Secondary Growth in Dicot Stem – A First Look
Think of a young dicot stem — say, a sunflower or a mango sapling. It's green, soft, and you can easily bend it. Now picture the same plant a year later. The stem is thicker, woody, and rigid. What happened? The plant didn't just "get bigger" in the same way a balloon inflates. It added new layers of tissue from the inside out. That process is secondary growth.
Primary growth (from apical meristems at the tips) makes the plant taller. Secondary growth makes it thicker. It's the reason a tree trunk has rings, and why wood exists at all.
The Core Idea: Two New Cambia
In a young dicot stem, the primary vascular bundles (xylem and phloem) are arranged in a ring. Between the xylem and phloem of each bundle lies a thin strip of fascicular cambium (intrafascicular cambium). Between the bundles, the ground tissue (medullary rays) also becomes meristematic, forming interfascicular cambium. These two join to form a complete, continuous ring of vascular cambium.
This ring is the engine of secondary growth. It's a lateral meristem — a layer of dividing cells that runs around the stem.
The vascular cambium is a single layer of cells that divides to produce secondary xylem (wood) toward the inside and secondary phloem (inner bark) toward the outside. This is the fundamental mechanism.
How the Vascular Cambium Works
The cambium cells divide periclinally (parallel to the surface). Each division produces two cells: one stays as cambium (the initial), the other differentiates. The cell on the inner side becomes a secondary xylem cell (vessel, tracheid, fibre, or xylem parenchyma). The cell on the outer side becomes a secondary phloem cell (sieve tube, companion cell, phloem fibre, or phloem parenchyma).
Because more xylem is produced than phloem, the stem grows mostly inward. Over years, this builds up the familiar annual rings — each ring corresponds to one growing season (spring wood + autumn wood).
Secondary xylem accumulates year after year. Secondary phloem gets crushed and pushed outward as new layers form inside it. That's why old bark is rough and dead — it's compressed, non-functional phloem.
The Cork Cambium and Periderm
As the stem thickens, the epidermis (the outer skin) can't stretch enough. It cracks and dies. To replace it, a second lateral meristem arises — the cork cambium (phellogen) . It usually forms in the cortex or epidermis.
The cork cambium divides to produce:
- Cork (phellem) toward the outside — dead, suberised cells that are waterproof and protective.
- Phelloderm toward the inside — living parenchyma cells.
Together, cork cambium + cork + phelloderm = periderm. This is the new "bark" that replaces the epidermis.
Don't confuse "bark" with periderm. In older stems, bark includes all tissues outside the vascular cambium — that is, secondary phloem (living and dead) plus all periderm layers. Periderm is just the protective outer part.
Lenticels: Breathing Holes
The periderm is impermeable to gases. So, at certain spots, the cork cambium produces loosely packed, unsuberised cells called complementary cells, which push through the cork to form lenticels. These are visible as small, raised, corky dots on the stem surface (like on a birch or guava stem). They allow gas exchange.
The Big Picture: What You End Up With
After many seasons of secondary growth, a dicot stem cross-section shows (from outside in):
| Layer | What it is |
|---|---| …