Chapter Introduction
Anatomy is the study of internal structure of organisms. While morphology deals with external form, anatomy deals with the arrangement of tissues and cells inside the plant body. In Chapter 6 of NCERT Class 11 Biology, you study how cells are organised into tissues, how tissues form tissue systems, and how these systems are arranged in the root, stem, and leaf of dicots and monocots. You also study secondary growth — how stems and roots increase in girth. This chapter connects directly to Chapter 5 (morphology) and is important for understanding plant physiology in later chapters.
Key Definitions
| Term | Definition |
|---|---|
| Anatomy | Study of internal structure of organisms |
| Meristem | Tissue with capacity to divide; cells are small, thin-walled, dense cytoplasm |
| Apical meristem | Meristem at growing tips (root and shoot apex); causes primary growth |
| Lateral meristem | Meristem along the sides of stem and root (vascular cambium, cork cambium); causes secondary growth |
| Intercalary meristem | Meristem at nodes or internodes; allows regrowth after grazing/cutting |
| Parenchyma | Simple tissue; thin-walled, living cells with intercellular spaces |
| Collenchyma | Simple tissue; unevenly thickened cell walls, living, no intercellular spaces |
| Sclerenchyma | Simple tissue; thick lignified walls, dead at maturity; fibres and sclereids |
| Xylem | Complex tissue for water/mineral conduction; tracheids, vessels, xylem fibres, xylem parenchyma |
| Phloem | Complex tissue for food conduction; sieve tube elements, companion cells, phloem parenchyma, phloem fibres |
| Vascular bundle | Strand of xylem and phloem together |
| Conjoint bundle | Xylem and phloem on the same radius |
| Collateral bundle | Phloem on outer side, xylem on inner side |
| Open bundle | Cambium present between xylem and phloem |
| Closed bundle | Cambium absent between xylem and phloem |
| Exarch xylem | Protoxylem towards outside, metaxylem towards centre (roots) |
| Endarch xylem | Protoxylem towards centre, metaxylem towards outside (stems) |
| Annual ring | One spring wood + one autumn wood layer formed in a year |
Meristematic Tissues
Meristems are regions of active cell division. They are classified based on position:
| Type | Position | Function | Example |
|---|---|---|---|
| Apical meristem | Root and shoot apex | Primary growth (length) | Root tip, shoot tip |
| Lateral meristem | Along sides of stem and root | Secondary growth (girth) | Vascular cambium, cork cambium |
| Intercalary meristem | At nodes and leaf bases | Regrowth after damage | Grasses, bamboo, mint |
Apical meristem gives rise to primary meristems (protoderm, ground meristem, procambium), which differentiate into the three tissue systems.
Tissue Systems
1. Epidermal Tissue System
- Outermost protective layer; single layer of parenchyma cells
- Covered by cuticle (waxy layer) in aerial parts to reduce transpiration
- Stomata: Small pores in epidermis; formed by two guard cells; regulate gas exchange and transpiration
- Guard cells: Bean-shaped (dicot) or dumbbell-shaped (monocot); contain chloroplasts; control stomatal opening
- Trichomes: Hair-like outgrowths on epidermis; can be simple, branched, stellate, or glandular
- Root hairs: Unicellular outgrowths from epidermal cells in the root hair zone
2. Ground Tissue System
All tissues between epidermis and vascular bundles form the ground tissue.
