Plants grow and adapt to their surroundings. In woody plants, a specialized tissue called cambium drives this growth. Cambium enables plants to increase in girth and develop robust structures. Understanding cambium provides insight into how trees and shrubs achieve their size and resilience.
Understanding Cambium
Cambium is a type of meristematic tissue, meaning it consists of undifferentiated cells that can divide and produce new cells. The cambium is generally located as a thin layer within the plant stem and root, positioned between the water-conducting xylem and nutrient-transporting phloem tissues. Its primary function is to facilitate active cell division, which drives the plant’s secondary growth or increase in diameter.
Different Types of Cambium
Plants possess two main types of cambium, each with distinct roles in growth and development. The vascular cambium is found within the vascular bundles of herbaceous plants and forms a continuous ring in woody stems and roots. This cambium produces secondary xylem, known as wood, towards the inside of the plant and secondary phloem towards the outside.
The cork cambium, also called phellogen, develops in the outer regions of stems and roots, beneath the epidermis. Its activity forms cork cells (phellem) on its outer side and secondary cortex cells (phelloderm) on its inner side. Together, the cork cambium, cork, and phelloderm form the periderm, which replaces the epidermis as the plant’s protective outer layer.
Cambium’s Role in Plant Growth
The vascular cambium increases plant girth, a process known as secondary growth. During the growing season, cells within the vascular cambium divide, generating new secondary xylem cells towards the stem’s center and new secondary phloem cells towards the periphery. The continuous accumulation of secondary xylem forms the bulk of the wood, leading to the characteristic annual growth rings visible in a tree’s cross-section. These rings reflect seasonal growth patterns, with wider rings indicating periods of more vigorous growth.
Over time, older secondary xylem cells in the stem’s center become non-functional for water transport and form heartwood, which provides structural support. The younger, outer secondary xylem, known as sapwood, remains active in water and mineral conduction. The cork cambium contributes to the thickening of the outer protective layers. It produces new cork cells that are suberized, containing a waxy substance that makes the outer bark waterproof and protective against physical damage and pathogens.
The Broader Importance of Cambium
Cambium activity is important for the structural integrity and survival of woody plants. By continuously adding new xylem, it ensures water and nutrient transport from roots to leaves, supporting the plant’s metabolic needs. Secondary growth also provides mechanical support, allowing trees to grow tall and withstand environmental stresses like wind. The periderm formed by the cork cambium offers a protective barrier against water loss, insect infestations, and fungal infections.
Beyond its functions for the plant, cambium is important for human applications. The wood produced by the vascular cambium is a primary resource for timber, used in construction, furniture, and paper production. Cambial layers are also used in horticultural practices such as grafting, where parts of plants are joined to grow as one. Understanding cambial activity is useful in managing tree health and assessing forest productivity.