Secondary Growth in Woody Plants

In woody plants, secondary thickening involves the growth of stems and roots in diameter. This occurs through the activity of lateral meristems, notably the vascular cambium and cork cambium.

Secondary thickening is facilitated by meristematic cells (cambium) located between phloem and xylem tissue in vascular bundles of the plants. The cambium cells divide radially to form a ring/cylinder of cambium tissue with xylem inside the ring and phloem outside the ring. Cells of the cambium divide to form secondary phloem outside. Later vascular cambium, cambium between the vascular bundles, divide to form secondary parenchyma thereby increasing the growth of the medullary rays. Much more xylem cells are formed than those of the phloem, thus pushing the phloem and the cambium ring outwards. The rate of growth is dependent on the seasons/rains, resulting in annual rings. Cork cambium divides to form new cork/bark to accommodate the expanding tissues and secondary cortex inside. This process enhances stem and root diameter, providing structural support and longevity in woody plants.