Topics to be Learn:

  • Reproduction
  • Asexual reproduction
  • Sexual reproduction
  • Microsporogenesis
  • Structure of anatropous ovule
  • Megasporogenesis
  • Pollination
  • Outbreeding devices — Contrivances
  • Pollen pistil interaction
  • Double fertilization
  • Development of endosperm
  • Development of embryo
  • Seed and fruit development
  • Apomixis
  • Parthenocarpy
Reproduction
  • Production of young ones like parents.
  • Essential process for continuity of species.
  • Offspring show similar characters to parents.

Types of Reproduction

  1. Asexual reproduction:

    • Does not involve fusion of two compatible gametes.
    • Produces genetically identical progeny, i.e., clones.
    • Progeny from single organism.
    • Inheritance of genes of parent by progeny.

Modes of Asexual Reproduction

  1. Fragmentation:

    • Occurs in lower plants, e.g., algae.
    • Multicellular organisms break into small pieces called fragments.
    • Fragments develop into new plants.
    • Reasons: accidental breakdown, death and decay of cells, etc.

  2. Budding:

    • Common in unicellular organism yeast.
    • Occurs in favourable conditions.
    • Mother cell produces small outgrowth known as bud.
    • Buds may be one or more and grow as new individuals on separation.
     

  3. Spore formation:

    • Common in lower plants.
    • Occurs by production of motile zoospores.
    • Zoospores grow independently into new individuals.
    • Biflagellate zoospores formed in algae Chlamydomonas.
     

  4. Fission:

    • Binary Fission.
    • Examples: Amoeba, Paramoecium.

  5. Conidia formation:

    • Example: Fungus Penicillium.

  6. Gemmule formation:

    • Example: Sponges.

    Vegetative Reproduction

  7. Definition:

    • Plants reproduce asexually from their vegetative plant parts.
    • New plants formed are genetically similar to parents.

  8. Usefulness:

    • Very useful in agriculture and horticulture.
    • Artificial methods like cutting and grafting propagate desired varieties.

Artificial Methods of Vegetative Reproduction

  1. Cutting:

    • Small pieces of plant parts with one or more buds selected for propagation.
    • Types:
      • Stem cutting (e.g., Rose, Bougainvillea)
      • Leaf cutting (e.g., Sansevieria)
      • Root cutting (e.g., Blackberry).

  2. Grafting:

    • Joining of two plant parts: stock and scion.
    • Stock: Rooted plant.
    • Scion: Joined plant.
    • Grow together as one plant.
    • Types:
      • Stem grafting (e.g., Apple)
      • Bud grafting (e.g., Rose).

  3. Tissue Culture:

    • A small amount of plant tissue grown aseptically to get plantlets.
    • Micropropagation: Modern method for plants from tissue culture (e.g., Orchids).

    Sexual Reproduction

  4. Definition:

    • Involves fusion of two compatible gametes.
    • Results in production of genetically dissimilar offspring.

  5. Variations:

    • Set in, important for survival and evolution of species.
    • Occurs after certain maturity.
    • Higher plants: Flowering indicates beginning of reproductive phase.

Flowers

  • Definition:
    • Specialized reproductive structure producing haploid gametes.
    • Ensures fertilization.
     

  1. Accessory Whorls:

    • Calyx: Sepals.
    • Corolla: Petals.

  2. Essential Whorls:

    • Androecium (Stamen): Filament, Connective, Anther lobe: Pollen grains.
    • Gynoecium (Pistil): Style, Stigma, Ovary: Ovule.

Sexual Reproduction - Two Major Events

  1. Meiosis:

    • Production of gametes (n).
    • Male gametes: In anther.
    • Female gametes: In ovule.

  2. Fusion of Gametes (Fertilization):

    • Diploid zygote -> Embryo -> New plant (2n) sporophyte.

    Dominant Plant Body:

    Diploid sporophyte.
    Meiosis -> Haploid spores -> Gametophyte -> Reduced structure -> Within flower -> Gametes.

    Structure of Anther - Mature Anther
  3. Immature Stage:
    • Group of parenchymatous tissue surrounded by single-layered epidermis.
    • Usually dithecous (having two lobes) -> tetrasporangiate (four pollen sacs).
    • Monothecous (single lobe) -> Bisporangiate (two pollen sacs).
    • Young: Homogeneous.
    • Parenchymatous with epidermis.
    • Heterogeneity appears with formation of archesporial cell.

Transverse Section of Anther (T.S. of Anther)

  • Internal Structure: Four chambers called microsporangia or pollen sacs.

Layers of Anther Wall

  1. Epidermis:

    Outermost layer, Made up of flattened cells for protection.

