Acorus calamus rhizome Light micrograph of a section through a rhizome from a sweet flag Acorus calamus plant The rhizome has an outer layer with scattered collateral vascular bundles and a central cylindrical core of larger vascular bundles The inner core has an outer ring endodermis and an inner layer pericycle The ground tissue is spongy aerenchyma with thinwalled cells and larger intercellular spaces to store oxygen The vascular bundles have a ring of outer xylem red which carries water and minerals to the stems and a surrounding inner phloem blue which carries sugar to the rhizome to be stored as starch Magnification x when printed centimetres wide