Class 12 Biology Notes Chapter 2 (Sexual reproduction in flowering plants) – Biology Book

Alright students, let's get straight into the crucial details of Chapter 2: Sexual Reproduction in Flowering Plants. This chapter is fundamental for understanding plant life cycles and frequently appears in various government examinations. Pay close attention to the terminology and processes.

Chapter 2: Sexual Reproduction in Flowering Plants - Detailed Notes

1. Introduction: The Flower - Site of Sexual Reproduction

• Flowering plants (Angiosperms) reproduce sexually via flowers.
• A typical flower has four whorls arranged on the thalamus: Calyx (sepals), Corolla (petals), Androecium (stamens), and Gynoecium/Pistil (carpels).
• Androecium and Gynoecium are the essential reproductive whorls.

2. Pre-fertilization: Structures and Events

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

• Stamen: Consists of a long, slender stalk called the filament and a terminal, typically bilobed structure called the anther.
• Anther Structure:
  • Bilobed, each lobe having two thecae (dithecous).
  • Appears tetragonal in transverse section, containing four microsporangia located at the corners (two in each lobe).
  • Microsporangia develop into pollen sacs, which contain pollen grains.
• Microsporangium Structure (Wall Layers): From outside to inside:
  • Epidermis: Outermost protective layer.
  • Endothecium: Layer below epidermis; cells have fibrous thickenings (α-cellulosic) that help in anther dehiscence.
  • Middle Layers: 2-3 layers below endothecium; degenerate at maturity.
  • Tapetum: Innermost layer, surrounds the sporogenous tissue.
    • Functions: Provides nourishment to developing pollen grains, secretes enzymes (like callase), hormones, and produces Ubisch bodies which contribute to sporopollenin formation for the exine. Cells are dense with cytoplasm and generally have more than one nucleus (polyploid).
• Sporogenous Tissue: Compactly arranged homogenous cells occupying the centre of each microsporangium when the anther is young.
• 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).
  • As the anther matures and dehydrates, the microspores dissociate from the tetrad and develop into pollen grains.
• Pollen Grain (Male Gametophyte):
  • Generally spherical (25-50 micrometers).
  • Has a two-layered wall:
    • 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 it. Exhibits fascinating patterns and designs (useful in taxonomy). It has prominent apertures called germ pores where sporopollenin is absent, facilitating pollen tube emergence.
    • Intine: Thin, continuous inner wall made of cellulose and pectin.
  • Cytoplasm: Surrounded by a plasma membrane. Contains stored food.
  • Mature Pollen Grain: Contains two cells:
    • Vegetative Cell (Tube Cell): Bigger, has abundant food reserve, large irregularly shaped nucleus. Responsible for pollen tube development.
    • Generative Cell: Small, spindle-shaped with dense cytoplasm and a nucleus, floats in the cytoplasm of the vegetative cell.
  • Development: In ~60% of angiosperms, pollen grains are shed at this 2-celled stage. In the remaining species, the generative cell divides mitotically to give rise to the two male gametes before pollen shedding (3-celled stage).
  • Pollen Viability: Period for which pollen grains remain viable (able to germinate). Highly variable (e.g., 30 mins in rice/wheat, months in Rosaceae, Leguminosae, Solanaceae). Can be stored for years in liquid nitrogen (-196°C) - cryopreservation in pollen banks.
  • Pollen Allergy: Pollen grains of some species (e.g., Parthenium or carrot grass) cause severe allergies and bronchial afflictions (asthma, bronchitis).
  • Pollen Products: Rich in nutrients, used as pollen tablets/syrups as food supplements. Claimed to enhance performance of athletes and racehorses.

