Quick Summary
Secondary growth increases the girth (diameter) of dicot stems and roots. It involves two lateral meristems: the vascular cambium (produces wood and phloem) and the cork cambium (produces bark). Secondary growth does NOT occur in monocots. Annual rings, heartwood, and sapwood are key concepts from this chapter.
Overview
Primary growth: Increase in length (apical meristem) Secondary growth: Increase in girth (lateral meristems) Occurs in: Dicot stems and roots, gymnosperms Absent in: Monocots (no cambium)
Vascular Cambium
Origin: Interfascicular cambium forms from parenchyma cells between vascular bundles; joins the fascicular (intrafascicular) cambium from within bundles → forms a complete ring.
Products:
- Inward → Secondary xylem (wood, accumulates)
- Outward → Secondary phloem (crushed over time)
Rays: Vascular rays (parenchyma) produced horizontally for lateral transport.
Secondary Xylem (Wood)
Spring wood (Early wood):
- Formed in spring (favourable conditions)
- Wide vessels, light colour, low density
Autumn wood (Late wood):
- Formed in autumn (less favourable)
- Narrow vessels, dark colour, high density
Annual ring: One spring wood + one autumn wood band = one year Dendrochronology: Counting annual rings to determine the age of a tree.
Heartwood and Sapwood
| Feature | Sapwood (Alburnum) | Heartwood (Duramen) |
|---|---|---|
| Position | Outer zone | Inner zone |
| Colour | Light | Dark (tannins, resins, oils) |
| Function | Active water conduction | Non-conducting (tyloses block vessels) |
| Durability | Less durable | More durable (resistant to decay) |
| Timber use | Less valued | Highly valued |
Tyloses: Outgrowths from adjacent parenchyma cells that block old xylem vessels → heartwood.
Cork Cambium (Phellogen)
Origin: Epidermis or outer cortex cells dedifferentiate → form cork cambium
Products:
- Outward → Cork (Phellem): Dead cells, suberin-impregnated, waterproof
- Inward → Phelloderm: Living parenchyma cells
Periderm = Cork + Cork cambium + Phelloderm
Bark: All tissues outside vascular cambium = secondary phloem + periderm (common definition)
Lenticels
- Loosely packed cells in cork that allow gaseous exchange
- Replace stomata (which are lost as epidermis is replaced by cork)
- Visible as raised dots on bark of trees (e.g., mango, cherry trunk)
Secondary Growth in Dicot Root
Pericycle → forms cork cambium (not epidermis/cortex as in stem) Endarch xylem in stem; Exarch xylem in root (xylogenesis direction differs)
Worked Examples
Q: A tree trunk cross-section shows 25 annual rings. How old is the tree? A: 25 years old (one ring = one year).
Q: Why is heartwood preferred for furniture? A: Heartwood is denser, darker (tannins/resins make it resistant to insects and decay), and non-conducting — ideal for durable timber.
Important Questions
One Mark
- What is dendrochronology?
- Name the two lateral meristems responsible for secondary growth.
- What is the function of lenticels?
- Define tyloses.
Short Answer (3 marks)
- Distinguish between heartwood and sapwood on the basis of four characters.
- What is the periderm? Name its three components and their functions.
- How are annual rings formed? What determines the width of each ring?
Long Answer (5 marks)
- Describe secondary growth in a dicot stem. Name the two lateral meristems and state what each produces.
- Draw a labelled diagram of a transverse section of a three-year-old dicot stem showing annual rings, heartwood, sapwood, and periderm.
Common Mistakes
- Secondary growth occurs in dicots, NOT monocots (monocots have no vascular cambium).
- Bark includes secondary phloem + periderm — NOT just the cork layer.
- Spring wood has wider vessels (formed when water is plentiful), autumn wood has narrower vessels — not the opposite.
- Lenticels are in the cork, not in the epidermis — they replace stomata after the bark forms.
Board Exam Tips
- "Describe secondary growth in dicot stem" is a classic 5-mark question — include both cambiums, their products, and the formation of periderm.
- Heartwood vs sapwood comparison (4 characters) is a reliable 3-mark question.
- Annual ring formation and dendrochronology appear as short-answer questions.
- NEET: Annual rings, heartwood formation, cork cambium products, and lenticels appear in 1–2 questions per paper.