Chapter 4: Anatomy of Flowering Plants

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The Tissues

Classification and Definition

• A tissue is a group of cells with common origin and function.


• Plants have two main tissue types: meristematic (dividing cells) and permanent (non-dividing cells).


• Term \"tissue\" coined by N. Grew (1682).

Memory Aids

• Meristematic = \"meristo\" means divide; active growth zones.


• Permanent = mature, fixed function cells.

Quick Checkpoints

• What defines a plant tissue?


• Difference between meristematic and permanent tissues?

Meristematic Tissues

Characteristics and Types

• Regions of active cell division; cells are immature and undifferentiated.


• Cells are isodiametric or polygonal, thin flexible walls, no intercellular spaces, dense cytoplasm, prominent nucleus.


• Classified by origin: Promeristem (embryonic), Primary meristem (from promeristem), Secondary meristem (from dedifferentiated permanent tissue).


• Classified by position: Apical (tips of roots/shoots), Intercalary (between mature tissues), Lateral (cylindrical, in mature regions for secondary growth).

Memory Aids

• Promeristem = embryo stage; Primary = apical growth; Secondary = secondary growth.


• Apical = tip; Intercalary = between; Lateral = side growth.

Quick Checkpoints

• What are the three origins of meristem?


• Where are apical meristems located?


• Function of lateral meristems?

Apical meristem

Features and Function

• Located at root and shoot tips; responsible for primary growth.


• Root apical meristem at root tip; shoot apical meristem at stem apex.


• Cells left behind form axillary buds, which can develop into branches or flowers.

Memory Aids

• Apical = apex; growth in length.

Quick Checkpoints

• What does apical meristem produce?


• Where are axillary buds formed?

Intercalary meristem

Features and Function

• Located between mature tissues, common in grasses.


• Enables regeneration of parts lost to grazing.


• Considered primary meristem as it appears early and contributes to primary plant body.

Memory Aids

• Intercalary = inserted between mature tissues.

Quick Checkpoints

• Where is intercalary meristem found?


• Role in plant regeneration?

Lateral meristem

Features and Function

• Located in mature regions of roots and shoots, especially woody plants.


• Also called secondary meristem; cylindrical in shape.


• Examples: vascular cambium, cork cambium.


• Responsible for secondary growth (increase in girth).


• Vegetative shoot apex is terminal meristem; root apex is sub-terminal and protected by root cap.


• Quiescent centre: group of cells with low mitotic activity at root tip.

Memory Aids

• Lateral = side growth; secondary thickening.


• Quiescent centre = resting cells at root tip.

Quick Checkpoints

• What is the role of lateral meristem?


• Where is the quiescent centre located?

Permanent Tissues

Definition and Types

• Cells that have lost the ability to divide and are specialized.


• Two types: simple (all cells similar) and complex (different cell types).

Memory Aids

• Simple = uniform cells; Complex = mixed cells.

Quick Checkpoints

• Difference between simple and complex permanent tissues?

Simple tissues

Types and Features

• Made of one cell type: parenchyma, collenchyma, sclerenchyma.

Memory Aids

• Parenchyma = basic tissue; Collenchyma = support; Sclerenchyma = strength.

Quick Checkpoints

• Functions of parenchyma?


• Where is collenchyma found?


• What is lignin's role in sclerenchyma?

Parenchyma

Structure and Function

• Cells are isodiametric, thin-walled, may have intercellular spaces.


• Functions: photosynthesis, storage, secretion.


• Chlorenchyma: parenchyma with chloroplasts in leaf mesophyll for photosynthesis.


• Aerenchyma: parenchyma with large air spaces in aquatic plants for buoyancy.

Memory Aids

• Chlorenchyma = chloroplasts + parenchyma.


• Aerenchyma = air spaces for floatation.

Quick Checkpoints

• What is the role of parenchyma in plants?


• Difference between chlorenchyma and aerenchyma?

Collenchyma

Structure and Function

• Supporting tissue below epidermis in dicots; cells thickened at corners with cellulose, hemicellulose, pectin.


