Boechera horizontalis

(Greene) Windham & Al-Shehbaz

Harvard Pap. Bot. 11: 266. 2007.

Basionym: Arabis horizontalis Greene Leafl. Bot. Observ. Crit. 2: 74. 1910
Synonyms: Arabis suffrutescens var. horizontalis (Greene) Rollins
Treatment appears in FNA Volume 7. Treatment on page 384. Mentioned on page 360, 361.

Perennials; long-lived; (cespitose); apomictic; caudex woody. Stems usually 1 per caudex branch, arising from center of rosette, near ground surface or somewhat elevated on woody base, or 1–3.5 dm, sparsely pubescent proximally, trichomes short-stalked, 3–6-rayed, 0.1–0.3 mm, glabrous distally. Basal leaves: blade narrowly oblanceolate, 1–5 mm wide, margins entire, ciliate near petiole base, trichomes (simple), to 0.4 mm, surfaces densely pubescent, trichomes short-stalked, 3–6-rayed, 0.1–0.3 mm. Cauline leaves: 3–13, often concealing stem proximally; blade auricles 0.5–1.5 mm, surfaces of distalmost leaves glabrous. Racemes 5–32-flowered, usually unbranched. Fruiting pedicels horizontal to descending, straight or slightly curved downward, 4–11 mm, glabrous. Flowers divaricate-ascending at anthesis; sepals pubescent; petals lavender to purple, 5–6 × 1.5–2 mm, glabrous; pollen spheroid. Fruits horizontal or descending, not appressed to rachis, secund, straight, edges slightly undulate (not parallel), 2–4 cm × 2–3 mm; valves glabrous; ovules 40–54 per ovary; style 0.2–0.5 mm. Seeds uniseriate, 2–2.5 × 1.7–2 mm; wing continuous, 0.5–1 mm wide.

Phenology: Flowering Jul–Aug.
Habitat: Dry pumice slopes


Morphological evidence suggests that Boechera horizontalis is an apomictic species that arose through hybridization between B. lemmonii and B. suffrutescens (M. D. Windham and I. A. Al-Shehbaz 2007). Boechera horizontalis is known only from the vicinity of Crater Lake in south-central Oregon.

Selected References


Lower Taxa