Vascular Cambium and Cork Cambium: Lateral Meristems

Vascular cambium and cork cambium are the two lateral meristems that make stems and roots grow thicker. This thickening is called secondary growth. Vascular cambium produces secondary xylem and phloem, while cork cambium produces protective cork.

These meristems matter because they create wood, bark, and healing tissue. Timber, cork stoppers, and the ability of trees to live for hundreds of years all depend on these thin cylinders of dividing cells.

Key Concepts

• Vascular cambium: A lateral meristem between primary xylem and phloem. It has fusiform initials (forming axial system) and ray initials (forming radial rays).
• Secondary xylem: Formed towards the inside; commonly called wood, usually more in amount.
• Secondary phloem: Formed towards the outside; part of bark tissues.
• Cork cambium (phellogen): Develops in outer cortex or epidermis; produces cork (phellem) outward and secondary cortex (phelloderm) inward. Together they form periderm.
• Lenticels: Openings in bark for gas exchange.

Annual rings arise because spring wood has larger vessels and autumn wood has smaller vessels.

How It Works

1. Vascular cambium cells divide tangentially.
2. Inner derivatives become secondary xylem; outer derivatives become secondary phloem.
3. As girth increases, outer tissues stretch and cork cambium forms.
4. Cork cambium produces suberised cork that protects and reduces water loss.

Worked Example

A tree trunk has 25 annual rings at breast height. Estimated age from rings = 25 years for that height level. If radius increased from 20 cm to 45 cm in that period, total radial increase = 45 − 20 = 25 cm. Average increase per year = 25 ÷ 25 = 1 cm per year. Ring counting gives a simple, correct age estimate.

Common Misconceptions

• Bark is only cork: Wrong. Bark includes all tissues outside vascular cambium, including secondary phloem and periderm.
• Wood is secondary phloem: Wrong. Wood is secondary xylem formed inward.
• Cork cells are living: Wrong. Mature cork cells are dead and filled with air, with suberin in walls.

Real-World Applications

Timber and firewood come from secondary xylem, commercial cork from Quercus suber periderm, and bark yields medicines like quinine and cinnamon. Grafting and wound healing in orchards succeed because cambium remains active and produces callus tissue.

Vascular cambium builds wood inside and phloem outside, while cork cambium seals the surface — together they turn a thin stem into a strong, protected trunk.