Chapter 4: Anatomy of Flowering Plants

Introduction

Plant anatomy is the study of internal structures and organization of plants. It involves examining the tissues, cells, and cellular arrangements within different plant organs. Understanding plant anatomy helps explain physiological processes and adaptations of plants to their environments.

Plant Tissues

Meristematic Tissues

• Composed of actively dividing cells
• Thin primary cell walls, dense cytoplasm, prominent nuclei
• Responsible for plant growth
• Types based on location:

Apical Meristems

• Located at the tips of roots and shoots
• Responsible for primary growth (increase in length)
• Give rise to primary tissues

Lateral Meristems

• Located parallel to the organ’s long axis
• Responsible for secondary growth (increase in girth)
• Include vascular cambium and cork cambium

Intercalary Meristems

• Present between mature tissues
• Common in monocots at the base of leaf blades and internodes
• Responsible for rapid elongation in grasses

Permanent Tissues

• Composed of cells that have lost the ability to divide
• Differentiated from meristematic tissues
• Specialized for specific functions

Simple Permanent Tissues

• Composed of one type of cells
• Types:

Parenchyma

• Living, thin-walled cells with intercellular spaces
• Isodiametric or elongated
• Functions: Photosynthesis, storage, secretion
• Forms the ground tissue in plants

Collenchyma

• Living cells with unevenly thickened cell walls
• Provide mechanical support to young growing parts
• Found in leaf stalks and stems of dicots

Sclerenchyma

• Dead cells with thick, lignified secondary walls
• Provide mechanical strength
• Types: Fibers (elongated) and sclereids (stone cells)
• Examples: Fibers in jute, sclereids in pear fruit

Complex Permanent Tissues

• Composed of different types of cells working together
• Types:

Xylem

• Water-conducting tissue
• Components:
  • Tracheids: Elongated cells with tapered ends
  • Vessels: Tube-like structures formed by vessel elements
  • Xylem parenchyma: Storage and lateral conduction
  • Xylem fibers: Mechanical support
• Primary xylem: Formed during primary growth
• Secondary xylem: Formed during secondary growth (wood)

Phloem

• Food-conducting tissue
• Components:
  • Sieve tubes: Conducting cells arranged end to end
  • Companion cells: Regulate sieve tube functions
  • Phloem parenchyma: Storage and lateral conduction
  • Phloem fibers: Mechanical support
• Primary phloem: Formed during primary growth
• Secondary phloem: Formed during secondary growth (inner bark)

Anatomy of Dicotyledonous and Monocotyledonous Plants

Root Anatomy

Dicot Root

• Epidermis with root hairs
• Cortex with parenchyma cells
• Endodermis with Casparian strips
• Pericycle (single layer)
• Xylem and phloem in separate bundles arranged in a ring
• Tetrarch, pentarch, or polyarch condition (4-5 or more xylem strands)
• Pith may be present or absent

Monocot Root

• Epidermis with root hairs
• Large cortex with parenchyma cells
• Distinct endodermis
• Pericycle (single layer)
• Xylem and phloem in alternate bundles arranged in a ring
• Polyarch condition (many xylem strands)
• Large central pith

Stem Anatomy

Dicot Stem

• Epidermis with cuticle and stomata
• Hypodermis made of collenchyma
• Cortex with parenchyma cells
• Endodermis (starch sheath)
• Pericycle with fibers
• Vascular bundles arranged in a ring, open (with cambium)
• Conjoint, collateral bundles (xylem and phloem on same radius)
• Large central pith

Monocot Stem

• Epidermis with cuticle
• Hypodermis with sclerenchyma
• Ground tissue with parenchyma (no distinct cortex and pith)
• Vascular bundles scattered throughout ground tissue
• Closed vascular bundles (no cambium)
• Each bundle surrounded by sclerenchymatous bundle sheath

Leaf Anatomy

Dorsiventral Leaf (Dicot Leaf)

• Upper and lower epidermis with cuticle
• Upper epidermis with fewer stomata
• Lower epidermis with more stomata
• Mesophyll differentiated into:
  • Palisade parenchyma (upper part)
  • Spongy parenchyma (lower part)
• Vascular bundles (veins) with bundle sheaths

Isobilateral Leaf (Monocot Leaf)

• Upper and lower epidermis with cuticle
• Stomata equally distributed on both surfaces
• Mesophyll not differentiated (uniform)
• Parallel venation with large and small vascular bundles
• Bulliform cells in upper epidermis (regulate leaf rolling)

Secondary Growth in Plants

Secondary Growth in Dicot Stem

• Increase in girth due to activity of lateral meristems
• Process:
  1. Formation of vascular cambium ring
  2. Production of secondary xylem (inward) and secondary phloem (outward)
  3. Formation of cork cambium (phellogen)
  4. Production of cork (phellem) outward and secondary cortex (phelloderm) inward
• Results in:
  • Formation of annual rings
  • Development of bark (all tissues outside vascular cambium)

Secondary Growth in Dicot Root

• Similar to stem but with differences:
  1. Vascular cambium forms from conjunctive tissue between xylem and phloem
  2. Cambium first forms opposite phloem then opposite xylem
  3. Wavy ring of cambium becomes circular with continued growth
  4. Secondary tissues produced similar to stem
• Results in loss of epidermis, cortex, endodermis, and pericycle

Special Anatomical Features

Anomalous Secondary Growth

• Deviations from normal secondary growth pattern
• Examples:
  • Bignonia: Xylem in four patches
  • Dracaena: Secondary growth despite being monocot
  • Boerhaavia and Mirabilis: Concentric vascular bundles

Periderm Formation

• Protective tissue replacing epidermis during secondary growth
• Composed of:
  • Cork cambium (phellogen)
  • Cork (phellem)
  • Secondary cortex (phelloderm)
• Functions: Protection, gas exchange through lenticels, healing of wounds

Heartwood and Sapwood

• Heartwood: Inner, darker, non-functional wood
  • Filled with gums, resins, and tannins
  • Provides mechanical support
• Sapwood: Outer, lighter, functional wood
  • Conducts water and minerals
  • Younger than heartwood

Adaptations in Anatomical Features

Xerophytic Adaptations

• Thick cuticle
• Sunken stomata
• Hypodermis with thick-walled cells
• Reduced leaf surface area
• Well-developed mechanical tissues

Hydrophytic Adaptations

• Reduced vascular tissues
• Poorly developed mechanical tissues
• Aerenchyma tissue (air spaces)
• Reduced root system
• Stomata on upper leaf surface

Halophytic Adaptations

• Succulent tissues for water storage
• Salt glands to excrete excess salt
• Thick cuticle
• Sunken stomata
• Well-developed mechanical tissues