ABSTRACT
The caatinga represents the largest and most isolated of the South American dry forests, occupying more than 850,000 km2 in the semi-arid region of north-eastern Brazil. Leguminosae is the best represented family in the caatinga, with 293 species in 77 genera, and comprises almost one-third of the total plant diversity there. The geographical distribution patterns of its taxa define seven major centres of endemism. Similarity analyses combined with these patterns reveal two major floristic groups with distinct composition and phenological traits. One group occupies c.70% of a mostly continuous surface of soils primarily derived from crystalline basement rocks. The other group occurs on disjunct and sandy sedimentary surfaces. The data support a scenario wherein the sedimentary areas became dissected during a huge process of pediplanation during the Tertiary period, which promoted both the vicariance of its flora and opened the way to the expansion of the flora related to the seasonally dry tropical forests.
