Specialization of the mesophyll in grasses is well known, especially with reference to its correlation with photosynthetic pathway. In cross-sectional view, the mesophyll has been classified as radiate or non-radiate, and several cell forms have been identified, including tabular cells, isodiametric cells, fusoid cells, and various lobed cells (e.g., arm cells). With the availability of a robust phylogeny of the grass family, we undertook a survey of the major clades in order to understand better the evolution of mesophyll in grasses. We included taxa of both forest and open habitats whenever possible and cross and longitudinal sections were examined. The results were mapped on the Grass Phylogeny Working Group phylogeny, but character distribution on an alternative topology was also explored. Our results suggest that arm cells, one of the cellular forms in which lobing/invagination shows up in cross-sectional view, is polymorphic in the Anomochlooideae, but is characteristic of Pharoideae, Puelioideae and Bambusoideae, and then is lost in the Pooideae and the PACCAD Clade. Fusoid cells show a similar evolutionary pattern. Rosette cells, previously lumped with arm cells, are interpreted as a potential synapomorphy for the Bistigmatic Clade (Puelioideae + [BEP+PACCAD]). Rosette cells are common in the mesophyll in Ehrhartoideae but usually occur in combination with arm cells in Bambusoideae. The presence of rosette cells in a few members of the Pooideae and the PACCAD clade may represent retention of this character. Longitudinally elongated lobed cells (LEL cells) previously known from a few taxa are here found to be a potential synapomorphy for the Pooideae + PACCAD Clade, under the alternative topology. We also report on some additional observations in the PACCAD Clade.

Key words: anatomy, arm cells, mesophyll, Poaceae, rosette cells