• Cells may contain chloroplasts; no intercellular spaces.


• Provides mechanical support and flexibility to growing parts.

Memory Aids

• Collenchyma = corner-thickened support.

Quick Checkpoints

• Where is collenchyma located?


• What substances thicken collenchyma cell walls?

Sclerenchyma

Structure and Function

• Cells with thick, lignified walls; usually dead at maturity.


• Provides mechanical strength.


• Two types: fibres (elongated, pointed ends) and sclereids (variable shapes, thick walls).


• Found in fruit walls, seed coats, leaves; coir is sclerenchyma fibre.

Memory Aids

• Sclerenchyma = strength; lignin thickening.


• Fibres = long; Sclereids = variable shape.

Quick Checkpoints

• Difference between sclerenchyma fibres and sclereids?


• Where is sclerenchyma commonly found?

Complex tissues

Definition and Types

• Made of more than one cell type working together.


• Also called vascular or conducting tissues.


• Two types: xylem and phloem.

Memory Aids

• Xylem = water transport; Phloem = food transport.

Quick Checkpoints

• What are the two complex tissues?


• Function of xylem and phloem?

Xylem

Structure and Function

• Conducts water and minerals from roots to shoots; provides mechanical support.


• Composed of tracheids, vessels, xylem fibres, and xylem parenchyma.


• Tracheids: elongated, lignified, dead cells with tapered ends and pits; primitive water conductors.


• Vessels: long tubes formed by vessel members with perforated end walls; characteristic of angiosperms.


• Xylem fibres: sclerenchyma fibres with thick walls, may be septate or aseptate.


• Xylem parenchyma: living, thin-walled cells storing food and aiding radial water conduction.


• Primary xylem: protoxylem (first formed) and metaxylem (later formed).


• Arrangement: endarch in stems (protoxylem inside), exarch in roots (protoxylem outside).


• Secondary xylem formed during secondary growth; not differentiated into protoxylem/metaxylem.

Memory Aids

• Protoxylem = first, annular/spiral thickening; Metaxylem = later, pitted thickening.


• Endarch = stem; Exarch = root.

Quick Checkpoints

• Components of xylem?


• Difference between protoxylem and metaxylem?


• What is endarch and exarch arrangement?

Phloem

Structure and Function

• Transports food from leaves to other parts.


• In angiosperms, composed of sieve tube elements, companion cells, phloem parenchyma, and phloem fibres.


• Gymnosperms have sieve cells and albuminous cells instead of sieve tubes and companion cells.


• Sieve tube elements: long tubes with sieve plates; lack nucleus; controlled by companion cells.


• Companion cells: specialized parenchyma; connected to sieve tubes; maintain pressure gradient.


• Phloem parenchyma: elongated cells with pits; store food and substances; absent in most monocots.


• Phloem fibres: thick-walled, dead at maturity; called bast fibres; commercial fibers include jute, hemp, flax.


• Primary phloem: protophloem (first formed, narrow sieve tubes) and metaphloem (later formed, larger sieve tubes).

Memory Aids

• Sieve tubes + companion cells = sister cells.


• Bast fibres = commercial phloem fibres.

Quick Checkpoints

• Components of phloem?


• Difference between protophloem and metaphloem?


• Role of companion cells?

Epidermal Tissue System

Epidermis

• Outermost single-layer covering of primary plant body.


• Composed of parenchymatous cells with thin cytoplasm and large vacuole.


• Outer surface often covered by waxy cuticle to prevent water loss; absent in roots.

Stomata

• Present in leaf epidermis; regulate transpiration and gas exchange.


• Each stoma has two guard cells (bean-shaped or dumbbell-shaped in grasses).


• Guard cells have thin outer walls and thick inner walls; contain chloroplasts.


• Opening and closing controlled by guard cells swelling or shrinking.


• Surrounded by subsidiary cells; together called stomatal apparatus.

Epidermal Appendages

• Root hairs: unicellular extensions aiding water and mineral absorption.


• Trichomes: multicellular hairs on stems; may be branched, unbranched, soft, stiff, or secretory.


