Write short notes on the following:

(a) Lateral meristems(b) Vessels(c) Subsidiary cells(d) Ground tissue

Solution:

(a) LATERAL MERISTEMS:

Definition: Lateral meristems are cylindrical meristems located in the mature regions of roots and stems, responsible for secondary growth (increase in girth).

Types:

1. Vascular Cambium:

• Location: Between xylem and phloem
• Origin:
  • Intrafascicular cambium (within vascular bundles - primary origin)
  • Interfascicular cambium (from medullary ray cells - secondary origin)
• Activity:
  • Produces secondary xylem (inward)
  • Produces secondary phloem (outward)
• Seasonal Activity: Forms annual rings in temperate climates

1. Cork Cambium (Phellogen):

• Location: Outer cortex or pericycle
• Origin: From dedifferentiation of parenchyma cells
• Activity:
  • Produces cork/phellem (outward) - dead, suberized cells
  • Produces phelloderm/secondary cortex (inward) - living parenchyma
• Forms:Periderm (cork + cork cambium + secondary cortex)

Distribution:

• Present: Most dicotyledons, gymnosperms
• Absent: Most monocotyledons (exceptions: Dracaena, Yucca)

Functions:

• Secondary xylem: Wood formation (timber, paper)
• Cork: Waterproofing, thermal insulation, commercial cork
• Repair: Wound healing via cork formation

(b) VESSELS:

Definition: Vessels (also called trachea) are long, cylindrical, tube-like structures formed by vertical fusion of multiple vessel members (vessel elements).

Structure:

Individual Vessel Member:

• Dead at maturity (no protoplasm)
• Lignified secondary walls with various thickening patterns:
  • Annular (ring-like)
  • Spiral (helical)
  • Scalariform (ladder-like)
  • Reticulate (net-like)
  • Pitted (with bordered pits)
• Large central cavity (lumen)

Vessel Formation:

• Vessel members align end-to-end
• End walls dissolve or perforate → Perforation plates
• Forms a continuous tube (can be meters long)

Perforation Plate Types:

Type	Description
Simple	End wall completely dissolved (single large opening)
Scalariform	Multiple parallel, elongated perforations
Reticulate	Net-like perforations
Foraminate	Multiple circular perforations

Function:

• Highly efficient water conduction (less resistance than tracheids)
• Mineral transport from roots to leaves

Evolutionary Significance:

• Characteristic feature of angiosperms
• More advanced than tracheids (found in all vascular plants)
• Xylem with vessels is more efficient for rapid water transport

NEET Tip: Distinguish vessels from tracheids based on perforation plates and multicellular origin.

(c) SUBSIDIARY CELLS:

Definition: Subsidiary cells (also called accessory cells) are specialized epidermal cells that surround guard cells and form part of the stomatal apparatus.

Characteristics:

Morphology:

• Differ in shape and size from regular epidermal cells
• Living cells with nucleus and cytoplasm
• Do not contain chloroplasts (unlike guard cells)
• Typically smaller than ordinary epidermal cells

Arrangement:

• Variable number (1-6 cells) depending on stomatal type
• Arranged in specific patterns (diagnostic for plant families)

Functions:

1. Mechanical Support:
   • Provide structural support to guard cells
   • Help maintain stomatal pore shape
2. Ion Regulation:
   • Act as ion reservoirs for guard cell osmotic changes
   • Facilitate K⁺ and Cl⁻ movement
3. Water Supply:
   • Supply water to guard cells during opening

Stomatal Classification Based on Subsidiary Cells:

Type	Subsidiary Cell Pattern
Paracytic	2 cells parallel to guard cells
Diacytic	2 cells perpendicular to guard cells
Anisocytic	3 cells of unequal size
Tetracytic	4 cells surrounding guard cells

Ecological Significance:

• Variation in subsidiary cell types represents adaptive evolution to different environments

(d) GROUND TISSUE:

Definition: Ground tissue comprises all tissues except epidermis and vascular bundles in the plant body.

Components:

In Stems:

1. Hypodermis (outer cortex)
2. General Cortex (middle cortex)
3. Endodermis (inner cortex)
4. Pericycle
5. Pith (central)
6. Medullary Rays (between vascular bundles)

In Roots:

1. Cortex
2. Endodermis
3. Pericycle
4. Pith (small or absent)
5. Conjunctive Tissue (between xylem and phloem)

In Leaves:

• Called Mesophyll (all tissue between upper and lower epidermis)

Tissue Composition: Ground tissue is primarily composed of simple tissues:

• Parenchyma (most abundant)
• Collenchyma (in young stems)
• Sclerenchyma (in mature stems, pericycle)

Functions:

Function	Responsible Tissue
Photosynthesis	Chlorenchyma (in cortex, mesophyll)
Storage	Parenchyma (starch, proteins, oils)
Mechanical Support	Collenchyma, sclerenchyma
Transport	Parenchyma (lateral conduction)
Anchorage	Sclerenchyma

Evolutionary Perspective:

• Ground tissue represents the fundamental tissue system
• Provides the basic framework of the plant body
• Highly adaptive (varies based on habitat - xerophytic, mesophytic, hydrophytic adaptations)

NEET Application:

• Ground tissue system questions often involve identification in cross-sections
• Understand positional relationships (cortex outside vascular bundles, pith inside)