Most American species of Arabis have been removed into the genus Boechera, a quite different evolutionary lineage characterized by a different base chromosome number (x = 7 rather than x = 8 in Arabis). The chromosome number is not very useful in the field, but most species of Arabis are Eurasian, represented in Michigan by garden escapes; the only native species is A. pycnocarpa, with narrow, erect and appressed fruits. Our Boechera species, all native, have either broader fruits (if tightly appressed) or spreading or even reflexed or pendent fruits. See Al-Shehbaz (2003), Kiefer et al. (2009), and the references under Boechera for more discussion.

1. Cauline leaves not clasping at the base; plant with well developed stolons; leaves all entire, at least the basal ones with appressed medifixed hairs on margins and midrib beneath.

A. procurrens

1. Cauline leaves clasping with auriculate or sagittate bases; plant without stolons; leaves ± toothed or lobed, pubescent or glabrous but without medifixed hairs.

2. Fruiting pedicels ± spreading or divaricate, the fruits clearly spreading from the axis; petals 10–19 mm long; plant a procumbent tufted escape from cultivation with numerous sterile rosettes; fibrous rooted.

A. caucasica

2. Fruiting pedicels strongly ascending to appressed, the fruits erect and closely appressed to the stem; petals 3.5–5.5 mm long; plant a tall native with few or no sterile rosettes, tap-rooted.

A. pycnocarpa

All species found in Arabis

Arabis caucasicaWALL ROCK CRESS 
Arabis procurrensHUNGARIAN ROCK CRESS 
Arabis pycnocarpaHAIRY ROCK CRESS 


MICHIGAN FLORA ONLINE. A. A. Reznicek, E. G. Voss, & B. S. Walters. February 2011. University of Michigan. Web. March 25, 2017.