• Trichomes reduce water loss by transpiration.

Memory Aids

• Epidermis = outer single layer + cuticle (except roots).


• Stomata = guard cells + subsidiary cells = stomatal apparatus.


• Root hairs absorb; trichomes protect.

Quick Checkpoints

• What is the function of the cuticle?


• Describe the shape and role of guard cells.


• Difference between root hairs and trichomes.

Ground Tissue System

Components and Structure

• Largest tissue system in plants.


• Includes hypodermis, cortex, endodermis, pericycle, medullary rays, and pith.


• Comprises simple tissues: parenchyma, collenchyma, sclerenchyma.


• In leaves, ground tissue is mesophyll with chloroplasts for photosynthesis.

Memory Aids

• Ground tissue = all tissues except epidermis and vascular bundles.


• Mesophyll = leaf ground tissue with chloroplasts.

Quick Checkpoints

• Which tissues form the ground tissue system?


• What is mesophyll and its function?

Vascular Tissue System

Structure and Types

• Continuous tissues from root to leaf called vascular bundles.


• Composed of complex tissues: xylem and phloem.


• Dicot stems have cambium between xylem and phloem; called open vascular bundles (can form secondary tissues).


• Monocot vascular bundles lack cambium; called closed vascular bundles (no secondary growth).


• Vascular bundles classified by xylem-phloem arrangement into radial, conjoint, and concentric types.

Memory Aids

• Open bundles = cambium present; secondary growth possible.


• Closed bundles = no cambium; no secondary growth.


• Radial bundles: xylem and phloem on different radii (roots).


• Conjoint bundles: xylem and phloem together (stems, leaves).


• Concentric bundles: one surrounds the other.

Quick Checkpoints

• What distinguishes open and closed vascular bundles?


• Describe the three types of vascular bundles.


• Where are radial vascular bundles typically found?

Dicotyledonous Root

Epidermis and Cortex

• Epidermis (epiblema) is a single outer layer with unicellular root hairs; hypodermis absent.


• Cortex has multiple layers of thin-walled parenchyma with intercellular spaces.

Endodermis and Pericycle

• Endodermis is the innermost cortex layer; cells are barrel-shaped with Casparian strips on radial and tangential walls.


• Pericycle lies next to endodermis; thick-walled parenchyma cells forming semi-lunar patches.


• Lateral roots develop from pericycle (endogenous origin).

Vascular Bundles and Stele

• Vascular bundles are radial; xylem usually 2-4 (diarch to tetrarch), exarch xylem.


• Xylem and phloem bundles are equal in number, separated by parenchymatous conjunctive tissue.


• Stele includes all tissues inside endodermis: pericycle, vascular bundles, and central pith (parenchymatous).

Memory Aids

• Pericycle produces lateral roots: \"Peri = around, roots arise from around vascular tissue.\"


• Exarch xylem: Protoxylem towards periphery, metaxylem towards center.

Quick Checkpoints

• What is the role of Casparian strips in endodermis?


• Origin of lateral roots in dicot root?


• Arrangement of vascular bundles in dicot root?

Monocotyledonous Root

Structure and Features

• Similar to dicot root: epidermis, cortex, endodermis, pericycle, vascular bundles, and pith present.


• More than six xylem bundles (polyarch); large, well-developed pith.


• No secondary growth; root thickness does not increase over time.

Memory Aids

• Polyarch xylem: \"Poly = many, more xylem bundles than dicots.\"


• Large pith in monocot root supports storage and structure.

Quick Checkpoints

• How many xylem bundles are typical in monocot root?


• Does monocot root undergo secondary growth?


• What is the function of pericycle in monocot root?

Regions of a Typical Root

Root Zones and Tissues

• Root cap protects growing tip during soil penetration.


• Zone of cell division: actively dividing small, thin-walled cells.


• Zone of elongation: cells elongate, increasing root length.


• Zone of maturation: cells differentiate into tissues like xylem, phloem.


• Endodermis controls substance entry into vascular system; contains Casparian strip.


