Secondary growth is the growth that results from cell division in the cambia (see below), or lateral meristems, and that causes the stems and roots to thicken. Primary growth is growth that occurs as a result of cell division at the tips of stems and roots, and that gives rise to primary tissue. Secondary growth occurs in most seed plants, but monocots usually lack secondary growth. If they do have secondary growth, it differs from the typical pattern of other seed plants.
In many vascular plants, secondary growth is the result of the activity of the two lateral meristems, the cork cambium and vascular cambium. Arising from lateral meristems, secondary growth increases the girth of the plant root or stem, rather than its length. As long as the lateral meristems continue to produce new cells, the stem or root will continue to grow in diameter. In woody plants, this process produces wood.
Because this growth usually ruptures the epidermis of the stem or roots, plants with secondary growth usually also develop a cork cambium. The cork cambium gives rise to thickened cork cells to protect the surface of the plant and reduce water loss. If this is kept up over many years, this process may produce a layer of cork. In the case of the cork oak it will yield harvestable cork.
Anomalous secondary growth 
Anomalous secondary growth does not follow the pattern of a single vascular cambium producing xylem to the inside and phloem to the outside. Some dicots have anomalous secondary growth, e.g. in Bougainvillea a series of cambia arise outside the oldest phloem.
Most monocots either have no secondary growth or else anomalous secondary growth of some type. For example, palm trees increase their trunk diameter due to division and enlargement of parenchyma cells, which is termed diffuse secondary growth. In some other monocot stems with anomalous secondary growth, a cambium forms, but it produces vascular bundles and parenchyma internally and just parenchyma externally. Some monocot stems increase in diameter due to the activity of a primary thickening meristem, which is derived from the apical meristem.
See also 
- Thompson, N.P. and Heimsch, C. 1964. Stem anatomy and aspects of development in tomato. American Journal of Botany 51: 7-19. 
- Ewers, F.W. 1982. Secondary growth in needle leaves of Pinus longaeva (bristlecone pine) and other conifers: Quantitative data. American Journal of Botany 69: 1552-1559. 
- Esau, K. and Cheadle, V.I. 1969. Secondary growth in bougainvillea. Annals of Botany 33: 807-819. 
- Esau, K. 1977. Anatomy of Seed Plants. New York: Wiley