How to Use This Page
Solve each Samacheer Kalvi exercise question on your own first, then compare your answer with the solutions below. Secondary Growth covers vascular cambium, cork cambium, annual rings, heartwood, sapwood, and bark — focus on understanding the process step by step.
Key Concepts (Quick Recap)
What is Secondary Growth?
Secondary growth is the increase in girth (diameter) of stems and roots due to the activity of lateral meristems: 1. Vascular cambium → produces secondary xylem (inside) and secondary phloem (outside) 2. Cork cambium (Phellogen) → produces cork (outside) and secondary cortex (inside)
Occurs in: Dicotyledons and Gymnosperms (not in monocots)
Formation of Vascular Cambium
The vascular cambium arises from two sources: 1. Intrafascicular cambium: Present between xylem and phloem in the vascular bundle (already present as a strip) 2. Interfascicular cambium: Formed from the dedifferentiation of parenchyma cells in the medullary rays (between vascular bundles)
The intrafascicular and interfascicular cambium join to form a complete ring of vascular cambium.
Activity of Vascular Cambium
The vascular cambium divides periclinally (tangentially) to produce:
- Secondary xylem (wood): Towards the inside (much more is produced)
- Secondary phloem: Towards the outside (less is produced)
The vascular cambium also divides anticlinally (radially) to increase its own circumference as the stem grows.
Fusiform initials: Produce secondary xylem and secondary phloem (axial system) Ray initials: Produce vascular rays (radial system — for lateral conduction and storage)
Secondary Growth in Stem — Step by Step
1. Vascular cambium forms a complete ring 2. Cambium produces secondary xylem towards the inside and secondary phloem towards the outside 3. More secondary xylem is produced than secondary phloem (xylem accumulates as wood) 4. Primary xylem gets pushed towards the centre; primary phloem gets crushed 5. Annual rings form due to seasonal activity of the cambium 6. The stem increases in girth
Annual Rings (Growth Rings)
| Feature | Spring Wood (Early Wood) | Autumn Wood (Late Wood) |
|---|
| Season | Spring (favourable) | Autumn (unfavourable) |
| Vessels | Large, numerous, thin-walled | Small, fewer, thick-walled |
| Density | Less dense, lighter colour | More dense, darker colour |
| Amount | More xylem produced | Less xylem produced |
| Growth | Active cambium, rapid growth | Slower cambium, slower growth |
- One annual ring = one spring wood band + one autumn wood band = one year of growth
- Dendrochronology: Counting annual rings to determine the age of a tree
Heartwood vs Sapwood
| Feature | Heartwood (Duramen) | Sapwood (Alburnum) |
|---|
| Location | Inner, older region | Outer, younger region |
| Colour | Dark | Light |
| Function | No water conduction (blocked) | Active water conduction |
| Tyloses | Present (block vessels) | Absent |
| Resins/oils | Present | Absent |
| Strength | Strong, durable | Weaker |
| Commercial value | High (furniture, construction) | Low |
Extrastelar Secondary Growth (Cork Cambium)
As secondary xylem accumulates and the stem grows in girth, the outer layers (epidermis, cortex) cannot keep up and get stretched and ruptured. To protect the inner tissues:
1. Cork cambium (phellogen) develops from the outer cortex or epidermis 2. It divides to produce:
3. Periderm = Cork + Cork cambium + Secondary cortex 4. Bark = All tissues outside the vascular cambium (including periderm and secondary phloem)
- Cork (phellem): Towards the outside — dead, suberised, waterproof cells
- Secondary cortex (phelloderm): Towards the inside — living, parenchymatous cells
Lenticels
- Small, raised pores on the bark
- Formed beneath stomata when cork cambium produces loosely arranged cells
- Function: Gas exchange (allow oxygen to reach inner living tissues)
- Visible as small dots/lines on tree bark
Important Terms
| Term | Definition |
|---|
| Phellogen | Cork cambium — produces cork and phelloderm |
| Phellem | Cork — dead, suberised, waterproof cells produced outward by cork cambium |
| Phelloderm | Secondary cortex — living parenchyma cells produced inward by cork cambium |
| Periderm | Phellem + Phellogen + Phelloderm (replaces epidermis in older stems) |
| Bark | All tissues outside the vascular cambium |
| Tyloses | Balloon-like ingrowths of parenchyma cells into vessel elements (block vessels in heartwood) |
| Wood | Secondary xylem accumulated over years |
| Sapwood | Outer, functional secondary xylem that conducts water |
| Heartwood | Inner, non-functional secondary xylem (blocked by tyloses) |
| Dendrochronology | Dating of tree age by counting annual rings |
I. Choose the Best Answer (MCQs)
Q1. Secondary growth occurs in (a) Monocots only (b) Dicots and gymnosperms (c) All plants (d) Algae
Answer(b) Dicots and gymnosperms — Secondary growth (increase in girth) occurs in dicotyledons and gymnosperms due to the activity of vascular cambium and cork cambium. Monocots generally lack lateral meristems and do not show secondary growth.
