Class 12 Notes

Class 12 Biology Notes Chapter 2 (Secual Reproduction in Flowering Plants) – Examplar Problems Book

Philoid

Philoid

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Examplar Problems
Alright, let's begin our detailed study of Chapter 2: Sexual Reproduction in Flowering Plants. This is a crucial chapter for your exams, focusing on the intricate processes angiosperms use to reproduce. We'll cover the structures involved, the key events, and related concepts.

Chapter 2: Sexual Reproduction in Flowering Plants - Detailed Notes

1. Introduction: The Flower - Site of Sexual Reproduction

  • Flowers are modified shoots specifically adapted for sexual reproduction in angiosperms.
  • A typical flower has four whorls arranged on a swollen end of the stalk (pedicel) called the thalamus or receptacle:
    • Calyx: Outermost whorl, composed of sepals (usually green, protective in bud stage).
    • Corolla: Composed of petals (often brightly coloured to attract pollinators).
    • Androecium: Male reproductive whorl, composed of stamens.
    • Gynoecium (or Pistil): Female reproductive whorl, composed of one or more carpels.

2. Pre-fertilisation: Structures and Events

  • Involve the development of male and female gametes and pollination.

    A. Stamen, Microsporangium and Pollen Grain (Male)

    • Stamen: Consists of a long, slender filament and a terminal, typically bilobed structure called the anther.
    • Anther Structure:
      • Typically bilobed, with each lobe having two thecae (dithecous).
      • Each anther is tetrasporangiate, containing four microsporangia located at the corners, two in each lobe.
      • These microsporangia develop further and become pollen sacs, which contain pollen grains.
    • Microsporangium Structure:
      • Surrounded by four wall layers:
        • Epidermis: Outermost protective layer.
        • Endothecium: Layer below the epidermis; cells have fibrous thickenings that help in anther dehiscence.
        • Middle Layers: 2-3 layers below the endothecium, degenerate at maturity.
        • Tapetum: Innermost nutritive layer. Cells are dense with cytoplasm and generally have more than one nucleus (polyploid). It nourishes the developing pollen grains.
      • Centre contains sporogenous tissue.
    • Microsporogenesis:
      • The process of formation of microspores from a Pollen Mother Cell (PMC) or microspore mother cell (diploid, 2n) through meiosis.
      • Cells of the sporogenous tissue differentiate into PMCs.
      • Each PMC undergoes meiosis to form a cluster of four haploid (n) cells called a microspore tetrad.
      • As the anther matures and dehydrates, the microspores dissociate from the tetrad and develop into pollen grains.
    • Pollen Grain (Male Gametophyte):
      • Represents the male gametophyte. Generally spherical.
      • 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 that degrades sporopollenin is known. This helps in fossilization. Exine has prominent apertures called germ pores where sporopollenin is absent; these facilitate pollen tube emergence.
        • Intine: Thin, continuous inner wall made of cellulose and pectin.
      • The cytoplasm of a mature pollen grain contains two cells:
        • Vegetative Cell (or Tube Cell): Larger, has abundant food reserves and a large, irregularly shaped nucleus. Responsible for pollen tube growth.
        • Generative Cell: Smaller, spindle-shaped with dense cytoplasm and a nucleus. Floats in the cytoplasm of the vegetative cell. It divides mitotically to give rise to the two male gametes either before pollen shedding (in ~60% of angiosperms, 2-celled stage) or after pollination in the pollen tube (in ~40% angiosperms, 3-celled stage).
    • Pollen Viability: Period for which pollen grains retain the ability to germinate. Varies greatly (e.g., 30 minutes in rice/wheat, several months in Rosaceae, Leguminosae, Solanaceae). Can be stored for years in liquid nitrogen (-196°C) for crop breeding programs (pollen banks).
    • Pollen Allergy & Products: Pollen grains of some species (e.g., Parthenium or carrot grass) cause severe allergies and bronchial afflictions. Pollen grains are rich in nutrients and used as food supplements (pollen tablets/syrups).

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

    • Gynoecium (Pistil): Female reproductive part. May consist of a single carpel (monocarpellary) or more than one carpel (multicarpellary).
      • If multicarpellary, carpels may be fused (syncarpous) or free (apocarpous).
    • Parts of a Pistil:
      • Stigma: Terminal receptive part for pollen grains.
      • Style: Elongated slender part beneath the stigma.
      • 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 ovule except at the tip where a small opening called the micropyle is present.
      • Opposite the micropylar end is the chalaza, representing the basal part.
      • Enclosed within the integuments is a mass of cells called the nucellus (diploid, 2n), which has abundant reserve food materials. The embryo sac is located within the nucellus.
    • Megasporogenesis:
      • The process of formation of megaspores from the Megaspore Mother Cell (MMC) (diploid, 2n).
      • Generally, a single MMC 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 megaspore (usually the chalazal one) remains functional while the other three degenerate. This is monosporic development.
      • The nucleus of the functional megaspore divides mitotically three times to form 8 nuclei without immediate cell wall formation.
      • These 8 nuclei arrange themselves within the megaspore cytoplasm.
      • Cell walls are then laid down, resulting in a typical female gametophyte or embryo sac, which is 7-celled and 8-nucleate at maturity.
      • Structure of Embryo Sac:
        • Egg Apparatus (at micropylar end): Consists of one egg cell (n) and two synergids (n). Synergids have special cellular thickenings at the micropylar tip called the filiform apparatus, which guides the pollen tube into the synergid.
        • Antipodal Cells (at chalazal end): Three cells (n). Their function is uncertain, often degenerate after fertilization.
        • Central Cell: Large central cell containing two polar nuclei (n + n). Prior to fertilization, these may fuse to form a diploid secondary nucleus (2n).

