Class 12 Biology Notes Chapter 2 (Sexual Reproduction in Flowering Plants) – Biology Book

Okay, here are detailed notes for Chapter 2: Sexual Reproduction in Flowering Plants, from the NCERT Class 12 Biology textbook, focusing on aspects relevant for government exam preparation.

Chapter 2: Sexual Reproduction in Flowering Plants

Introduction:

• Flowers are the sites of sexual reproduction in angiosperms (flowering plants).
• Sexual reproduction involves the formation of male and female gametes and their fusion (fertilisation) to form a zygote, which develops into an embryo.

I. Pre-fertilisation: Structures and Events

This phase includes gametogenesis (formation of gametes) and gamete transfer (pollination).

A. Stamen, Microsporangium and Pollen Grain (Male Reproductive Structures)

1. Stamen:

   • The male reproductive unit of a flower.
   • Consists of two parts:
     • Filament: Long and slender stalk. Its proximal end is attached to the thalamus or petal.
     • Anther: Terminal, typically bilobed structure. Each lobe has two thecae (dithecous). Often, a longitudinal groove separates the thecae.

2. Structure of Anther (Transverse Section):

   • Appears tetragonal (four-sided) in T.S. due to the presence of four microsporangia located at the corners, two in each lobe.
   • Microsporangia develop further and become pollen sacs, which contain pollen grains.
   • Anther Wall Layers (from outside inwards):
     • Epidermis: Outermost single protective layer.
     • Endothecium: Layer below the epidermis. Cells have fibrous thickenings (alpha-cellulosic) which help in anther dehiscence. Absent at the stomium region.
     • Middle Layers: 2-3 layers of cells below the endothecium. Degenerate at maturity.
     • Tapetum: Innermost layer surrounding the sporogenous tissue. Cells are dense with cytoplasm and generally have more than one nucleus (polyploid).
       • Functions of Tapetum: Provides nourishment to developing pollen grains, secretes enzymes (like callase), hormones (IAA), Ubisch bodies (which contribute to exine formation), and pollenkitt material.

3. Sporogenous Tissue:

   • Compactly arranged homogenous cells occupying the centre of each microsporangium when the anther is young.

4. Microsporogenesis:

   • The process of formation of microspores from a Pollen Mother Cell (PMC) or microspore mother cell through meiosis.
   • Cells of the sporogenous tissue differentiate into PMCs (diploid, 2n).
   • Each PMC undergoes meiosis to form four haploid (n) microspores, arranged in a microspore tetrad (commonly tetrahedral). Callose wall surrounds the tetrad.
   • As the anther matures and dehydrates, the microspores dissociate from the tetrad and develop into pollen grains. The enzyme callase (secreted by tapetum) dissolves the callose wall.

5. Pollen Grain (Male Gametophyte):

   • Represents the male gametophyte. Generally spherical (25-50 micrometers).
   • Wall Layers (Sporoderm):
     • Exine: Hard outer layer made of sporopollenin (one of the most resistant organic materials known). It can withstand high temperatures, strong acids, and alkali. No enzyme can degrade sporopollenin known so far. This helps in fossilization.
       • Exhibits fascinating patterns and designs (useful in taxonomy).
       • Has prominent apertures called germ pores where sporopollenin is absent. Pollen tube emerges through a germ pore. (Monocots typically have 1 germ pore; Dicots typically have 3).
     • Intine: Thin, continuous inner layer made of cellulose and pectin. It is the outer wall of the pollen cell.
   • Cytoplasm: Surrounded by a plasma membrane. Contains stored food.
   • Cells within Pollen Grain (at maturity):
     • Most angiosperms shed pollen grains at the 2-celled stage:
       • Vegetative Cell (Tube Cell): Bigger, has abundant food reserve, large irregularly shaped nucleus. Responsible for pollen tube growth.
       • Generative Cell: Small, spindle-shaped with dense cytoplasm and a nucleus. Floats in the cytoplasm of the vegetative cell. Divides mitotically to form two male gametes (either in the pollen grain before shedding or in the pollen tube after germination).
     • In about 40% of species, pollen is shed at the 3-celled stage (vegetative cell + two male gametes).
   • Pollen Viability: The period for which pollen grains remain viable (able to germinate). Highly variable, depends on temperature and humidity.
     • Examples: Cereals like rice and wheat lose viability within 30 minutes. Some members of Rosaceae, Leguminosae, Solanaceae maintain viability for months.
   • Pollen Allergy: Pollen grains of many species (e.g., Parthenium or carrot grass) cause severe allergies and bronchial afflictions (hay fever, asthma).
   • Pollen Products: Rich in nutrients. Used as pollen tablets/syrups as food supplements (claimed to enhance performance of athletes, racehorses).
   • Pollen Banks/Cryopreservation: Pollen grains can be stored for years in liquid nitrogen (-196°C) for crop breeding programs.

