Cladistic parsimony analysis of internal transcribed spacer (ITS) region nrDNA sequences indicates a sister relationship between the North American chloridoid grass genera Hilaria (8 or 9 spp.) and Pleuraphis (3 spp.). Introgression between P. jamesii and P. rigida, previously inferred from morphology, is evidenced in the ITS phylogeny. Hilaria is composed of three main lineages: (A) H. semplei, (B) H. belangeri, H. cenchroides, and H. swallenii, and (C) H. annua, H. ciliata, H. hintonii, and one, possibly two, undescribed species. Characters from morphology and leaf blade transectional anatomy support the Hilaria and Pleuraphis clades and the three lineages within Hilaria. However, species circumscription within Hilaria clade B remains uncertain. Much variation exists in the leaf blade structure of these genera. Pleuraphis possesses a suite of anatomical and cytological features that suggests the NAD malic enzyme (NAD-ME) photosynthetic pathway. Hilaria species in clade C exhibit a blade structure indicative of the PEP carboxykinase (PCK) pathway, whereas H. semplei and species in clade B are intermediate between clade C and Pleuraphis with respect to the anatomical/cytological characters that predict the subtype of C4 photosynthesis.

Key words: Chloridoideae, Gramineae, Hilaria, leaf anatomy, phylogenetics, Pleuraphis