Cambial

Cambial refers to cambium, a plant meristem responsible for the secondary (lateral) growth that thickens stems and roots in many vascular plants. The vascular cambium forms a continuous cylindrical layer between the secondary xylem (toward the inside) and the secondary phloem (toward the outside) in the stems and roots of most woody plants.

The cambium is composed of two main types of initial cells. Fusiform initials are elongated and produce longitudinal elements of the vascular tissues, including secondary xylem on the inside and secondary phloem on the outside. Ray initials are shorter and give rise to ray parenchyma that stores nutrients and facilitates radial transport. In many plants, the cambial region also includes interfascicular cambium, which connects the fascicular cambia (within vascular bundles) to form a continuous ring around the stem or root.

Functionally, cambial cells divide mainly periclinally to add new layers of xylem internally and phloem externally, contributing to wood formation and the transport tissues of the plant. Anticlinal divisions expand the cambial circumference, enabling the ring to widen as the plant grows. Seasonal variations in cambial activity generate growth rings in temperate species, reflecting periods of wood production and dormancy.

Variations exist among plants. Many monocots lack a true vascular cambium and show limited secondary growth, while many dicots and gymnosperms possess a well-developed vascular cambium. The term cambial or cambial tissue is central to discussions of woody growth, radial expansion, and the formation of annual rings in trees.