any of the 120 species of the genus Arabis, herbs belonging to the mustard family (Brassicaceae), found throughout the Northern Hemisphere and in mountainous areas of Africa. Some are cultivated as ornamentals for their white, pink, or purple four-petalled flowers. Rock cresses are either erect or form mounds and bear long, narrow seedpods. Wall rock cress, or garden arabis (A. caucasica), a perennial from southeastern Europe, reaches 30 cm (1 foot) in height and bears fragrant white flowers in early spring; it has double, pink, dwarf, and variegated varieties. Other members of the genus range in size from the tower mustard (A. glabra), which is nearly 1.7 metres (5.5 feet) tall, to the bluish rock cress (A. caerulea), an alpine perennial only 10 cm (4 inches) tall.