Unit 1: Reproduction

Chapter 1.1: Sexual Reproduction in Flowering Plants

Flower Structure

• Definition: A flower is the reproductive organ of angiosperms (flowering plants), specialized for sexual reproduction.
• Parts of a Typical Flower:
  • Calyx: Outermost whorl, composed of sepals. Sepals are typically green, leaf-like structures that protect the flower in its bud stage.
  • Corolla: Whorl inside the calyx, composed of petals. Petals are often brightly colored and scented to attract pollinators.
  • Androecium: Male reproductive whorl, composed of stamens. Each stamen consists of a filament (stalk) and an anther (pollen-producing part).
  • Gynoecium (Pistil/Carpel): Female reproductive whorl, composed of one or more carpels. Each carpel typically has three parts:
    • Ovary: Swollen basal part containing ovules.
    • Style: Elongated tube connecting the ovary to the stigma.
    • Stigma: Receptive tip for pollen grains.
• Pedicel: The stalk of the flower.
• Thalamus (Receptacle): The swollen tip of the pedicel on which the floral parts are borne.

Male Gametophyte: Microsporangium Structure, Microsporogenesis, Pollen Grain Development

• Microsporangium (Pollen Sac) Structure:

  • Located within the anther, typically four microsporangia per anther lobe.
  • Wall Layers (from outermost to innermost):
    • Epidermis: Outermost protective layer.
    • Endothecium: Layer below epidermis, develops fibrous thickenings, helps in dehiscence of anther.
    • Middle Layers: 2-3 layers, ephemeral, provide nourishment.
    • Tapetum: Innermost nutritive layer, surrounds the sporogenous tissue. Provides nourishment to developing pollen grains. Cells are dense with cytoplasm and often have more than one nucleus.
  • Sporogenous Tissue: Homogenous mass of cells occupying the center of each microsporangium, which will undergo meiosis to form microspores.

• Microsporogenesis:

  • Definition: The process of formation of microspores (pollen mother cells) from a microspore mother cell (MMC) through meiosis.
  • Process:
    1. Cells of the sporogenous tissue differentiate into microspore mother cells (MMCs) or pollen mother cells (PMCs), which are diploid (2n).
    2. Each MMC undergoes meiosis (reductional division).
    3. Meiosis results in the formation of a tetrad of haploid microspores (n).
    4. As the anther matures, the microspores dissociate from the tetrad and develop into pollen grains.

• Pollen Grain Development:

  • Pollen Grain (Microspore): Represents the male gametophyte.
  • Structure:
    • Exine: Outer, hard, protective layer made of sporopollenin (one of the most resistant organic materials known, enabling pollen to be preserved as fossils). It has prominent apertures called germ pores where sporopollenin is absent.
    • Intine: Inner, thin, continuous layer made of pectocellulose.
    • Cytoplasm: Surrounded by a plasma membrane.
    • Nucleus: Initially a single nucleus.
  • Development (Mitotic Divisions):
    1. The nucleus of the microspore divides mitotically to form two cells: a larger vegetative cell and a smaller generative cell.
    2. The vegetative cell (tube cell) is larger, has abundant food reserve, and an irregularly shaped nucleus. It is responsible for forming the pollen tube.
    3. The generative cell is small, floats in the cytoplasm of the vegetative cell, and has a dense cytoplasm and nucleus. It divides mitotically to form two male gametes (sperms).
    4. In over 60% of angiosperms, pollen grains are shed at this 2-celled stage (vegetative and generative cell).
    5. In the remaining species, the generative cell divides further to form two male gametes before shedding, thus shedding at the 3-celled stage.

Female Gametophyte: Structure of Ovule (Anatropous), Megasporogenesis, Embryo Sac Development

• Ovule (Megasporangium) Structure (Anatropous):

  • Definition: The ovule is the integumented megasporangium, which after fertilization develops into a seed.
  • Anatropous Ovule: The most common type of ovule, where the body of the ovule is inverted, bringing the micropyle close to the funicle.
  • Parts:
    • Funicle (Funiculus): The stalk that attaches the ovule to the placenta.
    • Hilum: The point of attachment of the funicle to the body of the ovule.
    • Raphe: The ridge formed by the fusion of the funicle with the body of the ovule.
    • Integuments: Protective envelopes surrounding the nucellus, leaving a small opening at the apex called the micropyle.
    • Micropyle: A small opening at the apex of the integuments, through which the pollen tube enters the ovule.
    • Chalaza: The basal part of the ovule, opposite to the micropyle, representing the basal part of the nucellus.
    • Nucellus: The central mass of parenchymatous tissue forming the body of the ovule. It contains the embryo sac and provides nourishment.
    • Embryo Sac (Female Gametophyte): Located within the nucellus, typically formed from a single megaspore.

