▢ Definition: Increase in thickness of plant body due to addition of secondary tissues by vascular cambium + cork cambium
▢ Cambium Activity:
Table 1: Cambium involved in secondary growth
Cambium | Region | Secondary tissue formed |
|---|---|---|
Vascular cambium | Stelar region | Secondary xylem + secondary phloem |
Cork cambium / phellogen | Extrastelar region | Cork / phellem + secondary cortex / phelloderm |
▢ Occurrence: Mainly dicots
📚
SECONDARY GROWTH IN DICOT STEM
📖
Intrastellar secondary growth
Occurs by vascular cambium
▢ Cambium Ring Formation:
- •Intrafascicular cambium is already present
- •Medullary ray cells become meristematic
- •Interfascicular cambium formed
- •Interfascicular cambium joins intrafascicular cambium
- •Complete cambium ring formed
▢ Cambium Ring Cells:
Table 1: Cells of cambium ring
Cell type | Forms |
|---|---|
Fusiform initials | Secondary phloem externally + secondary xylem internally |
Ray initials | Parenchymatous medullary rays |
▢ Secondary Rays:
Table 1: Secondary rays
Location | Name |
|---|---|
In secondary xylem | Wood ray / xylem ray |
In secondary phloem | Phloem ray |
▢ Seasonal Activity of Cambium: 
Table 1: Spring wood vs autumn wood
Feature | Spring wood / Early wood | Autumn wood / Late wood |
|---|---|---|
Season | Spring / favourable season | Winter / unfavourable season |
Cambial activity | High | Low / dormant |
Conducting elements | Wider diameter | Narrow diameter |
Vessel lumen | Wide lumen | Narrow lumen |
▢ Annual Rings / Growth Rings:
❖ Definition: Concentric rings representing total wood formed in one year
❖ Cause:
- •Unequal cambial activity in spring and autumn
- •Sudden transition from autumn wood → next spring wood
- •Spring wood → autumn wood transition is gradual
❖ Important Points:
- •Annual rings absent in plants growing in uniform climate
- •Annual rings used to study climate → dendroclimatology
- •Age determination by counting annual rings → dendrochronology
▢ Types of Wood:
❖ Hard Wood / Porous Wood vs Soft Wood / Non-porous Wood:
Table 1: Hard wood vs soft wood
Feature | Hard wood / porous wood | Soft wood / non-porous wood |
|---|---|---|
Pores | Present due to vessels | Absent; vessels absent |
Vessels | Present | Absent |
Example | Dicots | Gymnosperms |
❖ Ring Porous Wood vs Diffuse Porous Wood: 
Table 1: Porous wood types
Feature | Ring porous wood | Diffuse porous wood |
|---|---|---|
Vessel distribution | Vessels concentrated in spring wood | Vessels evenly distributed |
Cause | Seasonal climatic variation | No marked climatic variation |
Occurrence | Seasonal regions | Evergreen tropics |
❖ Heart Wood / Duramen vs Sap Wood / Alburnum: 
Table 1: Heart wood vs sap wood
Feature | Heart wood / duramen | Sap wood / alburnum |
|---|---|---|
Position | Inner central wood | Outer peripheral wood |
Colour | Dark | Light |
Function | Mechanical support | Water conduction |
Status | Dead + non-functional | Living/functional xylem |
Water content | Dry | Wetter |
Durability | More durable; resistant to microbes, insects, pests | Less durable; decays faster |
Deposits | Oil, aromatic substances, gums, resins, tannins | Less deposits |
Collective name of deposits | Extractives | |
Diameter during secondary growth | Increases | May gradually convert into heart wood |
Vessels | Plugged by tyloses | Functional vessels |
Older stems e.g., Dalbergia show outer sapwood + inner heartwood
▢ Tyloses:
- •Balloon-like ingrowths of adjacent parenchyma into vessel cavities
- •Plug vessels of heart wood
- •Found in secondary xylem
▢ Knots: Stumps of broken branches embedded in secondary xylem
▢ Lignification: Process by which plant becomes woody
📖
Extrastellar secondary growth
Occurs by Cork cambium
▢ Cork Cambium / Phellogen:
- •Formed due to pressure by addition of secondary tissues
- •Meristematic zone in outer extrastelar region
- •Origin → outer cortical cells / collenchyma / epidermis
▢ Products of Cork Cambium:
Table 1: Phellogen products
Side | Tissue formed | Other name |
|---|---|---|
Inner side | Secondary cortex | Phelloderm |
Outer side | Cork | Phellem |
▢ Periderm:
Table 1: Periderm components
Component | Position / Nature |
|---|---|
Phellem / cork | Outer protective tissue |
Phellogen / cork cambium | Meristematic layer |
Phelloderm / secondary cortex | Inner living tissue |
Periderm = phellem + phellogen + phelloderm
▢ Bark:
- •All tissues outside vascular cambium
- •Mostly dead outer tissues
- •Contains lenticels for gaseous exchange
▢ Lenticels: Openings in periderm/bark for gaseous exchange
▢ Cork Cells:
- Major component → suberin
- It is waterproof due to the presence of Suberin. (CEE 2025)
Q1.
