Cyperus japonicus Mak., which has a widespread distribution in subtropical Asia and extends northwards into Europe, was found to be a C4 species based upon its Kranz leaf anatomy, low CO2 compensation concentration and isotopic composition of leaf carbon. A curious variant of the anatomical arrangement of photosynthetic celis is developed in this wetland species. Connected by veins, two groups of Kranz units, one undemeath the abaxial epidermis and the other in the middle of the blade, form elliptical mesophyll channels. This arrangement of Kranz units has been defined as Kranzkette (literally "the chain of garland"). Like Cyperus longus, another C4 species in the genus, the structure and arrangement of chloroplasts in the bundle sheath celis show it to be a NADP-ME type. Starch grains, however, are formed in both mesophyll and bundle sheath celis. This modified C4 Kranz anatomy with large intercellular air spaces within the chlorenchyma suggests that the arrangement of assimilatory celis may be related to gas transport through the large air-spaces.