• Megasporogenesis:

  • Definition: The process of formation of megaspores from a megaspore mother cell (MMC) through meiosis.
  • Process:
    1. A single cell in the micropylar region of the nucellus differentiates into a large megaspore mother cell (MMC), which is diploid (2n).
    2. The MMC undergoes meiosis I, forming two haploid cells.
    3. These two cells undergo meiosis II, resulting in a linear tetrad of four haploid megaspores.
    4. In most flowering plants (e.g., Polygonum type), three of the four megaspores degenerate (usually the ones towards the micropylar end), and only one functional megaspore (usually the chalazal one) remains.

• Embryo Sac Development (Monosporic Development - Polygonum Type):

  • Definition: The development of the female gametophyte (embryo sac) from a single functional megaspore.
  • Process:
    1. The nucleus of the functional megaspore undergoes three successive free nuclear mitotic divisions.
    2. First mitosis: Forms two nuclei, which move to opposite poles, forming a 2-nucleate embryo sac.
    3. Second mitosis: Each of the two nuclei divides, forming a 4-nucleate embryo sac.
    4. Third mitosis: Each of the four nuclei divides, forming an 8-nucleate embryo sac. These divisions are free nuclear, meaning cell walls are not formed immediately after nuclear division.
    5. After the 8-nucleate stage, cell walls are laid down, leading to the organization of the typical 7-celled, 8-nucleate embryo sac.
  • Organization of Embryo Sac:
    • At the micropylar end: The egg apparatus consists of one large egg cell (female gamete) and two synergids. Synergids have special cellular thickenings at the micropylar tip called filiform apparatus, which guide the pollen tube into the synergid.
    • At the chalazal end: Three antipodal cells are present. Their function is generally nutritive, but they often degenerate before or after fertilization.
    • In the center: A large central cell is present, containing two polar nuclei (which later fuse to form a diploid secondary nucleus or definitive nucleus before fertilization).

Pollination: Types, Agents, Adaptations

• Definition: The transfer of pollen grains from the anther to the stigma of a flower.

• Types of Pollination:

  • Autogamy (Self-pollination): Pollen grains are transferred from the anther to the stigma of the same flower.
    • Cleistogamous flowers: Flowers that do not open at all, ensuring self-pollination (e.g., Viola, Oxalis, Commelina). They produce assured seed-set even in the absence of pollinators.
    • Chasmogamous flowers: Flowers with exposed anthers and stigma, similar to flowers of other species.
  • Geitonogamy: Pollen grains are transferred from the anther of one flower to the stigma of another flower on the same plant.
    • Genetically similar to autogamy (as it comes from the same plant), but ecologically it is cross-pollination as it involves a pollinating agent.
  • Xenogamy (Cross-pollination): Pollen grains are transferred from the anther of one flower to the stigma of a flower on a different plant of the same species.
    • Genetically and ecologically true cross-pollination, leading to genetic variation.

• Agents of Pollination:

  • Abiotic Agents (Non-living):
    • Wind (Anemophily):
      • Adaptations: Flowers are small, inconspicuous, not brightly colored, lack nectar and fragrance. Produce large amounts of light, non-sticky pollen. Well-exposed stamens and large, feathery stigmas to trap airborne pollen (e.g., grasses, corn).
    • Water (Hydrophily):
      • Adaptations: Rare, limited to about 30 genera, mostly monocots. Pollen grains are protected from wetting by a mucilaginous covering. Flowers are small, inconspicuous. (e.g., Vallisneria, Hydrilla - surface pollination; Zostera - submerged pollination).
  • Biotic Agents (Living):
    • Insects (Entomophily): Most common biotic agents (bees, butterflies, moths, beetles, flies, ants).
      • Adaptations: Flowers are large, brightly colored, fragrant, and produce nectar. Pollen grains are sticky or spiny to adhere to insect bodies. Some flowers provide safe places for egg-laying (e.g., Amorphophallus).
    • Animals (Zoophily): Birds (ornithophily), bats (chiropterophily), primates, arboreal rodents, reptiles.
      • Adaptations: Vary depending on the animal. Bird-pollinated flowers are often red, tubular, and produce abundant nectar. Bat-pollinated flowers are typically dull-colored, large, and open at night, producing strong fermentation-like odors.

