difference between

protoxylem and metaxylem

protophloem and metaphloem

exarchxylem and endarchxylem

The first formed primary xylem is called protoxylem and the later formed primary xylem is called metaxylem.

The first formed primary phloem consists of narrow sieve tubes and is referred to as protophloem and the later formed phloem has bigger sieve tubes and is referred to as metaphloem. 

In stems, the protoxylem lies towards the centre (pith) and the metaxylem lies towards the periphery of the organ. This type of primary xylem is called endarch.  In roots, the protoxylem lies towards periphery and metaxylem lies towards the centre. Such arrangement of primary xylem is called exarch.