The unusual edaphic habitats of late Tertiary lacustrine deposits in Sonoran Desert basins of central Arizona have previously been shown to harbor endemic taxa and disjunct taxa from other floristic regions, which inhabited the Sonoran Desert during previous climatic regimes. The infertile limestone soils contrast sharply with the surrounding volcanic soils, excluding the dominant Sonoran Desert vegetation and thereby providing an ecological opening for the disjuncts. The disjunctions provide clues for interpreting the biogeographical history of Arizona. Here, thirteen additional examples are documented. Taxonomic changes to two of the earlier examples, Eriogonum apachense and Hymenoxys acaulis var. arizonica, are discussed. Erigonum apachense from the San Carlos Basin is revealed to be a disjunct population of the northern E. heermannii var. argense, not a separate endemic species; and, a recently named taxon, Tetraneuris verdiensis, from the Verde Valley, is shown to be a rayless form of T. acaulis var. arizonica, not a separate endemic species. The biogeography of Ericameria nauseosa var. juncea and Quercus havardii are considered in more detail. The type locality of Ericameria nauseosa var. juncea is one of the disjunct localities, not from the main range of the variety. An unusual thicket-forming oak in the Verde Valley is determined to be a disjunct population of Quercus havardii from Staked Plains of New Mexico and Texas and the Four Corners of the Colorado Plateau.
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Vol. 58 • No. 2