(i) Protoxylem: Vessel cells with narrow pores are called protoxylem.
(ii) Metaxylem: Vessel cells with large pores are called metaxylem.
(iii) Exarc: If protoxylem is towards the periphery and metaxylem towards the centre, it is called exarc. Root xylem is exarc in nature.
(iv) Endarc: If protoxylem is towards the center and metaxylem towards the periphery, it is called endarc. The xylem of stem is endarc in nature.
(v) Mesarc : When protoxylem and metaxylem are arranged on both sides, it is called mesarc. Mesarc xylem is found in the leaves of ferns and cycads.
(vi) Centrach : When protoxylem is in the center and metaxylem is located around it, it is called centrach. Centarch xylem is found in fern plants.