  2. Endothecium:

    • Lies internal to epidermis.
    • Single layer of cells.
    • Fibrous thickenings on radial walls.

  3. Middle Layers:

    • 1 to 3 layers of parenchymatous cells.
    • Degenerate during microspore formation.

  4. Tapetum:

    • Innermost nutritive layer.
    • Single layer of cells surrounding sporogenous tissue.

 

Microsporangium Content

  • Contains microspore mother cells (2n).
  • Undergo meiosis to form microspore tetrad (n).
  • Microspores separate from tetrad.
  • Development of wall around microspores -> pollen grain.
  • From archesporial cell:
    • Primary parietal cell forms anther wall.
    • Sporogenous cell forms sporogenous tissue

Microsporogenesis
  • Definition:
    • Formation of Microspores by meiosis from MMC (Microspore Mother Cell).
    • Each MMC divides meiotically to form tetrad of haploid microspores (pollen grains).

Pollen Grain

  • Non-motile with single nucleus, Haploid.
  • Pollen wall, double-layered - Sporoderm.

Structure of Microspore

  • Typical pollen grain: Non-motile, haploid, unicellular body with single nucleus.
  • Surrounded by two-layered wall called sporoderm.

Exine (Outer Layer):

  • Thick, Made up of complex, non-biodegradable substance called sporopollenin.
  • Smooth or with sculptured pattern.
  • Resistant to chemicals.
  • Thin areas known as germ-pores for pollen tube growth during germination.

Intine (Inner Wall Layer): Consists of cellulose and pectin, Forms pollen tube smooth.

Pollen Viability

  • Functional ability of pollen grain to form male gametophyte by germination.
  • Factors Affecting Germination: Temperature and humidity.
  • Duration of Viability:
    • Low up to 30 minutes in rice and wheat.
    • Up to months in some plants of families Leguminosae, Rosaceae, and Solanaceae.

Development of Male Gametophyte

 

Develops in flower.
  • First Mitotic Division: Two unequal cells: (i) Vegetative cell, (ii) Generative cell.
  • Second Mitotic Division:In generative cell -> equal cells.
  • Process:
    • Pollen grain is first cell of male gametophyte.
    • Protoplast of pollen grain divides mitotically to form two unequal cells:
      • Small thin-walled generative cell.
      • Large naked vegetative or tube cell.
    • Generative cell separates and floats in cytoplasm of vegetative cell.
    • Vegetative cell possesses thick cytoplasm, irregular-shaped nucleus, and reserved food.
    • In most angiosperms, pollen grains are liberated at two-celled stage after anther dehiscence.
    • Generative cell divides by mitosis to form two male non-motile gametes, either in pollen grain or in pollen tube.
    • Pollen grains shed in either two- or three-celled stage, lodged on stigma of pistil.

Structure of Anatropous Ovule (Most Common Type)

  • Definition:

    Ovule with bent axis and downwardly directed micropyle.

  • Common Type:

    Most common type of ovule in angiosperms.

Parts of Anatropous Ovule

  1. Funiculus:

    Stalk of the ovule attaching it to placenta.

  2. Hilum:

    Point of attachment of funiculus to ovule body.

  3. Nucellus:

    Made up of diploid parenchymatous cells.

  4. Integuments:

    • Protective coverings of nucellus.
    • Outer and inner layers.

  5. Micropyle:

    • Narrow opening at apex.
    • Integuments do not completely cover nucellus, leaving a small opening.

  6. Chalaza:

    Basal part of nucellus.

  7. Female Gametophyte or Embryo Sac:

    • Oval-shaped structure towards micropylar end.
    • Embedded in nucellus.
    • Oval, elongated, multicellular seven-celled structure.
     

Presence of Ovules

  • Uniovulate:

    Examples: Mango, Wheat, Rice.

  • Multiovulate:

    Examples: Tomato, Lady's finger. 

Megasporogenesis
  • Definition: Process where diploid megaspore mother cell undergoes meiosis to form a tetrad of haploid megaspores.

Development of Female Gametophyte

  1. Formation of Megaspores:

    • Megaspore mother cell undergoes meiosis forming linear tetrad of four haploid megaspores.
    • Chalazal Megaspore: Generally becomes functional megaspore, others degenerate.

  2. Chalazal Megaspore:

    • First cell of female gametophyte.
    • Enlarges and develops into female gametophyte.

  3. Mitotic Divisions:

    • Diploid nucellus of chalazal megaspore undergoes three successive free nuclear mitotic divisions.
    • First mitotic division forms two nuclei.
    • Both nuclei undergo two successive mitotic divisions forming four nuclei at poles.