Q2. The vascular cambium produces (a) Secondary xylem outside and secondary phloem inside (b) Secondary xylem inside and secondary phloem outside (c) Primary xylem and primary phloem (d) Cork and secondary cortex
Answer(b) Secondary xylem inside and secondary phloem outside — The vascular cambium produces secondary xylem (wood) towards the inside and secondary phloem towards the outside. More secondary xylem is produced than secondary phloem.
Q3. Annual rings are formed due to (a) Difference in day and night temperature (b) Seasonal activity of vascular cambium (c) Activity of cork cambium (d) Difference in water availability
Answer(b) Seasonal activity of vascular cambium — Annual rings are formed because the vascular cambium is more active in spring (producing large, thin-walled spring wood) and less active in autumn (producing small, thick-walled autumn wood). The alternating bands of spring and autumn wood form annual rings.
Q4. Heartwood differs from sapwood in being (a) Lighter in colour (b) Conducting water (c) Non-conducting and darker (d) Outer region of wood
Answer(c) Non-conducting and darker — Heartwood (duramen) is the inner, older, darker region of secondary xylem that does not conduct water (vessels are blocked by tyloses). Sapwood (alburnum) is the outer, lighter, functional region that actively conducts water.
Q5. Cork cambium is also called (a) Phelloderm (b) Phellem (c) Phellogen (d) Periderm
Answer(c) Phellogen — Cork cambium is called phellogen. It produces phellem (cork) towards the outside and phelloderm (secondary cortex) towards the inside. Periderm = phellem + phellogen + phelloderm.
Q6. Lenticels are involved in (a) Water absorption (b) Photosynthesis (c) Gas exchange (d) Translocation of food
Answer(c) Gas exchange — Lenticels are small, raised pores on the bark formed by loosely arranged cork cells. They allow gas exchange (oxygen in, CO₂ out) for the inner living tissues.
Q7. The science of determining the age of a tree by counting annual rings is called (a) Anatomy (b) Dendrochronology (c) Taxonomy (d) Palynology
Answer(b) Dendrochronology — Dendrochronology is the scientific method of dating tree age by counting the number of annual rings in the wood. Each annual ring represents one year of growth.
II. Fill in the Blanks
1. Secondary growth increases the girth (diameter) of stems and roots. 2. Secondary growth occurs in dicots and gymnosperms but not in monocots. 3. The vascular cambium produces secondary xylem towards the inside and secondary phloem towards the outside. 4. Cork cambium is also called phellogen. 5. Cork cells are dead and filled with suberin (a waterproof substance). 6. The tissue formed by cork cambium towards the outside is called phellem (cork). 7. The tissue formed by cork cambium towards the inside is called phelloderm (secondary cortex). 8. Periderm consists of phellem + phellogen + phelloderm. 9. Alternating bands of spring wood and autumn wood form annual rings (growth rings). 10. The inner, darker, non-conducting region of secondary xylem is called heartwood (duramen). 11. The outer, lighter, conducting region of secondary xylem is called sapwood (alburnum). 12. Tyloses are balloon-like ingrowths of parenchyma cells into vessel elements that block them. 13. The science of counting annual rings to determine the age of a tree is called dendrochronology. 14. Lenticels are small pores on the bark that allow gas exchange. 15. All tissues outside the vascular cambium constitute the bark.
III. True or False (with corrections for false statements)
Q1. Secondary growth occurs in monocot stems.
AnswerFalse — Correction: Secondary growth occurs in DICOT stems and gymnosperms. Monocot stems generally lack vascular cambium and cork cambium, so they do not show typical secondary growth.
Q2. More secondary phloem is produced than secondary xylem.
AnswerFalse — Correction: More SECONDARY XYLEM is produced than secondary phloem. The vascular cambium produces much more xylem towards the inside than phloem towards the outside. That is why wood (secondary xylem) accumulates over years.
Q3. Heartwood conducts water.
AnswerFalse — Correction: Heartwood does NOT conduct water. The vessels in heartwood are blocked by tyloses (balloon-like ingrowths) and filled with resins, oils, and tannins. Only SAPWOOD (outer region) conducts water.
Q4. Cork cells are living.
AnswerFalse — Correction: Cork cells are DEAD at maturity. They are impregnated with suberin (a waterproof substance) and lack protoplasm. They provide protective, waterproof covering.
Q5. Lenticels help in gas exchange.
