Herbs, annual, glabrous, puberulent, or hoary. Stems usually erect, rarely decumbent, usually unbranched, sometimes branched, proximally (branches opposite or whorled), branched distally. Leaves falling early (persistent in H. drymarioides), in whorls of 4 at basal nodes, usually becoming irregularly whorled, opposite, or alternate proximally distal to basal nodes on main stem, alternate or opposite distally; stipular glands usually present (exudate often red), sometimes absent; blade threadlike to linear, oblong-lanceolate, ovate, or orbiculate, margins glandular-toothed, stipitate-glandular, or entire. Inflorescences open or dense, monochasial or dichasial cymes, sometimes helicoid or scorpioid. Pedicels articulated. Flowers: sepals persistent, 5, connate at base, equal or unequal in size, margins entire, stipitate-glandular or eglandular, surfaces glabrous or hairy; petals 5, distinct, attached near rim of cup between filament bases, yellow, white, or pink, base with 0 or 2 auricles flanking central ligule; stamens 5; staminodes 0; pistil 2–3-carpellate, ovary 4- or 6-locular by intrusion of incomplete false septa; styles 2–3, distinct; stigmas minute, linear, ± equal in width to styles. Fruits capsules, dehiscing into 4 or 6 segments. Seeds 4 or 6, oblong to clavate, triangular in cross section. x = 18.

Distribution

w United States, nw Mexico.

Discussion

Species 13 (13 in the flora).

Hesperolinon was monographed by H. K. Sharsmith (1961); the treatment here is based primarily on that work.

Hesperolinon taxa are winter annuals; the plants flower in late spring, after most other associated species are in fruit. H. K. Sharsmith (1961) was first to describe the complex flower morphology. Appendages are formed at the base of the petal by expansion into a pair of triangular lobes bearing pockets or pouches in their margins. The pouches are usually swollen on their adaxial surfaces with glandular-papillate cells that form horizontal crests (the lateral appendages), sometimes meeting near the center of the claw and connecting with the base of the ligule, a central, erect, greenish yellow, fleshy, often hairy or glandular-papillate, tonguelike protuberance. The ligule arises over the petal midvein, usually just proximal to the point where the two lateral veins arise, or the ligule may be a longitudinal, glandular, sometimes hairy, thickening or fold in the lamina.

Flowers in Hesperolinon often are pseudocleistogamous with the anthers dehiscing and depositing pollen on the stigma in bud, which may account for the morphological uniformity within populations, and the large differences between populations of the same species (H. K. Sharsmith 1961). In areas where two or more species grow sympatrically, there usually is a difference in phenology, with one flowering before another, or the species occupying different microhabitats.

C. M. Rogers (1975) noted that whorled leaves, common in Hesperolinon, were found in Linum only in his L. schiedeanum complex (the most primitive group) in sect. Linopsis. Linum schiedeanum and Hesperolinon also share other character states, including stipular glands and distinct styles. Rogers noted that features shared with the southwestern L. neomexicanum include annual habit, distinct styles, auricles at the base of petals, some floral pigments (D. E. Giannasi and C. M. Rogers 1970), and petals attached at the top of the filament cup. McDill (2009) suggested that Hesperolinon is related to a clade including the Mexican endemic L. mexicanum Kunth and L. guatalamense Bentham of Guatemala and San Salvador, in what he termed the L. mexicanum group in subsect. Linopsis, series Linopsis.

All species of Hesperolinon except H. micranthum are endemic to California, mostly restricted to the North and South Coast ranges, usually in chaparral of the inner Coast Ranges. H. K. Sharsmith (1961) considered Lake and Napa counties to be the center of diversity for the genus, and, except for H. californicum and H. micranthum, all species are found on the Jurassic Franciscan formation, most often on exposed serpentine components. Hesperolinon micranthum occurs on serpentine soil in the Coast Ranges and on volcanic flows in the northern Sierra Nevada and Modoc Plateau. At least eight of the species Sharsmith studied were obligate or near-obligate serpentine endemics and the remaining four species were facultative serpentine species.

Y. P. Springer (2009) compared occurrences of Hesperolinon on serpentine and nonserpentine soil types with a phylogeny based on gene sequences of four chloroplast loci from multiple populations of each species. For five of the species, sequences from different populations were assigned to different clades. Springer suggested this was due to either chloroplast capture or recent divergence of the species in Hesperolinon.