B. The Pistil, Megasporangium (Ovule) and Embryo Sac (Female Reproductive Structures)

1. Pistil/Gynoecium:

   • Female reproductive part of the flower.
   • May consist of a single pistil (monocarpellary) or more than one pistil (multicarpellary).
   • Multicarpellary conditions:
     • Syncarpous: Pistils are fused together (e.g., Papaver, Hibiscus).
     • Apocarpous: Pistils are free (e.g., Michelia, Rose).
   • Parts of a Pistil:
     • Stigma: Terminal receptive part for pollen grains. May be sticky, feathery etc.
     • Style: Elongated slender part beneath the stigma, connecting it to the ovary. Can be solid or hollow.
     • Ovary: Basal bulged part containing the ovarian cavity (locule). The placenta is located inside the ovarian cavity, bearing the megasporangia or ovules. Number of ovules can be one (wheat, paddy, mango) to many (papaya, watermelon, orchids).

2. Megasporangium (Ovule):

   • Structure attached to the placenta by a stalk called the funicle.
   • The body of the ovule fuses with the funicle in the region called the hilum (represents the junction).
   • Integuments: One or two protective envelopes surrounding the ovule body, except at the tip.
   • Micropyle: Small opening at the tip where integuments are absent. Facilitates pollen tube entry.
   • Chalaza: Basal part of the ovule, opposite the micropylar end. Represents the origin point of integuments.
   • Nucellus: Mass of parenchymatous cells enclosed within the integuments. Has abundant reserve food materials. The embryo sac is located within the nucellus.

3. Megasporogenesis:

   • The process of formation of megaspores from the Megaspore Mother Cell (MMC).
   • Generally, a single MMC (large cell, dense cytoplasm, prominent nucleus, diploid-2n) differentiates in the micropylar region of the nucellus.
   • The MMC undergoes meiosis to produce four haploid (n) megaspores, usually arranged in a linear tetrad.

4. Embryo Sac (Female Gametophyte):

   • In a majority of flowering plants, only one of the megaspores (usually the one towards the chalazal end) remains functional, while the other three degenerate.
   • The functional megaspore develops into the female gametophyte (embryo sac). This method of embryo sac formation from a single megaspore is termed monosporic development.
   • Development of Embryo Sac (Polygonum type - common):
     • The nucleus of the functional megaspore divides mitotically three times, without immediate cell wall formation.
     • 1st division -> 2 nuclei (move to opposite poles)
     • 2nd division -> 4 nuclei (two at each pole)
     • 3rd division -> 8 nuclei (four at each pole)
     • Cell walls are laid down, organizing the typical female gametophyte/embryo sac.
   • Structure of a Mature Embryo Sac:
     • Typically 8-nucleate and 7-celled.
     • Egg Apparatus (at micropylar end): Consists of:
       • One Egg Cell (female gamete).
       • Two Synergids: Have special cellular thickenings at the micropylar tip called the filiform apparatus, which guides the pollen tube into the synergid.
     • Antipodal Cells: Three cells at the chalazal end. Role is nutritive, degenerate after fertilisation.
     • Central Cell: Large central cell with two polar nuclei. Prior to fertilization, the polar nuclei often fuse to form a single diploid secondary nucleus.

C. Pollination

• The transfer of pollen grains from the anther to the stigma of a pistil.

