Flora of North America species comparison

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Arabis

rock-cress
Habit 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).
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).
Fruiting pedicels

erect, ascending, or divaricate, (not reflexed or secund), slender.
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.

Arabis
Distribution
from USDA
North America; Europe; Asia; n, alpine; c Africa; e Africa
[BONAP county map]
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.

(Discussion copyrighted by Flora of North America; reprinted with permission.)
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.
A. 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.
A. caucasica
4. Fruits divaricate to erect-ascending (not appressed to rachises); ovules 16-28 per ovary.
A. 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.
A. 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.
A. 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.
A. 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.
A. 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.
A. 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.
A. 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.
A. nuttallii
11. Basal leaf blades: surfaces with short-stalked, stellate trichomes.
A. 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.
A. 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.
A. 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.
A. oregana
14. Plants without 3-rayed, stellate trichomes; cauline leaves 0.4-1(-1.5) cm; fruits not torulose.
A. aculeolata
Source FNA vol. 7, p. 257. Author: Ihsan A. Al-Shehbaz.
Parent taxa Brassicaceae > tribe Arabideae
Subordinate taxa
A. aculeolata, A. alpina, A. blepharophylla, A. caucasica, A. crucisetosa, A. eschscholtziana, A. furcata, A. georgiana, A. mcdonaldiana, A. modesta, A. nuttallii, A. olympica, A. oregana, A. patens, A. pycnocarpa
Name authority Linnaeus: Sp. Pl. 2: 664. (1753): Gen. Pl. ed. 5, 298. (1754)
Web links 
Unless otherwise noted, all data comes from the online version of the Flora of North America, who have made most of their content available via a Creative Commons license (CC BY 4.0).



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