Vascular cambium is the thin engine that thickens woody stems
In dicots such as oak, gymnosperms such as pine, and many other plants with secondary growth, vascular cambium is the main stem and root growth tissue. It lays secondary xylem inward toward the pith and secondary phloem outward toward the bark—usually making more wood than bast.
In herbs the cambium sits inside vascular bundles like beads on a necklace; in woody plants it forms a continuous cylinder of unspecialised meristem cells. It does not itself carry water or food. Fascicular cambium between primary xylem and phloem joins interfascicular sheets from medullary rays to complete the ring. Trees also keep a cork cambium, and grafting succeeds only when stock and scion cambia align.
Two cell types dominate: tall fusiform initials and smaller ray initials. Hormones—auxin, ethylene, gibberellins, cytokinins, abscisic acid—plus short peptides keep feedback loops balanced from both xylem and phloem sides. Auxin especially stimulates mitosis and cambial patterning.
Auxin's power shows in a simple experiment: smearing it on a tree stump let beheaded shoots keep thickening, whereas mutants lacking it space their interfascicular cambium farther apart and make less vascular tissue. Age leaves its mark too. A study of Styphnolobium japonicum trees aged 80, 500 and 1,000 years found the oldest cambium thinner, with fewer cell layers and less auxin and gibberellin, while bark grew thicker and defence-linked hormones such as jasmonic and salicylic acid rose. In the millennial trees, cell-division genes were dialled down and phenolic acids, flavonoids and lignin-related compounds piled up.
Seed plants almost all retain the tissue except five angiosperm lineages that lost it independently, including monocots and water-lily relatives. Scandinavians once milled edible cambium into bark bread. That microscopic sheet is why trunks fatten and why wood and bark keep parting along a living line.
Source: Vascular cambium
