The limits of Arabidopsis have been the subject of long controversy, and more than 50 species were placed in the genus (I. A. Al-Shehbaz et al. 1999). The delimitation of the genus was based primarily on the presence of branched trichomes, linear fruits, and accumbent or incumbent cotyledons. That combination of characteristics evolved independently multiple times in Brassicaceae. Extensive molecular data and critical evaluation of morphology have shown that Arabidopsis is polyphyletic (S. L. O´Kane and Al-Shehbaz 2003). Nine of the ten species in the genus are native to Europe; only A. arenicola is endemic to North America.