• Kinds of Pollination:

  1. Autogamy (Self-pollination): Transfer of pollen grains from the anther to the stigma of the same flower.
     • Requires synchrony in pollen release and stigma receptivity.
     • Anthers and stigma should lie close to each other.
     • Occurs in chasmogamous flowers (open flowers with exposed anthers and stigma) and cleistogamous flowers.
     • Cleistogamous flowers: Flowers which do not open at all (e.g., Viola (common pansy), Oxalis, Commelina). Anthers and stigma lie close, ensuring self-pollination even in the absence of pollinators. Assured seed-set. Disadvantage: leads to inbreeding depression.
     • Commelina produces both chasmogamous and cleistogamous flowers.
  2. Geitonogamy: Transfer of pollen grains from the anther to the stigma of another flower of the same plant.
     • Functionally cross-pollination (involves a pollinating agent).
     • Genetically similar to autogamy (since pollen comes from the same plant).
  3. Xenogamy (Cross-pollination): Transfer of pollen grains from the anther to the stigma of a different plant of the same species.
     • Genetically different pollen grains are brought to the stigma. Promotes genetic variation.

• Agents of Pollination:

  1. Abiotic Agents (Non-living):
     • Wind (Anemophily): More common.
       • Pollen grains: Light, non-sticky, sometimes winged. Produced in large numbers.
       • Flowers: Small, inconspicuous, often unisexual, without nectar or fragrance. Well-exposed stamens (for easy dispersal). Large, often feathery stigma (to trap airborne pollen).
       • Examples: Grasses, Maize, Wheat, Rice, Sugarcane, Bamboo, Pinus.
     • Water (Hydrophily): Quite rare (about 30 genera, mostly monocots).
       • Pollen grains: Often long, ribbon-like, protected from wetting by mucilaginous covering.
       • Flowers: Small, inconspicuous, no nectar/fragrance.
       • Examples:
         • Vallisneria (Epihydrophily): Female flower reaches water surface by long stalk, male flowers/pollen released onto surface, carried by water currents to female flower.
         • Zostera (Sea-grass) (Hypohydrophily): Female flowers submerged. Pollen grains long, ribbon-like, released inside water, carried passively to stigma.
         • Hydrilla (Epihydrophily).
       • Note: In most aquatic plants like water hyacinth (Eichhornia) and water lily (Nymphaea), flowers emerge above water level and are pollinated by insects or wind.
  2. Biotic Agents (Living):
     • Insects (Entomophily): Majority of flowering plants use insects (Bees are dominant). Moths, butterflies, flies, beetles, wasps, ants also involved.
       • Flowers: Usually large, colourful, fragrant, rich in nectar (floral rewards).
       • Pollen grains: Often sticky (due to pollenkitt) or spiny to adhere to insect body.
       • Stigma: Often sticky.
       • Floral Rewards: Nectar, pollen grains, safe places to lay eggs (e.g., Amorphophallus - tallest flower, Yucca and Pronuba moth - obligate mutualism).
     • Other Animals: Birds (Ornithophily - e.g., Sunbirds, Hummingbirds), Bats (Chiropterophily), Lemurs, arboreal rodents, reptiles (gecko lizard, garden lizard) have also been reported as pollinators.

• Outbreeding Devices: Mechanisms to discourage self-pollination and encourage cross-pollination.

  1. Non-synchronization: Pollen release and stigma receptivity are not synchronised (Dichogamy: Protandry - anthers mature first; Protogyny - stigma matures first).
  2. Different Positioning: Anther and stigma are placed at different positions so pollen cannot contact the stigma of the same flower (Herkogamy).
  3. Self-incompatibility: Genetic mechanism preventing self-pollen (from the same flower or other flowers of the same plant) from fertilising the ovules by inhibiting pollen germination or pollen tube growth in the pistil.
  4. Production of Unisexual Flowers (Dicliny): If both male and female flowers are on the same plant (monoecious, e.g., Castor, Maize), it prevents autogamy but not geitonogamy. If male and female flowers are on different plants (dioecious, e.g., Papaya, Date palm), it prevents both autogamy and geitonogamy.

• Pollen-Pistil Interaction:

  • All events from pollen deposition on the stigma until the pollen tube enters the ovule.
  • Dynamic process involving recognition (compatibility/incompatibility) between pollen and pistil, followed by acceptance or rejection. Mediated by chemical components.
  • If compatible: Pistil accepts pollen, promotes post-pollination events. Pollen germinates on the stigma to produce a pollen tube through a germ pore.
  • Pollen tube grows down through the tissues of the stigma and style, enters the ovary.
  • If pollen shed at 2-celled stage, generative cell divides into two male gametes during pollen tube growth. If shed at 3-celled stage, pollen tube carries two male gametes from the beginning.
  • Pollen tube enters the ovule (usually through micropyle), then enters one of the synergids through the filiform apparatus. Synergid degenerates after entry.

