Anatomy of Flowering Plants

Introduction

Anatomy is the study of internal structure. Plant anatomy reveals how tissues are organised inside roots, stems, and leaves. These internal arrangements are directly linked to the plant's functions — transport, support, photosynthesis, and growth.

Plant Tissues: Overview

• All plant tissues arise from meristems (actively dividing cells). Tissues are broadly of two types:
• Meristematic tissues: capable of cell division (apical, lateral, intercalary meristems)
• Permanent tissues: mature, non-dividing cells; simple or complex

Meristematic Tissues

• Apical meristem: at shoot and root tips; responsible for primary (elongation) growth
• Lateral meristem (Cambium): along sides of stem/root; responsible for secondary (girth) growth; includes vascular cambium and cork cambium (phellogen)
• Intercalary meristem: at leaf bases and internodes; e.g., grass stem elongation

Simple Permanent Tissues

• Parenchyma: thin-walled, living; large central vacuole; stores starch/water; chlorenchyma (with chloroplasts) for photosynthesis; aerenchyma (with air spaces) in aquatic plants
• Collenchyma: living; unevenly thickened cell walls (at corners); provides mechanical support with flexibility; found in hypodermis of dicot stems and leaf midrib
• Sclerenchyma: dead at maturity; heavily lignified walls; provides rigidity and strength. Two types: · fibres · (long, tapering) and · sclereids/stone cells · (isodiametric — found in hard seed coats, · Pyrus · pulp)

Complex Permanent Tissues

• Xylem: conducts water and minerals upward; composed of — · tracheids · (elongated with pits), · vessels/tracheae · (wider, vessel members with perforation plates), · xylem fibres · , · xylem parenchyma · . Vessels: found only in angiosperms (and a few gymnosperms); tracheids in all vascular plants.
• Phloem: conducts food (mainly sucrose) in both directions; composed of — · sieve tube elements · (with sieve plates; no nucleus at maturity), · companion cells · (nucleated, with dense cytoplasm; regulate sieve tube), · phloem fibres · , · phloem parenchyma · . Companion cells are absent in gymnosperms (replaced by albuminous cells).

Tissue Systems

1. 1.Three tissue systems in a plant organ:
2. 2.Epidermal tissue system: outermost layer; cuticle; stomata (guard cells); trichomes; root hairs
3. 3.Ground tissue system: all tissue between epidermis and vascular bundles (cortex, pith, mesophyll)
4. 4.Vascular tissue system: xylem and phloem organised as vascular bundles

Internal Structure of Dicot Root (e.g., mustard)

Outermost to innermost: Epiblem (Rhizodermis) → Cortex (parenchyma with intercellular spaces) → Endodermis (with Casparian strips — waterproof, suberin-thickened band in radial and transverse walls, controls water movement) → Pericycle → Vascular bundles (radial arrangement; xylem and phloem ALTERNATE — exarch xylem; 2–6 xylem poles) → Pith (well-developed)

Internal Structure of Monocot Root (e.g., maize)

Similar to dicot root but: many xylem poles (polyarch — 8 or more); large, well-developed pith; no secondary growth.

Internal Structure of Dicot Stem (e.g., sunflower)

Epidermis (with cuticle, trichomes) → Cortex (3 layers: chlorenchyma/hypodermis of collenchyma, cortical parenchyma, endodermis/starch sheath) → Pericycle (alternating patches of sclerenchyma and parenchyma) → Vascular bundles (in a ring; conjoint, collateral, open — cambium present between xylem and phloem) → Pith (large, parenchymatous, with intercellular spaces) → Medullary rays (pith rays connecting pith to cortex)

Internal Structure of Monocot Stem (e.g., maize/Zea mays)

Epidermis → Hypodermis (sclerenchyma — provides rigidity) → Ground tissue (no differentiation into cortex and pith) → Vascular bundles (scattered throughout ground tissue; conjoint, collateral, closed — no cambium; surrounded by sclerenchymatous bundle sheath). No secondary growth.

Internal Structure of Dicot Leaf (e.g., sunflower)

Upper epidermis (cuticle; no or fewer stomata) → Mesophyll: · Palisade parenchyma · (cylindrical, chloroplast-rich, near upper surface — main photosynthetic layer) + · Spongy parenchyma · (irregular, air spaces, lower mesophyll) → Lower epidermis (more stomata; guard cells; cuticle) → Vascular bundles in veins (xylem toward upper surface, phloem toward lower).

Internal Structure of Monocot Leaf (e.g., maize)

Epidermis (bulliform/motor cells in upper epidermis — cause leaf rolling in dry conditions) → Mesophyll (NOT differentiated into palisade and spongy layers; all cells similar) → Vascular bundles (both large and small; complete bundle sheath of large green parenchyma cells — Kranz anatomy — characteristic of C4 plants).

Secondary Growth in Dicot Stem

1. 1.Vascular cambium (fascicular cambium + interfascicular cambium) produces secondary xylem (wood) inward and secondary phloem outward.
2. 2.Cork cambium (Phellogen) produces cork (phellem) outward and phelloderm inward. Cork is impermeable (suberin-coated); replaces epidermis. Cork + Phellogen + Phelloderm = Periderm.
3. 3.Annual rings (growth rings): alternating early wood (spring wood — wider vessels) and late wood (autumn wood — narrow vessels). Count rings to estimate age.