what is Protoxylem, Metaxylem, Exarch and Endarch.

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.

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