A. C. Schneider et al. (unpubl.) compared results from ITS and a broader geographical sampling with Y. P. Springer’s (2009) cpDNA analysis. The resulting weakly supported ITS tree placed all species except Hesperolinon drymarioides, the more morphologically divergent taxon, in more than one clade. More study is needed to determine how genetic relationships relate to distribution and morphology in Hesperolinon.

Selected References

O’Donnell, R. 2010. The genus Hesperolinon (Linaceae): An introduction. Four Seasons 13(4): 1–61.

Sharsmith, H. K. 1961. The genus Hesperolinon. Univ. Calif. Publ. Bot. 32: 235–314.

Lower Taxa

Hesperolinon adenophyllum, Hesperolinon bicarpellatum, Hesperolinon breweri, Hesperolinon californicum, Hesperolinon clevelandii, Hesperolinon congestum, Hesperolinon didymocarpum, Hesperolinon disjunctum, Hesperolinon drymarioides, Hesperolinon micranthum, Hesperolinon sharsmithiae, Hesperolinon spergulinum, Hesperolinon tehamense

Key

1	Petals usually yellow, sometimes fading to white, often veined or tinged orange or reddish; anthers yellow.	> 2
2	Leaf bases keeled, clasping, margins with stalked glands on teeth in 1–2 rows.	Hesperolinon adenophyllum
2	Leaf bases flat, not clasping, margins without stalked glands.	> 3
3	Styles 2(–3) (plants sometimes with both 2- and 3-carpellate flowers).	Hesperolinon bicarpellatum
3	Styles (2–)3 (sometimes some 2-carpellate flowers on otherwise 3-carpellate plants).	> 4
4	Styles 0.5–1(–1.8) mm, included; petals not or slightly spreading.	Hesperolinon clevelandii
4	Styles 2–6(–8) mm, exserted; petals widely spreading.	> 5
5	Flowers clustered at inflorescence tips; stipular glands usually present at all nodes; petals (4–)6–7(–10) mm; filaments (4–)6–8 mm.	Hesperolinon breweri
5	Flowers widely scattered throughout inflorescence; stipular glands absent or present only at proximal nodes; petals (2–)3–5.5 mm; filaments 1.5–4 mm.	> 6
6	Plants usually hoary throughout, sometimes glabrous except on stems; petals 3.5–5.5 mm; styles 3.5–4.5(–5) mm.	Hesperolinon tehamense
6	Plants glabrous, or stems puberulent only immediately distal to nodes; petals (2–)3–4.5 mm; styles 2–3.5 mm.	> 7
7	Pedicels (2–)10–12 mm (to 40 mm in fruit); sepals 1.5–2(–3) mm.	Hesperolinon bicarpellatum
7	Pedicels 0.5–5 mm; sepals (1.5–)3 mm.	Hesperolinon sharsmithiae
1	Petals white to pink, often veined or tinged darker pink or lavender to purple; anthers white to pink, rose, or purple.	> 8
8	Proximal leaves whorled (usually in 4s) on main stem, blades ovate or orbiculate, 3–6 mm wide, margins minutely stipitate-glandular.	Hesperolinon drymarioides
8	Proximal leaves alternate on main stem, blades threadlike to linear or narrowly oblong, 0.5–2.5(–3) mm wide, margins eglandular (except sometimes in H. congestum and in H. disjunctum).	> 9
9	Styles 2.	Hesperolinon didymocarpum
9	Styles (2–)3 (sometimes some 2-carpellate flowers on otherwise 3-carpellate plants).	> 10
10	Inflorescences dense; pedicels 0.5–2(–5) mm at anthesis, to 10 mm in fruit; stipular glands present.	> 11
11	Sepals glabrous; petals white or partly pink to white, irregularly veined or flushed with pink or rose pink.	Hesperolinon californicum
11	Sepals hairy; petals pink to rose.	Hesperolinon congestum
10	Inflorescences open; pedicels 1–15(–25) mm at anthesis, to 45 mm in fruit; stipular glands absent or minute, or present only at proximal nodes.	> 12
12	Styles included; petals not or only slightly spreading, 1.5–3.5 mm (shorter than to equaling sepals).	Hesperolinon micranthum
12	Styles exserted; petals widely spreading to reflexed, (3–)4–7 mm (longer than sepals).	> 13
13	Pedicels bent, pendent in bud, 5–15(–25) mm in flower.	Hesperolinon spergulinum
13	Pedicels straight, not pendent in bud, 1–5(–8) mm in flower.	Hesperolinon disjunctum