Arabis horizontalis Greene, Leafl. Bot. Observ. Crit. 2: 74. 1910; A. suffrutescens S. Watson var. horizontalis (Greene) Rollins

Perennials; long-lived; (ces-pitose); 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.

Flowering Jul-Aug. Dry pumice slopes; Oreg.

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.