Anatomy of Flowering PlantsNEET Botany · Class 11 · NCERT Chapter 5

Introduction

Anatomy is the study of the internal structure of living organisms. In flowering plants, the internal organisation is remarkably precise: specific cell types are grouped into tissues, tissues are grouped into tissue systems, and these systems are arranged in predictable patterns depending on whether the plant is a dicot or a monocot.

Expect 2 to 3 NEET questions from this chapter each year. The most frequently tested areas are: identifying tissue type from cell features, distinguishing dicot from monocot root and stem cross-sections, xylem and phloem elements, and secondary growth including annual rings, heartwood, and bark formation.

Meristematic Tissues

Meristems are regions of actively dividing cells. They are the growth centres of the plant. Meristematic cells are small, thin-walled, densely cytoplasmic, with a large nucleus and no vacuoles. They divide continuously to add new cells to the plant body.

1. Apical Meristems

Found at the tips of roots and shoots (apices). Responsible for primary growth: increase in the length of roots and shoots. Apical initials divide and differentiate into primary permanent tissues.

2. Lateral Meristems

Run parallel to the long axis of the organ. Responsible for secondary growth: increase in girth (thickness). Two types:

• Vascular cambium: located between primary xylem and phloem in dicot stems and roots. Produces secondary xylem (wood) inward and secondary phloem outward.
• Cork cambium (Phellogen): forms in cortex of dicot stems. Produces cork (phellem) outward and phelloderm inward, together forming the periderm (bark).

3. Intercalary Meristems

Portions of meristematic tissue left behind between mature tissues. Found at the base of internodes in grasses (Poaceae) and Equisetum. They allow the plant to regrow after grazing or mowing. Important NEET distinction: intercalary meristems cause primary growth (elongation), not secondary growth.

Permanent Tissues

When meristematic cells differentiate and lose the ability to divide, they form permanent tissues. Permanent tissues are of two broad types: simple and complex.

• Simple permanent tissues: made of one type of cell (parenchyma, collenchyma, sclerenchyma).
• Complex permanent tissues: made of more than one type of cell (xylem, phloem).

Simple Permanent Tissues

1. Parenchyma

The most common and versatile tissue. Features: thin cellulose cell wall, living, large central vacuole, isodiametric cells (roughly spherical), loosely packed with intercellular spaces.

Functions and modifications:

• Storage: starch, water, oils in cortex and pith.
• Chlorenchyma: parenchyma containing chloroplasts; performs photosynthesis; forms the mesophyll of leaves.
• Aerenchyma: parenchyma with very large air spaces (lacunae); provides buoyancy to aquatic plants like lotus (Nelumbo nucifera) and Hydrilla. Found in stems and petioles of hydrophytes.

2. Collenchyma

Features: unevenly thickened cell walls (cellulose and pectin deposited at corners), living, elongated cells. Found in: hypodermis of dicot stems, petioles, and leaf margins. Provides flexible mechanical support, allowing the organ to bend without breaking. NEET trap: collenchyma is a LIVING support tissue.

3. Sclerenchyma

Features: heavily lignified secondary cell walls, dead at maturity (protoplast disappears), provides rigidity and strength. Two types:

• Fibres: long, tapering, narrow cells. Found in bundles. Commercial sources: jute (Corchorus), hemp (Cannabis sativa), flax (Linum usitatissimum).
• Sclereids (stone cells): short, isodiametric or varied shape (branched in some). Found in: hard seed coats, shell of coconut (Cocos nucifera), grit cells (brachysclereids) of pear (Pyrus), and leaves of tea (Camellia sinensis).

Complex Permanent Tissues

Complex tissues contain more than one type of cell and work as a unit. The two main complex tissues are xylem and phloem, which together form the vascular tissue.

Xylem

Conducts water and dissolved minerals from roots to all parts of the plant (upward flow). Made of four elements:

• Tracheids: elongated, tapering, dead, thick walls with pits. Main conducting element in gymnosperms (Pinus, Cycas) and pteridophytes (ferns). Also provides mechanical support.
• Vessel members (Vessels): cylindrical or barrel-shaped, dead, perforated end walls (perforation plates). Wider than tracheids; allow faster water conduction. Main element in most angiosperms. Absent in gymnosperms (exception: Gnetum has vessels).
• Xylem parenchyma: the only living element of xylem. Stores food (starch, fats). Conducts water radially (ray cells). Living cells with thin walls.
• Xylem fibres: dead, thick lignified walls, provide mechanical support. Not directly involved in water conduction.

Memory tip: Xylem has 3 dead elements (tracheids, vessels, xylem fibres) and 1 living element (xylem parenchyma).

