Sexual Reproduction in Flowering Plants

Sexual Reproduction in Flowering Plants

• Beauty and Purpose of Flowers
  • Flowers are essential for sexual reproduction in plants.
  • They provide scents, colors, and beauty to aid in this process.
  • All flowering plants reproduce sexually.
• Diversity and Adaptations
  • Flowers, inflorescences, and floral parts have diverse structures.
  • These adaptations ensure the formation of fruits and seeds.

1.1 Flower – A Fascinating Organ of Angiosperms

• Human Connection with Flowers
  • Flowers have been important in aesthetics, ornamentation, social, religious, and cultural contexts.
  • They symbolize love, affection, happiness, grief, and mourning.

• Floriculture
  • Floriculture refers to the cultivation of flowers for decorative purposes.
• Biological Significance of Flowers
  • To biologists, flowers are sites of sexual reproduction.
  • Parts of a typical flower include various structures that aid in reproduction.
  • Key reproductive parts: the stamens (male) and pistils (female).

1.2 Pre-Fertilisation: Structures and Events

• Preparation for Flowering
  • Plants decide to flower before the actual flower appears.
  • Hormonal and structural changes lead to the development of the floral primordium.
  • Inflorescences form and bear floral buds and flowers.
  • Male and female reproductive structures develop in the flower.

1.2.1 Stamen, Microsporangium, and Pollen Grain

• Structure of Stamen
  • Stamen has two parts: filament (stalk) and anther (bilobed structure).
    • Anther Structure: The anther has two lobes, each with two thecae (dithecous), (thecae = compartment).
    • Theca and Microsporangia: Each theca contains two microsporangia at the corners, so in total, an anther has four microsporangia (a Tetragonal structure).
    • Microsporangia in the anther develop into pollen sacs, where microspores mature into and are stored as pollen grains.

• Microsporangium Structure
  • Four wall layers: epidermis (outermost), endothecium, middle layers, and tapetum (Innermost).
  • Tapetum nourishes developing pollen grains.
  • Sporogenous tissue (also called MMC) in the center undergoes meiosis to form microspore tetrads (4 cell cluster).
• Microsporogenesis (process)
  • Sporogenous tissue cells divide to form microspore tetrads.
  • Microspores develop into pollen grains.
  • Thousands of pollen grains are released when the anther dehisces (splits open).
  • So, Microsporogenesis is the process in which microspore mother cells(MMC) undergo meiosis to produce haploid(n) microspores, which eventually develop into pollen grains in seed plants.

• Pollen Grain
  • Pollen grains are male gametophytes.
  • Spherical, 25-50 micrometers in diameter.
  • Two-layered wall: exine (hard, with sporopollenin) and intine (thin, with cellulose and pectin).
    • A germ pore is an opening in the exine, foms pollen tube.
  • Contains two cells: vegetative cell (larger, with food reserve) and generative cell (smaller, with nucleus thus fuses with egg during fertilisation).
• Pollen Viability and Storage
  • Pollen grains must land on stigma before losing viability.
  • Viability varies: 30 minutes in rice and wheat, months in some other plants.
  • Pollen can be stored in liquid nitrogen (-196°C) for years for use in crop breeding.

Importance, Effects and Uses of Pollen

• Pollen Allergies
  • Pollen grains can cause allergies and respiratory issues like asthma and bronchitis.
  • Parthenium (carrot grass) causes pollen allergy.
• Nutritional Value
  • Pollen grains are rich in nutrients.
  • Used as food supplements in tablets and syrups.
  • Claimed to enhance athletic performance and racehorse speed.

