Arabis

Linnaeus

Sp. Pl. 2: 664. 1753. 1753

,

Gen. Pl. ed. 5, 298. 1754. 1754

Common names: Rockcress
Etymology: Latin Arabia
Treatment appears in FNA Volume 7. Treatment on page 257. Mentioned on page 226, 234, 242, 258, 264, 348, 349, 449, 450, 492, 561, 720.

Annuals, biennials, or perennials; (sometimes stoloniferous with vegetative rosettes, or caudex simple or branched); not scapose; often pubescent or hirsute, sometimes glabrous or glabrate, trichomes stalked, stellate, sometimes mixed with fewer, simple or forked ones. Stems erect, ascending, or decumbent, unbranched or branched distally. Leaves basal and cauline; petiolate or sessile; basal rosulate, petiolate or sessile, blade margins usually entire or dentate to denticulate, rarely lyrate-pinnatifid; cauline usually sessile, rarely shortly petiolate, blade (base often auriculate, sagittate, or amplexicaul), margins entire or dentate. Racemes (sometimes paniculate, usually simple, sometimes branched). Fruiting pedicels erect, ascending, or divaricate, (not reflexed or secund), slender. Flowers: sepals erect or ascending, ovate or oblong, lateral pair saccate or not basally, (margins membranous); petals white, pink, or purple, usually spatulate, oblong, or oblanceolate, rarely obovate, claw differentiated from blade, (shorter than sepals, apex obtuse or rounded); stamens tetradynamous; filaments usually not dilated basally; anthers ovate, oblong, or linear, (apex obtuse); nectar glands confluent, subtending bases of stamens, lateral glands semiannular or annular, median glands rarely absent, (sometimes toothlike and distinct). Fruits siliques, usually sessile, rarely shortly stipitate, linear, smooth or torulose, (usually straight, sometimes slightly curved), flattened, latiseptate; valves (papery), each with obscure or prominent midvein, glabrous; replum (visible), rounded; septum complete, (membranous, translucent, veinless); ovules 10–86[–110] per ovary; style obsolete or distinct; stigma capitate, (sometimes slightly 2-lobed). Seeds uniseriate, flattened, winged or margined, oblong or orbicular; seed coat (smooth or minutely reticulate), not mucilaginous when wetted; cotyledons accumbent. x = 8.

Distribution

North America, Europe, Asia, n, alpine, c, e Africa.

Discussion

Species ca. 70 (15 in the flora).

As treated by most North American authors (e.g., M. Hopkins 1937; R. C. Rollins 1941, 1993; G. A. Mulligan 1996), Arabis was so broadly delimited that it included species presently assigned to six genera in five tribes (I. A. Al-Shehbaz et al. 2006). These are: Arabidopsis and Turritis (Camelineae), Arabis (Arabideae), Boechera (Boechereae), Pennellia (Halimolobeae), and Streptanthus (Thelypodieae) (see Al-Shehbaz 2003b; M. D. Windham and Al-Shehbaz 2006, 2007).

R. C. Rollins (1993) recognized 80 species of Arabis in North America, of which 24 were divided into 40 varieties. By contrast, G. A. Mulligan (1996) recognized 30 species and six varieties in Canada alone. The combination of characteristics that they used to circumscribe the genus (e.g., linear and latiseptate fruits, accumbent cotyledons, and often branched trichomes) evolved repeatedly in Brassicaceae and cannot be relied on in the delimitation of genera (O. Appel and I. A. Al-Shehbaz 2003). The vast majority of North American species have been transferred to Boechera (x = 7), a genus distinct morphologically, cytologically, and molecularly (Al-Shehbaz 2003b; M. D. Windham and Al-Shehbaz 2006, 2007, 2007b). As currently circumscribed, Arabis (x = 8) is a primarily Eurasian genus with only 15 species in North America.

