Paksuuskasvu

Paksuuskasvu refers to the secondary growth in plants, a process that increases the diameter of stems and roots. This growth is primarily responsible for the formation of wood and bark in woody plants like trees and shrubs. Unlike primary growth, which elongates the plant, secondary growth adds girth.

The main tissues responsible for paksuuskasvu are the vascular cambium and the cork cambium (also known as the phellogen). The vascular cambium is a layer of actively dividing cells located between the xylem and phloem. It produces secondary xylem towards the inside, which accumulates as wood, and secondary phloem towards the outside, which becomes part of the bark. The cork cambium, typically formed in the outer cortex or epidermis of older stems and roots, produces cork cells (phellem) outwards, which form the protective outer bark, and phelloderm inwards.

Paksuuskasvu is a defining characteristic of woody perennials, allowing them to grow large and strong, supporting their structures against gravity and environmental stresses. The annual rings seen in the cross-section of a tree trunk are a direct result of seasonal variations in vascular cambium activity, with lighter, wider spring wood and darker, denser summer wood. This process is crucial for the long-term survival and ecological role of many plant species.