Cork cells are impermeable to water due to deposition of a waxy substance called:
📅CEE MBBS 2025
Q2.
Grafting is generally not possible in monocot plants because of:
📅CEE MBBS 2025
Q3.
Cork, cork cambium and secondary cortex are collectively called:
📅CEE MBBS 2024B
Q4.
Conventional grafting is generally not possible in monocots due to:
📅CEE MBBS 2024a
Q5.
If a complete ring of bark containing phloem is removed from a tree trunk, which part is deprived of food first?
📅CEE MBBS 2022
Q6.
The openings in corky stems that allow gaseous exchange are called:
📅CEE MBBS 2021
Q7.
The periderm includes:
📅CEE BDS 2024
Q8.
A woody trunk shows ten annual growth rings, each comprising spring wood and autumn wood. Its approximate age is:
📅CEE BDS 2024
Q9.
Xylem vessels in plants are commonly blocked by:
📅IOM 2005
Q10.
Which of the following monocots exhibits anomalous secondary growth?
📅IOM 2003
Q11.
Which meristem produces extrastelar secondary tissues in a dicot stem?
Q12.
Secondary growth in girth is brought about mainly by the activity of:
📅BPKIHS 2005
Q13.
Vascular cambium chiefly helps in:
📅MOE 2063
Q14.
The youngest secondary xylem in a woody stem lies immediately:
📅MOE 2000, 2055
Q15.
In a dicot root, vascular cambium develops from cells of the:
📅MOE 2000
Q16.
Periderm consists of:
📅MOE 2054
Q17.
For successful grafting, stock and scion should have their:
📅BPKIHS 1994
Q18.
During prolonged secondary growth of a tree, which wood region generally increases in diameter as sapwood is converted to heartwood?
📅BPKIHS 1995
Q19.
Suberin deposited in cork cell walls primarily makes them:
📅BPKIHS 2004
Q20.
Cork and secondary cortex develop through the activity of:
📅IE 2005
Q21.
Cork tissue is also called:
📅IE 2008
Q22.
Phellogen in a typical dicot stem develops from:
📅IE 2009
Q23.
Complete removal of a ring of bark from a woody tree eventually kills it because it:
📅BPKIHS 2016
Q24.
A typical vascular cambium is absent in the stem of:
📅BPKIHS 2013
Q25.
Determination of the age of trees using annual growth rings is called:
📅BPKIHS 2012
Q26.
Increase in circumference of the stem or root is mainly due to:
📅BPKIHS 2011
Q27.
Balloon-like outgrowths of parenchyma entering the lumen of xylem vessels are called:
📅BPKIHS 2011
Q28.
Successful conventional grafting requires the presence and alignment of:
📅BPKIHS 2011
Q29.
The study of tree growth rings in relation to past climate is:
📅IOM 2014
Q30.
Annual growth rings can be used to determine the approximate:
📅IOM 2016
Q31.
Which monocot shows anomalous secondary growth?
📅MOE 2014
Q32.
Annual growth rings in dicot stems result from seasonal activity of the:
📅MOE 2013