The anatomy of Carex roots was generally considered to be rather uniform. The aim of this work was to provide an overview of the root anatomy of Carex in a phylogenetic context with an even sampling across the different clades. It was tested, if the observed anatomical characters show a relation to the habitat of the species, i.e. the soil moisture conditions. Further, it was asked if the anatomical data may provide useful characters for the comprehension and circumscription of clades. Root sections of 145 species were studied for 8 characters. Relations of root anatomical characters were inferred by linear regressions of phylogenetic independent contrasts. Phylogenetic ANOVAs were calculated to reveal anatomical characters that may have a relation to growth conditions. The anatomical data were mapped on the phylogeny to infer if they may provide useful characters for systematics. The root anatomy of Carex revealed a considerable diversity. Size differences of the roots were in orders of magnitude; particularly apparent were differences in the development of the aerenchyma and the sclerenchymatic tissues of the cortex. Most measures of the anatomy were positively related, only the sclerenchyma shows no significant correlation to other anatomical characters. The size of the sclerenchyma is significantly related to soil moisture, whereas the size of aerenchyma shows only a weak correlation. The development of the sclerenchyma was revealed to be a possible functional trait, whose development is related to dry soils. Unexpectedly, the size of the aerenchyma shows only a weak relation to soil moisture; considerable aerenchyma may be observed in species of all growth conditions. The development of the sclerenchyma and the arrangement of vessels in the central cylinder of the root may be useful characters for the systematics of Carex.