14Secondary Growth

▢ 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

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SECONDARY GROWTH IN DICOT STEM

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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

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Extrastellar secondary growth

Occurs by Cork cambium

▢ Cork Cambium / Phellogen:

•
Formed due to pressure by addition of secondary tissues
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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
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Contains lenticels for gaseous exchange

▢ Lenticels: Openings in periderm/bark for gaseous exchange

▢ Cork Cells: Major component → suberin

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MCQ POINTS

Q1.

Which decays faster if exposed freely to air?

Q2.

Tyloses are found in

Q3.

Callose is found in

Q4.

Periderm is formed from

Q5.

Periderm includes

Q6.

Cork cells have a major component of

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