• Artificial Hybridisation:

  • Used in crop improvement programs to cross different varieties/species. Desired pollen grains are used for pollination. Stigma is protected from contamination (unwanted pollen).
  • Steps:
    1. Emasculation: Removal of anthers from the bisexual flower bud before the anther dehisces (if the female parent bears bisexual flowers). Not required if female parent is unisexual.
    2. Bagging: Emasculated flower is covered with a bag (usually butter paper) to prevent contamination of its stigma with unwanted pollen.
    3. Pollination: When the stigma attains receptivity, mature pollen grains collected from desired male parent are dusted onto the stigma.
    4. Rebagging: Flower is rebagged and fruits allowed to develop.

II. Double Fertilisation

• Unique characteristic event of Angiosperms. Discovered by Nawaschin in Lilium and Fritillaria.
• After entering one of the synergids, the pollen tube releases the two male gametes into the cytoplasm of the synergid.
• Events:
  1. Syngamy: One male gamete (n) fuses with the egg cell nucleus (n) to form the Zygote (2n).
  2. Triple Fusion: The other male gamete (n) fuses with the two polar nuclei (or the diploid secondary nucleus formed by their fusion) (n+n or 2n) located in the central cell to produce the Primary Endosperm Nucleus (PEN) (3n).
• Since two types of fusions (syngamy and triple fusion) occur in the embryo sac, the phenomenon is termed Double Fertilisation.
• The central cell after triple fusion becomes the Primary Endosperm Cell (PEC) and develops into the endosperm.
• The zygote develops into the embryo.

III. Post-fertilisation: Structures and Events

• Includes development of endosperm and embryo, maturation of ovule(s) into seed(s), and ovary into fruit.

A. Endosperm Development:

• Develops from the Primary Endosperm Cell (PEC). Usually precedes embryo development.
• PEC divides repeatedly to form a triploid (3n) nutritive tissue, the endosperm.
• Provides nutrition to the developing embryo.
• Types of Endosperm Development:
  1. Nuclear Endosperm: Most common type. PEN undergoes repeated nuclear divisions without cytokinesis (free-nuclear stage). Cell wall formation occurs later. (e.g., Coconut water is free-nuclear endosperm, surrounding white kernel is cellular endosperm).
  2. Cellular Endosperm: Cell wall formation occurs immediately after each nuclear division. (e.g., Petunia, Datura).
  3. Helobial Endosperm: Intermediate type (common in monocots).
• Endosperm may be completely consumed by the developing embryo before seed maturation (e.g., pea, groundnut, beans - non-albuminous/ex-albuminous seeds) or it may persist in the mature seed (e.g., castor, coconut, wheat, maize, barley - albuminous/endospermic seeds).

B. Embryo Development (Embryogeny):

• Develops from the zygote (2n) at the micropylar end of the embryo sac.
• Most zygotes divide only after some amount of endosperm is formed (ensures nutrition).
• Early stages of embryo development (embryogeny) are similar in both monocots and dicots.
• Stages in Dicot Embryo Development: Zygote -> Proembryo -> Globular Embryo -> Heart-shaped Embryo -> Mature Embryo.
• Mature Dicot Embryo:
  • Embryonal Axis: Main axis of the embryo.
  • Cotyledons: Two seed leaves attached laterally to the embryonal axis. Store food or are photosynthetic.
  • Epicotyl: Portion of embryonal axis above the level of cotyledons, terminates in the plumule (shoot tip).
  • Hypocotyl: Cylindrical portion below the level of cotyledons, terminates at its lower end in the radicle (root tip).
  • Root Cap: Covers the radicle tip.
  • Example: Bean, Pea, Gram.
• Mature Monocot Embryo:
  • Possess only one cotyledon. In the grass family (Poaceae), it is called the scutellum (shield-shaped, situated towards one side - lateral - of the embryonal axis).
  • Embryonal Axis: Has the radicle and root cap enclosed in an undifferentiated sheath called coleorhiza at its lower end.
  • The portion above the level of attachment of the scutellum is the epicotyl.
  • Epicotyl has a shoot apex and a few leaf primordia enclosed in a hollow foliar structure, the coleoptile.
  • Epiblast: Remnant of the second cotyledon (sometimes present).
  • Example: Grasses, Maize, Rice.