3. Pollination

  • The transfer of pollen grains from the anther to the stigma of a pistil.
  • Types:
    • Autogamy: 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.
      • Cleistogamous flowers: Flowers which do not open at all (e.g., Oxalis, Viola, Commelina). Assured seed-set even without pollinators. Disadvantage: leads to inbreeding depression.
    • Geitonogamy: Transfer of pollen from the anther of one flower to the stigma of another flower on the same plant. Functionally cross-pollination (involves pollinating agent), but genetically similar to autogamy (pollen comes from the same plant).
    • Xenogamy (Cross-pollination): Transfer of pollen from the anther of one flower to the stigma of a flower on a different plant of the same species. Genetically different pollen types are brought to the stigma. Promotes genetic variation.
  • Agents of Pollination:
    • Abiotic: Wind and Water.
      • Wind Pollination (Anemophily): Pollen grains are light, non-sticky, produced in large numbers. Well-exposed stamens, large feathery stigma. Common in grasses, maize.
      • Water Pollination (Hydrophily): Rare (about 30 genera, mostly monocots). E.g., Vallisneria, Hydrilla (freshwater), Zostera (seagrass). Pollen grains often long, ribbon-like, protected by mucilaginous covering. In Vallisneria, female flower reaches the surface, male pollen released onto the surface. In seagrasses, pollen released inside water. Not all aquatic plants are water-pollinated (e.g., water hyacinth, water lily are insect/wind pollinated).
    • Biotic: Animals (Insects, Birds, Bats, Snails, etc.).
      • Insect Pollination (Entomophily): Majority of flowering plants. Flowers are large, colourful, fragrant, rich in nectar. Pollen grains often sticky or spiny. Plants provide floral rewards (nectar, pollen). Some show specific adaptations (e.g., Yucca and moth, Ophrys and bee - pseudo-copulation).
  • Outbreeding Devices: Mechanisms to discourage self-pollination and encourage cross-pollination:
    • Non-synchronization of pollen release and stigma receptivity.
    • Different positioning of anther and stigma.
    • Self-incompatibility: Genetic mechanism preventing self-pollen (from the same flower or another flower of the same plant) from fertilizing the ovules by inhibiting pollen germination or pollen tube growth.
    • 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.

4. Pollen-Pistil Interaction

  • All events from pollen deposition on the stigma until the pollen tube enters the ovule.
  • Involves recognition of compatible pollen by the pistil (a chemical dialogue).
  • If compatible, pollen germinates on the stigma to produce a pollen tube through one of the germ pores.
  • The contents of the pollen grain (vegetative nucleus, two male gametes) move into the pollen tube.
  • Pollen tube grows through the tissues of the stigma and style and reaches the ovary.
  • Enters the ovule usually through the micropyle (porogamy), guided by the filiform apparatus of the synergids. Enters one of the synergids.

5. Double Fertilisation

  • A unique event characteristic of angiosperms. Discovered by Nawaschin.
  • After entering one of the synergids, the pollen tube releases the two male gametes into the cytoplasm of the synergid.
  • Syngamy: One male gamete (n) fuses with the egg cell (n) to form the zygote (2n).
  • Triple Fusion: The other male gamete (n) fuses with the two polar nuclei (or the secondary nucleus, 2n) located in the central cell to produce the Primary Endosperm Nucleus (PEN) (triploid, 3n).
  • Since two types of fusion (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.

6. Post-fertilisation: Structures and Events

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

    A. Endosperm Development:

    • PEC divides repeatedly (usually mitosis) to form the triploid endosperm tissue.
    • Provides nourishment to the developing embryo.
    • Types of Development:
      • Nuclear: PEN undergoes repeated nuclear divisions without cytokinesis, forming free nuclei. Later, cell walls develop (e.g., coconut water is free-nuclear endosperm, surrounding white kernel is cellular endosperm). Most common type.
      • Cellular: Each nuclear division is followed by cytokinesis (e.g., Petunia).
      • Helobial: Intermediate type.
    • Endosperm may be completely consumed by the developing embryo before seed maturation (e.g., pea, bean, groundnut - non-albuminous/exalbuminous seeds) or it may persist in the mature seed (e.g., castor, coconut, wheat, maize - albuminous seeds).

