Sekundaarikasvu

Sekundaarikasvu, also known as secondary growth, is a biological process that leads to an increase in the diameter of stems and roots of woody plants. This growth occurs in certain vascular plants, primarily gymnosperms and dicotyledonous angiosperms, and is responsible for the formation of wood and bark. Secondary growth is initiated by the activity of two lateral meristems: the vascular cambium and the cork cambium.

The vascular cambium is a cylindrical layer of meristematic cells located between the xylem and phloem. Its activity produces secondary xylem towards the inside of the stem or root and secondary phloem towards the outside. Secondary xylem is commonly referred to as wood, and its characteristic annual rings are a result of seasonal variations in cambial activity. Secondary phloem contributes to the bark by transporting sugars produced during photosynthesis.

The cork cambium, or phellogen, arises in the outer tissues of the stem or root. It produces cork cells towards the outside, which form a protective layer that replaces the epidermis. Cork cells are typically dead at maturity and are impregnated with suberin, a waxy substance that makes them impermeable to water and gases. The periderm, consisting of the cork cambium, cork, and sometimes phelloderm, collectively forms the outer bark.

Secondary growth allows plants to develop greater structural support, increasing their height and resistance to mechanical stress. It also enhances water and nutrient transport capabilities through the extensive network of xylem tissues. The presence of secondary growth is a key characteristic that distinguishes woody plants from herbaceous ones.