Cell Modification and Specialized Cell Structures

Cell Modification and Specialized Cell Structures

Introduction

• United masses of cells are distinct structurally and functionally.
• Modifications to basic plant cell structure involve:
  • Cell wall composition
  • Cell shape
  • Cell contents
• These modifications provide microscopical characters useful for:
  • Drug plant identification
  • Adulteration detection
• Cell groupings form tissues, which develop into simpler or complex cellular organizations.

Tissue System

• Three tissue systems in the plant body:
  1. Dermal Tissue System:
     • Outermost part of the plant.
     • Forms protective covering.
     • Includes epidermis and periderm.
  2. Vascular Tissue System:
     • Transmits materials within the plant.
     • Includes stellar structures like xylem and phloem.
  3. Ground Tissue System:
     • Consists of simple cells, potentially strengthened by thickened cells.
     • Parenchyma, collenchyma, and sclerenchyma cells are common.

Plant Epidermis

• Covers the outer surface of all plant tissues, from roots to tips.
• Made of epidermal cells.
• Functions:
  • Protective layer against microorganisms entering underlying tissues of leaves and stems.
  • Prevents excess water loss.
• Epidermis consists of different cell types with varied morphology and functions.

Plant Epidermal Cells

• Show great variety in form, creating characteristic patterns, especially in leaves.
• In transection:
  • Often flattened parallel to the surface.
  • Square or rectangular shape.
  • Outer walls (anticlinal wall) are markedly thickened, may be straight, wavy, or beaded.
  • Often covered with a cuticle layer made of cutin.
• Examples:
  • Straight-walled epidermal cells: Pilocarpus jaborandi, Erythroxylum coca, Senna acutifolia leaves.
  • Wavy-walled epidermal cells: Datura stramonium, Hyoscyamus niger, Atropa belladonna.
  • Beaded walls: Lobelia inflata, Digitalis lanata.
  • Papillose epidermis: Coca leaf.

Cuticle

• Presence/absence and form are characteristic features of the epidermis.
• Cutin: A fatty substance forming a separate outer layer (cuticle).
• Variations:
  • Striated (e.g., Belladonna leaf).
  • Thick (e.g., Aloe leaf).

Stomata

• Minute openings usually in the epidermis of leaves, young green stems, flowers, and fruits.
• Surrounded by a pair of kidney-shaped guard cells.
• Guard cells control opening and closing of the pore by changing shape.
• Subsidiary cells: Epidermal cells surrounding the guard cells, differing in size, arrangement, and shape from other epidermal cells.
• Function: Gaseous exchange and transpiration.

Stomata Distribution

• Distribution varies between upper (adaxial) and lower (abaxial) epidermis.
• Most abundant in the lower epidermis of dorsiventral leaves, less abundant on the upper epidermis.
• Isobilateral leaves: Stomata confined to the upper epidermis.
• Examples:
  • Stomata confined to the lower epidermis: Ficus, coca, neem.
  • Stomata on both surfaces: Belladonna, Datura, Senna.
  • Submerged leaves: No stomata.
• Floating leaves of aquatics: Stomata confined to the upper epidermis.
• Xerophytic adaptations (e.g., Ephedra, Agave, Oleander):
  • Stomata situated in grooves or pits to reduce excessive evaporation.
  • Sunken stomata are protected from wind.

Types of Stomata

• Classification based on arrangement of epidermal cells:
  • Anomocytic (irregular celled):
    • Stoma surrounded by a limited number of cells indistinguishable from other epidermal cells.
    • Examples: Digitalis, Lobelia, Neem, Eucalyptus.
  • Anisocytic (unequal celled):
    • Stoma surrounded by 3 or 4 subsidiary cells, one distinctly smaller than the others.
    • Examples: Belladonna, Datura, Hyoscyamus, Stramonium, Tobacco.
  • Paracytic (Parallel celled):
    • Stoma has 1 or more subsidiary cells parallel to the long axis of the pore and guard cell on each side.
    • Examples: Senna, other Rubiaceae.
  • Diacytic (cross celled):
    • Stoma enclosed by 2 subsidiary cells whose common wall is at right angles to the guard cell.
    • Examples: Mentha piperita (Peppermint), spearmint.
  • Actinocytic (radiate celled):
    • Stoma surrounded by a circle of radiating cells.
    • Examples: Pilocarpus jaborandi

Diagnostic Characteristics of the Epidermis

• Nature and distribution of wall thickenings (straight, wavy, beaded, papillose).
• Presence/absence of cuticle and its form (striated, thick).
• Distribution and structure of stomata, including location and type (Anisocytic, paracytic, etc.).
• Presence/absence, size, and distribution of epidermal trichomes.