• Exodermis (in older monocot roots) is suberised outer cortex layers for protection.


• Passage cells in endodermis opposite protoxylem allow fluid conduction.

Memory Aids

• Root cap = \"soil shield\" for root tip.


• Casparian strip = \"water gatekeeper\" in endodermis.

Quick Checkpoints

• What is the function of root cap?


• Where are passage cells located and their role?


• Define zones of root growth.

Dicotyledonous Stem

Tissue Layers and Arrangement

• Epidermis: single layer with cuticle and multicellular trichomes.


• Cortex divided into hypodermis (multilayered collenchyma with chloroplasts), general cortex (parenchyma with intercellular spaces), and endodermis (single layer rich in starch grains, called starch sheath).


• Pericycle: semi-lunar patches of sclerenchyma between endodermis and vascular bundles.


• Vascular bundles arranged in a ring; conjoint, collateral, open; xylem is endarch.


• Pith: central parenchymatous cells with large intercellular spaces.

Memory Aids

• Ring arrangement of vascular bundles is characteristic of dicot stem.


• Endodermis = starch sheath in stem.

Quick Checkpoints

• What are the three sub-zones of cortex in dicot stem?


• Describe vascular bundle arrangement in dicot stem.


• What is the role of pericycle in dicot stem?

Monocotyledonous Stem

Structure and Features

• Epidermis: single layer with thick cuticle, no trichomes.


• Hypodermis: 2-3 layers of sclerenchyma providing mechanical support.


• Ground tissue undifferentiated; no distinct cortex, endodermis, pericycle, or pith.


• Vascular bundles: many, scattered, oval-shaped; conjoint, collateral, closed (no secondary growth).


• Each vascular bundle surrounded by sclerenchymatous bundle sheath.


• Protoxylem cavity (lacuna) formed by dissolution of protoxylem vessels during growth.

Memory Aids

• Scattered vascular bundles = monocot stem hallmark.


• Closed vascular bundles mean no secondary growth.

Quick Checkpoints

• What is the arrangement of vascular bundles in monocot stem?


• Why is secondary growth absent in monocot stem?


• What is the function of bundle sheath?

Dorsiventral (Dicotyledonous) Leaf

Leaf Structure

• Epidermis: single layer on upper (adaxial) and lower (abaxial) surfaces; abaxial has more stomata; covered by thick cuticle.


• Mesophyll differentiated into palisade (elongated, tightly packed cells for photosynthesis) and spongy tissue (loosely arranged with air spaces for gas exchange).


• Vascular bundles: conjoint, collateral, closed; xylem towards upper epidermis, phloem towards lower epidermis; surrounded by bundle sheath with parenchymatous extensions.


• No cambium in leaf vascular bundles; no secondary growth.

Memory Aids

• Palisade mesophyll = main photosynthetic tissue.


• Spongy mesophyll = facilitates gas exchange.


• Reticulate venation typical of dicot leaves.

Quick Checkpoints

• Difference between palisade and spongy mesophyll?


• Where are stomata more abundant in dorsiventral leaf?


• Describe vascular bundle arrangement in dicot leaf.

Isobilateral (Monocotyledonous) Leaf

Leaf Structure

• Stomata present on both surfaces (amphistomatic).


• Mesophyll not differentiated into palisade and spongy parenchyma.


• Bundle sheath extensions: patches of sclerenchyma above and below vascular bundles extending to epidermis.


• Bulliform cells (large, colorless motor cells) present in adaxial epidermis; regulate leaf folding to reduce water loss.


• Parallel venation reflected in similar-sized vascular bundles.

Memory Aids

• Bulliform cells control leaf rolling in grasses.


• Amphistomatic leaves have stomata on both sides.

Quick Checkpoints

• What is the function of bulliform cells?


• How is mesophyll arranged in monocot leaf?


• Describe venation pattern in monocot leaves.

Secondary Growth

General Concept

• Primary growth: elongation of roots and stems by apical meristem.


• Secondary growth: increase in girth (thickness) mainly in dicots, by lateral meristems.


