Class 11 Biology Notes Chapter 6 (Anatomy of flowering plants) – Biology Book

Alright students, let's focus on Chapter 6, 'Anatomy of Flowering Plants'. Understanding the internal structure of plants is crucial, not just for your Class 11 syllabus, but also forms a significant part of various government exams where Biology is a subject. We'll break down the key concepts systematically.

Anatomy of Flowering Plants - Detailed Notes

1. Introduction

• Anatomy: The study of the internal structure of organisms.
• Plant Anatomy: Focuses on the internal structure of plants, primarily the arrangement and types of tissues.
• Tissue: A group of cells having a common origin and usually performing a common function.

2. Plant Tissues: Classification
Plant tissues are broadly classified into two main groups based on their ability to divide:
* Meristematic Tissues: Actively dividing cells responsible for growth.
* Permanent Tissues: Mature cells derived from meristems that have lost the ability to divide (or divide only under specific conditions) and are specialized for specific functions.

3. Meristematic Tissues (Meristems)

• Characteristics:
  • Cells are actively dividing (mitosis).
  • Cells are generally small, isodiametric (roughly spherical or polygonal).
  • Thin cellulosic cell walls.
  • Dense cytoplasm.
  • Prominent nuclei.
  • Vacuoles are small or absent.
  • No intercellular spaces.
• Types based on Location/Origin:
  • Apical Meristems: Located at the tips of roots (Root Apical Meristem - RAM) and shoots (Shoot Apical Meristem - SAM). Responsible for primary growth (increase in length). Forms primary tissues.
  • Intercalary Meristems: Located between mature tissues, often at the base of internodes or leaf bases (e.g., in grasses). They contribute to elongation and regeneration of parts removed by grazing herbivores. They are derived from apical meristems.
  • Lateral Meristems: Located along the sides (lateral axis) of stems and roots. Responsible for secondary growth (increase in girth). Examples: Vascular Cambium and Cork Cambium (Phellogen). Appear later in a plant's life, primarily in dicots and gymnosperms.

4. Permanent Tissues

• Cells derived from primary or secondary meristems.
• They lose the ability to divide and attain permanent shape, size, and function (differentiation).
• Types:
  • Simple Permanent Tissues: Composed of only one type of cell.
  • Complex Permanent Tissues: Composed of more than one type of cell, working together as a unit.

5. Simple Permanent Tissues

• Parenchyma:
  • Most abundant tissue.
  • Cells are generally isodiametric, thin-walled (cellulose).
  • May be spherical, oval, round, polygonal, or elongated.
  • Often have small intercellular spaces.
  • Living cells.
  • Functions: Photosynthesis (chlorenchyma), storage (food, water, waste products), secretion, buoyancy (aerenchyma in aquatic plants).
• Collenchyma:
  • Found in layers below the epidermis in dicot stems and petioles. Absent in roots and monocots generally.
  • Cells are often elongated, thickened at the corners due to deposition of cellulose, hemicellulose, and pectin.
  • Intercellular spaces are generally absent.
  • Living cells.
  • Function: Provides mechanical support to growing parts like young stems and petioles, allows bending without breaking.
• Sclerenchyma:
  • Composed of long, narrow cells with thick, lignified secondary walls.
  • Cells are usually dead at maturity (without protoplasts).
  • Provide mechanical strength and rigidity.
  • Types:
    • Fibres: Thick-walled, elongated, pointed cells, occurring in groups. Provide mechanical strength (e.g., Jute, Flax fibres).
    • Sclereids: Short, irregular shape (spherical, oval, cylindrical), highly thickened lignified walls with narrow lumen (cavity). Found in fruit walls (nuts), pulp of fruits (guava, pear - cause grittiness), seed coats (legumes), leaves (tea). Provide stiffness.

