The cambium is a thin, cylindrical layer of actively dividing cells found in the stems and roots of many plants, particularly woody species. This layer of tissue, known as a lateral meristem, is responsible for increasing the plant’s girth or thickness, a process called secondary growth. This function is distinctly different from primary growth, which is caused by apical meristems and results in the plant growing taller or longer. The cambium’s continuous cell production allows a tree to expand its diameter year after year, providing the structural strength needed to support increasing height and canopy size.
The Two Primary Cambium Types
Woody plants rely on two types of cambium tissue for continuous outward expansion. The vascular cambium is situated in the center of the stem, forming a ring between the wood (xylem) and the inner bark (phloem) tissues. Its purpose is to produce the plant’s internal transport systems: secondary xylem and secondary phloem. This position allows it to create new conducting cells added to both the inside and the outside of the ring.
Closer to the plant’s surface is the cork cambium, also known as the phellogen, which develops beneath the epidermis or outer cortex. As the stem thickens, the outer layers of the plant rupture and crack. The cork cambium generates the protective outer layers, collectively called the periderm or bark, to replace these damaged tissues. This renewal insulates the interior of the plant from environmental damage, pathogens, and water loss.
The Mechanics of Secondary Growth
The vascular cambium consists of meristematic cells that retain the ability to divide continuously throughout the plant’s life. When these cells divide, they add new tissue both toward the center of the stem and toward the outer edge, driving the increase in a tree’s diameter. One daughter cell remains in the cambium layer to maintain the meristem, while the other differentiates into a specialized cell type.
Cells produced toward the inside of the stem mature into secondary xylem, the dense, structural material that makes up wood. This secondary xylem transports water and dissolved minerals from the roots up to the leaves. The accumulation of this strong, lignified tissue provides the mechanical support required for the weight and height of mature trees. Since the cambium is generally more active in producing inward-facing cells, the vast majority of a tree’s bulk is composed of secondary xylem.
Cells produced toward the outside of the stem differentiate into secondary phloem, which forms the inner layer of the bark. The secondary phloem transports sugars created during photosynthesis from the leaves to other parts of the plant. While new phloem is continually generated, the older, outermost phloem cells are gradually crushed and sloughed off as the stem expands. This renewal ensures the inner transport system maintains its function.
How Cambium Activity Creates Tree Rings
The cell production of the vascular cambium is regulated by seasonal cycles, which result in the formation of visible tree rings. In temperate climates, the cambium is active in the spring and early summer when water is abundant. During this period, it produces earlywood (or springwood), which consists of large, thin-walled xylem cells efficient at transporting high volumes of water. The resulting wood tissue is porous and appears lighter in color.
As the growing season progresses into late summer and fall, water availability decreases, and the rate of cambium division slows. The cambium then produces latewood (or summerwood), characterized by smaller xylem cells with thicker, denser walls. This denser latewood is darker in color and provides greater structural strength to the stem. The difference between the dense latewood formed at the end of one season and the light earlywood formed at the beginning of the next creates the annual boundary.
Each pair of light earlywood and dark latewood represents one full year of growth, forming the annual growth ring. Counting these rings allows scientists to accurately determine the age of a tree. The width of each ring provides a historical record of environmental conditions, with wide rings indicating favorable conditions and narrow rings suggesting drought or stress. This application, known as dendrochronology, allows researchers to reconstruct past climate patterns and ecological events.