Plants forming tufts or mats, green to grayish, obscurely (and minutely) glandular, resinously aromatic, often strongly so. Stems decumbent or ascending to erect, (0.5–)1–10(–12) dm. Basal leaves usually planar, sometimes ± cylindric; stipules usually entire, sometimes basally lobed; leaflets (1–)3–16(–20) per side, separate to overlapping, divided ± 1/6–3/4+ to midrib into 3–30(–60) teeth or lobes not restricted to apex. Inflorescences open to congested, flowers arranged individually, in usually non-capitate glomerules, and/or in corymbiform clusters. Pedicels remaining straight, 1–30(–40) mm. Flowers: epicalyx bractlets narrowly elliptic-lanceolate to broadly ovate, 0.5–3 mm wide, usually entire, sometimes toothed; hypanthium interior pilose or glabrous; sepals acute; petals white, oblong-oblanceolate to round, apex obtuse to truncate to emarginate; filaments white, glabrous, anthers longer than wide; carpels 10–200(–220). Achenes 0.8–2 mm, usually smooth or slightly rugose, sometimes merely roughened.

Distribution

Calif., nw Mexico.

Discussion

Species 9 (9 in the flora).

Section Horkelia encompasses the species that are most commonly encountered in heavily populated areas of California. Plants are notably glandular-viscid (unless obscured by dense vestiture) and have a distinctive resinous odor.

Previous revisions (for example, P. A. Rydberg 1908c; D. D. Keck 1938) have placed Horkelia frondosa (here treated as H. californica var. frondosa) at the beginning, implying that this is the least derived expression within the genus. Such an assumption is based on its gross resemblance to sympatric members of Drymocallis; molecular evidence (T. Eriksson et al. 1998; M. Lundberg et al. 2009; C. Dobeš and J. Paule 2010) confirms that this similarity is superficial. If, as speculated above, species composing sect. Hispidulae are relicts of the original radiation, then H. californica var. frondosa is actually one of the more derived members of the genus. Species within sect. Horkelia have been ordered here according to that interpretation.

Selected References

Ertter, B. 1993c. A re-evaluation of the Horkelia bolanderi (Rosaceae) complex, with the new species Horkelia yadonii. Syst. Bot. 18: 137–144.

Lower Taxa

Horkelia bolanderi, Horkelia californica, Horkelia clevelandii, Horkelia cuneata, Horkelia marinensis, Horkelia rydbergii, Horkelia tenuiloba, Horkelia truncata, Horkelia yadonii

Key

1	Leaflets 1–3 per side, sparsely short-pilose to glabrate, (15–)20–30-toothed 1/6–1/4 to midrib; petals obovate to round, (3–)4–7 mm wide.	Horkelia truncata
1	Leaflets (3–)5–16(–20) per side, hairy, 3–15(–30)-toothed or -lobed (collectively 10–60-toothed in H. californica) 1/6–3/4+ to midrib; petals narrowly obovate or oblanceolate to elliptic or oblong, 1–3(–4) mm wide	> 2
2	Hypanthia 3–5.5 mm, 1/2 to nearly as deep as wide; epicalyx bractlets 4–6(–8) mm, ± equal to sepals, entire or toothed.	Horkelia californica
2	Hypanthia 1–2(–3) mm, less than 1/2 as deep as wide (except H. clevelandii); epicalyx bractlets 1–4(–5) mm, shorter than sepals, entire	> 3
3	Leaflets ± elliptic to obovate, divided less than 1/3 to midrib into (5–)10–15 teeth; epicalyx bractlets 1.5–3 mm wide.	Horkelia cuneata
3	Leaflets cuneate or flabellate to ovate or nearly round, divided (1/5–)1/3–3/4+ to midrib into 3–10(–30 in H. yadonii) teeth or lobes; epicalyx bractlets 0.5–2(–2.5) mm wide	> 4
4	Stems ± decumbent to ascending, 1–3.5(–4.5) dm; leaflets 5–10(–12) per side, cuneate, ± overlapping; inflorescences usually congested; plants matted; coastal.	Horkelia marinensis
4	Stems ascending to erect, (0.5–)1–7 dm; leaflets 6–16(–20) per side, cuneate to flabellate or nearly round, separate to ± overlapping; inflorescences open to ± congested; plants tufted to matted; coastal or interior	> 5
5	Epicalyx bractlets 1–2(–2.5) mm wide, lanceolate to ovate; anthers 0.8–1.2 mm; hypanthia 3–6.5 mm diam.	Horkelia yadonii
5	Epicalyx bractlets 0.5–1(–1.5) mm wide, narrowly elliptic to broadly lanceolate; anthers 0.4–1 mm; hypanthia 2–4.5(–6) mm diam	> 6
6	Stems: hairs ascending to appressed; plants usually grayish to grayish green; hypanthium interior pilose	> 7
6	Stems: hairs ± spreading; plants green to grayish; hypanthium interior glabrous or pilose	> 8
7	Carpels 20–50(–120); styles 2–4 mm; basal leaves (4–)8–20(–30) cm; Transverse Ranges, s California.	Horkelia rydbergii
7	Carpels 10–20(–27); styles (1–)1.5–2 mm; basal leaves 3–8(–9) cm; Coast Ranges, n California.	Horkelia bolanderi
8	Leaflets divided 1/2–3/4+ to midrib into linear to narrowly oblanceolate or narrowly elliptic lobes; hypanthium interior pilose; n Coast Ranges, California.	Horkelia tenuiloba
8	Leaflets divided 1/3 to midrib into acute to obtuse teeth; hypanthium interior glabrous; Peninsular Ranges, s California.	Horkelia clevelandii