The Carex straminea complex (section Ovales) consists of five species limited to hydric communities of eastern North America. Based upon gross morphology and perigynium structure, these species are closely aligned with the Carex albolutescens complex but not the western species Carex feta. An SEM investigation of the micromorphology of achene and style epidermis from 15 species of Carex section Ovales demonstrated much interspecific variation. As a result, these data could not confirm the relationship between the C. straminea complex and the C. albolutescens complex but did support the delimitation of section Ovales. Numerical analysis showed that characters of the inflorescence and perigynium are satisfactory for separating C. straminea from Carex hormathodes, a species limited to the Atlantic coast. Likewise, characters of inflorescence, pistillate scale, and perigynium could clearly separate the geographically widespread Carex alata from the Florida endemic species Carex vexans. A fifth and relatively distinctive species, Carex suberecta, was found to have a narrow ecological preference for fens and a distribution limited to the upper Midwest, with outlying populations in western Virginia. Putative natural hybrids are documented between Carex scoparia and Carex longii and some species in the C. straminea complex. Key words: Carex, Cyperaceae, section Ovales, taxonomy, SEM, biogeography, hybrid.
