Question: where do secondary xylem and phloem cells arise from?
Answer: the vascular cambium, The vascular cambium

Other names for the vascular cambium are the main cambium , wood cambium , or bifacial cambium . In more detail , the vascular cambium is a plant tissue located between the xylem and the phloem in the stems and roots of certain vascular plants . It is a cylinder of unspecialized meristem cells that divide to form secondary vascular tissues . It is the source of both secondary xylem growth inwards towards the pith , and secondary phloem growth outwards to the bark . Unlike the xylem and phloem , it does not transport water , minerals or food through the plant .