| Component | Description |
|---|---|
| Cortex | Outer region; parenchyma with intercellular spaces; stores food |
| Endodermis | Innermost layer of cortex; barrel-shaped cells with Casparian strips (suberin deposits); regulates flow |
| Pericycle | Layer inside endodermis; gives rise to lateral roots; may be sclerenchymatous |
| Pith | Central region; parenchymatous; stores food |
3. Vascular Tissue System
- Xylem: Conducts water and minerals upward; consists of tracheids, vessels, xylem fibres, xylem parenchyma
- Phloem: Conducts food (sugars) bidirectionally; consists of sieve tube elements, companion cells, phloem parenchyma, phloem fibres (only dead element in phloem)
Vascular bundle types:
| Bundle Type | Description | Found in |
|---|---|---|
| Conjoint, collateral, open | Xylem + phloem on same radius; cambium present | Dicot stem |
| Conjoint, collateral, closed | Xylem + phloem on same radius; cambium absent | Monocot stem |
| Conjoint, bicollateral, open | Phloem on both sides of xylem; cambium between each | Cucurbita (pumpkin) |
| Radial | Xylem and phloem on different radii | Roots (dicot and monocot) |
Anatomy of Dicot Root
1. Epidermis (epiblema): Outermost layer; thin-walled; root hairs present; no cuticle 2. Cortex: Thick; parenchymatous with intercellular spaces; stores food 3. Endodermis: Barrel-shaped cells; Casparian strips present; controls water flow 4. Pericycle: Single layer; gives rise to lateral roots 5. Vascular bundles: Radial arrangement; 2–4 xylem bundles (diarch to tetrarch); exarch xylem (protoxylem towards pericycle) 6. Pith: Small or absent 7. Conjunctive tissue: Parenchyma between xylem and phloem
Anatomy of Monocot Root
1. Epidermis (epiblema): Same as dicot root 2. Cortex: Thick; parenchymatous 3. Endodermis: With Casparian strips 4. Pericycle: Outer layer of vascular cylinder 5. Vascular bundles: Radial; many xylem bundles (polyarch — more than 6); exarch xylem 6. Pith: Well-developed, large; parenchymatous 7. Conjunctive tissue: Sclerenchymatous (thick-walled)
Dicot Root vs Monocot Root
| Feature | Dicot Root | Monocot Root |
|---|---|---|
| Number of xylem bundles | 2–4 (diarch to tetrarch) | Many (polyarch, usually 8+) |
| Pith | Small or absent | Well-developed, large |
| Conjunctive tissue | Parenchymatous | Sclerenchymatous |
| Secondary growth | Present | Absent |
| Cortex | Relatively thinner | Thicker |
| Example | Gram, pea | Maize, rice |
Anatomy of Dicot Stem
1. Epidermis: Outermost; covered with cuticle; bears trichomes; protective 2. Cortex: Below epidermis; 3 zones — outer hypodermis (collenchyma), middle cortical parenchyma, inner endodermis (starch sheath) 3. Pericycle: Above phloem; sclerenchymatous bundles and parenchyma 4. Vascular bundles: Arranged in a ring; conjoint, collateral, open (cambium present) 5. Pith: Central; large; parenchymatous; stores food 6. Medullary rays: Radial rows of parenchyma between vascular bundles; help in lateral conduction
Anatomy of Monocot Stem
1. Epidermis: Outermost; cuticle present 2. Hypodermis: Sclerenchymatous (not collenchyma as in dicot) 3. Ground tissue: Uniform parenchyma; no distinction of cortex, endodermis, pericycle 4. Vascular bundles: Scattered throughout ground tissue; conjoint, collateral, closed (no cambium) 5. Pith: Not distinct; ground tissue is continuous 6. Bundle sheath: Sclerenchymatous sheath around each vascular bundle
Dicot Stem vs Monocot Stem
| Feature | Dicot Stem | Monocot Stem |
|---|---|---|
| Hypodermis | Collenchymatous | Sclerenchymatous |
| Cortex & Endodermis | Distinct | Not distinct; ground tissue uniform |
| Vascular bundle arrangement | In a ring | Scattered |
| Vascular bundle type | Conjoint, collateral, open | Conjoint, collateral, closed |
| Cambium | Present (open bundle) | Absent (closed bundle) |
| Bundle sheath | Absent | Present (sclerenchymatous) |
| Pith | Distinct, central | Not distinct |
| Medullary rays | Present | Absent |
| Secondary growth | Present | Absent |