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

• Gynoecium/Pistil: Female reproductive part. Can be:
  • Monocarpellary: Single pistil.
  • Multicarpellary: More than one pistil.
    • Syncarpous: Pistils fused together (e.g., Papaver, Hibiscus).
    • Apocarpous: Pistils free (e.g., Michelia, Rose).
• Parts of a Pistil:
  • Stigma: Terminal receptive part for pollen grains.
  • Style: Elongated slender part beneath the stigma, connecting it to the ovary.
  • Ovary: Basal bulged part containing the ovarian cavity (locule) where ovules (megasporangia) are located, attached to the placenta.
• 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.
  • Each ovule has one or two protective envelopes called integuments, which encircle the nucellus except at the tip where a small opening called the micropyle is present.
  • Opposite the micropylar end is the chalaza, representing the basal part of the ovule.
  • Nucellus: Mass of cells enclosed within the integuments, containing abundant reserve food materials. The embryo sac is located within the nucellus.
  • Most common type of ovule in Angiosperms: Anatropous (inverted ovule where micropyle lies close to the funicle).
• Megasporogenesis:
  • The process of formation of megaspores from the Megaspore Mother Cell (MMC).
  • 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.
• Female Gametophyte (Embryo Sac) Development:
  • In a majority of flowering plants, only one of the four 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 (Polygonum type is most common).
  • Development Steps:
    1. The nucleus of the functional megaspore (n) divides mitotically into two nuclei which move to opposite poles.
    2. Two more sequential mitotic divisions occur at each pole, resulting in a total of 8 nuclei (four at each pole). These divisions are strictly free nuclear (karyokinesis not immediately followed by cytokinesis).
    3. Cell walls start forming:
       • Three nuclei at the micropylar end organize into the egg apparatus (one central egg cell and two flanking synergids). Synergids have special cellular thickenings at the micropylar tip called the filiform apparatus, which guides the pollen tube into the synergid.
       • Three nuclei at the chalazal end organize into the antipodal cells.
       • The remaining two nuclei (polar nuclei) move to the centre, below the egg apparatus, and are located in the large central cell.
  • Mature Embryo Sac: Typically 7-celled and 8-nucleate at maturity. (3 antipodals + 3 egg apparatus cells + 1 large central cell = 7 cells; 3 antipodal nuclei + 1 egg nucleus + 2 synergid nuclei + 2 polar nuclei = 8 nuclei).

3. Pollination

• The transfer of pollen grains from the anther to the stigma of a pistil.
• Types of Pollination:
  • Autogamy (Self-Pollination): Transfer within the same flower. Requires synchrony in pollen release and stigma receptivity, and close proximity of anther and stigma.
    • Chasmogamous flowers: Flowers with exposed anthers and stigma (typical flowers). Autogamy is possible but cross-pollination is also favoured.
    • Cleistogamous flowers: Flowers which do not open at all (e.g., Oxalis, Viola, Commelina). Anthers and stigma lie close, ensuring self-pollination even in the absence of pollinators. Assured seed-set. Disadvantage: leads to inbreeding depression.
  • Geitonogamy: Transfer of pollen from the anther to the stigma of another flower of the same plant. Functionally cross-pollination (involves pollinating agent), but genetically similar to autogamy since pollen comes from the same plant.
  • Xenogamy (Cross-Pollination): Transfer of pollen from anther to the stigma of a different plant of the same species. Genetically different pollen types are brought to the stigma. Promotes genetic variation.
• Agents of Pollination:
  • Abiotic Agents:
    • Wind (Anemophily): More common.
      • Characteristics: Pollen grains are light, non-sticky, winged (sometimes); well-exposed stamens for easy dispersal; large, often feathery stigma to trap airborne pollen. Flowers are often inconspicuous, without nectar or fragrance. Single ovule in each ovary, numerous flowers packed into an inflorescence (e.g., grasses, maize, wheat).
    • Water (Hydrophily): Quite rare (~30 genera, mostly monocots).
      • Characteristics: Pollen grains often long, ribbon-like, protected from wetting by mucilaginous covering. Flowers small, inconspicuous. Nectar and odour absent.
      • Epihydrophily: Pollination on the surface of water (e.g., Vallisneria - female flower reaches surface by long stalk, male flowers/pollen released onto surface).
      • Hypohydrophily: Pollination beneath the water surface (e.g., Zostera - seagrass).
      • Note: Not all aquatic plants use water pollination (e.g., water hyacinth, water lily are insect/wind pollinated).
  • Biotic Agents: Majority of flowering plants use animals.
    • Insects (Entomophily): Most common biotic agent (bees, butterflies, flies, beetles, wasps, ants, moths).
      • Characteristics: Flowers are often large, colourful, fragrant, rich in nectar. Pollen grains often sticky or spiny. Stigma often sticky. Flowers provide floral rewards (nectar, pollen grains). Some offer safe places to lay eggs (e.g., Amorphophallus - tallest flower, Yucca-moth relationship).
    • Other Animals: Birds (Ornithophily - e.g., sunbirds, hummingbirds), Bats (Chiropterophily), reptiles (geckos, garden lizards), primates (lemurs), arboreal rodents.
• Outbreeding Devices: Mechanisms to discourage self-pollination and encourage cross-pollination.
  • Non-synchronization: Pollen release and stigma receptivity are not synchronized.
  • Different Positions: Anther and stigma placed at different positions so pollen cannot easily contact the stigma of the same flower.
  • Self-incompatibility: Genetic mechanism preventing self-pollen (from the same flower or other flowers of the same plant) from fertilizing the ovules by inhibiting pollen germination or pollen tube growth in the pistil.
  • Production of Unisexual Flowers (Dicliny):
    • Monoecious plants: Bear both male and female flowers on the same plant (e.g., Castor, Maize). Prevents autogamy but not geitonogamy.
    • Dioecious plants: Bear male and female flowers on different plants (e.g., Papaya, Date palm). Prevents both autogamy and geitonogamy.
• Pollen-Pistil Interaction:
  • Dynamic process involving pollen recognition followed by promotion or inhibition of the pollen.
  • Recognition of compatible pollen (right type) by the pistil. If compatible, the pistil accepts the pollen and promotes post-pollination events. If incompatible (wrong type), the pistil rejects the pollen.
  • Mediated by chemical components of the pollen and the pistil.
  • Events: Pollen germination on stigma -> Pollen tube growth through style tissues -> Pollen tube reaches ovary -> Enters ovule -> Enters embryo sac.
• Artificial Hybridization: Crop improvement technique. Desired pollen grains are used for pollination and fertilization.
  • Emasculation: Removal of anthers from the flower bud before the anther dehisces (if the female parent bears bisexual flowers).
  • Bagging: Covering the emasculated flower with a bag (usually butter paper) to prevent contamination of its stigma with unwanted pollen. When the stigma attains receptivity, mature pollen grains collected from the desired male parent are dusted on the stigma, and the flower is rebagged.

