Q.How secondary growth takes place in plants?
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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 |
|---|---| …
Secondary growth is the increase in the girth (thickness) of stems and roots by activity of lateral meristems. …
Secondary growth increases girth via the vascular cambium (secondary xylem and phloem) and cork cambium (bark).
Secondary growth (increase in thickness) in dicot stems and roots:
- A vascular cambium is formed as a complete ring (from intrafascicular and interfascicular cambium). It divides to add new cells — cutting off secondary xylem towards the inside and secondary phloem towards the outside. Much more xylem is formed than phloem, so the stem thickens; annual rings appear in temperate wood. …
- CBSE 2026Set ANNUAL1 markMCQQ.What is the another name for cork cambium ?(a) Vascular cambium(b) Phellogen(c) Pericycle(d) Phelloderm
›Reveal solutionSolution
Cork cambium is also called phellogen, so the answer is (B).
During secondary growth in dicot stems, a lateral meristem develops in the cortex called the cork cambium or phellogen. It cuts off cells:
- Outward — cork or phellem (dead, suberised, protective).
- Inward — secondary cortex or phelloderm (living). …
- CBSE 2026Set ANNUAL1 markMCQQ.Which of the following in dicot stem, originates due to the activity of the vascular cambium ?(a) Primary xylem(b) Medullary rays(c) Secondary xylem(d) Pith
›Reveal solutionSolution
The vascular cambium produces secondary xylem (inward) during secondary growth, so the answer is (C).
In a dicot stem, secondary growth is brought about by the vascular cambium. It divides to add:
- Secondary xylem toward the inside (much more of it, forming the bulk of wood).
- Secondary phloem toward the outside. …
- CBSE 2025Set ANNUAL1 markMCQQ.Read the different components from(i) to(iv) in the list given below and tell the correct order of the components with reference to their arrangement from outer side to inner side in a woody dicot stem.(i) Phellem(ii) Secondary Phloem(iii) Wood(iv) Secondary Cortex(a) iv, i, ii, iii(b) ii, iv, i, iii(c) i, ii, iv, iii(d) i, iv, ii, iii
›Reveal solutionSolution
From outside to inside, a woody dicot stem shows: Phellem (cork) -> Secondary Cortex (phelloderm) -> Secondary Phloem -> Wood (secondary xylem).
As a dicot stem undergoes secondary growth, the cork cambium (phellogen) cuts off cells on both sides:
- Outward, it forms phellem (cork), a protective layer of dead, suberised cells — the outermost tissue.
- Inward, it forms a few layers of secondary cortex (phelloderm), living parenchymatous cells.
Deeper than the periderm lies the vascular cylinder, produced by the vascular cambium:
- Outward from the cambium ring lies secondary phloem.
- Inward from the cambium ring lies the bulk of the stem as wood (secondary xylem), which forms the innermost and most extensive tissue. …
- CBSE 2025Set ANNUAL1 markQ.Fill in the blank: In dicotyledons, secondary growth is present because of activation of ___________.
›Reveal solutionSolution
Secondary growth in dicots results from the activity of the vascular cambium (produces secondary xylem and phloem) and the cork cambium (produces periderm).
Unlike monocots, dicotyledonous stems and roots possess lateral meristems in addition to the apical meristem. The vascular cambium, present between the primary xylem and primary phloem (and also forming in the medullary ray regions), becomes a continuous ring and cuts off secondary xylem toward the inside and secondary phloem toward the outside, increasing the girth of the axis. As the axis widens, the outer cortical/epidermal layers cannot cope with the stress, so a cork cambium (phellogen) develops in the outer cortex a …
- CBSE 2023Set ANNUAL1 markMCQQ.Secondary growth does not take place in monocot due to lack of :(a) xylem(b) phloem(c) cambium(d) hormones
›Reveal solutionSolution
Monocots lack cambium, so no secondary growth.
Secondary growth (increase in thickness) is brought about by the vascular cambium. In monocot stems the vascular bundles are closed (no cambium between xylem and phloem), so there is no vascular cambium and hence no no …
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