6. Complex Permanent Tissues

• Xylem:
  • Function: Conduction of water and minerals from roots to stem and leaves (unidirectional), mechanical support.
  • Components:
    • Tracheids: Elongated, tube-like dead cells with thick, lignified walls and tapering ends. Water conduction occurs through pits. Main water-conducting elements in pteridophytes and gymnosperms.
    • Vessels: Long, cylindrical, tube-like structures made of vessel members connected end-to-end through perforations in their common walls. Dead cells with lignified walls and a large central lumen. Characteristic feature of Angiosperms (absent in most gymnosperms). More efficient water conductors than tracheids.
    • Xylem Parenchyma: Living cells with thin cellulosic walls. Store food (starch, fat) and tannins. Help in radial conduction of water.
    • Xylem Fibres: Sclerenchymatous fibres associated with xylem. Thick walls, obliterated lumen. Provide mechanical strength. Dead cells.
  • Types based on Origin:
    • Protoxylem: First formed primary xylem elements (smaller).
    • Metaxylem: Later formed primary xylem elements (larger).
  • Arrangement:
    • Endarch: Protoxylem towards the center (pith), Metaxylem towards the periphery (Stems).
    • Exarch: Protoxylem towards the periphery, Metaxylem towards the center (Roots).
• Phloem:
  • Function: Transport of food materials (sugars, primarily sucrose) from leaves to other parts (bidirectional - translocation).
  • Components (in Angiosperms):
    • Sieve Tube Elements: Long, tube-like structures arranged longitudinally. Associated with companion cells. End walls are perforated (sieve plates). Mature sieve tube elements have peripheral cytoplasm, a large vacuole, but lack a nucleus. Function controlled by the companion cell nucleus.
    • Companion Cells: Specialized parenchymatous cells closely associated with sieve tube elements (originate from the same mother cell). Connected by pit fields. Maintain pressure gradient in sieve tubes. Have dense cytoplasm and a prominent nucleus. Found only in Angiosperms.
    • Phloem Parenchyma: Elongated, tapering cylindrical cells with dense cytoplasm and nucleus. Store food, resins, latex, mucilage. Absent in most monocots.
    • Phloem Fibres (Bast Fibres): Sclerenchymatous cells. Elongated, unbranched, pointed needle-like apices. Thick cell walls. Dead at maturity. Provide mechanical support. Commercially important (e.g., Jute, Flax, Hemp).
  • Types based on Origin: Protophloem (narrow sieve tubes) and Metaphloem (bigger sieve tubes).

7. The Tissue System
Based on structure and location, tissues are grouped into three systems:

• Epidermal Tissue System:
  • Outermost covering of the plant body.
  • Components:
    • Epidermis: Single layer of parenchymatous cells. Outer walls often covered by a waxy thick layer called the Cuticle (prevents water loss).
    • Stomata: Structures present in the epidermis of leaves (and sometimes stems). Regulate transpiration and gaseous exchange. Each stoma has two bean-shaped Guard Cells (dumbbell-shaped in grasses) enclosing a stomatal pore. Guard cells contain chloroplasts and regulate opening/closing. Sometimes associated with specialized Subsidiary Cells.
    • Epidermal Appendages:
      • Root Hairs: Unicellular elongations of epidermal cells in roots, increase surface area for water and mineral absorption.
      • Trichomes: Epidermal hairs on the stem (multicellular or unicellular, branched or unbranched, soft or stiff). May be secretory. Help in preventing water loss and protection.
• Ground Tissue System:
  • All tissues except epidermis and vascular bundles.
  • Consists of simple tissues like parenchyma, collenchyma, and sclerenchyma.
  • Forms: Cortex, Pericycle, Pith, and Medullary rays in stems and roots.
  • In leaves, it consists of Mesophyll (parenchymatous cells containing chloroplasts). Differentiated into Palisade and Spongy parenchyma in dicot leaves.
• Vascular Tissue System:
  • Consists of complex tissues: Xylem and Phloem, forming Vascular Bundles.
  • Cambium: In dicot stems, cambium (lateral meristem) is present between xylem and phloem (Open vascular bundles - capable of secondary growth). In monocots, cambium is absent (Closed vascular bundles - no secondary growth).
  • Arrangement:
    • Radial: Xylem and Phloem occur in different radii, alternating with each other (Roots).
    • Conjoint: Xylem and Phloem are situated at the same radius. Phloem is located on the outer side of the xylem (Stems and Leaves).
      • Collateral: Phloem on the outer side only (most common).
      • Bicollateral: Phloem on both outer and inner sides of xylem (e.g., Cucurbitaceae).

8. Anatomy of Dicotyledonous and Monocotyledonous Plants

9. Secondary Growth

• Increase in girth/thickness of stems and roots, primarily in Dicots and Gymnosperms.

• Involves two lateral meristems: Vascular Cambium and Cork Cambium.