C. Seed:

• The final product of sexual reproduction in angiosperms. Fertilised ovule develops into a seed.
• Consists of:
  • Seed Coat(s): Develop from integument(s) of the ovule. Outer testa (from outer integument) and inner tegmen (from inner integument). Hard protective covering. Micropyle persists as a small pore (facilitates entry of O2 and water during germination).
  • Cotyledon(s): Usually thick and swollen due to food storage (as in legumes) or thin (as in cereals).
  • Embryonal Axis: Contains plumule and radicle.
• Types of Seeds (based on persistence of endosperm):
  • Non-albuminous/Ex-albuminous: No residual endosperm as it is completely consumed during embryo development (e.g., Pea, Groundnut, Bean). Food stored in cotyledons.
  • Albuminous/Endospermic: Retain a part of endosperm as it is not completely used up (e.g., Wheat, Maize, Barley, Castor, Coconut, Sunflower). Food stored in endosperm.
• Perisperm: In some seeds (e.g., Black pepper, Beet), remnants of nucellus are persistent. This residual, persistent nucellus is the perisperm.
• Seed Dormancy: State of inactivity of the embryo within the seed. Seed does not germinate even under favourable external conditions. Caused by impermeable/hard seed coat, chemical inhibitors, immature embryo etc. Broken by various means (scarification, stratification, hormones).
• Seed Viability: Ability of a seed to germinate. Varies greatly.
  • Few months to several years.
  • Oldest viable seed: Lupinus arcticus (Arctic Lupin) - excavated from Arctic Tundra, germinated after estimated 10,000 years of dormancy.
  • Recent record: 2000 years old viable seed of Date Palm (Phoenix dactylifera) discovered during archaeological excavation at King Herod’s palace near the Dead Sea.
• Advantages of Seeds:
  • Better adaptive strategies for dispersal to new habitats.
  • Have food reserves, nourish young seedlings.
  • Hard seed coat provides protection.
  • Being products of sexual reproduction, generate new genetic combinations leading to variation.
  • Basis of agriculture - dehydration and dormancy allow storage for later use.

D. Fruit:

• Mature or ripened ovary, developed after fertilisation.
• If formed without fertilisation of the ovary, it is called a parthenocarpic fruit (e.g., Banana, Grapes - seedless). Can be induced by growth hormones.
• Fruit Wall (Pericarp): Develops from the ovary wall. Can be dry or fleshy. Fleshy pericarp is often differentiated into outer epicarp, middle mesocarp, and inner endocarp.
• Types of Fruits:
  • True Fruits: Develop only from the ovary (e.g., Mango, Pea, Tomato).
  • False Fruits (Pseudocarps): Besides the ovary, other floral parts like thalamus also contribute to fruit formation (e.g., Apple, Strawberry, Cashew - fleshy part is thalamus).
• Fruits protect the seeds and help in their dispersal.

IV. Apomixis and Polyembryony

• Apomixis:

  • A form of asexual reproduction that mimics sexual reproduction.
  • Development of seeds without fertilisation. (From Greek: apo = away from, mixis = mixing).
  • Common in Asteraceae and grasses.
  • Ways it can occur:
    • Diploid egg cell (formed without reduction division) develops into embryo without fertilisation.
    • Some nucellar or integument cells (diploid) surrounding the embryo sac protrude into the embryo sac and develop into embryos.
  • Genetically identical to the parent plant (clones).
  • Importance: Hybrid varieties offer increased yield, but hybrid seeds need to be produced every year (costly). If hybrid characters can be fixed using apomixis, farmers can keep using hybrid seeds year after year without loss of characters. Active research is ongoing.

• Polyembryony:

  • Occurrence of more than one embryo in a seed.
  • First observed by Leeuwenhoek in Citrus seeds.
  • Can arise due to:
    • Cleavage of zygote/proembryo.
    • Development of more than one egg cell in the embryo sac.
    • Formation of embryos from cells of embryo sac other than the egg (e.g., synergids, antipodals - usually haploid).
    • Development of embryos from cells outside the embryo sac (e.g., nucellus, integuments - adventive embryony, common in Citrus, Mango). Nucellar embryos are diploid and clones of the parent.

Note: This summary covers the key concepts from NCERT Chapter 2. For government exams, it's crucial to remember specific examples, definitions, processes (like microsporogenesis, megasporogenesis, double fertilisation), and the significance of various structures and phenomena. Refer to the NCERT textbook for diagrams and detailed explanations. Good luck with your preparation!