Field & Lab. 19: 71. 1951.
Perennials, (10–) 20–70 (–110) cm; taprooted. Stems 1–40+, decumbent to ascending or erect (sometimes ± brittle, sometimes reddish-brown), moderately to densely hispido-strigose (long-spreading hairs often broken off), sometimes moderately hairy and densely stipitate-glandular distally (axillary leaf fascicles sometimes present). Leaves: margins flat to strongly undulate; basal and proximal usually withering by flowering; proximal cauline subsessile, blades oblanceolate, (8–) 15–40 (–60) × (2.4–) 4–8 (–11) mm, bases cuneate, margins entire, strigoso-ciliate (proximal hairs longer, spreading), apices acute, faces moderately to densely hispido-strigose; distal sessile, blades usually narrowly to broadly lanceolate, sometimes oblanceolate, 6.5–40 × 1.5–8 mm, usually reduced distally, bases rounded, faces sparsely to densely hispido-strigose or short-strigose (5–245 hairs/mm2), sparsely to densely glandular (0–50 glands/mm2). Heads (1–) 17–36 (–126) in corymbiform or paniculiform arrays, branches ascending. Peduncles 2–100 mm, densely hispid, strigose, or glandular; bracts 2–5+, proximal lanceolate, leaflike, distal reduced, phyllary-like, (2–) 2.5–8.5 (–13) × (0.3–) 0.5–1.8 (–4) mm, sometimes a few, large, leafy bracts proximal to heads. Involucres cylindric, turbinate, or campanulate (campanulate upon drying), (6–) 7.5–11 (–15) mm. Phyllaries in 4–6 series, mid narrowly triangular, unequal (outer lengths 1/5–1/4 inner), margins hyaline, fimbriate-ciliate apically, faces very sparsely to moderately strigose, very sparsely to moderately stipitate-glandular. Ray-florets (4–) 7–15 (–24); laminae (3.5–) 4.5–10.5 (–18.5) × (0.7–) 1–1.7 (–2.4) mm. Disc-florets (9–) 20–50 (–81); corollas ± ampliate, (4–) 5.5–7 (–9.5) mm, glabrous to glabrate, lobes 0.4–1 mm, sparsely pilose (hairs 0.1–1 mm, osteolate-celled ones often fragile). Cypselae monomorphic, obconic, compressed, (1.3–) 2–3 (–4.5) mm, ribs 6–10, faces sparsely to moderately strigose; pappi off-white, outer of linear scales 0.25–0.5 mm, inner of 25–45 bristles 5–8 (–10) mm, longest attenuate to weakly clavate. 2n = 18, 36.
Calif., nw Mexico
Subspecies 4 (4 in the flora).
Except for Heterotheca monarchensis, H. sessiliflora is the only species in the section with long (more than 0.25 mm), fragile, osteolate-celled hairs on the corolla lobes, and it is the only species native to the central and southern coastal ranges and valleys of California. The species is divided into four subspecies and five varieties differentiated on the basis of indument features, degree of waviness of the leaf margins, and to a lesser extent, stem height and leaf shape. The treatment here is based on J. C. Semple (1996), which includes a key to and descriptions, illustrations, and distribution maps of the varieties.
|1||Distal leaf margins distinctly undulate; disc corolla lobe hairs 0.1–1 mm||> 2|
|1||Distal leaf margins weakly undulate to flat; disc corolla lobe hairs 0.2–2.5 mm||> 3|
|2||Distal leaves green, not stiff, usually densely glandular in arrays; heads often subtended by 1–3 large, leaflike bracts equaling or surpassing them, peduncle bracts sometimes phyllary-like distally; coast, dunes of south coastal ranges, 0–100 m, s California, nw Baja California (Mexico)||Heterotheca sessiliflora subsp. sessiliflora|
|2||Distal leaves whitish, stiff; heads not subtended by leaflike bracts surpassing them, bracts reduced distally; (150–)300–1800(–2200) m, Ventura e to e San Bernadino Mountains, Mt. Palomar||Heterotheca sessiliflora subsp. fastigiata|
|3||Proximal stems hispid; distal leaves elliptic to lanceolate, reduced distally (in tall plants), margins sometimes somewhat undulate, faces strigoso-hispid; inland hills and valleys, c California||Heterotheca sessiliflora subsp. echioides|
|3||Proximal stems sparsely hispido-strigose; distal leaves oblanceolate, margins flat, faces long-strigose, little reduced distally; dunes, headlands, Mendocino to Santa Cruz (rarely to s of Monterey), San Francisco Bay area at 10–150 m||Heterotheca sessiliflora subsp. bolanderi|
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