73. Boechera pinetorum (Tidestrom) Windham & Al-Shehbaz, Harvard Pap. Bot. 11: 271. 2007.
Arabis pinetorum Tidestrom, Proc. Biol. Soc. Wash. 36: 182. 1923; A. divaricarpa A. Nelson var. pinetorum (Tidestrom) B. Boivin; A. holboellii Hornemann var. pinetorum (Tidestrom) Rollins; Boechera holboellii (Hornemann) Á. Löve & D. Löve var. pinetorum (Tidestrom) Dorn
Perennials; short-lived; apomictic; caudex not woody. Stems usually 1 per caudex branch, arising from center of rosette near ground surface, 2-8(-9.6) dm, densely pubescent proximally, trichomes simple and short-stalked, 2-4-rayed, to 1 mm, sparsely pubescent or glabrous distally. Basal leaves: blade oblanceolate, 2-8(-11) mm wide, margins entire or denticulate, ciliate proximally, trichomes (simple), 0.5-1 mm, surfaces sparsely to densely pubescent, trichomes short-stalked, 2-4 (or 5)-rayed, 0.2-0.5 mm, these mixed with fewer simple ones. Cauline leaves: 7-33, often concealing stem proximally; blade auricles 1-3 mm, surfaces of distalmost leaves sparsely pubescent or glabrous. Racemes 15-63 -flowered, usually unbranched. Fruiting pedicels reflexed, abruptly recurved near base, otherwise straight, 4-12 mm, sparsely pubescent or glabrous, trichomes spreading, simple and 2-rayed. Flowers divaricate to pendent at anthesis; sepals pubescent; petals white to lavender, 5-6 × 1-1.5 mm, glabrous; pollen spheroid. Fruits reflexed to closely pendent, not appressed to rachis, not secund, straight or curved, edges parallel, (4.5-)5.5-8.5 cm × 1.5-2 mm; valves glabrous; ovules 70-110 per ovary; style 0.05-0.4 mm. Seeds uniseriate, 1.5-1.8 × 1.2-1.5 mm; wing continuous, 0.15-0.3 mm wide.
Flowering May-Jul. Rock outcrops and gravelly soil in meadows and open conifer forests; 1100-3200 m; Calif., Nev., Oreg.
Most authors (e.g., R. C. Rollins 1993; R. D. Dorn 2001; S. L. Welsh et al. 2003; N. H. Holmgren 2005b) have treated Boechera pinetorum as a variety of Arabis (Boechera) holboellii. Under this guise, the name has been applied to a vast array of plants collected throughout western North America. This includes a diversity of sexual diploids plus nearly every hybrid containing a genome from B. retrofracta. Based on re-examination of the type collection, we have adopted a much narrower concept of the species. Morphological evidence suggests that B. pinetorum is an apomictic triploid hybrid containing three different genomes, derived from B. rectissima, B. retrofracta, and B. sparsiflora. Plants closely resembling the type of B. pinetorum are confined to the northern Sierra Nevada and adjacent southern Cascades. The majority of collections previously associated with the epithet pinetorum represent B. pauciflora (see M. D. Windham and I. A. Al-Shehbaz 2007 for detailed comparison).