Phloem

Conducts food (sucrose, amino acids) from leaves to all parts of the plant (bidirectional flow). Made of four elements:

• Sieve tube elements: elongated cells with large perforations (sieve pores) on end walls (sieve plates). Lose their nucleus at maturity (alive but without nucleus). Stack end-to-end forming sieve tubes. Main food-conducting element.
• Companion cells: adjacent to sieve tube elements, with dense cytoplasm and large nucleus. Living. Control the function of sieve tubes. Present only in angiosperms. In gymnosperms, albuminous cells (Strasburger cells) serve this function.
• Phloem parenchyma: living, stores food. Present in primary phloem of dicots. Absent in monocot phloem.
• Phloem fibres (bast fibres): dead, thick walls. Mechanical support. Sources of commercial fibres: jute, flax, hemp.

Memory tip: Phloem has 2 living elements (sieve tubes and companion cells, plus phloem parenchyma) and 1 clearly dead element (phloem fibres). Sieve tubes are alive but nucleus-free at maturity.

Tissue Systems

N.C. Esau grouped all plant tissues into three tissue systems based on their position and function:

1. Epidermal Tissue System

The outermost layer protecting the plant. Components:

• Epidermis: single layer of cells; covered by a waxy cuticle (cutin) on aerial parts; no cuticle on roots. Function: reduces water loss.
• Trichomes (hairs): epidermal outgrowths. Multicellular in stems (help in reducing water loss, trapping insects). Unicellular and elongated in roots (root hairs, increase surface area for water absorption).
• Stomata: pores for gas exchange and transpiration. Flanked by two guard cells. Guard cells contain chloroplasts and control stomatal opening. Shape: kidney-shaped in dicots, dumbbell-shaped in grasses (monocots). Subsidiary cells surround guard cells in many species.

2. Ground Tissue System

Everything except epidermis and vascular tissue. Includes parenchyma, collenchyma, and sclerenchyma. In the leaf, ground tissue = mesophyll:

• Dorsiventral leaf (dicot): mesophyll is differentiated into palisade parenchyma (elongated, packed cells below upper epidermis; rich in chloroplasts; main photosynthetic region) and spongy parenchyma (loosely packed, irregular cells, large air spaces, near lower epidermis).
• Isobilateral leaf (monocot): mesophyll is NOT differentiated into palisade and spongy layers. Bulliform (motor) cells on upper epidermis help in leaf rolling.

3. Vascular Tissue System

Xylem and phloem together. Types of vascular bundle arrangement:

• Radial: xylem and phloem present on alternating radii (different positions). Found in roots.
• Conjoint: xylem and phloem present in the same bundle. Found in stems and leaves. Two subtypes: (a) Collateral: phloem only on the outer side of xylem. (b) Bicollateral: phloem on both outer and inner sides of xylem (e.g., Cucurbita/pumpkin family).
• Open vs Closed: open bundle has cambium between xylem and phloem (dicots, can do secondary growth); closed bundle has no cambium (monocots, cannot do secondary growth).

Dicot Root Anatomy

Example: gram (Cicer arietinum) or bean. Cross-section from outside to inside:

1. Epiblema (root epidermis): outermost single layer; no cuticle; root hairs (elongated unicellular) increase surface area for water absorption.
2. Cortex: several layers of parenchyma; large intercellular spaces; store food and water; helps in lateral transport of water.
3. Endodermis: single layer, innermost layer of cortex; cells have Casparian strips (bands of suberin on radial and transverse walls); control movement of water and ions; passage cells (thin-walled cells without suberin) present in some areas for selective ion entry.
4. Pericycle: one or few layers of parenchyma immediately inside endodermis; origin of lateral roots; contributes to vascular cambium formation during secondary growth.
5. Vascular bundles: 2 to 6 xylem poles (diarch to hexarch);exarch protoxylem (first-formed xylem at periphery, differentiates centripetally inward); phloem alternates with xylem arms (radially arranged); conjunctive tissue (parenchyma) between xylem and phloem.
6. Pith: small or absent in dicot root (centre occupied by xylem).

Monocot Root Anatomy

Example: maize (Zea mays). Cross-section from outside to inside:

1. Epiblema: with root hairs; no cuticle (same as dicot).
2. Cortex: wider than dicot; parenchymatous.
3. Endodermis: well-developed with Casparian strips; passage cells present in certain regions allowing selective entry.
4. Pericycle: single layer.
5. Vascular bundles: more than 6 xylem poles (polyarch); exarch protoxylem (same as dicot root); phloem alternates with xylem.
6. Pith: large, well-developed parenchymatous pith at centre (key distinguishing feature from dicot root).

Key comparison: Dicot root: 2-6 xylem poles, small/absent pith, can undergo secondary growth. Monocot root: many xylem poles (polyarch), large pith, no secondary growth.

