Cambialen

Cambialen are a category of meristematic tissues in vascular plants that drive secondary growth. They form a thin, living layer between the xylem (toward the interior) and the phloem (toward the exterior) and are essential for the widening of stems and roots in many woody species.

The vascular cambium is the most important type, occurring as a cylindrical ring around stems and roots in many dicots and gymnosperms. It contains two main cell populations: fusiform initials, which are elongated cells that primarily produce secondary xylem to the inside and secondary phloem to the outside, and ray initials, which generate parenchyma cells that form vascular rays for transport and storage. A separate lateral meristem, the cork cambium (phellogen), is sometimes grouped with cambial tissue; it produces the periderm (including cork) to replace the epidermis as the stem or root thickens.

Functionally, cambialen enable radial growth, contributing to the thickening of wood and bark. They produce growth rings that reflect seasonal variation in activity. In stems and roots with active cambial tissue, the balance of xylem and phloem production shapes the plant’s hydraulic and structural properties. Cambial activity is influenced by hormonal signals, especially auxin and cytokinins, and responds to environmental factors such as temperature, water availability, and age. In many herbaceous plants and some monocots, true vascular cambium is reduced or absent, leading to little or no secondary growth.

In summary, cambialen are key drivers of secondary growth in many vascular plants, enabling increased girth and the formation of wood and bark through regulated production of new vascular tissues.