Chapter 4: Anatomy of Flowering Plants

Introduction

Overview

• Plant anatomy is the study of internal plant structure.
• Plants have cells organized into tissues, which form organs.
• Monocots and dicots differ anatomically within angiosperms.
• Katherine Esau showed that curly top virus spreads via phloem.
• She authored key books: "Plant Anatomy" (1954) and "The Anatomy of Seed Plants" (1960).

Memory Aids

• Esau = Phloem virus spread + Plant anatomy books.
• Cells → Tissues → Organs hierarchy.

Quick Checkpoints

• What is plant anatomy?
• How do monocots and dicots differ anatomically?
• Which tissue spreads the curly top virus?

The Tissues

Classification

• Tissue: group of cells with common origin and function.
• Two main types: meristematic (dividing) and permanent (non-dividing).
• Term coined by N. Grew in 1682.

Memory Aids

• Meristematic = dividing cells.
• Permanent = mature, non-dividing cells.

Quick Checkpoints

• Define tissue in plants.
• What are the two main tissue types?

Meristematic Tissues

Characteristics

• Regions of active cell division called meristems (meristos = divide).
• Cells are immature, isodiametric or polygonal, with thin, flexible walls.
• No intercellular spaces; cells compactly arranged.
• Vacuoles absent or small; large nucleus and dense cytoplasm.
• Term coined by C. Nageli in 1858.

Memory Aids

• Meristem = dividing, immature cells with thin walls.
• No vacuoles or intercellular spaces.

Quick Checkpoints

• What defines meristematic tissue?
• Describe meristematic cell features.

Classification of Meristem

Types by Origin and Position

• Based on origin: promeristem, primary meristem, secondary meristem.
• Promeristem: embryonic meristem forming the plant embryo.
• Primary meristem: develops from promeristem; forms primary plant body; found at apices.
• Secondary meristem: arises by dedifferentiation from primary permanent tissue; forms secondary plant body.
• Based on position: apical (tips), intercalary (between mature tissues), lateral (cylindrical, in mature regions).

Memory Aids

• Promeristem = embryo stage.
• Primary = apical, early growth.
• Secondary = dedifferentiated, secondary growth.
• Apical = tips; Intercalary = between tissues; Lateral = cylindrical.

Quick Checkpoints

• What are the three meristem types by origin?
• Where are apical meristems located?
• What is the role of secondary meristem?

Apical Meristem

Features

• Located at root and shoot tips; produces primary tissues.
• Root apical meristem at root tip; shoot apical meristem at stem apex.
• Cells left behind from shoot apical meristem form axillary buds (can form branches or flowers).

Memory Aids

• Apical = apex (tips) growth.
• Axillary buds = leftover cells from shoot apex.

Quick Checkpoints

• Where is the shoot apical meristem located?
• What forms axillary buds?

Intercalary Meristem

Features

• Located between mature tissues, common in grasses.
• Enables regeneration after grazing.
• Both apical and intercalary meristems are primary meristems.

Memory Aids

• Intercalary = between mature tissues.
• Found in grasses for regrowth.

Quick Checkpoints

• Where is intercalary meristem found?
• Why is it important in grasses?

Lateral Meristem

Features

• Occurs in mature regions of roots and shoots, especially woody plants.
• Also called secondary meristem; cylindrical in shape.
• Examples: fascicular vascular cambium, interfascicular cambium, cork cambium.
• Responsible for secondary growth (thickening).
• Vegetative shoot apex is terminal meristem at stem tips; root apex is sub-terminal, protected by root cap.
• Quiescent centre: group of distal root cells with low mitotic activity, acting as a reserve.

Memory Aids

• Lateral = secondary growth, cylindrical cambium.
• Quiescent centre = root cell reserve.

Quick Checkpoints

• What is the function of lateral meristem?
• Where is the quiescent centre located?

Permanent Tissues

Features

• Cells lose division ability, become specialized.
• Two types: simple (all cells similar) and complex (different cell types).

Memory Aids

• Permanent = mature, non-dividing cells.
• Simple = uniform cells; Complex = mixed cells.

Quick Checkpoints

• Define permanent tissue.
• Difference between simple and complex tissues?

Simple Tissues

Types

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

Memory Aids

• Simple = one cell type.

Quick Checkpoints

• Name the simple tissues in plants.

Parenchyma

Features

• Major component of organs; cells isodiametric, spherical, oval, polygonal or elongated.
• Thin cellulose walls; may have small 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

• Parenchyma = basic tissue, thin walls, various shapes.
• Chlorenchyma = photosynthetic parenchyma.
• Aerenchyma = air spaces for buoyancy.

Quick Checkpoints

• What are the functions of parenchyma?
• Define chlorenchyma and aerenchyma.