  4. Formation of Secondary Nucleus:

    • One nucleus from each pole moves towards centre, fuses to form diploid nucleus, secondary nucleus.

  5. Organization of Nuclei:

    • Three nuclei at micropylar end form three-celled egg apparatus.
    • Three nuclei at chalazal end form three antipodal cells.
    • Egg Apparatus:
      • Central cell: Egg cell or female gamete.
      • Flanked by two lateral cells: Synergids.
       

Composition of Female Gametophyte

  • Egg apparatus, secondary nucleus, and three antipodal cells.
  • Seven-celled, eight-nucleated structure. 

Pollination
  • Pollination: Transfer of pollen grains from anther to stigma.

Types of Pollination

1. Self-pollination: 

  • Pollen from anther to stigma of same/different flower with same genetic makeup.
  • Occurs in single or two flowers of same plant.
  • Autogamy: Bisexual flower pollinated by its own pollen.
  • Offspring genetically identical to parents (e.g., Pea).

2. Cross-pollination (Outbreeding): 

  • Pollen from anther of one flower to stigma of another flower with different genetic makeup.
  • Xenogamy: Involves two different plants.
  • Requires pollinating agent.
  • Genetically varied offspring (e.g., food and fibre crops).

Types of Cross Pollination

(a) Chasmogamy: Flowers open and expose sex organs.
(b) Homogamy: Anther and stigma mature simultaneously.
(c) Cleistogamy: Flowers remain closed (e.g., Viola, Commelina).
(d) Geitonogamy: Transfer of pollen grains to stigma of other flower produced on same plant (e.g., unisexual flowers of Cucurbita). 

Agents of Pollination

(1) Abiotic (Non-living)

  • Wind (Anemophily)

    • Transfer of pollen grains through wind.
    • Anemophilous plants:
      • Small, inconspicuous flowers.
      • No bright colors, fragrance, or nectar.
      • Flowers produced in large numbers.
      • Stamens are long with versatile anthers.
      • Feathery stigma exposed to receive pollen grains.
      • e.g., Grasses, maize, Jowar, Palms.

  • Water (Hydrophily)

    • Transfer of pollen grains with the help of water.
    • Hydrophilous plants:
      • Small, inconspicuous unisexual flowers.
      • Lack fragrance, nectar, and bright colors.
      • Pollen grains and floral parts protected from getting wet.
      • Long, sticky stigma.
      • e.g., Zostera, Vallisneria.

(2) Biotic (Living)

  • Insects (Entomophily)

    • Pollination with the help of insects.
    • Entomophilous flowers:
      • Large and attractive.
      • Brightly colored with pleasant smell.
      • Produce nectar.
      • Spiny and sticky pollen grains.
      • e.g., Rose, Jasmine, Cestrum, Salvia.

  • Birds (Ornithophily)

    • Pollination through birds.
    • Ornithophilous plants:
      • Large and showy flowers.
      • Brightly colored, lack fragrance.
      • Sticky and spiny pollen grains.
      • e.g., Callistemon, Bignonia, Bombax, Butea.

  • Bats (Chiropterophily)

    • Pollination with the help of bats.
    • Chiropterous flowers:
      • Large and stout.
      • Nocturnal, emit fruity odors.
      • Produce copious nectar and edible pollen grains.
      • e.g., Anthocephalus, Kigellia, Adansonia.

Outbreeding Devices - Contrivances
  • Mechanisms to prevent self-pollination and promote cross-pollination.
  • Self-pollination leads to inbreeding depression; cross-pollination is needed.
  • Observed in plants.

Devices Observed in Plants

  1. Unisexuality

    • Plants bear either male or female flowers.
    • Avoids self-pollination.
    • Plants are either dioecious (e.g., Maize) or monoecious (e.g., Papaya, Mulberry).

  2. Dichogamy

    • Stamens and carpels mature at different times in a bisexual flower.
    • Avoids self-pollination and favors cross-pollination.
    • Protandry: Anthers mature before stigma (e.g., Sunflower).
    • Protogyny: Stigma matures before anthers (e.g., Gloriosa).

  3. Prepotency

    • Rapid pollen germination on other stigma of the same type (e.g., Apple).

  4. Heteromorphy

    • Different types of flowers in the same plant.
    • Stigmas and anthers at different levels.
    • Types: Heterostyly, Heteroanthy
    • Prevents self-pollination (e.g., Primrose).

  5. Herkogamy

    • Mechanical device in bisexual flowers to prevent self-pollination.
    • Natural physical barrier between sex organs.
    • Prevents contact of pollen with stigma (e.g., Calotropis).

