Are Floristic Areas Hierarchically Arranged?

Floristic geographers have devised hierarchical classifications of floristic areas (choria) as a useful means of organizing information. The objective of this study was to determine whether floristic areas are actually arranged in any sort of natural hierarchy. For the western U.S.A. I de- veloped two sets of three choria of variable sizes, one based on ranges of species (narrow, intermediate and wide- spread), and one based on taxonomic rank (species, genera and families). Each set of choria showed a high degree of hierarchical structure, and the two higher levels of the