Outbreeding Devices, Pollen-Pistil Interaction, Artificial Hybridization

• Outbreeding Devices (Contrivances for Cross-Pollination): Mechanisms evolved by plants to discourage self-pollination and encourage cross-pollination, promoting genetic diversity.

  • Dichogamy: Anthers and stigma mature at different times.
    • Protandry: Anthers mature earlier than the stigma (e.g., sunflower, cotton).
    • Protogyny: Stigma matures earlier than the anthers (e.g., Ficus, Aristolochia).
  • Herkogamy: Physical barrier between anther and stigma, preventing self-pollination (e.g., Gloriosa - stigma and anthers are at different positions).
  • Heterostyly: Flowers have different lengths of styles and stamens (e.g., Primula).
  • Self-incompatibility (Self-sterility): A genetic mechanism that prevents self-pollen (from the same plant or genetically similar plants) from germinating on the stigma or growing through the style. It is a physiological barrier (e.g., Nicotiana, Brassica).
  • Unisexuality (Dioecy/Monoecy):
    • Monoecious plants: Both male and female flowers are present on the same plant (e.g., castor, maize). Prevents autogamy but not geitonogamy.
    • Dioecious plants: Male and female flowers are on different plants (e.g., papaya, date palm). Prevents both autogamy and geitonogamy, ensuring only xenogamy.

• Pollen-Pistil Interaction:

  • Definition: The entire sequence of events from pollen deposition on the stigma until the entry of the pollen tube into the ovule.
  • Process:
    1. Recognition: The pistil has the ability to recognize compatible (right type) pollen from incompatible (wrong type) pollen. This recognition is mediated by chemical components exchanged between the pollen and the stigma.
    2. Germination: If the pollen is compatible, it absorbs moisture and nutrients from the stigma and germinates, producing a pollen tube through one of the germ pores.
    3. Pollen Tube Growth: The pollen tube grows through the tissues of the stigma and style, guided by chemical signals (e.g., calcium-boron-inositol complex) from the ovule.
    4. Entry into Ovule: The pollen tube typically enters the ovule through the micropyle (porogamy). In some cases, it may enter through the chalaza (chalazogamy) or through the integuments (mesogamy).
    5. Entry into Embryo Sac: After entering the ovule, the pollen tube usually enters one of the synergids through the filiform apparatus. The filiform apparatus guides the pollen tube and facilitates the release of male gametes.
  • Significance: Ensures that only compatible pollen leads to fertilization, preventing interspecific crosses and promoting species integrity.

• Artificial Hybridization:

  • Definition: A crop improvement program where desired pollen grains are used for pollination to obtain superior varieties.
  • Steps:
    1. Emasculation: The process of removing the anthers from a bisexual flower bud before they dehisce (release pollen). This is done to prevent self-pollination in bisexual flowers.
    2. Bagging: The emasculated flowers are immediately covered with a butter paper bag to prevent contamination of the stigma with unwanted pollen. When the stigma becomes receptive, desired pollen is dusted on it.
    3. Rebagging: The flower is rebagged after pollination until the fruit develops.
  • For unisexual female flowers: Emasculation is not required. The female flower buds are bagged before they open, and when the stigma becomes receptive, desired pollen is dusted on it, and the flower is rebagged.

