When the protoxylem or first formed xylem lies in contact with pericycle while metaxylem or later formed xylem is present towards the centre, such type. of xylem is called exarch, eg, dicot root, and monocot root. In dicot root bundles of xylem and phloem arranged alternately. The root may be diarch (two xylem bundles-tomato) triarch (Pea) tetrarch (Buttercup, Gram) pentarch or hexarch. Due to presence of numerous xylem bundles and exarch condition, xylem of monocot root is polyarch.