1.2.2 The Pistil, Megasporangium (Ovule), and Embryo Sac

• Gynoecium
  • Female reproductive part of the flower.
  • Can have one (monocarpellary) or more pistils (multicarpellary).
  • Pistils can be fused (syncarpous) or free (apocarpous).
• Parts of a Pistil
  • Stigma: Landing platform for pollen grains.
  • Style: Slender part below the stigma.
  • Ovary: Basal bulged part with an ovarian cavity (locule).
• Ovule (Megasporangium)
  • Small structure attached to placenta by funicle (a stalk).
  • Hilum – Junction between ovule and funicle is the hilum.
  • Protected by integument (envelop), except at the tip, where there is a micropyle.
  • Chalaza – Opposite the micropyle is the chalaza.
  • Nucellus – Contains nucellus (2n) with abundant food reserve.
  • Inside nucellus is the embryo sac (female gametophyte/reproductive structure).
    • The embryo sac is the female gametophyte of seed plants, containing the egg cell and other nuclei necessary for fertilization and embryo development.
• Megasporogenesis (process)
  • Formation of megaspores from megaspore mother cell (MMC).
  • MMC undergoes meiosis to produce four megaspores.
  • Usually, one megaspore is functional, others degenerate.
  • Functional megaspore develops into the embryo sac.
• Embryo Sac Formation
  • Functional megaspore undergoes mitotic divisions.
  • Forms 2-nucleate, 4-nucleate, then 8-nucleate embryo sac.
  • Mitotic divisions are free nuclear (no cell wall formation initially).
  • Cell walls form after 8-nucleate stage, creating a 7-celled structure.
• Structure of Embryo Sac
  • Egg Apparatus: 3 cells at micropylar end (1 egg cell, 2 synergids).
    • Synergids: Have filiform apparatus to guide pollen tubes.
  • Antipodals: 3 cells at chalazal end.
  • Central Cell: Large with 2 polar nuclei.
• Summary
  • Mature angiosperm embryo sac is 8-nucleate but 7-celled.

1.2.3 Pollination

What is Pollination?

• Pollination is how plants transfer pollen grains from the anther (male part) to the stigma (female part).
• This helps male and female gametes meet for fertilization.
• Flowering plants use various external agents to help with pollination.

Types of Pollination

1. Self Pollination
   • Pollen transfer within the same plant.
   • 2 types; Autogamy & Geitonogamy
   • Autogamy
     • Pollen from the anther goes to the stigma of the same flower.
     • Seen in Cleistogamous flowers (Self-pollinating, closed flowers), Example: like Viola and Oxalis don’t open, ensuring self-pollination.
     • Rare in flowers that open and expose anthers and stigma i.e.- Open flowers (Chasmogamous flowers).
   • Geitonogamy
     • Pollen goes from one flower to another flower on the same plant.
     • Functionally like cross-pollination but genetically like self-pollination.
2. Cross Pollination
   • Pollen transfer between flowers of different plants of same species.
   • only 1 type; Xenogamy
   • Xenogamy
     • Pollen goes from one plant to a different plant.
     • Brings genetically different pollen to the stigma.

Agents of Pollination

• Abiotic (Non-living)
  • Wind Pollination:
    • Pollen grains are light and non-sticky and in very large number.
    • Anther – well exposed for easy pollen dispersal.
    • Stigma – large & feathery to easily traps pollens.
    • Flowers lack color and nectar.
    • Example: Corn., Cotton ,Grasses.
    • Water Pollination:
      • Flowers lack color and nectar but produce pollens in very large quantity.
      • Rare in seed plants, mostly in plants like Vallisneria and Hydrilla and see grass like Zostera.
      • But exclusive mode for lower plants like algae, bryophytes and pteridophytes.
• Biotic (Living)
  • Animals like bees, butterflies, birds, and even some reptiles help with pollination.
  • Flowers are often large, colorful, fragrant, and rich in nectar to attract animals.
  • Some plant species offer a safe place for egg-laying to pollinators as a reward.
    • example – Amorphophallus Plant’s flower (tallest flower, 6 feet) , Yucca Plant.

Outbreeding Devices (v. important)

• Various mechanism or tactics to prevent self-pollination and encourage cross-pollination:
  • Different timing for pollen release and stigma receptivity.
  • Different positions of anther and stigma.
  • Self-incompatibility: Genetic mechanism preventing self-pollen from fertilizing.
  • Unisexual Flowers: Either male or female flowers on different plants (dioecy).

