Secondary Thickening

In many vascular plants, secondary thickening arises from the activity of the vascular cambium, a type of meristematic tissue. The vascular cambium divides to produce secondary xylem cells on its inner side and secondary phloem cells on its outer side. This process increases the diameter of plant roots or stems, rather than their length, which is why it is referred to as "secondary thickening". As long as the vascular cambium continues to generate new cells, the stem or root will keep growing in thickness. In woody plants, this results in the formation of wood.

Secondary thickening leads to an increase in the plant's diameter. Obstructions, such as foreign objects or parts of the plant like stubs, can be incorporated into the growing plant structure over time.

Because secondary thickening often causes the epidermis of stems or roots to rupture, plants with secondary growth typically develop a cork cambium. This cambium produces thickened cork cells that protect the plant's surface and minimise water loss. Over many years, this process can result in a layer of cork, as seen in cork oaks, which can be harvested for cork.

Secondary growth is also observed in various non-woody plants, such as tomatoes, potato tubers, carrot taproots, and sweet potato tuberous roots. Some long-lived leaves also exhibit secondary growth.

Primary growth, which increases the length of roots and stems, is a characteristic of all vascular plants. Secondary growth, however, mainly occurs in many dicots and gymnosperms. Monocots generally lack secondary growth, but when it does occur, it differs from the process described above.