Unit 2: Tissue Systems and Adaptations

Syllabus Reference: Tissue systems (Epidermal, Ground, Vascular); Vascular cambium; Secondary growth in root and stem, heart wood and sapwood; [cite_start]Anatomical adaptations in xerophytes and hydrophytes [cite: 551-552].

1. Tissue Systems

Sachs classified tissues into three systems:

1. Epidermal Tissue System: Outermost protection. Includes Epidermis, Stomata, Cuticle, Trichomes (hairs), and Root hairs.
2. Ground Tissue System: Bulk of the plant body. Includes Cortex (Hypodermis, General cortex, Endodermis), Pericycle, Pith, and Medullary rays.
3. Vascular Tissue System: Conducting tissues. Xylem and Phloem arranged in bundles.

2. Vascular Cambium

A lateral meristem responsible for secondary growth (thickness).

• Structure: A single layer of meristematic cells located between primary xylem and primary phloem.
• Types of Cells:
  • Fusiform Initials: Elongated cells; produce Xylem/Phloem (Tracheary elements, sieve tubes).
  • Ray Initials: Isodiametric cells; produce Secondary Medullary Rays (radial transport).
• Seasonal Activity: In temperate regions, cambium activity varies, leading to Annual Rings.
  Spring Wood (Early Wood): Active cambium, large vessels, light color.
  Autumn Wood (Late Wood): Less active, small vessels, dark color.

3. Secondary Growth

Increase in girth due to the activity of Vascular Cambium and Cork Cambium.

In Dicot Stem:

1. Formation of Cambium Ring: Intrafascicular cambium (inside bundle) joins with Interfascicular cambium (between bundles).
2. Activity: Cuts off Secondary Xylem towards the inside (pith) and Secondary Phloem towards the outside (periphery).
3. Formation of Periderm (Bark): Cork cambium (Phellogen) develops in the cortex. It produces Cork (Phellem) outside and Secondary Cortex (Phelloderm) inside.

Wood Types:

• Sapwood (Alburnum): Outer, functional, light-colored secondary xylem. Conducts water.
• Heartwood (Duramen): Central, non-functional, dark-colored xylem. Blocked by tyloses and tannins. Provides mechanical support and resistance to microbes.

4. Anatomical Adaptations

A. Hydrophytes (Water Plants, e.g., Hydrilla, Nymphaea)

• Aerenchyma: Large air cavities in parenchyma for buoyancy and gas exchange.
• Cuticle/Stomata: Absent or reduced (except in floating leaves like Lotus where stomata are on the upper surface).
• Vascular Tissue: Poorly developed (especially Xylem) as water absorption is by body surface.
• Mechanical Tissue: Reduced or absent (water provides support).

B. Xerophytes (Dry Habitat Plants, e.g., Nerium, Casuarina)

• Thick Cuticle: Reduces transpiration.
• Sunken Stomata: Stomata in pits/cavities to reduce air flow and water loss (e.g., Nerium).
• Sclerenchyma: Well-developed mechanical tissue.
• Multiple Epidermis: e.g., Nerium has multi-layered epidermis.
• Leaves: Reduced to spines or scales (e.g., Opuntia).