Pollen-Pistil Interaction

• The pistil can recognize if pollen is the right type.
• If compatible, the pollen germinates and grows a pollen tube to reach the ovary.
• This interaction ensures successful fertilization.

Artificial Hybridisation

• Used in crop improvement by using desired pollen to produce superior varieties.
  • Emasculation: Removing anthers from bisexual flowers to prevent unwanted pollen.
    • If the female parent produces unisexual flowers, there is no need for emasculation.
  • Bagging: Covering flowers to protect from contamination, then pollinating with desired pollen.

1.3 DOUBLE FERTILISATION

What is Double Fertilisation?

• A unique event in flowering plants.
• Involves two types of fusions in the embryo sac, resulting in a zygote and a triploid (3n) endosperm.

How it Happens

1. Pollen Tube Entry
   • Pollen tube enters one of the synergids (special cells near the egg).
   • Releases two male gametes into the synergid’s cytoplasm.
2. First Fusion: Syngamy
   • One male gamete fuses with the egg cell’s nucleus.
   • Forms a diploid cell called the zygote.
3. Second Fusion: Triple Fusion
   • The other male gamete fuses with the two polar nuclei in the central cell.
   • Forms a triploid cell called the primary endosperm nucleus (PEN).

Results of Double Fertilisation

• Zygote: Develops into the embryo.
• Primary Endosperm Cell (PEC): Develops into the endosperm, which provides nourishment to the developing embryo.

Double fertilisation ensures that both the embryo and its food supply (endosperm) are formed simultaneously, which is a special feature of flowering plants.

1.4 POST-FERTILISATION : STRUCTURES AND EVENTS

What Happens After Fertilisation?

• Development of endosperm and embryo.
• Ovule turns into a seed.
• Ovary becomes a fruit.
• and all these collectively called as post-fertilisation events.

1.4.1 Endosperm

Endosperm Development

• Happens before embryo development.
• The primary endosperm cell (PEC) divides to form triploid endosperm tissue.
• Provides nutrition to the embryo.

Types of Endosperm

• Free-Nuclear Endosperm: Many nuclei form before cell walls. Example: coconut water.
• Cellular Endosperm: Forms after cell walls develop. Example: white kernel of coconut.

Endosperm Usage

• Completely consumed in: Pea, groundnut, beans. Thus called non -Albuminous endosperm.
• Persists in mature seeds: Castor, coconut. Thus called Albuminous endosperm.

2.4.2 Embryo

Embryo Development

• Starts at the micropylar end of the embryo sac.
• Zygote divides after some endosperm forms.

Stages of Embryo Development

• Proembryo
• Globular stage
• Heart-shaped stage
• Mature embryo

Dicot Embryo

• Has two cotyledons.
• Parts: Epicotyl (stem tip), hypocotyl (root tip).

Monocot Embryo

• Has one cotyledon called scutellum.
• Parts: Epicotyl (shoot apex), coleoptile, coleorrhiza (root cap).

1.4.3 Seed

What is a Seed?

• A fertilized ovule.
• Contains seed coat, cotyledons, and embryo axis.

Types of Seeds

• Non-Albuminous: No endosperm remains. Example: pea, groundnut.
• Albuminous: Endosperm remains. Example: wheat, maize.

Seed Parts

• Seed Coat: Protective outer layer.
• Micropyle: Small pore for oxygen and water.
• Hilum: The seed coat’s scar.
• Perisperm: Remnants of persistent nucellus, as seen in black pepper.

Seed Maturity

• Water content reduces.
• Embryo becomes dormant or germinates if conditions are right.

Fruit Formation

• Ovary develops into fruit.
• The ovary wall transforms into the fruit wall known as the pericarp.
• True Fruits: Develop from ovary.
• False Fruits: Include other parts like thalamus. Example: apple, strawberry.

Parthenocarpic Fruits

• Develop without fertilisation. Example: banana.