4. Double Fertilization

• Unique event in Angiosperms.
• After entering one of the synergids (guided by filiform apparatus), the pollen tube releases the two male gametes into the cytoplasm of the synergid.
• Syngamy: One male gamete (n) fuses with the egg cell nucleus (n) to form the zygote (2n).
• Triple Fusion: The other male gamete (n) moves towards the two polar nuclei located in the central cell and fuses with them to produce a triploid (3n) Primary Endosperm Nucleus (PEN).
• Since two types of fusions (syngamy and triple fusion) take place in the embryo sac, the phenomenon is termed Double Fertilization.
• The central cell after triple fusion becomes the Primary Endosperm Cell (PEC), which develops into the endosperm.
• The zygote develops into the embryo.

5. Post-fertilization: Structures and Events

• Following double fertilization, development of endosperm and embryo, maturation of ovule(s) into seed(s), and ovary into fruit occur collectively termed post-fertilization events.

A. Endosperm Development:

• Develops from the Primary Endosperm Cell (PEC). Usually precedes embryo development.
• Provides nourishment to the developing embryo.
• PEC undergoes repeated mitotic divisions to form triploid endosperm tissue.
• Types of Endosperm Development:
  • Nuclear Endosperm: PEN undergoes repeated nuclear divisions without cytokinesis (free-nuclear stage). Cell wall formation occurs later. (Most common type, e.g., Coconut water is free-nuclear endosperm, surrounding white kernel is cellular endosperm).
  • Cellular Endosperm: Each nuclear division is immediately followed by cytokinesis. (e.g., Petunia, Datura).
  • Helobial Endosperm: Intermediate type. First division is unequal, creating a large micropylar chamber and a small chalazal chamber. Further development is mostly free-nuclear in the micropylar chamber. (Common in monocots).
• Fate of Endosperm: May be completely consumed by the developing embryo (e.g., pea, groundnut, beans - non-albuminous/exalbuminous seeds) or it may persist in the mature seed (e.g., castor, coconut, wheat, maize - albuminous/endospermic seeds).

B. Embryo Development (Embryogeny):

• Develops from the zygote (2n) at the micropylar end of the embryo sac.
• Zygote divides only after some amount of endosperm is formed (to ensure nutrition).
• Early stages of embryo development (embryogeny) are similar in both monocots and dicots.
• Zygote -> Proembryo -> Globular -> Heart-shaped -> Mature embryo.
• Dicot Embryo:
  • Consists of an embryonal axis and two cotyledons.
  • Portion of embryonal axis above the level of cotyledons is the epicotyl, which terminates with the plumule (shoot tip).
  • Portion below the level of cotyledons is the hypocotyl, which terminates at its lower end in the radicle (root tip). The root tip is covered with a root cap.
• Monocot Embryo:
  • Possesses only one cotyledon, called the scutellum (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.
  • Portion of the axis above the level of attachment of scutellum is the epicotyl. Epicotyl has a shoot apex and a few leaf primordia enclosed in a hollow foliar structure, the coleoptile.

