Anatomy of Flowering Plants
'The internal structure of plants reveals the elegant design of the plant body.' — Plant Anatomy
1. Chapter Overview
ANATOMY is the study of INTERNAL structure. This chapter examines the TISSUES that make up the plant body, the TISSUE SYSTEMS (dermal, ground, vascular), the INTERNAL ORGANISATION of roots, stems, and leaves, and SECONDARY GROWTH (increase in girth) in dicot plants. Understanding anatomy is CRUCIAL for understanding how plants function — how water travels, food is stored, and support is maintained.
2. Plant Tissues
Meristematic Tissues (Actively Dividing Cells)
Based on Position
| Type | Location | Function |
|---|---|---|
| Apical meristem | Root and shoot tips | PRIMARY growth (length) |
| Intercalary meristem | Base of internodes/leaves | Growth in grasses |
| Lateral meristem | Sides (cambium) | SECONDARY growth (girth) |
Based on Origin
- Primary meristem: From EMBRYO (apical, intercalary)
- Secondary meristem: From PERMANENT tissues (cork cambium, fascicular cambium)
Permanent Tissues (Mature, Differentiated)
Simple Tissues (Single Cell Type)
| Tissue | Cell Type | Function | Location |
|---|---|---|---|
| Parenchyma | Living, thin-walled | Photosynthesis, STORAGE | Cortex, pith |
| Collenchyma | Living, uneven thickened | FLEXIBLE support | Young stems, petioles |
| Sclerenchyma | DEAD, thick (lignin) | RIGID support | Fibres, sclereids |
Complex Tissues (Multiple Cell Types)
Xylem (Water Conduction)
- Components: TRACHEIDS (dead), VESSELS (dead), Xylem fibres (dead), Xylem parenchyma (living)
- Function: UPWARD water + mineral transport
- Movement: Transpiration pull
Phloem (Food Conduction)
- Components: SIEVE TUBES (living, enucleated), COMPANION cells, Phloem fibres, Phloem parenchyma
- Function: DOWNWARD food transport (source → sink)
- Movement: Pressure flow hypothesis
3. Tissue Systems (Sachs, 1875)
| System | Components | Function |
|---|---|---|
| Epidermal (Dermal) | Epidermis, stomata, trichomes, root hairs | PROTECTION, gas exchange, absorption |
| Ground | Cortex, endodermis, pericycle, pith, medullary rays | FILLING, storage, support |
| Vascular | Xylem + Phloem (in bundles) | TRANSPORT — water (xylem) and food (phloem) |
4. Anatomy of Dicot vs Monocot Root
Dicot Root (Example: Gram)
- Epidermis: Single layer (with root hairs)
- Cortex: PARENCHYMA cells (large, thin-walled)
- Endodermis: Single layer with CASPARIAN STRIPS (water-impermeable)
- Pericycle: GIVES rise to LATERAL ROOTS
- Vascular bundles: RADIAL (xylem and phloem on ALTERNATE radii)
- Pith: SMALL or ABSENT
- Number of xylem bundles: 2-6 (Diarch to Hexarch)
Monocot Root (Example: Maize)
- Similar to dicot root but:
- Pith: LARGE and prominent
- Number of xylem bundles: MORE than 6 (Polyarch)
- Pericycle: Also gives rise to ROOT HAIRS
5. Anatomy of Dicot vs Monocot Stem
Dicot Stem (Example: Sunflower)
| Layer | Characteristics |
|---|---|
| Epidermis | Single layer, covered by CUTICLE |
| Cortex | Hypodermis (collenchyma), general cortex, starch sheath (endodermis) |
| Vascular bundles | CONJOINT, OPEN, COLLATERAL, arranged in a RING |
| Cambium | PRESENT between xylem and phloem (secondary growth possible) |
| Pith | CENTRAL, large |
Monocot Stem (Example: Maize)
- Epidermis: Cutinised
- Hypodermis: Sclerenchyma (not collenchyma)
- Vascular bundles: CONJOINT, CLOSED, SCATTERED (not in a ring)
- Cambium: ABSENT (NO secondary growth)
- Pith: NOT distinct (ground tissue fills stem)
6. Anatomy of Leaf (Dicot)
- Epidermis: Upper + lower layers, cuticle, stomata (MORE on LOWER surface)
- Mesophyll:
