Formation of Secondary Tissues — Revision Notes

⚡ 30-Second Revision

Secondary Growth — Increase in girth by lateral meristems.
Lateral Meristems — Vascular Cambium, Cork Cambium (Phellogen).
Vascular Cambium — Forms from intrafascicular + interfascicular cambium.

- Produces: Secondary Xylem (inward), Secondary Phloem (outward). - Cells: Fusiform initials (axial system), Ray initials (radial system).

Cork Cambium (Phellogen) — Forms from cortex/hypodermis.

- Produces: Phellum (cork, outward), Phelloderm (secondary cortex, inward).

Periderm — Phellogen + Phellum + Phelloderm (replaces epidermis).
Bark — All tissues outside vascular cambium (includes secondary phloem + periderm).
Lenticels — Pores in periderm for gaseous exchange.
Annual Rings — Seasonal activity of vascular cambium.

- Early wood (spring wood): Wider vessels, more active cambium. - Late wood (autumn wood): Narrower vessels, less active cambium.

2-Minute Revision

Secondary tissue formation is the process of increasing plant girth, driven by two lateral meristems. The vascular cambium forms a continuous ring from existing intrafascicular cambium and newly formed interfascicular cambium (from medullary rays).

It actively divides, producing secondary xylem (wood) towards the inside and secondary phloem towards the outside. Secondary xylem is far more abundant, providing structural support and water transport.

The seasonal variation in its activity leads to annual rings, with distinct early wood (spring wood) and late wood (autumn wood) layers, allowing for tree age determination.

Simultaneously, as the stem expands, the epidermis ruptures. To replace it, the cork cambium (phellogen) develops, typically from cortical cells. It produces cork (phellum) towards the outside, which is dead and suberized for protection, and secondary cortex (phelloderm) towards the inside, which is living parenchyma.

The phellogen, phellum, and phelloderm together form the periderm, the new protective outer layer. Lenticels are specialized pores in the periderm that facilitate gaseous exchange. All tissues outside the vascular cambium, including secondary phloem and periderm, constitute the bark.

5-Minute Revision

Secondary growth is the process by which dicotyledonous stems and roots, and gymnosperms, increase in girth. This is distinct from primary growth, which increases length. The key players are two lateral meristems: the vascular cambium and the cork cambium.

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Vascular Cambium — This meristem originates from the intrafascicular cambium (present between primary xylem and phloem in vascular bundles) and the interfascicular cambium (formed by dedifferentiation of medullary ray cells). These join to form a complete ring. The vascular cambium has two types of initials: fusiform initials (elongated, forming axial system like tracheids, vessels, sieve tubes, fibres) and ray initials (isodiametric, forming radial system like vascular rays). It divides to produce secondary xylem towards the pith (inside) and secondary phloem towards the cortex (outside). Secondary xylem accumulates significantly more, forming the bulk of the wood. The seasonal variation in its activity (more active in spring, less in autumn/winter) leads to the formation of annual rings, comprising early wood (spring wood - wider vessels, less dense) and late wood (autumn wood - narrower vessels, denser). These rings are crucial for dendrochronology (tree dating).

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Cork Cambium (Phellogen) — As the stem expands, the epidermis and outer cortex rupture. The phellogen develops, usually from cortical cells. It is also a lateral meristem that divides to produce cells. Towards the outside, it forms phellum (cork), which are dead, suberized, and compactly arranged cells providing excellent protection against water loss, pathogens, and mechanical injury. Towards the inside, it forms phelloderm (secondary cortex), which are living parenchymatous cells. The phellogen, phellum, and phelloderm collectively form the periderm, which replaces the epidermis as the plant's outer protective layer. Lenticels are specialized, lens-shaped openings in the periderm where the phellogen produces loosely arranged complementary cells instead of compact cork, allowing for gaseous exchange. The term bark refers to all tissues external to the vascular cambium, including secondary phloem and the entire periderm. Understanding the precise origin, products, and functions of these cambia and their derivatives is essential for NEET.

Prelims Revision Notes

Formation of Secondary Tissues: NEET Revision Notes

1. Secondary Growth Basics:

Definition: — Increase in girth/diameter of stem and root.
Occurrence: — Dicot stems and roots, Gymnosperms. Generally absent in monocots.
Meristems Involved: — Lateral meristems (Vascular Cambium, Cork Cambium).

2. Vascular Cambium:

Origin:

* Intrafascicular cambium: Primary meristematic strip between primary xylem and primary phloem within vascular bundles. * Interfascicular cambium: Formed by dedifferentiation of medullary ray cells between vascular bundles. * These join to form a continuous cambial ring.

Activity: — Divides periclinally.

* Inward: Produces Secondary Xylem (wood) – more abundant. * Outward: Produces Secondary Phloem – less abundant.

Cell Types:

* Fusiform initials: Elongated, form axial system (tracheids, vessels, sieve tubes, fibres, parenchyma). * Ray initials: Isodiametric, form radial system (vascular rays/medullary rays for radial conduction and storage).

3. Annual Rings (Growth Rings):

Cause: — Seasonal variation in vascular cambium activity in temperate regions.
Early Wood (Spring Wood): — Formed in spring; cambium highly active, wider vessels, less dense, lighter color.
Late Wood (Autumn Wood): — Formed in autumn/winter; cambium less active, narrower vessels, more fibres, denser, darker color.
Significance: — Each ring = one year's growth; used for dendrochronology (tree age determination).

4. Cork Cambium (Phellogen):

Origin: — Develops in outer cortical cells, hypodermis, or sometimes pericycle (in roots) as epidermis ruptures.
Activity: — Divides periclinally.

* Outward: Produces Phellum (Cork) – dead, suberized, compactly arranged, protective, impermeable to water/gases. * Inward: Produces Phelloderm (Secondary Cortex) – living parenchymatous cells, storage.

5. Periderm:

Components: — Phellogen + Phellum + Phelloderm.
Function: — Replaces ruptured epidermis and cortex, provides protection.

6. Lenticels:

Structure: — Lens-shaped, raised pores on bark, formed by loosely arranged complementary cells from phellogen.
Function: — Facilitate gaseous exchange between internal tissues and atmosphere.

7. Bark:

Definition: — All tissues exterior to the vascular cambium.
Components: — Includes secondary phloem and periderm (phellogen, phellum, phelloderm).
Types: — Early bark (soft bark), Late bark (hard bark).

Key Differences to Remember:

Primary vs. Secondary Growth: — Length vs. Girth.
Vascular Cambium vs. Cork Cambium: — Products and functions.

Vyyuha Quick Recall

To remember the components of the PERIDERM and their positions:

Please Eat Ripe Indian Dates, Every Ripe Mango.

Phellum (Cork) - External (Outward)
Root (Phellogen / Cork Cambium) - In Division (Meristematic)
Endodermis (Phelloderm / Secondary Cortex) - Reaching Medulla (Inward)

This helps recall Phellum (cork) is outside, Phellogen is the middle dividing layer, and Phelloderm (secondary cortex) is inside.