Conceptual Overview

The vascular cambium is a particularly significant tissue in both gymnosperms and dicotyledons where it plays the important role of providing for secondary growth resulting in an increase in the diameter(or girth) of stems and roots. Secondary growth results in the formation of both secondary xylem and secondary phloem. Cells of the vascular cambium are originally derived from procambium, and develop into either fascicular (within vascular bundles) or interfascicular (developing in the ground tissues between vascular bundles) layers of cells that form a continuous ring. The vascular cambium is sometimes referred to as a lateral meristem, along with the cork cambium, or phellogen, and it normally assumes a cylindrical form within stems and roots. In some plants (i.e. cucurbits), or in leaf petioles and major leaf veins from plants that have secondary growth in their leaves, the cambium is strip-like. In roots where the initial vascular cambium is organized into a series of radial projections from a ring, it "rounds out" due to greater mitotic activity at the less extended sites.

In most cases, the vascular cambium is only one cell thick, but viewing with microscopy may make it appear to be several layers in thickness due to the fact that the immediate derivatives look similar. In fact, the thickness may be greater at times of the year when activity is greater. This range of apparent thickness has given rise to the use of the term cambial zone for the microscopic view of the cambium tissue and its immediate derivatives. Cells of the vascular cambium divide mostly periclinally and are said to be bifacial meristematic since they produce derivatives in two directions (the secondary phloem to the outside,and the secondary xylem to the inside).

Vascular cambium cells are of two types: fusiform initials, which are elongated but broadly flattened on the tangential face, and ray initials, which are shorter cells, somewhat elongated on their radial axis. Fusiform initials produce axial elements of secondary vascular tissues, such as fibers, tracheids, vessel members, companion cells, sieve-tube elements and axial parenchyma. Ray initials give rise to phloem and xylem rays--the radial (or horizontal) system of the secondary plant body. Vascular cambium initials have only primary cell walls that are never lignified. Their tangential walls are thinner than radial walls and lack primary pit fields. The radial walls, however, often have numerous orderly arranged primary pit fields that give the walls a "beaded" appearance. When the cells are dormant they have few and small vacuoles, but in the active state they become highly vacuolated and a central vacuole develops. Mitotic division and cytokinesis may take several days for completion from an initial.

Vascular cambium cells have no intercellular spaces and they grow intrusively. Sometimes, that intrusive growth goes through rays that have already been established. Cross divisions of fusiform initials may shorten the vertical axis of the cells to shorter cells that can change into ray initials. The growth of both secondary tissues and rays may be storied (stratified) in which the cells are arranged in uniform horizontal series, or nonstoried (nonstratified) in which such a uniform arrangement does not exist.

Growth in vascular cambium is stimulated by plant growth regulators (aka, hormones), primarily auxin, but also including cytokinins and gibberellins. In gymnosperms, during the maturation of tracheids in which the living cell contents decompose, plant growth regulators are thought to be derived from materials of the cellular breakdown.