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The sectional delimitation of Carex section Rhomboidales sensu Kükenthal (1909) is problematic and, here, we provide contributions to a systematic understanding of nutlet micromorphology in section Rhomboidales. Nutlet micromorphology of 87 samples representing 71 taxa was investigated using scanning electron microscopy and nine nutlet types were recognized. Although some closely related taxa show similarities, nutlet morphology in section Rhomboidales is stable at the species level and is reliable for species identification. Based on the nutlet characters, section Rhomboidales is delimited and emended as having nutlet rhombic-ovoid, trigonous; beaks conspicuous, erect, rarely curved or coiled; periclinal walls of epidermal cells straight, with zero to two silica bodies. The species with nutlets of the C. harlandii-type and C. thibetica-type are included, and species of the C. chinensis-type and C. macrandrolepsis-type are ascribed to section Mitratae and section Infossae, respectively. Carex longirostrata and C. pseudolongirostrata (= C. nodaeana), formerly ascribed to section Careyanae or section Depauperatae, are well supported as members of section Rhomboidales. © 2014 The Linnean Society of London, Botanical Journal of the Linnean Society, 2014, 175, 123-143. [ABSTRACT FROM AUTHOR]