    B. Embryo Development (Embryogeny):

    • Zygote (2n) develops into the embryo (usually at the micropylar end).
    • Most zygotes divide only after some amount of endosperm is formed (to ensure nutrition).
    • Early stages of development (embryogeny) are similar in monocots and dicots.
    • Zygote -> Proembryo -> Globular -> Heart-shaped -> Mature Embryo.
    • Dicot Embryo: Consists of an embryonal axis and two cotyledons.
      • Portion of axis above cotyledons: epicotyl, terminates in plumule (shoot tip).
      • Portion below cotyledons: hypocotyl, terminates in radicle (root tip), covered by a root cap.
    • Monocot Embryo: Possesses only one cotyledon (called scutellum in grasses, located laterally).
      • Radicle and root cap enclosed in an undifferentiated sheath called coleorhiza.
      • Epicotyl has a shoot apex and 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): Develop from the integuments of the ovule (outer testa, inner tegmen). Micropyle persists as a small pore facilitating entry of oxygen and water during germination.
      • Cotyledon(s): Embryonic leaves, often thick and swollen due to food storage (as in legumes) or thin (as in cereals).
      • Embryonal axis.
    • Seed Dormancy: A state of inactivity, allows seeds to survive unfavourable conditions. Can be overcome by favourable conditions (moisture, oxygen, temperature) or specific treatments.
    • Seed Viability: Ability of a seed to germinate. Varies greatly (few months to many years). Oldest viable seed: Lupinus arcticus (10,000 years). Recent record: Date palm (Phoenix dactylifera) - 2000 years old.

    D. Fruit Development:

    • The ovary matures into the fruit after fertilisation.
    • The transformation of ovules into seeds and ovary into fruit proceeds simultaneously.
    • The wall of the ovary develops into the pericarp (fruit wall), which can be fleshy (guava, mango) or dry (mustard, groundnut).
    • True Fruits: Develop only from the ovary (e.g., mango, tomato).
    • False Fruits: Develop from other floral parts (like thalamus) along with the ovary (e.g., apple, strawberry, cashew - thalamus contributes to fruit formation).
    • Parthenocarpic Fruits: Fruits that develop without fertilisation (hence, seedless). Can be induced artificially (e.g., bananas, grapes).

7. Apomixis and Polyembryony

  • Apomixis: A form of asexual reproduction that mimics sexual reproduction; production of seeds without fertilisation (e.g., some species of Asteraceae, grasses). Apomictic seeds are genetically identical to the parent plant. It's a way of producing clones through seeds. Importance: Preservation of desirable hybrid characters indefinitely.
  • Polyembryony: Occurrence of more than one embryo in a seed (e.g., citrus, mango). Can arise due to fertilisation of more than one egg, development of synergids or nucellar/integumentary cells into embryos, or cleavage of zygote.

Multiple Choice Questions (MCQs)

  1. The nutritive tissue nourishing the developing microspores in an anther is the:
    a) Endothecium
    b) Epidermis
    c) Tapetum
    d) Middle layers

  2. 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

  3. Double fertilisation involves the fusion of:
    a) One male gamete with egg and the other with a synergid
    b) One male gamete with egg and the other with the central cell
    c) Both male gametes with the egg cell
    d) One male gamete with a synergid and the other with an antipodal cell

  4. The ploidy level of the Primary Endosperm Nucleus (PEN) in angiosperms is typically:
    a) Haploid (n)
    b) Diploid (2n)
    c) Triploid (3n)
    d) Tetraploid (4n)

  5. Which of the following structures develops into the seed coat?
    a) Ovary wall
    b) Nucellus
    c) Integuments
    d) Chalaza

  6. Geitonogamy involves:
    a) Fertilisation of a flower by the pollen from another flower of the same plant.
    b) Fertilisation of a flower by the pollen from the same flower.
    c) Fertilisation of a flower by the pollen from a flower of another plant of the same species.
    d) Fertilisation of a flower by the pollen from a flower of another plant of a different species.

  7. Sporopollenin, a highly resistant organic material, is found in the:
    a) Intine of pollen grain
    b) Exine of pollen grain
    c) Filiform apparatus
    d) Endothecium of anther wall

  8. In albuminous seeds, the food is stored primarily in the:
    a) Cotyledons
    b) Endosperm
    c) Perisperm
    d) Embryo axis

  9. Which of the following is an example of a false fruit?
    a) Mango
    b) Tomato
    c) Apple
    d) Brinjal

  10. Apomixis is a type of:
    a) Sexual reproduction involving fertilisation
    b) Asexual reproduction mimicking sexual reproduction (seed formation without fertilisation)
    c) Vegetative propagation
    d) Parthenocarpy

Answer Key:

  1. (c)
  2. (c)
  3. (b)
  4. (c)
  5. (c)
  6. (a)
  7. (b)
  8. (b)
  9. (c)
  10. (b)

Make sure you understand the sequence of events, the ploidy levels of different structures (PMC, microspore, pollen grain cells, MMC, megaspore, embryo sac cells, zygote, PEN, endosperm, embryo), and the specific terminology. This chapter forms the basis for understanding plant breeding techniques as well. Good luck with your preparation!

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