The Stipeae is a widely distributed tribe of grasses of 400-500 species. Nassella is one of the largest genera in the Stipeae, comprising 112 species, of which 104 are native to South America. This study is part of a larger project being undertaken by the Stipeae Working Group to explore the diversity within the Stipeae with the ultimate goal of developing a strongly supported taxonomic revision of the tribe. We compared the cross-sectional leaf anatomy of 28 species of Nassella with species from Aciachne, Jarava, Piptochaetium and Stipa subg. Pappostipa. The species of Nassella were chosen to reflect the ecological and geographic distribution of the genus and include all 6 species native to North America. Anatomically, the species of Nassella fall into two major groups, those with valvate leaves and those with flat leaves. The species with valvate leaves have a cross-sectional leaf anatomy similar to that found in the 15 species Stipa subg. Pappostipa examined and approach that of Aciachne. The leaf anatomy of the species with flat leaves was more similar to that of species of Jarava [= Stipa subg. Jarava]. The anatomy of Nassella lepida is intermediate between that of the two groups. The cross-section of Nassella planaltima is similar in some respects to that of Piptochaetium but differs in having upper and lower girders associated with its vascular bundles and in having no sclerenchyma in its phloem.

Key words: ecology., leaf anatomy, Nassella, South America, Stipoid