Unit-V: Adaptive and Protective Anatomy

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1. Epidermal Tissue System: Cuticle, Waxes, and Trichomes

The epidermal tissue system forms the outermost protective covering of the plant body.

• Cuticle: A fatty layer of cutin on the outer wall of epidermal cells that prevents excessive water loss.
• Waxes: Often deposited on the surface of the cuticle to provide additional waterproofing and protection against UV radiation.
• Trichomes: Epidermal outgrowths (hairs) that can be unicellular or multicellular; they help in reducing transpiration, reflecting light, or secreting chemicals.

2. Stomata and Hydathodes

These are specialized structures in the epidermis for gas exchange and water regulation.

• Stomata: Pores surrounded by guard cells that regulate the exchange of gases (CO2, O2) and water vapor.
• Hydathodes: Specialized structures, usually at leaf margins or tips, that allow the discharge of water in liquid form through a process called guttation.

3. Cavities, Lithocysts, and Laticifers

Internal secretory and specialized structures used for storage and protection.

• Cavities: Internal spaces (e.g., oil glands in Citrus) formed by the breakdown or separation of cells to store secretions.
• Lithocysts: Enlarged epidermal cells containing cystoliths (calcium carbonate crystals).
• Laticifers: Specialized cells or tubes that contain latex (a milky or clear fluid) used for defense and wound healing.

4. Periderm, Rhytidome, and Lenticels

As plants undergo secondary growth, the epidermis is replaced by a more robust protective system.

• Periderm: Composed of phellogen (cork cambium), phellem (cork), and phelloderm (secondary cortex).
• Rhytidome: The technical term for "bark," consisting of the periderm and all dead tissues outside it.
• Lenticels: Lens-shaped openings in the periderm that allow gas exchange in woody stems and roots.

5. Ergastic Substances

These are non-protoplasmic products of metabolism found within cells.

Examples: Starch grains, oil droplets, crystals (calcium oxalate), and tannins that are stored for future use or as waste products.

6. Anatomical Features of Hydrophytes and Xerophytes

Plants adapt their internal anatomy to survive in specific environmental conditions.

A. Hydrophytes (Water Plants)

• Aerenchyma: Large air spaces to provide buoyancy and internal gas exchange.
• Reduced Vascular Tissue: Minimal xylem development as water is readily available.
• Absence of Cuticle: Often missing or very thin to allow direct absorption.

B. Xerophytes (Desert Plants)

• Thick Cuticle: To minimize water loss through the epidermis.
• Sunken Stomata: Stomata located in pits or grooves to reduce transpiration by creating a humid micro-environment.
• Multiple Epidermis: To provide extra protection against intense heat and light.
• Well-developed Sclerenchyma: To prevent wilting and provide mechanical support.