Vascular cambium

The vascular cambium is the primary tissue responsible for growth in thickness in the stems and roots of many plants that undergo secondary growth. This includes dicots like buttercups and oaks, gymnosperms like pines, and certain other vascular plants. It generates secondary xylem toward the inside (toward the pith) and secondary phloem toward the outside (toward the bark), with secondary xylem typically produced in greater quantity. In herbaceous plants, the vascular cambium is located within vascular bundles that form an interrupted ring around the stem, resembling beads on a string.

In woody plants, it exists as a continuous cylinder of unspecialized meristem cells, a ring from which new tissues develop. Unlike xylem and phloem, it does not transport water, minerals, or food. It is also known as the main cambium, wood cambium, or bifacial cambium.

Occurrence

Vascular cambia occur in all seed plants except for five angiosperm lineages that have independently lost this structure: Nymphaeales, Ceratophyllum, Nelumbo, Podostemaceae, and monocots. In dicot and gymnosperm trees, the vascular cambium forms the visible boundary between bark and wood; these trees also possess a cork cambium. For grafting to succeed, the vascular cambia of the rootstock and scion must be aligned so they can fuse and grow together.

Structure and function

The cambium located between primary xylem and primary phloem is called fascicular cambium (found within vascular bundles). During secondary growth, cells of the medullary rays—which lie in a line (or, in three dimensions, a sheet) between adjacent vascular bundles—become meristematic and create new interfascicular cambium (between bundles). These fascicular and interfascicular cambia then unite to form a ring (a tube in three dimensions) that separates the primary xylem from the primary phloem, known as the cambium ring. The vascular cambium then produces secondary xylem on the inner side of this ring and secondary phloem on the outer side, pushing the primary xylem and phloem apart.

The vascular cambium typically contains two cell types: fusiform initials (tall and oriented along the axis) and ray initials (smaller, round to angular).

Maintenance of cambial meristem

A network of interacting signal feedback loops maintains the vascular cambium. Hormones and short peptides act as information carriers in these systems. Although similar regulation occurs in other plant meristems, the cambial meristem receives signals from both the xylem and phloem sides. Signals from outside the meristem downregulate internal factors, which promotes cell proliferation and differentiation.

Hormonal regulation

Phytohormones involved in vascular cambial activity include auxins, ethylene, gibberellins, cytokinins, abscisic acid, and likely others yet to be discovered. Each is vital for regulating cambial activity, and the combination of their concentrations is important in plant metabolism. Auxin stimulates mitosis, cell production, and regulates both interfascicular and fascicular cambium. Applying auxin to a tree stump allowed decapitated shoots to continue secondary growth.

Without auxin, a plant suffers detrimental effects; mutants lacking auxin show increased spacing between interfascicular cambia and reduced vascular bundle growth, leading to decreased transport of water, nutrients, and photosynthates and eventual death. Auxin also regulates the two cell types (ray and fusiform initials), ensuring connections and communication between xylem and phloem for nutrient translocation and sugar storage. Ethylene levels are high in plants with an active cambial zone, though its role is still under study.

Gibberellin stimulates cambial cell division and regulates xylem tissue differentiation, without affecting phloem differentiation rate. In poplar trees, high gibberellin concentrations correlate with increased cambial cell division and increased auxin in cambial stem cells. Gibberellin also drives xylem expansion via a signal traveling from shoot to root. Cytokinin regulates the rate of cell division rather than the direction of differentiation; studies show that cytokinin mutants have reduced stem and root growth, but the secondary vascular pattern of bundles remains unaffected.

The cambium of most trees is edible. In Scandinavia, it was historically ground into flour to make bark bread.