C. Seed Development:

• The fertilized ovule matures into a seed.
• Consists of:
  • Seed coat(s): Formed from integuments of the ovule. Outer testa (from outer integument) and inner tegmen (from inner integument). Micropyle persists as a small pore in the seed coat (facilitates entry of O2 and water during germination).
  • Cotyledon(s): Often thick and swollen due to storage of food reserves (as in legumes) or thin (as in cereals).
  • Embryonal axis.
• 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. Maturation dehydrates the seed (10-15% moisture by mass). Metabolic activity slows down. Favourable conditions (adequate moisture, oxygen, suitable temperature) are required for germination.
• Seed Viability: Ability of seeds to germinate. Varies greatly. Some lose viability within months, others remain viable for hundreds of years (e.g., Lupinus arcticus - 10,000 years; Date Palm - Phoenix dactylifera - 2000 years).

D. Fruit Development:

• The ovary matures/ripens into a fruit.
• Transformation of ovules into seeds and ovary into fruit proceeds simultaneously.
• The wall of the ovary develops into the wall of the fruit, called the pericarp.
• Types of Fruits:
  • True Fruits: Develop only from the ovary (e.g., Mango, Tomato).
  • False Fruits: Develop from the ovary along with other floral parts like thalamus (e.g., Apple, Strawberry, Cashew).
  • Parthenocarpic Fruits: Develop from the ovary without fertilization. These fruits are seedless (e.g., Banana). Can be induced by growth hormones.

6. Apomixis and Polyembryony

• Apomixis:
  • A form of asexual reproduction that mimics sexual reproduction by producing seeds without fertilization. (From Greek: apo = away from, mixis = mixing).
  • A way of producing clones through seeds.
  • Mechanisms: Diploid egg cell forms without meiosis and develops into embryo without fertilization; cells of nucellus or integuments (2n) surrounding embryo sac protrude into it and develop into embryos.
  • Importance: Preservation of desirable hybrid characters indefinitely (no segregation of characters in hybrid progeny). Production of hybrid seeds is costly, apomixis offers potential for large-scale production. (e.g., Asteraceae, Grasses).
• Polyembryony:
  • Occurrence of more than one embryo in a seed.
  • Can arise due to: fertilization of more than one egg cell, development of synergids or antipodals into embryos, cleavage of zygote/proembryo, development of nucellar/integumentary cells into embryos (adventive embryony - common in Citrus, Mango).

Multiple Choice Questions (MCQs)

1. The innermost layer of the anther wall, which provides nourishment to developing pollen grains, is the:
   a) Epidermis
   b) Endothecium
   c) Middle layers
   d) Tapetum

2. Sporopollenin, a highly resistant organic material, constitutes the:
   a) Intine of pollen grain
   b) Exine of pollen grain
   c) Generative cell wall
   d) Vegetative cell wall

3. A typical mature angiosperm embryo sac is:
   a) 8-nucleate, 8-celled
   b) 7-nucleate, 8-celled
   c) 8-nucleate, 7-celled
   d) 7-nucleate, 7-celled

4. Double fertilization involves the fusion of:
   a) Two male gametes with the egg cell
   b) One male gamete with the egg cell and the other with a synergid
   c) One male gamete with the egg cell and the other with the central cell (polar nuclei)
   d) Both male gametes with the polar nuclei

5. The primary endosperm nucleus (PEN) in angiosperms is typically:
   a) Haploid (n)
   b) Diploid (2n)
   c) Triploid (3n)
   d) Tetraploid (4n)

6. Which of the following adaptations is characteristic of wind-pollinated (anemophilous) flowers?
   a) Large, colourful petals
   b) Production of nectar
   c) Light, non-sticky pollen grains
   d) Sticky stigma

7. In artificial hybridization experiments, the removal of anthers from a bisexual flower bud is called:
   a) Bagging
   b) Emasculation
   c) Pollination
   d) Tagging

8. Fruits that develop from the ovary along with other floral parts like the thalamus are called:
   a) True fruits
   b) Parthenocarpic fruits
   c) Apomictic fruits
   d) False fruits

9. The phenomenon of formation of seeds without fertilization is known as:
   a) Polyembryony
   b) Parthenocarpy
   c) Apomixis
   d) Syngamy

10. In a monocot embryo, the radicle is enclosed within a protective sheath called the:
    a) Coleoptile
    b) Scutellum
    c) Coleorhiza
    d) Epiblast

Answer Key:

1. d) Tapetum
2. b) Exine of pollen grain
3. c) 8-nucleate, 7-celled
4. c) One male gamete with the egg cell and the other with the central cell (polar nuclei)
5. c) Triploid (3n)
6. c) Light, non-sticky pollen grains
7. b) Emasculation
8. d) False fruits
9. c) Apomixis
10. c) Coleorhiza

Study these notes thoroughly. Focus on the definitions, processes (like microsporogenesis, megasporogenesis, double fertilization), structures and their functions, and the examples provided. Good luck with your preparation!