AnswerTrue — Lenticels are small, raised pores on the bark composed of loosely arranged cork cells. They allow gas exchange (oxygen entry and carbon dioxide exit) for the inner living tissues.
IV. Match the Following
| Column A | Column B | Answer |
|---|
| 1. Vascular cambium | (a) Produces cork and phelloderm | (b) Produces secondary xylem and phloem |
| 2. Cork cambium | (b) Produces secondary xylem and phloem | (a) Produces cork and phelloderm |
| 3. Heartwood | (c) Outer, functional wood | (e) Inner, non-functional, dark wood |
| 4. Sapwood | (d) Gas exchange pores | (c) Outer, functional wood |
| 5. Lenticels | (e) Inner, non-functional, dark wood | (d) Gas exchange pores |
| 6. Annual rings | (f) Dating tree age by counting rings | (g) Spring wood + autumn wood bands |
V. Answer Briefly (2-mark questions)
Q1. What is secondary growth? Where does it occur?
Answer
Secondary growth is the increase in girth (diameter) of stems and roots due to the activity of lateral meristems — vascular cambium and cork cambium. It occurs in dicotyledonous plants and gymnosperms. Monocots generally do not show secondary growth because they lack lateral meristems.
Q2. Differentiate between heartwood and sapwood.
Answer
| Feature | Heartwood (Duramen) | Sapwood (Alburnum) | |---|---|---| | Location | Inner, older region | Outer, younger region | | Colour | Dark | Light | | Function | Non-conducting (vessels blocked by tyloses) | Actively conducts water and minerals | | Strength | Hard, durable | Softer | | Commercial value | High (furniture, construction) | Low |
Q3. What are annual rings? How are they formed?
Answer
Annual rings (growth rings) are alternating bands of spring wood and autumn wood visible in a cross-section of wood. They are formed due to the seasonal activity of the vascular cambium: - Spring wood (early wood): Formed in spring when cambium is active; large, thin-walled vessels; lighter in colour - Autumn wood (late wood): Formed in autumn/winter when cambium is less active; small, thick-walled vessels; darker in colour One spring wood band + one autumn wood band = one annual ring = one year of growth.
Q4. What is cork cambium? What does it produce?
Answer
Cork cambium (phellogen) is a lateral meristem that develops from the outer cortex or epidermis. It produces: - Phellem (cork): Towards the outside — dead, suberised, waterproof cells that replace the epidermis - Phelloderm (secondary cortex): Towards the inside — living, parenchymatous cells Together, phellem + phellogen + phelloderm form the periderm, which replaces the epidermis in older stems.
VI. Answer in Detail (5-mark questions)
Q1. Describe the process of secondary growth in a dicot stem.
Answer
Secondary growth in a dicot stem involves the formation of secondary tissues by lateral meristems: Step 1: Formation of Vascular Cambium Ring - The intrafascicular cambium (present between xylem and phloem in vascular bundles) becomes active - Parenchyma cells in the medullary rays dedifferentiate into interfascicular cambium - Intrafascicular and interfascicular cambium join to form a complete ring of vascular cambium Step 2: Activity of Vascular Cambium - The vascular cambium divides periclinally (tangentially): - Produces secondary xylem (wood) towards the inside (much more produced) - Produces secondary phloem towards the outside (less produced) - The cambium also divides anticlinally (radially) to increase its own circumference - Fusiform initials produce axial elements (vessels, fibres, sieve tubes) - Ray initials produce vascular rays (radial rows for lateral conduction) Step 3: Changes in the Stem - Primary xylem is pushed towards the centre - Primary phloem gets crushed and compressed (becomes non-functional) - Secondary xylem accumulates as wood (more in spring, less in autumn → annual rings) - The stem increases in girth Step 4: Extrastelar Secondary Growth (Cork Cambium) - As the stem grows, the epidermis ruptures - Cork cambium (phellogen) develops from the outer cortex - Produces phellem (cork) outward — dead, suberised, waterproof - Produces phelloderm (secondary cortex) inward — living parenchyma - Periderm (phellem + phellogen + phelloderm) replaces epidermis Step 5: Formation of Lenticels - Lenticels form at places where cork cambium produces loosely arranged cells - They appear as raised pores on the bark - Function: Gas exchange for inner living tissues
Q2. Describe annual rings, heartwood, and sapwood.