• Lateral meristems involved: vascular cambium and cork cambium (phellogen).

Memory Aids

• Vascular cambium produces secondary xylem and phloem.


• Cork cambium produces protective outer layers (periderm).

Quick Checkpoints

• What tissues are produced by vascular cambium?


• Role of cork cambium in secondary growth?


• Difference between primary and secondary growth?

Secondary Growth in Stem

Dicot Stem

• Vascular cambium forms a continuous ring by joining intrafascicular and interfascicular cambium.


• Cambium produces secondary xylem (inside) and secondary phloem (outside).


• More secondary xylem formed than phloem; phloem gets crushed over time.


• Secondary medullary rays (vascular rays) develop; conduct food radially.


• Periderm formed by cork cambium replaces epidermis; cork cells are suberised and waterproof.

Memory Aids

• Secondary xylem = wood; secondary phloem = inner bark.


• Annual rings formed by variation in cambium activity.

Quick Checkpoints

• What forms the vascular cambium ring?


• Difference between spring wood and autumn wood?


• Function of medullary rays?

Spring Wood and Autumn Wood

Formation and Characteristics

• Annual rings formed due to seasonal variation in vascular cambium activity.


• Spring wood (early wood): formed in spring; vessels with wide cavities; lighter color, lower density.


• Autumn wood (late wood): formed in winter; vessels with narrow cavities; darker color, higher density.


• Annual rings help estimate tree age (dendrochronology).

Memory Aids

• Spring wood = fast growth, wide vessels.


• Autumn wood = slow growth, narrow vessels.

Quick Checkpoints

• What causes formation of annual rings?


• How do spring and autumn wood differ?


• What is dendrochronology?

Heartwood and Sapwood

Definitions and Functions

• Heartwood (duramen): central, dark brown, contains deposited organic compounds (tannin, resin); dead, lignified cells; provides mechanical support; does not conduct water.


• Reasons for water conduction absence in heartwood: vessels filled with deposits and tyloses formation.


• Sapwood (alburnum): peripheral, lighter color; conducts water and minerals from root to leaf; active xylem.


• Heartwood is durable and resistant to decay; best quality wood.

Memory Aids

• Heartwood = \"dead support\"; sapwood = \"living transport\".


• Tyloses block water conduction in heartwood.

Quick Checkpoints

• Why does heartwood not conduct water?


• What substances accumulate in heartwood?


• Role of sapwood in plants?

Cork Cambium

Structure and Function

• Cork cambium (phellogen) is a lateral meristem producing cork (phellem) outward and secondary cortex (phelloderm) inward.


• Cork cells are dead, suberised, and impermeable to water.


• Periderm = cork cambium + cork + phelloderm.


• Commercial cork obtained from Quercus suber (oak).


• New cork cambium forms annually beneath old one; old layers slough off.


• Bark includes all tissues outside vascular cambium, including secondary phloem.


• Lenticels: lens-shaped openings in bark formed by ruptured epidermis; allow gas exchange.

Memory Aids

• Periderm replaces epidermis during secondary growth.


• Lenticels = \"breathing pores\" in bark.

Quick Checkpoints

• What tissues constitute periderm?


• Function of lenticels?


• Source of commercial cork?

Secondary Growth in Dicot Root

Process and Features

• Vascular cambium in dicot root is entirely secondary in origin.


• Originates from pericycle tissue below phloem and above protoxylem, forming a continuous wavy ring that becomes circular.


• Cambium produces secondary xylem inward and secondary phloem outward, similar to dicot stem.


• Primary phloem gets crushed by secondary growth.


• Secondary phloem contains sieve tubes, companion cells, phloem parenchyma; secondary xylem has vessels, tracheids, xylem parenchyma.

Memory Aids

• Secondary growth in root adds girth and vascular capacity.


• Pericycle contributes to vascular cambium formation.

Quick Checkpoints

• Origin of vascular cambium in dicot root?


• What tissues are produced by vascular cambium?


• Effect of secondary growth on primary phloem?

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