| Example | Sunflower, mustard | Maize, wheat |
Anatomy of Dicot Leaf (Dorsiventral Leaf)
1. Upper epidermis (adaxial): Single layer; thick cuticle; no stomata or fewer stomata 2. Lower epidermis (abaxial): Single layer; thin cuticle; many stomata; more transpiration 3. Mesophyll: Between the two epidermis layers
4. Vascular bundles: Midrib and veins; xylem on upper side, phloem on lower side 5. Bundle sheath: Parenchymatous or sclerenchymatous covering around vascular bundles
- Palisade parenchyma: Upper side; elongated cells; more chloroplasts; main photosynthetic tissue
- Spongy parenchyma: Lower side; round cells; large intercellular spaces; gas exchange
Anatomy of Monocot Leaf (Isobilateral Leaf)
1. Upper epidermis: Single layer; cuticle; stomata present on both surfaces (amphistomatic) 2. Lower epidermis: Single layer; cuticle; stomata present 3. Mesophyll: Undifferentiated — no distinct palisade and spongy parenchyma; uniform spongy cells 4. Vascular bundles: Similar to dicot leaf; xylem on adaxial side, phloem on abaxial side 5. Bundle sheath: Double-layered — inner sclerenchymatous, outer parenchymatous (in some grasses) 6. Bulliform cells: Large, thin-walled, empty cells in upper epidermis; help leaf rolling in dry conditions (grasses)
Dicot Leaf vs Monocot Leaf
| Feature | Dicot Leaf | Monocot Leaf |
|---|---|---|
| Stomata distribution | More on lower surface (hypostomatic) | On both surfaces (amphistomatic) |
| Mesophyll | Differentiated into palisade and spongy | Undifferentiated |
| Bundle sheath | Single-layered parenchyma | Double-layered |
| Bulliform cells | Absent | Present (in grasses) |
| Venation | Reticulate | Parallel |
| Shape | Dorsiventral | Isobilateral |
Secondary Growth
Secondary growth is the increase in girth of stems and roots in dicot plants, produced by lateral meristems — vascular cambium and cork cambium. It is absent in most monocots.
Vascular Cambium
- Formed from intrafascicular cambium (within vascular bundle) and interfascicular cambium (between bundles)
- Together they form a complete ring of cambium
- Cambium divides periclinally: cells towards inside form secondary xylem (wood); cells towards outside form secondary phloem
- More secondary xylem is produced than secondary phloem
Annual Rings
| Feature | Spring Wood (Early Wood) | Autumn Wood (Late Wood) |
|---|---|---|
| Season | Spring (favourable) | Autumn/summer (unfavourable) |
| Cell size | Larger | Smaller |
| Cell wall | Thinner | Thicker |
| Lumen | Wide | Narrow |
| Colour | Lighter | Darker |
| Amount formed | More | Less |
| Density | Less dense | More dense |
One spring wood + one autumn wood = one annual ring. Counting annual rings at the base of a tree gives its age.
Heartwood vs Sapwood
| Feature | Heartwood (Duramen) | Sapwood (Alburnum) |
|---|---|---|
| Position | Inner, central part of wood | Outer, peripheral part of wood |
| Colour | Dark | Light |
| Function | No longer conducts water; stores waste (resins, tannins, oils) | Actively conducts water and minerals |
| Strength | Hard, strong | Soft |
| Durability | More durable, resistant to decay | Less durable |
| Dead/Alive | Dead cells | Living cells |
Cork Cambium (Phellogen)
- As secondary xylem increases, the outer layers (epidermis, cortex) are stretched and ruptured
- Cork cambium (phellogen) arises from the cortex
- Phellogen produces cork (phellem) on the outside and secondary cortex (phelloderm) on the inside
- Phellem + phellogen + phelloderm = Periderm (protective secondary covering)
- Cork cells are dead, suberised, and impervious to water
- Lenticels: Lens-shaped openings in cork; permit gas exchange
Bark
- All tissues outside the vascular cambium are collectively called bark
- Early bark: Formed early in the season
- Late bark: Formed late in the season
- Bark may be smooth, rough, scaly, or peeling depending on the species
Important Diagrams to Practise