Trichomes

• Hair-like or thorn-like projections from the epidermis.
• Parts:
  • Foot or root embedded in the epidermis.
  • Free projecting portion (body).
• Location: Usually leaves but also other plant parts (Nux vomica seeds, Belladonna stem, Lady’s finger fruits).
• Functions:
  • Physical and chemical protection against microbial organisms and insects.
  • Prevent clogging of stomata due to dust accumulation.
  • Aids seed dispersal (Milkweed, Calotropis).
  • Secretion of volatile oils (Peppermint, Rosemary).
  • Maintaining still-air layer on leaf surface, preventing excessive transpiration.

Types of Trichomes

• Broad classification:
  1. Covering trichomes (non-glandular).
  2. Glandular trichomes.
• Further classification based on structure, shape, and number of cells:

Covering Trichomes

1. Unicellular trichomes:
   • Short, conical trichomes: Tea
   • Short, conical, warty trichomes: Senna
   • Large, conical, longitudinally striated trichomes: Lobelia
   • Lignified trichomes: Nux vomica
   • Short, sharp, pointed, curved, conical trichomes: Cannabis
2. Multicellular unbranched trichomes:
   • Uniseriate, bicellular, conical: Datura
   • Biseriate: Calendula officinalis
   • Multiseriate: Male fern
3. Multicellular branched trichomes:
   • Stellate (star shaped): Hamamelis, Kamala
   • Peltate (shield-like structure): Cascarilla
   • Candelebra (branched): Rosemary, Verbascum Thapsus
   • T-shaped trichomes: Pyrethrum

Glandular Trichomes

• Composed of a stalk and a glandular head (aromatic).

• Types:

  • Unicellular glandular trichomes
    *Sessile trichomes (without stalk): Piper betel
  • Unicellular stalk with a single spherical secreting cell at the apex.
  • Uniseriate, multicellular stalk with a single spherical cell at the apex.
  • Uniseriate stalk and bicellular head.
  • Multicellular, uniseriate stalk and multicellular head.
  • Biseriate stalk and biseriate secreting head.
  • Short, unicellular stalk and head formed by a rosette of two to eight club-shaped cells.
  • Multiseriate, multicellular cylindrical stalk and a secreting head of about eight radiating club-shaped cells.

Periderm

• Protective tissue of secondary origin, replacing the epidermis in stems and roots.
• Composed of:
  • Cork tissue (Phellem).
  • Cork cambium (Phellogen).
  • Parenchyma (Phelloderm).
• Equation: $Phellem + Phellogen + Phelloderm = Periderm$
• Phellem (cork) is particularly diagnostic.
• Cork cells:
  • Suberification of walls.
  • Rectangular brick-shaped or polygonal.
• Presence in powdered drugs indicates adulteration or low-quality drug (e.g., cinnamon, ginger, liquorice).
• Cork formation puts stomatal apparatus out of action, leading to the formation of lenticels (special breathing pores).

Periderm - Lenticels and Cork Cells

• Lenticels are larger in size and smaller in number than stomata.
• Mostly present in the bark of old plants.
• Cork cell walls:
  • Vary in thickness.
  • May be colored brown or yellow.
  • Lumen may contain resinous or tanniferous materials.
• Commercial cork (bottle cork):
  • Thin walls.
  • Highly impervious to water due to suberin.
  • Light in weight.
  • Thermal insulating properties.

Types of Cork Cells

• Thick-walled
• Thin-walled flattened
• Thin-walled polygonal
• Stratified cork