Cruciferae (Brassicaceae), in the mustard family of the caper order (Capparales), are found on all continents except Antarctica. The cruciferae, so named because of the uniform, four-petaled flowers suggestive of a Greek cross, are an example of a natural family and demonstrate a large amount of diversity. Although most are weeds, the family includes significant food crop plants such as broccoli, cabbage, turnip, and radish. Cruciferae are most abundant in areas north of the equator and exhibit greatest variety in temperate and arid regions. The Mediterranean region is generally considered the site of the family’s origination. Nonetheless, many of these cultigens appear to be native to northern Europe, and Reed C. Rollins (1993: 1) contends that the Irano–Turanian region of Eastern Europe and western Asia was the birthplace of at least some members of this plant family. A precise number of species and genera of the cruciferae is undetermined, although estimates range from 340 to 400 genera and 3,000 to 3,500 species (Vaughan, Macleod, and Jones 1976: vii; Rollins 1993: 2).

Taxonomy

The classification of cruciferae presents a challenge because of the large number of members and their unusually homogeneous nature (Hedge and Rechinger 1968: 1). But basic characteristics, both macroscopic and microscopic, mark the family as a whole. Typically the radial flower is characterized by the uniformity of its structure. This already mentioned flower type, four petals in the shape of a Greek cross, is common to a large majority of this family’s species. However, this pattern is altered by deviations, particularly in the structure of the stamen, f lowers, and calyx. This is true of genera such as Romanschulzia, Stanleya, and Warea, and some species of Streptanthus, Lepidium, and Megacarpaea (Hedge and Rechinger 1968: 1–2; Rollins 1993: 2).