Anatomy of Flowering Plants

• The Tissues

  • Meristematic Tissues
    • Characteristics: Actively dividing cells
    • Classification by Position:
      • Apical meristems: Primary growth, root/shoot tips
      • Intercalary meristems: Between mature tissues, leaf base/internodes
      • Lateral meristems: Secondary growth, vascular cambium, cork cambium
    • Classification by Origin:
      • Promeristem (Primary)
      • Primary meristem
      • Secondary meristem (De-differentiated)
  • Permanent Tissues
    • Characteristics: Non-dividing, specialized cells
    • Simple Tissues:
      • Parenchyma: Packing tissue, photosynthesis, storage
      • Colenchyma: Living mechanical tissue, uneven corner thickening
      • Sclerenchyma: Dead mechanical tissue, lignified walls (fibers and sclereids)
    • Complex Tissues:
      • Xylem: Water and mineral transport
        • Elements: Tracheids, Vessels, Xylem fibers, Xylem parenchyma
      • Phloem: Organic food transport
        • Elements: Sieve tube elements, Companion cells, Phloem parenchyma, Phloem fibers

• The Tissue System

  • Epidermal Tissue System
    • Outermost covering
    • Components: Epidermis, cuticle, stomata, epidermal appendages (trichomes, hairs)
  • Ground Tissue System
    • All tissues except epidermis and vascular bundles
    • Components: Cortex, pericycle, pith, medullary rays (parenchyma)
  • Vascular Tissue System
    • Transport channels
    • Types of Vascular Bundles:
      • Radial: Xylem and phloem on alternate radii (Roots)
      • Conjoint: Xylem and phloem on same radius (Stems and Leaves)
        • Closed: No cambium, no secondary growth (Monocots)
        • Open: With cambium, secondary growth present (Dicots)

• Anatomy of Dicotyledonous and Monocotyledonous Plants

  • Dicotyledonous Root
    • Epidermis with root hairs
    • Cortex: Multilayered parenchyma
    • Endodermis: Casparian strips present
    • Pericycle: Gives rise to lateral roots and vascular cambium
    • Vascular bundles: Radial, usually tetrarch
    • Pith: Small or inconspicuous
  • Monocotyledonous Root
    • Similar to dicot root in general plan
    • Vascular bundles: Polyarch (more than six)
    • Pith: Large and well-developed
    • No secondary growth
  • Dicotyledonous Stem
    • Epidermis with cuticle and trichomes
    • Cortex zones: Hypodermis (collenchymatous), cortical layers, endodermis (starch sheath)
    • Pericycle: Semi-lunar patches of sclerenchyma
    • Vascular bundles: Conjoint, collateral, open, arranged in a ring
    • Pith: Well-developed center
  • Monocotyledonous Stem
    • Epidermis without trichomes
    • Hypodermis: Sclerenchymatous
    • Vascular bundles: Conjoint, closed, scattered in ground tissue (surrounded by sclerenchymatous bundle sheath)
  • Dorsiventral (Dicot) Leaf
    • Reticulate venation
    • Epidermis: Upper (adaxial) and Lower (abaxial) with more stomata on lower side
    • Mesophyll: Differentiated into palisade parenchyma and spongy parenchyma
    • Vascular bundles: Reticulate network, enclosed in bundle sheath
  • Isobilateral (Monocot) Leaf
    • Parallel venation
    • Stomata present on both upper and lower epidermis equally
    • Mesophyll: Not differentiated into palisade and spongy parenchyma
    • Special feature: Bulliform cells (motor cells) for leaf curling during water stress

• Secondary Growth

  • Occurs in dicot roots and stems (increases girth)
  • Vascular Cambium
    • Formation: Interfascicular cambium + intrafascicular cambium forms a continuous ring
    • Activity: Cuts off secondary xylem inwards and secondary phloem outwards
    • Spring wood (Early wood): Vessels with wider cavities, lighter color
    • Autumn wood (Late wood): Vessels with narrow cavities, darker color
    • Annual rings: Formed by alternating spring and autumn wood; used for dendrochronology (age determination)
    • Heartwood (Duramen) vs. Sapwood (Alburnum):
      • Heartwood: Central, dark, dead, filled with tannins/resins, non-conductive (mechanical support)
      • Sapwood: Peripheral, light, living, conducts water and minerals
  • Cork Cambium (Phellogen)
    • Formation: Develops in cortex region
    • Activity: Cuts off cork (pelleme) outwards and secondary cortex (phelloderm) inwards
    • Periderm: Phellogen + Phellem + Phelloderm
    • Bark: All tissues outside vascular cambium (including secondary phloem and periderm)
    • Lenticels: Lens-shaped openings in periderm for gaseous exchange