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How does the vascular cambium make a tree trunk grow wider?
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wood
Temperate softwoods (left column) and hardwoods (right column), selected to highlight natural variations in colour and figure: (A) Douglas fir, (B) sugar pine, (C) redwood, (D) white oak, (E) American sycamore, and (F) black cherry. Each image shows (from left to right) transverse, radial, and tangential surfaces. Click on an individual image for an enlarged view.
The vascular cambium is a lateral meristemāa layer of actively dividing cellsāresponsible for the secondary growth that increases the thickness of a tree trunk. It functions as a cylinder of cells located between the primary xylem and primary phloem.1234
To increase the trunk's girth, the vascular cambium undergoes tangential cell divisions:54
- Secondary Tissue Production: The cambium produces new cells toward the inside of the stem, which differentiate into secondary xylem (wood), and toward the outside, which differentiate into secondary phloem (inner bark). Because the cambium produces significantly more xylem internally than phloem externally, the trunk accumulates wood over time.1356
- Expansion of Circumference: To accommodate the increasing diameter of the stem, the cambial cells themselves also divide radially. This allows the vascular cambium cylinder to expand in circumference, ensuring it can continue to produce new layers of tissue as the tree grows wider.54
As this secondary growth progresses, the pressure from the expanding inner tissues often crushes or obliterates the older primary tissues located outside the cambium.54