Nevada crest, Kern and Tulare counties.||Horkeliella purpurascens|