Advantages of Seeds

• Independent of water for reproduction.
• Aid in Plant species dispersal or propagation.
• Provide food reserves for seedlings.
• Protection from hard seed coat.

Seed Storage and Viability

• Can be stored for long periods.
• Some seeds remain viable for years or even centuries.

Interesting Facts

• Oldest viable seed: Lupine, 10,000 years old.
• Date palm seed: 2,000 years old.

Key Questions

• How many eggs, ovules, ovaries, and flowers are there in a plant?
• Think of plants with many seeds. Example: orchids, Ficus tree.

1.5 APOMIXIS AND POLYEMBRYONY

What is Apomixis?

• Apomixis is a special way some plants produce seeds without fertilisation.
• It’s like asexual reproduction but mimics sexual reproduction.

How Does Apomixis Work?

• In some plants, a diploid egg cell forms without reduction division(meiosis) and becomes an embryo without fertilisation.
• In others, like Citrus and Mango, cells around the embryo sac divide and turn into embryos.

Why Apomixis is Important?

• Helps produce hybrid seeds that maintain their traits year after year.
• Farmers can reuse hybrid seeds without needing to buy new ones each year.

Research on Apomixis

• Scientists are studying apomixis to help make farming more cost-effective and productive.

What is Polyembryony?

• Polyembryony is when a seed contains more than one embryo.
• This can be observed in seeds of certain fruits like oranges (citrus fruit).
  • In plants like orange, each seed can have many embryos.
  • Try squeezing an orange seed to see the different embryos.

Chapter Summary:

• Flowers are the seat of sexual reproduction in angiosperms.
• Androecium (stamens) represents male reproductive organs.
• Gynoecium (pistils) represents female reproductive organs.
• A typical anther is bilobed, dithecous, and tetrasporangiate.
• Pollen grains develop inside microsporangia.
• Microsporangium has four wall layers: epidermis, endothecium, middle layers, and tapetum.
• Sporogenous tissue cells undergo meiosis (microsporogenesis) to form microspore tetrads.
• Microspores mature into pollen grains.
• Pollen grains represent the male gametophytic generation.
• Pollen grains have a two-layered wall: outer exine and inner intine.
• Exine is made of sporopollenin and has germ pores.
• Pollen grains may have two cells (vegetative cell and generative cell) or three cells (vegetative cell and two male gametes).
• The pistil has three parts: stigma, style, and ovary.
• Ovules are present in the ovary.
• Ovules have a stalk called funicle, protective integument(s), and an opening called micropyle.
• The central tissue is the nucellus, where archesporium differentiates.
• Megaspore mother cell divides meiotically, and one megaspore forms the embryo sac.
• The mature embryo sac is 7-celled and 8-nucleate.
• Egg apparatus (two synergids and an egg cell) is at the micropylar end.
• Three antipodals are at the chalazal end.
• Central cell has two polar nuclei.
• Pollination transfers pollen grains from the anther to the stigma.
• Pollination agents can be abiotic (wind, water) or biotic (animals).
• Pollen-pistil interaction includes pollen landing on the stigma to pollen tube entering the embryo sac.
• Pollen grain germinates on the stigma, and the pollen tube grows through the style.
• Pollen tube enters the ovule and discharges two male gametes in one synergid.
• Double fertilisation in angiosperms involves two fusion events: syngamy and triple fusion.
• Syngamy produces a diploid zygote, and triple fusion produces a triploid primary endosperm nucleus.
• Zygote develops into the embryo.
• Primary endosperm cell forms endosperm tissue.
• Endosperm development precedes embryo development.
• Embryo development stages: proembryo, globular, heart-shaped, and maturation.
• Mature dicotyledonous embryo has two cotyledons, an embryonal axis with epicotyl and hypocotyl.
• Monocotyledonous embryos have a single cotyledon.
• Post-fertilisation: ovary develops into fruit, ovules develop into seeds.
• Apomixis results in seed formation without fertilisation, common in some angiosperms (especially grasses).
• Apomixis is beneficial in horticulture and agriculture.
• Polyembryony is the formation of more than one embryo in a seed.