  6. Self-Sterility

    • Genetic mechanism preventing pollen germination on the same flower's stigma.
    • E.g., Tobacco, Thea.

Pollen-Pistil Interaction
  • Self-Incompatibility
    • Genetic mechanism preventing pollen germination on the same flower's stigma.
    • Favors cross-pollination (e.g., Tobacco).
  • Pollen-Pistil Interaction
    • Pistil recognizes and allows germination of compatible pollen.
    • Special proteins on stigmatic surface determine compatibility.
    • Physiological Mechanism:
      • Compatible pollen absorbs water and nutrients from stigma.
      • Pollen tube emerges, grows through style to ovule.
      • Tip enters synergid.
      • Growth determined by specific chemicals.
      • In vitro Tube Growth: Pollen grains induced to grow on artificial medium (sucrose, boric acid).

Artificial Hybridization

  • Approach in Crop Improvement : Major approach in plant breeding programs.
  • Procedure
    • Only desired pollen grains are hand-pollinated.
    • Emasculation: Removal of anthers before flower opening.
    • Bagging: Protecting emasculated flowers. 

Double Fertilization
  • Definition: Fertilization where both male gametes participate in the complex mechanism seen in angiosperms.


Importance of Double Fertilization

  • Ensures seed formation with food storage for embryo.
  • Diploid zygote develops into embryo, forming a new plant.
  • Triploid PEN forms endosperm, nutritive tissue for embryo.
  • Restoration of diploid condition by syngamy.

Characteristic Features of Angiosperms

  • Porogamy: Pollen tube entering ovule through micropyle.
    • Most common type: Enters synergid and releases content (2 non-motile male gametes).
  • Chalazogamy: Entry of pollen tube through chalaza.
  • Mesogamy: Entry of pollen tube through integuments.
  • Siphonogamy: Non-motile gametes present in pollen tube.

 

Process of Double Fertilization

  • Two male gametes participate in fertilization.
  • Steps:
    1. Pollen tube enters embryo sac through micropyle, bursts in synergid.
    2. Two male gametes released.
    3. One unites with egg, other with secondary nucleus.
    4. Formation of triploid primary endosperm nucleus (PEN).
    5. Fusion with egg nucleus forms diploid zygote (syngamy).
    6. Reproductive process: Non-motile male nuclei carried to egg cell through pollen tube (siphonogamy).
    7. Zygote develops into embryo, ovule changes leading to seed development.

 Process of Double Fertilization in Embryo Sac

(1) Syngamy (Generative Fertilization)

  • 1 male gamete unites with egg cell.
  • (n) + (n) = 2n -> Zygote (2n) -> Embryo.

(2) Triple Fusion (Vegetative Fertilization)

  • Out of two male gametes, first fuses with egg, second with secondary nucleus.
  • Called triple fusion or second fertilization.
  • First fusion: Male gamete + Egg -> Zygote (2n).
  • Second fusion: Male gamete + Secondary nucleus -> PEN (3n) -> Endosperm (Nutritive tissue).

Significance of Double Fertilization

  • Unique feature of angiosperms.
  • Concerned with seed production.
  • Zygote develops into embryo, forming new plant.
  • Triploid PEN forms endosperm, nutritive tissue.
  • Restoration of diploid chromosome number.
  • Avoids polyembryony.

Development of Endosperm
  • Nutritive tissue nourishing embryo.
  • Develops from PEN.
  • Post-fertilization tissue.
  • Types: Endospermic (e.g., Castor, Maize) and Non-endospermic (e.g., Bean, Pea).

 Types of Endosperm

 

  1. Nuclear Endosperm

    • Most common type.
    • Formation:
      • Primary Endosperm Nucleus (PEN) undergoes free nuclear division.
      • Triploid nuclei remain suspended in central cell cytoplasm.
      • Nuclei pushed towards periphery by central vacuole.
      • Wall formation around nuclei to form cellular mass.
    • Seen in plants like maize, sunflower, wheat, coconut.

  2. Cellular Endosperm

    • Formation:
      • PEN undergoes nuclear divisions followed by cytokinesis.
      • Endosperm develops in cellular form.
    • Less common, seen in dicots like Datura, Petunia, Balsam, Adoxa.

  3. Helobial Endosperm

    • Formation:
      • First division of PEN followed by cell wall formation.
      • Central cell divided into large micropylar cell and small chalazal cell.
      • Further development in both chambers:
        • Micropylar chamber: Nuclear type, walls develop between nuclei.
        • Chalazal chamber: Nuclear type.
    • Seen in plants of order Helobiales of Monocots, e.g., Asphodelus.

