Describe the process of secondary thickening.

secondary growth is the result of the activity of the vascular cambium. The latter is a meristem that divides to produce secondary xylem cells on the inside of the meristem (the adaxial side) and secondary phloem cells on the outside (the abaxial side). This growth increases the girth of the plant root or stem, rather than its length, hence the phrase "secondary thickening". As long as the vascular cambium continues to produce new cells, the stem or root will continue to grow in diameter. In woody plants, this process produces wood.