Secondary xylem and phloem in dicot stem are produced by
The vascular cambium and cork cambium are secondary meristems that are formed in stems and roots after the tissues of the primary plant body have differentiated.
In dicot stems, the vascular cambium initially differentiates from procambial cells within the vascular bundles. This fascicular cambium may contribute additional cells to both the xylem and the phloem of the bundle. At some point the cambium expands into the ground tissue between the vascular bundles, forming an interfascicular cambium, completing the ring of the vascular cambium. Cell division by the cambium produces cells that become secondary xylem and phloem. As secondary phloem and xylem tissue accumulate, it both increases the girth of the stem and forms wood and bark.