Dicot Stem Anatomy

Example: sunflower (Helianthus annuus). Cross-section from outside to inside:

1. Epidermis: single layer; outer cuticle; some stomata; multicellular trichomes (leaf hairs) may be present.
2. Hypodermis: 2-4 layers of collenchyma below epidermis; provides flexible mechanical support.
3. Cortex: layers of parenchyma; may be chlorenchymatous in young stems; large intercellular spaces.
4. Endodermis (starch sheath): single layer; cells rich in starch grains (starch sheath); less prominent than in roots; no Casparian strips in most dicot stems.
5. Pericycle: made of sclerenchyma (cap-like strands) and parenchyma.
6. Vascular bundles: arranged in a ring; each bundle is conjoint, collateral, and open (cambium present between xylem and phloem);endarch protoxylem (first-formed xylem toward centre/pith).
7. Medullary rays: radial bands of parenchyma between vascular bundles; lateral transport of materials.
8. Pith: large, central mass of parenchyma; stores starch and water.

Monocot Stem Anatomy

Example: maize (Zea mays). Cross-section from outside to inside:

1. Epidermis: single layer with a thick cuticle.
2. Hypodermis: 2-4 layers of sclerenchyma (thick-walled, dead); provides rigidity to grass stems.
3. Ground tissue: parenchymatous; no distinct cortex or pith(undifferentiated ground tissue throughout).
4. Vascular bundles: scattered throughout the ground tissue (not in a ring); each bundle is conjoint, collateral, and closed (no cambium); each surrounded by a sclerenchymatous bundle sheath; endarch protoxylem.

Key comparison: Dicot stem: bundles in ring, open (cambium present), collenchyma hypodermis, distinct pith. Monocot stem: bundles scattered, closed (no cambium), sclerenchyma hypodermis, no distinct pith.

Leaf Anatomy

Dorsiventral Leaf (Dicot, e.g., mango)

Cross-section from upper to lower surface:

• Upper epidermis: single layer, cuticle, few or no stomata, no chloroplasts.
• Mesophyll: differentiated into two regions:
  • Palisade parenchyma: elongated cells, arranged perpendicular to epidermis, densely packed, rich in chloroplasts, main site of photosynthesis.
  • Spongy parenchyma: loosely arranged, irregular cells, large air spaces (facilitate gas exchange), fewer chloroplasts than palisade.
• Vascular bundles (veins): midrib and smaller veins; each bundle surrounded by bundle sheath.
• Lower epidermis: many stomata; cuticle present.

Isobilateral Leaf (Monocot, e.g., maize)

• Epidermis: both upper and lower epidermis similar in appearance. Dumbbell-shaped guard cells. Bulliform (motor) cells on upper epidermis; large, thin-walled, vacuolated; when turgid, leaf remains flat; when flaccid, leaf rolls to reduce transpiration.
• Mesophyll: NOT differentiated into palisade and spongy; uniform parenchyma with chloroplasts.
• Vascular bundles: larger bundles surrounded by a double bundle sheath (Kranz anatomy in C4 plants like maize).

Secondary Growth

Secondary growth increases the girth of a plant body. It occurs in dicots and gymnosperms. Monocots generally do not show secondary growth (closed bundles, no cambium).

1. Secondary Growth by Vascular Cambium

In dicot stems, after primary growth, a complete ring of vascular cambium forms:

• Intrafascicular cambium: cambium originally present between xylem and phloem within vascular bundles.
• Interfascicular cambium: formed by dedifferentiation of medullary ray parenchyma cells (between bundles).

This complete cambium ring then divides: secondary xylem (wood) is formed toward the pith (inside) and secondary phloem toward the cortex (outside). More secondary xylem is produced than phloem.

Annual Rings

In temperate regions, vascular cambium activity varies with season:

• Spring wood (early wood): formed in spring/early summer when water is abundant; large-diameter vessels; light-coloured and less dense.
• Autumn wood (late wood): formed in late summer/autumn when water is limited; small-diameter vessels; dark-coloured and dense.

One ring = one year of growth. Count the rings (each consisting of one spring + one autumn band) to determine the age of the tree (dendrochronology).

Heartwood vs Sapwood

• Sapwood (alburnum): outer, lighter-coloured secondary xylem; still functional; conducts water and minerals; living parenchyma cells are active.
• Heartwood (duramen): inner, dark-coloured old xylem; non-functional for water conduction; vessels blocked by tyloses (outgrowths of adjacent parenchyma cells into vessel lumen); impregnated with resins, tannins, oils, gums; very hard and durable; provides mechanical support; used in furniture and construction (e.g., teak, rosewood).

2. Secondary Growth by Cork Cambium (Phellogen)

As the stem increases in girth, the epidermis is replaced by the periderm:

• Phellogen (cork cambium): arises from cortex cells just below epidermis.
• Phellem (cork): produced outward by phellogen; cells are dead, impregnated with suberin (fatty substance), waterproof, prevents water loss and pathogen entry.
• Phelloderm: produced inward by phellogen; living parenchyma cells.
• Periderm = Phellogen + Phellem + Phelloderm: replaces epidermis during secondary growth.
• Lenticels: lens-shaped openings in the periderm filled with loosely arranged cells (complementary tissue) allowing gas exchange. Equivalent to stomata in the epidermis.

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