Collenchyma

Features

• Supporting tissue below epidermis in dicots; in layers or patches.
• Cells thickened at corners by cellulose, hemicellulose, pectin.
• Cells oval, spherical or polygonal; may contain chloroplasts.
• No intercellular spaces; provides mechanical support to young stems and petioles.

Memory Aids

• Collenchyma = unevenly thickened corners, flexible support.

Quick Checkpoints

• Where is collenchyma found?
• What is the function of collenchyma?

Sclerenchyma

Features

• Cells with thick lignified walls; provide mechanical support.
• Usually dead at maturity; walls have pits.
• Two types: fibres (elongated, pointed ends, with lumen and pits) and sclereids (short, thick-walled, narrow lumen).
• Found in fruit walls, seed coats, leaves, and coir (coconut fibre).

Memory Aids

• Sclerenchyma = thick lignified walls, dead cells.
• Fibres = long; Sclereids = short, irregular.

Quick Checkpoints

• What are the two types of sclerenchyma?
• Where is sclerenchyma commonly found?

Complex Tissues

Features

• Made of different cell types working as a unit.
• Also called vascular or conducting tissues.
• Two types: xylem and phloem.

Memory Aids

• Complex = multiple cell types, vascular tissues.

Quick Checkpoints

• What are complex tissues?
• Name the two complex tissues.

Xylem

Components and Functions

• Conducts water and minerals from roots to shoots.
• Provides mechanical strength.
• 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 of vessel members with perforated end walls; characteristic of angiosperms.
• Xylem fibres: sclerenchyma fibres with thick walls and obliterated lumen.
• Xylem parenchyma: living 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; no protoxylem/metaxylem differentiation.

Memory Aids

• Xylem = water conduction + support.
• Tracheids = primitive; Vessels = advanced.
• Protoxylem inside stem, outside root.

Quick Checkpoints

• What are the four elements of xylem?
• Difference between protoxylem and metaxylem?
• What is endarch and exarch arrangement?

Phloem

Components and Functions

• Transports food from leaves to other parts.
• In angiosperms: sieve tube elements, companion cells, phloem parenchyma, phloem fibres.
• In gymnosperms: sieve cells and albuminous cells; lack sieve tubes and companion cells.
• Sieve tube elements: long tubes with sieve plates; lack nucleus; controlled by companion cells.
• Companion cells: specialized parenchyma; maintain sieve tube function; connected by pit fields.
• 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 from jute, hemp, flax.
• Primary phloem: protophloem (first formed, narrow sieve tubes) and metaphloem (later formed, wider sieve tubes).
• Special cells in pteridophytes and gymnosperms: albuminous or Strasburger cells.

Memory Aids

• Phloem = food transport.
• Sieve tubes + companion cells = functional unit.
• Bast fibres = commercial fibers.

Quick Checkpoints

• What are the components of phloem in angiosperms?
• Difference between sieve tube elements and companion cells?
• What are bast fibres?

Epidermal Tissue System

Epidermis

• Single-layered outermost covering of primary plant body.
• Composed of parenchymatous cells with large vacuole and thin cytoplasm.
• Outer surface 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 of stomata controlled by guard cells.
• Stomatal apparatus = stomatal aperture + guard cells + subsidiary cells.

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 = 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 structure of guard cells.
• What forms the stomatal apparatus?
• 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.
• All tissues except epidermis and vascular bundles.
• Made of simple tissues: parenchyma, collenchyma, sclerenchyma.
• In leaves, ground tissue = mesophyll (chloroplast-rich cells).

Memory Aids

• Ground tissue = all non-epidermal, non-vascular tissues.
• Mesophyll = leaf ground tissue with chloroplasts.

Quick Checkpoints

• Which tissues form the ground tissue system?
• What is mesophyll?
• What simple tissues are found in ground tissue?

Vascular Tissue System

Structure and Types

• Continuous tissues from root to leaf called vascular bundles.
• Complex tissues: xylem (water transport) and phloem (food transport).
• Dicot stems have cambium between xylem and phloem; called open vascular bundles (secondary growth possible).
• Monocot vascular bundles lack cambium; called closed vascular bundles (no secondary growth).
• Vascular bundles classified by xylem-phloem arrangement:
  • Radial: xylem and phloem on different radii (roots).
  • Conjoint: xylem and phloem together on same radius (stems, leaves).
    • Collateral: phloem outside, xylem inside; open or closed.
    • Bicollateral: xylem between two phloem layers with cambium.
  • Concentric: one tissue surrounds the other.
    • Amphivasal: xylem surrounds phloem.
    • Amphicribral: phloem surrounds xylem.

Memory Aids

• Open bundles = cambium present; closed bundles = no cambium.
• Radial = separate xylem and phloem; conjoint = together; concentric = one surrounds other.

Quick Checkpoints

• What defines open and closed vascular bundles?
• List types of vascular bundles by arrangement.
• Where are radial bundles found?