Double Fertilization: Triple Fusion, Significance

• Definition: A unique event in flowering plants where both the male gametes released by the pollen tube participate in fertilization.
• Process:
  1. After the pollen tube enters the synergid, its tip ruptures, releasing the two male gametes into the cytoplasm of the synergid.
  2. Syngamy (Generative Fertilization): One male gamete fuses with the egg cell to form a diploid zygote (2n). This fusion is also called true fertilization.
  3. Triple Fusion (Vegetative Fertilization): The other male gamete fuses with the two polar nuclei (or the already fused diploid secondary nucleus) in the central cell to form a triploid primary endosperm nucleus (PEN) (3n).
• Significance:
  • Ensures the formation of a diploid zygote, which develops into the embryo.
  • Ensures the formation of a triploid endosperm, which provides nourishment to the developing embryo. This prevents wastage of energy and resources on forming nutritive tissue if fertilization does not occur.
  • It is a characteristic feature of angiosperms.

Post-Fertilization Events: Endosperm, Embryo Development, Seed Formation, Fruit Types, Albuminous vs. Non-Albuminous Seeds

• Post-Fertilization Events: Changes in the flower after double fertilization, leading to the formation of seed and fruit.

  • Ovary develops into the fruit.
  • Ovules develop into seeds.
  • Zygote develops into the embryo.
  • Primary Endosperm Nucleus (PEN) develops into the endosperm.

• Endosperm:

  • Definition: The nutritive tissue formed from the primary endosperm nucleus (PEN) after triple fusion. It provides nourishment to the developing embryo.
  • Types of Endosperm Development:
    • Nuclear Endosperm: The PEN undergoes successive free nuclear divisions without immediate cell wall formation, forming a large number of free nuclei. Later, cell walls may form (e.g., coconut water is free nuclear endosperm, while the white kernel is cellular endosperm; wheat, rice).
    • Cellular Endosperm: The PEN undergoes divisions immediately followed by cell wall formation, leading to the formation of cellular tissue from the beginning (e.g., Datura, Petunia).
    • Helobial Endosperm: Intermediate type, where the first division of PEN is followed by cell wall formation, but subsequent divisions in the two chambers may be free nuclear (e.g., monocots like Asphodelus).

• Embryo Development (Embryogeny):

  • Definition: The process of development of the embryo from the zygote.

  • Process: The zygote divides mitotically to form the embryo. The development of the embryo occurs at the micropylar end of the embryo sac where the zygote is situated.

  • Dicot Embryo Development:

    1. The zygote first divides transversely to form a larger suspensor cell (towards the micropyle) and a smaller embryonal cell (towards the chalaza).
    2. The suspensor cell divides to form a filamentous suspensor, which pushes the embryo into the endosperm for nourishment. The first cell of the suspensor towards the micropyle is the haustorium, and the last cell towards the embryo is the hypophysis.
    3. The embryonal cell divides to form a proembryo, then a globular embryo, followed by a heart-shaped embryo, and finally a mature embryo.
    4. Mature Dicot Embryo: Consists of an embryonal axis and two cotyledons.
       • Embryonal Axis: Has a plumule (future shoot) at one end and a radicle (future root) at the other end.
       • Hypocotyl: The cylindrical portion below the level of cotyledons that terminates at the radicle.
       • Epicotyl: The cylindrical portion above the level of cotyledons that terminates at the plumule.

  • Monocot Embryo Development:

    1. The zygote divides to form a large scutellum (single cotyledon) and a smaller portion that forms the plumule and radicle.
    2. Mature Monocot Embryo (e.g., Grass embryo):
       • Scutellum: A single, large, shield-shaped cotyledon situated towards one side of the embryonal axis.
       • Embryonal Axis: Has a plumule and a radicle.
       • Coleoptile: A protective sheath enclosing the plumule.
       • Coleorhiza: An undifferentiated sheath enclosing the radicle and root cap.

• Seed Formation:

  • Definition: The final product of sexual reproduction in angiosperms, consisting of an embryo, stored food (endosperm/cotyledons), and a protective seed coat.
  • Parts of a Seed:
    • Seed Coat: Developed from the integuments of the ovule. The outer integument forms the testa (outer seed coat), and the inner integument forms the tegmen (inner seed coat).
    • Hilum: A scar on the seed coat where the ovule was attached to the funicle.
    • Micropyle: A small pore on the seed coat, above the hilum, which facilitates the entry of oxygen and water during germination.
    • Embryo: Consists of an embryonal axis and cotyledon(s).

