Boechera arcuata |
Boechera retrofracta |
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arching rockcress, elegant rockcress |
dangling suncress, reflexed rock-cress, secund rock-cress |
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Habit | Perennials; usually long-lived; sexual; caudex often woody (well-developed). | Biennials or perennials; short-lived; sexual; caudex present or absent. |
Stems | usually 1 per caudex branch, arising from center of rosette, elevated on woody base or from ground surface, (2–)3–8 dm, densely pubescent proximally, trichomes short-stalked, 2-rayed and simple, to 1 mm, pubescent distally. |
usually 1 per caudex branch, arising from center of rosette near ground surface, 1.5–7(–10.5) dm, densely pubescent proximally, trichomes short-stalked, 2–8-rayed, 0.1–0.2 mm, sparsely pubescent distally. |
Basal leaves | blade linear to oblanceolate, 2–7(–12) mm wide, margins usually entire, rarely denticulate, ciliate along petiole, trichomes to 1.5 mm, surfaces densely pubescent, trichomes usually short-stalked, 2–5-rayed (rarely some simple), 0.4–0.8 mm. |
blade oblanceolate, 2–7 mm wide, margins entire or shallowly dentate, sometimes ciliate at petiole base, surfaces densely pubescent, trichomes short-stalked, 5–10-rayed, 0.05–0.2 mm. |
Cauline leaves | 10–30(–45), often concealing stem proximally; blade auricles 2–5(–6) mm, surfaces of distalmost leaves pubescent. |
15–40, concealing stem proximally; blade auricles 0.5–2.5 mm or, rarely, absent, surfaces of distalmost leaves pubescent. |
Racemes | 12–50(–70)-flowered, usually unbranched. |
15–80(–140)-flowered, usually unbranched. |
Flowers | ascending at anthesis; sepals pubescent; petals purple, 9–14 × 2–4 mm, glabrous or sparsely pubescent (trichomes abaxially); pollen ellipsoid. |
pendent at anthesis; sepals pubescent; petals white to lavender, 4–8 × 0.8–2.2 mm, glabrous; pollen ellipsoid. |
Fruiting pedicels | usually divaricate-ascending, rarely horizontal, gently recurved or straight, 8–22 mm, pubescent, trichomes subappressed, 2–4-rayed. |
reflexed, abruptly recurved at base, otherwise usually straight, 7–12(–18) mm, pubescent, trichomes appressed, branched. |
Fruits | usually divaricate-ascending, rarely horizontal, not appressed to rachis, not secund, usually curved, edges parallel, (6–)8–13 cm × 1.5–2.2 mm; valves glabrous or trichomes relatively few, scattered; ovules 90–250 per ovary; style 0.01–0.5 mm. |
strongly reflexed, usually appressed to rachis, sometimes somewhat secund, straight, edges parallel, 3.5–9 cm × 0.9–1.8 mm; valves glabrous or sparsely pubescent throughout; ovules 60–116 per ovary; style 0.5–2 mm. |
Seeds | uniseriate or sub-biseriate, 1.5–1.7 × 1–1.2 mm; wing continuous, 0.1–0.2 mm wide. |
uniseriate, 1–1.4 × 0.9–1.2 mm; wing continuous, to 0.1 mm wide. |
2n | = 14. |
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Boechera arcuata |
Boechera retrofracta |
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Phenology | Flowering Mar–Jun. | Flowering Apr–Aug. |
Habitat | Rocky hillsides and cliffs in pine forests and chaparral | Rock outcrops, open hillsides, gravel bars and sandy banks in grassland, sagebrush, oak woodlands, and open conifer forests |
Elevation | 300-1800 m (1000-5900 ft) | 300-3300 m (1000-10800 ft) |
Distribution |
CA
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AK; CA; ID; MI; MT; NV; OR; UT; WA; WY; AB; BC; MB; NT; ON; QC; SK
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Discussion | Although usually treated as a variety of Arabis (Boechera) sparsiflora (e.g., R. C. Rollins 1993), B. arcuata is easily distinguished from that species by having rachises and fruiting pedicels pubescent with subappressed, 2–4-rayed trichomes and a geographic range limited to southern and western California. By contrast, B. sparsiflora has rachises and fruiting pedicels with spreading, usually simple trichomes (sometimes glabrescent) and an allopatric distribution north and east of the Sierra Nevada. (Discussion copyrighted by Flora of North America; reprinted with permission.) |
Though often treated as a variety of Arabis (Boechera) holboellii (e.g., R. C. Rollins 1993; G. A. Mulligan 1996; N. H. Holmgren 2005b), B. retrofracta is easily distinguished from that species by having narrower (0.9–1.8 versus 2–2.5 mm), mostly non-secund fruits that are almost always appressed to rachises. The two taxa have allopatric distributions, with B. retrofracta found on the North American continent (mostly west of the Great Plains) and B. holboellii apparently confined to Greenland. Boechera retrofracta has formed hybrids with at least 12 other species. Besides differing in macromorphological characters, all those hybrids are distinct from B. retrofracta in the strict sense in having wider (20–30 versus 13–16 µm), spheroid pollen grains with asymmetric colpi. Arabis kochii Blankinship is an illegitimate name, sometimes found in synonymy with Boechera retrofracta. (Discussion copyrighted by Flora of North America; reprinted with permission.) |
Source | FNA vol. 7, p. 365. | FNA vol. 7, p. 402. |
Parent taxa | Brassicaceae > tribe Boechereae > Boechera | Brassicaceae > tribe Boechereae > Boechera |
Sibling taxa | ||
Synonyms | Streptanthus arcuatus, Arabis holboellii var. arcuata, Arabis maxima, Arabis sparsiflora var. arcuata | Arabis retrofracta, Arabis arcuata var. secunda, Arabis exilis, Arabis holboellii var. retrofracta, Arabis holboellii var. secunda, Arabis lignipes, Arabis retrofracta var. multicaulis, Arabis secunda, Arabis sparsiflora subsp. secunda, Arabis tenuis, B. exilis, B. holboellii var. secunda |
Name authority | (Nuttall) Windham & Al-Shehbaz: Harvard Pap. Bot. 11: 64. (2006) | (Graham) Á. Löve & D. Löve: Taxon 31: 125. (1982) |
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