Define periderm.

Periderm is the secondary protective tissue that replaces the epidermis in stems and roots undergoing secondary growth.

Comprehensive Definition:

Periderm is a complex tissue formed during secondary growth, comprising three components:

1. Phellogen (Cork Cambium) - Meristematic layer
2. Phellem (Cork) - Outer protective layer
3. Phelloderm (Secondary Cortex) - Inner parenchymatous layer

Formation Process:

In Stems:

1. Due to secondary vascular growth, stem girth increases
2. Epidermis and outer cortex layers stretch and rupture
3. Parenchymatous cells in outer cortex or epidermis become meristematic
4. Form phellogen (cork cambium)
5. Phellogen undergoes periclinal divisions:
   • Outward: Produces phellem (cork)
   • Inward: Produces phelloderm (secondary cortex)

In Roots:

• Phellogen arises from pericycle cells
• All tissues outside phellogen (cortex, epidermis) slough off

Structural Features:

Component	Characteristics	Function
Phellem (Cork)	• Dead, suberized cells • No intercellular spaces • Impermeable to water and gases • Lenticels for gas exchange	Protection, waterproofing, thermal insulation
Phellogen	• Single layer of meristematic cells • Thin-walled, living	Production of phellem and phelloderm
Phelloderm	• Living parenchyma cells • Thin-walled, may have chloroplasts	Storage, photosynthesis (in green stems)

Functional Importance:

• Replaces epidermis as protective layer
• Forms bark (all tissues outside vascular cambium)
• Commercial source of cork (from Quercus suber - Cork Oak)

NEET Relevance: Questions on secondary growth often ask about periderm components and bark formation.