C4 plants are unique in possessing two types of photosynthetic cell . A layer of cells surrounding the vascular bundle, the bundle-sheath, is a common structural feature, but only in C4 plants does it contain chloroplasts. The bundle-sheath is thick-walled, sometimes suberized and there is no direct access from the intercellular spaces of the mesophyll. The appearance of a wreath of cells surrounding the vasculature gives rise to the term ‘Kranz’ anatomy. In contrast, the mesophyll is typical of the type of photosynthetic tissue found in leaves of most C4 plants and comprises thin walled cells with abundant intercellular spaces. The distance between bundle-sheath cells is normally only two or three mesophyll cells, so that no mesophyll cell is more than one cell away from a bundle-sheath cell. Mesophyll cells are also connected to bundle-sheath cells by large numbers of plasmodesmata. These features are both necessary for rapid fluxes of metabolites between the two cell types, which is an essential feature of the CO2 pump. There are also distinct anatomical features in the arrangement of chloroplasts and other organelles at the subcellular level, but the biochemical significance of these differences remains unclear.