You are given a fairly old piece of dicot stem and a dicot root. Which of the following anatomical structures will you  use to distinguish between the two?

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. Therefore, root and stem can be easily distinguished on the basis of positions of protoxylem in them.