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PROTOXYLEM:  The first formed primary xylem is called protoxylem METAXYLEM: The later formed primary xylem is considered as metaxylem. EXARCH: If protoxylem lies towards centre that is towards pith and metaxylem lies towards periphery then type of primary xylem is called exarch. ENDARCH: If protoxylem is towards periphery and metaxylem is towards centre(pith) then such arrangement of primary xylem is called as endarch.

PROTOPHLOEM: The phloem containing narrow sieve tubes. such first formed primary phloem is considered as protophloem. METAPHLOEM: Phloem containing larger sieve tubes. such later formed phloem is called metaphloem.