Answer
Annual Rings (Growth Rings): Annual rings are alternating bands of spring wood and autumn wood visible in the cross-section of a tree trunk. They are formed due to seasonal variation in cambial activity: Spring wood (Early wood): - Formed in spring/early summer when conditions are favourable - Cambium is very active → produces more secondary xylem - Vessels are large, numerous, and thin-walled - Tracheids are larger - Lighter in colour, less dense Autumn wood (Late wood): - Formed in autumn/winter when conditions are unfavourable - Cambium is less active → produces less secondary xylem - Vessels are small, fewer, and thick-walled - Tracheids are smaller - Darker in colour, more dense One annual ring = one spring wood band + one autumn wood band = one year of growth Dendrochronology = dating trees by counting annual rings Heartwood (Duramen): - Inner, older, darker region of secondary xylem - Vessels are blocked by tyloses (balloon-like ingrowths of parenchyma cells) - Filled with resins, oils, tannins, and gums - Does NOT conduct water - Very hard, durable, and resistant to decay and insects - Has high commercial value (furniture, construction, musical instruments) - Examples: Teak, Rosewood, Sandalwood Sapwood (Alburnum): - Outer, younger, lighter region of secondary xylem - Vessels are open and functional (no tyloses) - Actively conducts water and minerals from roots to leaves - Lighter in colour, softer - Less durable, lower commercial value - Over time, sapwood gradually converts into heartwood
Q3. Describe the extrastelar secondary growth (cork cambium and bark formation).
Answer
As the stem grows in girth due to secondary growth, the outer tissues (epidermis, cortex) cannot stretch and get ruptured. To protect the inner tissues, cork cambium develops: Formation of Cork Cambium: - Cork cambium (phellogen) develops from dedifferentiation of parenchyma cells in the outer cortex (or sometimes from the epidermis) - It is a lateral meristem that divides to produce: Products of Cork Cambium: 1. Phellem (Cork): Towards the outside - Dead at maturity - Cells are impregnated with suberin (a waterproof, waxy substance) - Suberin makes them impermeable to water and gases - Provides protection against water loss, mechanical injury, and pathogen attack 2. Phelloderm (Secondary cortex): Towards the inside - Living, parenchymatous cells - Contains chloroplasts in some species (can photosynthesise) - Stores food and provides aeration Periderm = Phellem + Phellogen + Phelloderm - This triple layer replaces the epidermis in older stems Bark: - Bark refers to all tissues outside the vascular cambium - Includes: Secondary phloem + Periderm + Remnants of cortex and epidermis - Bark provides protection against water loss, mechanical injury, herbivores, and pathogens Lenticels: - Small, raised pores on the bark - Formed where cork cambium produces loosely arranged, thin-walled cells (complementary cells) instead of tightly packed cork cells - Function: Allow gas exchange (O₂ enters, CO₂ exits) for inner living tissues - Visible as small dots or elongated marks on tree bark Multiple Cork Cambium Layers: - In some trees, the first cork cambium may become inactive - New cork cambium layers develop from deeper in the cortex or secondary phloem - This results in the formation of multiple layers of bark - Old bark layers may peel off (ex: Eucalyptus, Betula)
Common Mistakes to Avoid
- Saying secondary growth occurs in monocots: Secondary growth occurs in DICOTS and GYMNOSPERMS, not in monocots. Monocots lack vascular cambium and cork cambium.
- Confusing secondary xylem and secondary phloem amounts: More secondary XYLEM is produced (inside) than secondary phloem (outside). That is why wood accumulates.
- Confusing heartwood and sapwood: Heartwood = inner, dark, non-conducting (blocked by tyloses). Sapwood = outer, light, conducting water.
- Confusing phellogen, phellem, and phelloderm: Phellogen = cork cambium (the meristem). Phellem = cork (dead, suberised, outside). Phelloderm = secondary cortex (living, inside).
- Saying bark is just cork: Bark = ALL tissues outside the vascular cambium (including secondary phloem, periderm, and cortex remnants). Cork is only the phellem.
- Saying annual rings are formed by different tissues: Annual rings are formed by the SAME tissue (secondary xylem), but the seasonal difference in cambial activity creates alternating bands of spring wood and autumn wood.
- Confusing intrafascicular and interfascicular cambium: Intrafascicular cambium = within the vascular bundle (already present). Interfascicular cambium = between vascular bundles (formed from medullary ray parenchyma).
Exam Checklist
- [ ] Can you define secondary growth and state where it occurs?
- [ ] Can you explain how the vascular cambium ring is formed?
- [ ] Can you describe the products of vascular cambium (secondary xylem and phloem)?
- [ ] Can you explain how annual rings are formed (spring wood vs autumn wood)?
- [ ] Can you differentiate heartwood and sapwood (at least 5 points)?
- [ ] Can you explain the formation and function of cork cambium (phellogen)?
- [ ] Can you define phellem, phelloderm, and periderm?
- [ ] Can you explain what lenticels are and their function?
- [ ] Can you define bark and list what it includes?
- [ ] Can you explain what dendrochronology is?