Key

1 Cauline leaves: blade bases auriculate, subcordate, amplexicaul or subamplexicaul > 2
1 Cauline leaves: blade bases not auriculate or amplexicaul, sometimes cuneate > 7
2 Perennials, stoloniferous; lateral sepals conspicuously saccate basally; fruits ascending to spreading > 3
2 Biennials or perennials, not stoloniferous; lateral sepals not or slightly saccate basally, or not conspicuously so; fruits erect, erect-ascending, or nearly so (except A. patens) > 4
3 Petals 5-8(-9) × 2-3.5 mm; fruits 1.7-3.5(-4) cm; basal leaves: blade trichomes stellate with simple rays; sepals 2.5-4(-4.7) mm. Arabis alpina
3 Petals 10-19 × 4-8 mm; fruits (3-)4-7 cm; basal leaves: blade trichomes stellate with some rays branched; sepals 4.5-8 mm. Arabis caucasica
4 Fruits divaricate to erect-ascending (not appressed to rachises); ovules 16-28 per ovary. Arabis patens
4 Fruits usually erect, rarely erect-ascending (often appressed to rachises); ovules 38-86 per ovary > 5
5 Fruits 0.7-0.8 mm wide; petals 6-9 mm; basal leaf blades: abaxial surface with predominantly subsessile, stellate trichomes; Alabama, Georgia. Arabis georgiana
5 Fruits 0.8-1.8(-2) mm wide; petals 3.5-5(-5.5) mm (longer in A. eschscholtziana); basal leaf blades: abaxial surface usually with mix of simple, forked, and stalked, stellate trichomes, rarely subglabrate; not Alabama, not Georgia > 6
6 Petals 3.5-5(-5.5) mm; lateral sepals not saccate basally; fruits 0.8-1 (-1.2)mm wide; cauline leaves (7-)10-45(-61), overlapping or not. Arabis pycnocarpa
6 Petals (5.5-)6.5-9(-10) mm; lateral sepals saccate basally; fruits 1.2-1.8(-2) mm wide; cauline leaves (2-)4-12(-18), not overlapping or rarely so. Arabis eschscholtziana
7 Petals white, 3.5-11 mm > 8
7 Petals purple or pink, 8-18(-20) mm > 11
8 Trichomes of basal leaf blades short-stalked, stellate. Arabis crucisetosa
8 Trichomes of basal leaf blades simple and forked, or only simple > 9
9 Fruits and fruiting pedicels erect, (sub)appressed to rachises; petals 3.5-4 mm. Arabis olympica
9 Fruits and fruiting pedicels ascending to divaricate or suberect, not appressed to rachises; petals (5-)6-11 mm > 10
10 Fruits 1.7-2.2 mm wide; seeds winged, (1.5-)1.8-2.5(-3) mm; petals 7-11. Arabis furcata
10 Fruits 0.8-1.2(-1.5) mm wide; seeds not winged, 0.6-1.2(-1.5) mm; petals (5-)6-8 mm. Arabis nuttallii
11 Basal leaf blades: surfaces with short-stalked, stellate trichomes. Arabis modesta
11 Basal leaf blades: surfaces with simple, forked, and sometimes 2-, 3-, or 4-rayed trichomes, or, rarely, with fewer, stellate ones > 12
12 Seeds orbicular or nearly so, 2-2.5 mm diam.; fruits 2-3 mm wide. Arabis blepharophylla
12 Seeds oblong, 1-1.3 mm wide; fruits 1.5-2 mm wide > 13
13 Plants glabrous or with simple trichomes (terminating teeth of basal leaf blades); simple trichomes not bulbous-based; fruits 2-4 cm. Arabis mcdonaldiana
13 Plants moderately pubescent with simple and forked trichomes; simple trichomes often bulbous-based; fruits (3-)3.5-6.5 cm > 14
14 Plants with at least some 3-rayed, stellate trichomes; cauline leaves 1-3(-6) cm; fruits slightly torulose. Arabis oregana
14 Plants without 3-rayed, stellate trichomes; cauline leaves 0.4-1(-1.5) cm; fruits not torulose. Arabis aculeolata
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