White sand areas in northern South America have a distinct flora with many endemic species. These lowland areas are characteristically temporarily flooded, nutrient-poor savannas or caatingas that may have great temporal continuity. White sand areas have a scattered distribution in Amazonia, disrupted by more recent sedimentation, and are especially abundant in the upper Rio Negro basin. Similar nutrient-poor and isolated environments in this region are the summits of the tepuis in the Guayana Highlands. Using the Gentianaceae as an example we have studied the relationships of species and genera with disjunct white sand and highland distributions. Other species are widespread, and their relationships to the narrow endemics are investigated. We are focusing on the Irlbachia generic complex using both molecular (trnL intron and ITS sequences) and morphological data. Irlbachia sensu stricto has 5 white sand distributed species, 2 highland species, and one widespread species. Two recently discovered white sand species from Brazil fall within the Irlbachia complex, Aripuana cullmaniorum from the Rio Aripuanã region south of the Amazon, and an unnamed taxon from Roraima State north of the Amazon. Aripuana is unique among Gentianaceae in having partly valvate corolla lobes, and the unnamed plant has erect, tubular, orange corollas that are unlike those of any related genera. A preliminary analysis of trnL sequences places Aripuana between the basal most genus, Calolisyanthus (from the Brazilian Highland), and all other included taxa of the Irlbachia complex. Most of the latter are found to the north and west of the Amazon, which suggests that relationships of white sand endemics will be crucial to an understanding of the biogeographic history of the Amazon basin and the highlands at its northern and southern borders. This research is supported by the Lewis B. and Dorothy Cullman Foundation.

Key words: Amazonia, biogeography, Gentianaceae, molecular systematics, white sand