• Vascular Cambium:

  • Origin: In stems, partly primary (intrafascicular cambium within VBs) and partly secondary (interfascicular cambium from medullary rays). Forms a complete Cambial Ring. In roots, originates from pericycle and conjunctive tissue.
  • Activity: Cuts off cells on both sides. More active inwards.
    • Secondary Xylem: Formed towards the inside (pith). Forms the bulk of the tissue (wood).
    • Secondary Phloem: Formed towards the outside (periphery).
  • Annual Rings: In temperate regions, cambium activity varies seasonally.
    • Spring Wood (Early Wood): Formed during spring. Cambium active, produces more xylary elements with wider vessels. Lighter in colour, lower density.
    • Autumn Wood (Late Wood): Formed during autumn/winter. Cambium less active, fewer xylary elements, narrow vessels, thick walls. Darker in colour, higher density.
    • One annual ring = Spring wood + Autumn wood. Used to estimate tree age (Dendrochronology).
  • Heartwood: Central, older, darker part of secondary xylem. Dead elements filled with tannins, resins, oils, gums. Hard, durable, resistant to microbes. Non-conductive, provides mechanical support.
  • Sapwood: Peripheral, younger, lighter part of secondary xylem. Involved in water and mineral conduction. Living cells present (parenchyma).

• Cork Cambium (Phellogen):

  • Develops usually in the cortex region. Meristematic layer.
  • Activity: Cuts off cells on both sides.
    • Cork (Phellem): Formed towards the outside. Cells become dead, suberin deposited in walls, making them impervious to water. Protective layer.
    • Secondary Cortex (Phelloderm): Formed towards the inside. Parenchymatous cells.
  • Periderm: Collective term for Phellogen + Phellem + Phelloderm. Replaces the epidermis/cortex during secondary growth.
  • Bark: Non-technical term for all tissues exterior to the vascular cambium (including secondary phloem). Protective. Shed periodically.
  • Lenticels: Lens-shaped openings in the periderm, allowing gaseous exchange between the atmosphere and internal tissues. Formed by loosely arranged complementary cells produced by phellogen.

• Secondary Growth in Roots (Dicot): Similar process, vascular cambium formation is initially wavy, then becomes circular. Cork cambium arises from the pericycle.

Multiple Choice Questions (MCQs)

1. Which of the following tissues provides mechanical support to the growing parts of the plant like young stem and petiole of a leaf?
   (a) Parenchyma
   (b) Collenchyma
   (c) Sclerenchyma
   (d) Xylem

2. In dicot roots, the vascular bundles are radial and the number of xylem bundles is typically:
   (a) Polyarch (more than six)
   (b) Diarch to Hexarch (two to six)
   (c) Tetrarch only (four)
   (d) Always Monorch (one)

3. Vessels are the main water-conducting elements in:
   (a) Pteridophytes
   (b) Gymnosperms
   (c) Angiosperms
   (d) Bryophytes

4. Companion cells are closely associated with:
   (a) Tracheids
   (b) Vessel elements
   (c) Sieve tube elements
   (d) Sclereids

5. Which of the following statements is INCORRECT regarding monocot stems?
   (a) Vascular bundles are conjoint, collateral, and closed.
   (b) Vascular bundles are scattered in the ground tissue.
   (c) Vascular bundles are surrounded by a sclerenchymatous bundle sheath.
   (d) Vascular cambium is present, allowing for secondary growth.

6. Casparian strips, which are impervious to water, are found in the cell walls of:
   (a) Epidermis
   (b) Pericycle
   (c) Endodermis
   (d) Cortex

7. Heartwood differs from sapwood in:
   (a) Being the peripheral region of secondary xylem.
   (b) Containing living cells and being involved in conduction.
   (c) Being dark coloured due to deposition of organic compounds and non-functional in conduction.
   (d) Being lighter in colour and having lower density.

8. Bulliform cells, involved in the rolling and unrolling of leaves in response to water availability, are found in the epidermis of:
   (a) Dicot leaves
   (b) Grasses (Monocot leaves)
   (c) Dicot stems
   (d) Gymnosperm needles

9. Phellogen, Phellem, and Phelloderm collectively constitute the:
   (a) Vascular Cambium
   (b) Periderm
   (c) Stele
   (d) Bark (technical definition)

10. The arrangement where protoxylem lies towards the periphery and metaxylem lies towards the center is called:
    (a) Endarch
    (b) Exarch
    (c) Mesarch
    (d) Polyarch

Answer Key for MCQs:

1. (b) Collenchyma
2. (b) Diarch to Hexarch (two to six)
3. (c) Angiosperms
4. (c) Sieve tube elements
5. (d) Vascular cambium is present, allowing for secondary growth. (It is absent in monocots)
6. (c) Endodermis
7. (c) Being dark coloured due to deposition of organic compounds and non-functional in conduction.
8. (b) Grasses (Monocot leaves)
9. (b) Periderm
10. (b) Exarch (Characteristic of roots)

Make sure you revise these notes thoroughly. Focus on the differences between monocots and dicots, the components and functions of complex tissues, and the process of secondary growth. Understanding these fundamentals is key for your exams. Good luck!