1. T.S. of dicot stem — label: epidermis, hypodermis, cortex, endodermis, pericycle, vascular bundles in a ring, pith, medullary rays 2. T.S. of monocot stem — label: epidermis, hypodermis, ground tissue, scattered vascular bundles, bundle sheath 3. T.S. of dicot root — label: epiblema, cortex, endodermis, pericycle, radial vascular bundles (exarch), pith 4. T.S. of monocot root — label: same as dicot root but with polyarch xylem and large pith 5. T.S. of dicot leaf — label: upper epidermis, palisade parenchyma, spongy parenchyma, lower epidermis, stomata, vascular bundle 6. T.S. of monocot leaf — label: both epidermis, undifferentiated mesophyll, bulliform cells, bundle sheath 7. Secondary growth in dicot stem — label: annual rings, heartwood, sapwood, bark, cork, lenticels
Common Mistakes to Avoid
1. Confusing open and closed vascular bundles — open means cambium is present (dicot stem); closed means cambium is absent (monocot stem). This is the key difference in stem anatomy. 2. Mixing up exarch and endarch xylem — in roots, xylem is exarch (protoxylem towards periphery); in stems, xylem is endarch (protoxylem towards centre). This is a very common exam error. 3. Saying monocot root has no pith — monocot root has a well-developed large pith; it is the dicot root where pith is small or absent. 4. Confusing collenchyma and sclerenchyma positions — dicot stem hypodermis is collenchymatous; monocot stem hypodermis is sclerenchymatous. Reversing this loses marks. 5. Calling heartwood the conducting wood — heartwood does NOT conduct water; it is dead and filled with waste. Sapwood (alburnum) is the conducting wood. 6. Forgetting bulliform cells — they are a distinguishing feature of monocot (grass) leaves, not present in dicot leaves.
Exam Tips
- 5-mark questions often ask: "Describe the internal structure of a dicot stem" or "Describe secondary growth in dicot stem." Draw a labelled diagram alongside your written description.
- 3-mark questions commonly ask to differentiate: dicot vs monocot stem, dicot vs monocot root, spring wood vs autumn wood, or heartwood vs sapwood. Always use a table.
- 1-mark questions target definitions: meristem types, exarch/endarch, open/closed bundles, annual ring, periderm, lenticels.
- Diagram questions: T.S. of dicot stem and T.S. of monocot stem are the most frequently asked. Practise labels precisely.
- Use NCERT terminology: write "phellogen" not "cork cambium" when asked for the technical term; write "epiblema" not "epidermis" for the outermost layer of roots.
- For secondary growth questions, always mention both vascular cambium AND cork cambium. Students often forget cork cambium.
Quick Revision Notes
- Meristems: apical (primary growth), lateral (secondary growth), intercalary (regrowth)
- Tissue systems: epidermal (protection), ground (cortex, pith, storage), vascular (conduction)
- Simple tissues: parenchyma (thin-walled, living), collenchyma (unevenly thickened, living), sclerenchyma (lignified, dead)
- Complex tissues: xylem (water/minerals, endarch in stem, exarch in root), phloem (food, bidirectional)
- Dicot stem: ring of open bundles, collenchyma hypodermis, distinct cortex/endodermis/pith
- Monocot stem: scattered closed bundles, sclerenchyma hypodermis, uniform ground tissue, bundle sheath
- Dicot root: 2–4 xylem (diarch to tetrarch), exarch, small/absent pith, parenchymatous conjunctive tissue
- Monocot root: many xylem (polyarch), exarch, large pith, sclerenchymatous conjunctive tissue
- Dicot leaf (dorsiventral): palisade + spongy mesophyll, stomata on lower surface
- Monocot leaf (isobilateral): undifferentiated mesophyll, stomata on both surfaces, bulliform cells
- Secondary growth: vascular cambium (secondary xylem + phloem) and cork cambium (cork + secondary cortex)
- Annual rings: spring wood (light, wide) + autumn wood (dark, narrow) = 1 year
- Heartwood: inner, dark, dead, non-conducting; Sapwood: outer, light, living, conducting
- Bark: all tissues outside vascular cambium; Periderm: phellem + phellogen + phelloderm
- Lenticels: lens-shaped pores in bark for gas exchange