In gymnosperms and woody dicots the roots and stems continue to increase in diameter (whereas in monocots and most herbaceous dicots  growth ceases with the maturation of the primary tissues ) This is called secondary growth and encompasses all lateral growth, a major component of woody plant tissues. It results from the activity of two lateral meristems:

1. Vascular cambium - gives rise to secondary xylem (wood) to the inside and secondary phloem to the outside

2. Cork cambium - gives rise to the periderm (bark) to the outside and phelloderm to the inside which replaces the epidermis, found only in woody plants.