In many vascular plants, 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.
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.
Primary growth in roots and stems is growth in length and occurs in all vascular plants. Secondary growth occurs mainly in many dicots and gymnosperms. Monocots usually lack secondary growth. If they do have secondary growth, it differs from that described above.
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 have no secondary growth or 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.
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