Aristida is one of three genera in the grass tribe Aristideae along with Sartidia and Stipagrostis. Composed of ca. 330–360 species, Aristida is distributed in the tropics and subtropics, growing mostly on grassy plains, arid lands, and open ground. Studies of this taxonomically difficult genus have relied primarily on morphology, anatomy, and karyology. Taxonomic groupings have been based principally on inflorescence architecture and awn characters. Henrard (1926, 1929) recognized seven sections within the genus. Since Henrard’s contributions little revisionary work has been done. Towards an improved understanding of the relationships and evolution of Aristida, we are gathering data from macromorphology, lemma micromorphology, leaf blade anatomy, and the nuclear (internal transcribed spacers; ITS) and chloroplast (trnT-L-F region) genomes. All of the sections of Aristida and as well as the related genera are being sampled and, as well, one or a few of the problematic species complexes in North America (e.g., the A. purpurea complex) are being targeted. The overarching goal of this research is to test the existing taxonomic circumscriptions and gain insight into character evolution, phytogeography, and ecology of this widespread and diverse genus. Sequences from the ITS region of 34 New World species of Aristida have proven sufficiently variable for phylogenetic inference. Three of Henrard's sections are not monophyletic. However, some species complexes centered around A. adscensionis and A. purpurea are well supported by bootstrap analysis. Additional sampling of taxa from different sections of the genus Aristida, as well as Sartidia, and Stipagrostis, should provide much better resolution and confidence in the phylogenies and provide a better overview of the relationships within this group.

Key words: Aristida, Gramineae, internal transcribed spacers (ITS), phylogenetics