Vascular cambium

The vascular cambium is the primary tissue responsible for growth in thickness in the stems and roots of plants that undergo secondary growth. This includes dicots (like buttercups and oak trees), gymnosperms (like pine trees), and some other vascular plants. It generates secondary xylem toward the inside of the plant (near the pith) and secondary phloem toward the outside (near the bark), typically producing more xylem than phloem. In herbaceous plants, the vascular cambium is found within vascular bundles that are arranged like beads on a necklace, forming a broken ring inside the stem. In woody plants, it forms a continuous cylinder of unspecialized meristem cells from which new tissues develop. Unlike xylem and phloem, the vascular cambium does not transport water, minerals, or food. It is also known as the main cambium, wood cambium, or bifacial cambium.

Vascular cambia are present in all seed plants except for five angiosperm groups that have independently lost it: Nymphaeales, Ceratophyllum, Nelumbo, Podostemaceae, and monocots. In dicot and gymnosperm trees, the vascular cambium is the visible line separating bark from wood; these trees also have 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.

The cambium located between the primary xylem and primary phloem is called the fascicular cambium (found within vascular bundles). During secondary growth, cells of the medullary rays—arranged in a line (or, in three dimensions, a sheet) between adjacent vascular bundles—become meristematic and form the interfascicular cambium (between vascular bundles). The fascicular and interfascicular cambia then join to create a ring (a tube in three dimensions) that separates the primary xylem from the primary phloem, known as the cambium ring. The vascular cambium produces secondary xylem on the inside of this ring and secondary phloem on the outside, pushing the primary xylem and phloem apart. The vascular cambium typically contains two cell types: fusiform initials (tall and axially oriented) and ray initials (smaller, round to angular in shape).

The vascular cambium is maintained by a network of interacting signal feedback loops. Both 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, promoting cell proliferation and differentiation.

The 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 different hormone concentrations is crucial for plant metabolism. Auxins stimulate mitosis, cell production, and regulate both interfascicular and fascicular cambium. Applying auxin to a tree stump allows decapitated shoots to continue secondary growth. Without auxin, plants suffer; mutants lacking auxin show increased spacing between interfascicular cambiums and reduced vascular bundle growth, leading to decreased transport of water, nutrients, and photosynthates, eventually causing death. Auxin also regulates the two cell types in the vascular cambium—ray and fusiform initials—ensuring connections between xylem and phloem for nutrient transport and sugar storage. Ethylene levels are high in plants with an active cambial zone and are still under study. Gibberellin stimulates cambial cell division and regulates xylem tissue differentiation, without affecting phloem differentiation rate. In poplar trees, high gibberellin levels correlate with increased cambial cell division and higher auxin in cambial stem cells. Gibberellin also promotes xylem expansion via a signal traveling from shoot to root. Cytokinin regulates the rate of cell division rather than the direction of differentiation. Mutants treated with cytokinin show reduced stem and root growth, but the secondary vascular pattern of bundles remains unaffected.

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