Mosaic Endosperm

  • Endosperm containing tissues of two different types.
  • Seen in plants like corn, where endosperm contains patches of two different colors, forming a mosaic pattern.

Development of Embryo

  • Embryogenesis: Development of zygote into embryo.
  • Zygotic embryo situated towards micropylar end in embryo sac.
  • Growth of embryo begins when some amount of endosperm is formed.
  • Zygote develops wall, becomes oospore.

2-Celled Pro-Embryo

 

  • Formation:
    • Oospore divides transversely.
    • Large basal cell towards micropyle, small apical cell towards chalaza.
    • Basal cell forms multicellular suspensor.
    • Suspensor draws nutrition from endosperm.
    • Apical cell divides transversely, then vertically to form 8-celled octant stage.
    • Lower four cells become hypocotyl and radicle, upper four form plumule and cotyledons.
    • Hypophysis forms part of radicle and root cap.
    • Upper cells divide to form heart-shaped embryo, elongating into two cotyledons.
    • Enlargement results in curved, horseshoe-shaped embryo.

Embryogenesis in Monocots

  • Similar till octant stage.
  • Single cotyledon.
    • Scutellum: Shield-shaped cotyledon.
    • Coleorhiza: Protective covering of radicle.
    • Coleoptile: Protective covering of plumule.

  

Seed and Fruit Development

  • Goal of Reproduction: Create offspring for next generation.

  • Seed Formation: Initiated by fertilization.

  • Seed Coat Development:

    • Integuments: Form seed coat.
      • Testa: Outer integument.
      • Tegmen: Inner integument.

  • Perisperm: Remnants of nucellus in seed.

    • e.g., Black pepper, beet.

  • Pericarp: Ovary wall becomes fruit wall.

                                         

Types of Seeds

  • Endospermic or Albuminous:

    • e.g., Castor, Maize.

  • Non-Endospermic or Exalbuminous:

    • e.g., Bean, Pea.

  • Cotyledons: Store food materials.

  • Micropyle: Opening in seed coat for emergence of radicle.

Significance of Seed and Fruit Formation

  • Nourishment: Provides nourishment to seed inside fruit.
  • Protection: Protects seed.
  • Propagating Unit: Helps in reproduction.
  • Dispersal: Aids in spread of species.

Seed Dormancy

  • Definition: Metabolic arrest for survival in unfavorable conditions.
  • Adaptive Measures: Structure or physiological adaptations.
  • Dispersal: Seeds dispersed during dormancy.
  • Germination: Occurs only after dormancy period is completed. 

 Apomixis

  • Definition: Formation of embryo(s) through asexual reproduction without gamete formation.
  • Results in rapid production of genetically identical plants.

Types of Apomixis

  1. Apogamy:

    Gametophytic cell produces embryo without fertilization.

  2. Apospory:

    Sporophytic cell produces embryo without fertilization.

  3. Agamospermy:

    Seed produced but embryo forms without meiosis and syngamy.

Categories of Apomixis

  1. Recurrent Apomixis:

    • Diploid sporophytic cell, archesporial cell, or nucellus form embryos.
    • Diploid megaspore mother cell forms embryo sac (diplospory or apospory).

  2. Non-recurrent Apomixis:

    • Haploid embryo sac formed, embryos arise from egg cell or other haploid cell (apogamy).

  3. Adventive Embryony:

    • Additional embryos develop from nucellus or integuments.
    • Results in polyembryony. 

Parthenocarpy

  • Definition: Fruit developed without fertilization.
  • Natural Occurrence:
    • Observed in Pineapple and Banana.
    • Auxin (IAA) stimulus from placental tissues of unfertilized ovary.
    • Enlargement of ovary forms fruit.
  • Characteristics:
    • Parthenocarpic fruits are seedless.
    • e.g., Banana, Pineapple, Papaya.
  • Artificial Induction:
    • Gibberellins or physical methods.
    • e.g., Grapes.

Polyembryony

  • Definition: Development of more than one embryos inside seed.
  • Types:
    • Adventive Polyembryony:
      • Embryos from diploid cells of nucellus or integuments.
    • Cleavage Polyembryony:
      • Zygote divides into small units, developing into embryos.
  • Advantages:
    • Genetically uniform seedlings.
    • Emergence of multiple seedlings.
    • Beneficial in horticulture.

To know

  • Fibrous Endothecium: Helps in anther dehiscence.
  • Zoophily: Pollination by animals.
  • Malacophily: Pollination by snails and slugs.
  • Pollenkitt: Substance in insect-pollinated pollen grains from tapetum.
  • Raphe: Part of funiculus united with ovule wall.
  • Aleurone Layer: Outer part of starchy endosperm in maize grain, stores proteins.