• Fruit Types:

  • True Fruit: Develops only from the ovary of the flower (e.g., mango, tomato, pea).
  • False Fruit (Accessory Fruit): Develops from the ovary along with other floral parts like the thalamus, calyx, or receptacle (e.g., apple, strawberry, cashew, pear).

• Albuminous vs. Non-Albuminous Seeds:

  • Albuminous (Endospermic) Seeds: Retain a part of the endosperm as nutritive tissue in the mature seed. The endosperm is not completely consumed during embryo development (e.g., wheat, maize, barley, castor, coconut).
  • Non-Albuminous (Ex-albuminous) Seeds: Have no residual endosperm in the mature seed. The endosperm is completely consumed by the developing embryo, and food is stored in the cotyledons (e.g., pea, groundnut, beans).

Special Modes: Apomixis, Parthenocarpy, Polyembryony

• Apomixis:

  • Definition: A form of asexual reproduction that mimics sexual reproduction, where seeds are formed without fertilization.
  • Types:
    • Recurrent Apomixis: The embryo sac develops from a diploid cell (either nucellar cell or megaspore mother cell) without meiosis, resulting in a diploid embryo (e.g., Citrus, mango, Asteraceae).
    • Non-recurrent Apomixis: The embryo develops directly from a haploid egg cell without fertilization (parthenogenesis), but the resulting embryo is haploid and usually sterile.
  • Significance: Important in hybrid seed production, as it ensures the perpetuation of hybrid vigor without segregation in subsequent generations.

• Parthenocarpy:

  • Definition: The development of fruit without fertilization.
  • Characteristics: Parthenocarpic fruits are typically seedless (e.g., banana, grapes, pineapple).
  • Induction: Can be naturally occurring or induced artificially by the application of growth hormones (e.g., auxins).
  • Significance: Commercially important for producing seedless fruits.

• Polyembryony:

  • Definition: The occurrence of more than one embryo in a single seed.
  • Causes:
    • Development of multiple embryo sacs within the ovule.
    • Development of embryos from cells other than the egg cell (e.g., synergids, antipodals, nucellus, integuments).
    • Cleavage of the proembryo into multiple embryos.
  • Examples: Citrus, mango, onion.
  • Significance: In Citrus, nucellar embryos are genetically identical to the parent plant, providing a source of uniform seedlings for horticulture.

Significance of Seed and Fruit Dispersal

• Seed Dispersal: The movement or transport of seeds away from the parent plant.

  • Significance:
    • Prevents overcrowding: Reduces competition for resources (light, water, nutrients) among seedlings and between seedlings and the parent plant.
    • Colonization of new areas: Helps in the spread of the species to new habitats, increasing its geographical distribution.
    • Avoidance of pathogens/predators: Reduces the chances of all seeds being consumed by localized predators or infected by pathogens.
    • Genetic diversity: Promotes outcrossing and genetic mixing if seeds are dispersed to areas where they can cross-pollinate with other populations.

• Fruit Dispersal: The movement or transport of fruits (which contain seeds) away from the parent plant.

  • Significance: Similar to seed dispersal, as fruits often aid in seed dispersal.
    • Protection of seeds: Fruits protect the developing seeds from adverse environmental conditions and predators.
    • Attraction of dispersal agents: Fleshy, colorful, and fragrant fruits attract animals, which consume the fruit and disperse the seeds (endozoochory).

• Agents of Dispersal:

  • Wind (Anemochory): Light seeds with wings (e.g., Moringa, Dipterocarpus) or hairy appendages (e.g., Calotropis, dandelion).
  • Water (Hydrochory): Seeds/fruits with fibrous or spongy outer coverings that allow them to float (e.g., coconut).
  • Animals (Zoochory):
    • Epizoochory: Seeds/fruits with hooks, spines, or sticky surfaces that attach to animal fur/feathers (e.g., Xanthium, Urena).
    • Endozoochory: Fleshy fruits eaten by animals, and seeds are dispersed through their faeces (e.g., berries, tomatoes).
  • Explosive Mechanism (Autochory): Fruits burst open forcefully, scattering seeds (e.g., pea, castor, balsam).
  • Gravity: Heavy fruits fall directly to the ground (e.g., apple, mango).