- Palisade parenchyma: Columnar cells, RICH in chloroplasts (under upper epidermis)
- Spongy parenchyma: Irregular, AIR SPACES
- Vascular bundles: In MIDRIB (veins), surrounded by BUNDLE SHEATH
Monocot Leaf
- Stomata on BOTH surfaces
- Mesophyll NOT differentiated into palisade and spongy
- Bulliform cells: Present on UPPER epidermis (help leaf folding in water stress)
7. Secondary Growth (Increase in Girth)
In Dicot Stem
- Fascicular cambium: Between xylem and phloem (intrafascicular)
- Interfascicular cambium: Between bundles (from medullary rays)
- Vascular cambium ring: Fas + Inter → CONTINUOUS RING
- Secondary xylem: Formed INWARD (wood)
- Secondary phloem: Formed OUTWARD (bark)
- Annual rings: Rings of xylem — ONE per season (Dendrochronology — age determination)
Cork Cambium (Phellogen)
- Forms: CORK (phellem) outward + PHELLODERM inward
- Periderm: Cork + Cork cambium + Phelloderm
- Lenticels: Openings for GAS EXCHANGE (on bark)
Heartwood vs Sapwood
| Feature | Heartwood | Sapwood |
|---|---|---|
| Position | INNER region | OUTER region |
| Cells | DEAD | Living |
| Colour | DARK (due to tannins, resins) | LIGHT |
| Function | SUPPORT only | WATER conduction + support |
8. Common Mistakes
- All living cells are NOT parenchyma: Collenchyma is living too; sclerenchyma is dead
- Dicot stem has OPEN bundles (cambium present), monocot has CLOSED (cambium absent): Very important distinction
- Root endodermis has Casparian strips, NOT stem endodermis: In stems, the 'endodermis-like' layer is the starch sheath
- Bark is dead tissue, but lenticels allow gas exchange through it
- Annual rings are visible ONLY in WOODY dicots, not in monocots or non-woody dicots
9. CBSE Exam Focus
- Meristematic and permanent tissues — types and functions (3-mark)
- Dicot vs monocot: root, stem, leaf anatomy (5-mark)
- Secondary growth in dicot stem (5-mark)
- Tissue systems — dermal, ground, vascular (3-mark)
- Heartwood vs sapwood (3-mark)
10. Self-Test (5+ Q&A)
Q1: Differentiate between dicot stem and monocot stem anatomically. A: Dicot: Vascular bundles in RING, bundle sheath present, CAMBIUM present (open), pith distinct. Monocot: VBs SCATTERED, closed (no cambium), ground tissue undifferentiated, NO distinct pith.
Q2: What are Casparian strips? Where are they found? A: THICKENED, water-impermeable bands on RADIAL and TRANSVERSE walls of endodermal cells in roots. They FORCE water to pass through the SYMPLAST pathway.
Q3: Describe the role of cambium in secondary growth. A: Cambium (fascicular + interfascicular) forms a CONTINUOUS ring. It divides to produce SECONDARY XYLEM (inward) and SECONDARY PHLOEM (outward), causing INCREASE IN GIRTH.
Q4: What are lenticels? A: PORE-like openings on bark that allow GAS EXCHANGE between living stem tissues and the atmosphere.
Q5: Why are sieve tube cells enucleated at maturity? A: Losing the nucleus CREATES MORE SPACE for TRANSPORT of food materials and allows EASIER flow of sap through the sieve tube elements.
11. Conclusion
Plant anatomy REVEALS the sophisticated internal architecture of plants. Simple tissues (parenchyma, collenchyma, sclerenchyma) provide SUPPORT and STORAGE. Complex tissues (xylem, phloem) form a TRANSPORT network. Dicot and monocot anatomy differ SIGNIFICANTLY — in root (xylem bundles), stem (bundle arrangement, cambium), and leaf (mesophyll). Secondary growth produces WOOD and BARK, enabling trees to INCREASE in girth each year. Anatomy is ESSENTIAL for understanding plant physiology and the STRUCTURAL adaptations that allow plants to thrive in diverse environments.
