Sexual Reproduction in Flowering Plants

Sexual Reproduction in Flowering Plants

• Flowers are specialised structures meant for sexual reproduction in angiosperms.
• All flowering plants reproduce sexually.
• Flowers are not only biologically important but also valued for their beauty, colour, fragrance, and cultural significance.

Beauty and Purpose of Flowers

• Role in Reproduction:
  • Flowers are essential organs for sexual reproduction in flowering plants.
• Attractive Features:
  • Bright colours, fragrance, and nectar help in pollination.
• End Result:
  • Successful sexual reproduction leads to formation of fruits and seeds.

Diversity and Adaptations in Flowers

• Structural Diversity:
  • Flowers, inflorescences, and floral parts show wide variation in size, shape, and arrangement.
• Adaptive Significance:
  • These variations ensure effective pollination, fertilisation, and seed formation.

1.1 Flower – A Fascinating Organ of Angiosperms

• Nature of Flower:
  • A flower is a modified shoot with determinate growth.
  • Internodes are highly condensed and leaves are modified into floral organs.
• Biological Significance:
  • Flowers are the sites of sexual reproduction in angiosperms.

Human Connection with Flowers

• Cultural Importance:
  • Flowers are used in social, religious, and cultural activities.
• Symbolic Value:
  • They represent love, affection, happiness, grief, and mourning.

Floriculture

• Definition:
  • Floriculture is the cultivation of flowers for decorative and commercial purposes.

1.2 Pre-Fertilisation: Structures and Events

Preparation for Flowering

• Initiation of Flowering:
  • Plants decide to flower much before the actual flower appears.
• Developmental Changes:
  • Hormonal and structural changes lead to formation of floral primordium.
• Inflorescence Formation:
  • Floral buds develop on inflorescences and later mature into flowers.
• Development of Reproductive Organs:
  • Male (androecium) and female (gynoecium) reproductive structures differentiate and develop.

1.2.1 Stamen, Microsporangium, and Pollen Grain

Structure of Stamen

• Parts of Stamen:
  1. Filament – slender stalk.
  2. Anther – terminal, bilobed structure.

Anther Structure

• Lobes and Thecae:
  • Anther is bilobed.
  • Each lobe contains two thecae, hence anther is dithecous (thecae = compartment).
• Microsporangia:
  • Each theca has two microsporangia.
  • Total four microsporangia are present in an anther.
  • Thus, anther is tetragonal in structure.
• Pollen Sacs:
  • Microsporangia develop into pollen sacs where pollen grains are formed and stored.

Structure of Microsporangium

• Wall Layers:
  1. Epidermis – outermost protective layer.
  2. Endothecium – helps in anther dehiscence.
  3. Middle layers – temporary layers.
  4. Tapetum – innermost nutritive layer.

• Tapetum Function:
  • Provides nourishment to developing pollen grains.

• Sporogenous Tissue:
  • Located at the centre of microsporangium.
  • Also called microspore mother cells (MMC).

Microsporogenesis

• Definition:
  • Microsporogenesis is the process by which microspore mother cells undergo meiosis to form haploid (n) microspores.
• Process:
  • MMC undergoes meiosis.
  • Four haploid microspores are formed as a tetrad (4 cell cluster).
  • Microspores separate and develop into pollen grains.
• Anther Dehiscence:
  • Mature anther splits open (dehisces).
  • Thousands of pollen grains are released.

Pollen Grain

• Nature:
  • Pollen grain represents the male gametophyte.
• Shape and Size:
  • Generally spherical.
  • Diameter ranges from 25–50 micrometres.

Pollen Wall Structure

• Exine:
  • Outer thick wall.
  • Made of sporopollenin.
  • Highly resistant to chemical and biological degradation.
• Intine:
  • Inner thin wall.
  • Made of cellulose and pectin.
• Germ Pore:
  • Region where exine is thin or absent.
  • Pollen tube emerges through germ pore during germination.

Cellular Composition

• Vegetative Cell:
  • Larger cell.
  • Contains abundant food reserve.
  • Has a large, irregular nucleus.
• Generative Cell:
  • Smaller cell.
  • Contains dense cytoplasm and nucleus.
  • Ultimately forms male gametes.

Pollen Viability and Storage

• Viability Period:
  • Pollen grains must reach stigma before losing viability.
  • Viability varies with species and environment.
• Examples:
  • Rice and wheat – viable for about 30 minutes.
  • Some members of Rosaceae, Leguminosae, Solanaceae – viable for months.

• Artificial Storage:
  • Pollen grains can be stored in liquid nitrogen at –196°C.
  • Stored pollen can remain viable for years.
• Application:
  • Stored pollen is used in crop breeding programmes as pollen banks.

Importance, Effects and Uses of Pollen

Pollen Allergy

• Health Effects:
  • Some pollen grains cause allergies and respiratory problems.
  • May lead to asthma, bronchitis, and chronic respiratory disorders.
• Example:
  • Parthenium (carrot grass) causes severe pollen allergy.

Nutritional Value of Pollen

• Nutrient Rich:
  • Pollen grains contain proteins, vitamins, and minerals.
• Uses:
  • Used as food supplements in the form of tablets and syrups.
  • Claimed to improve athletic performance and stamina.

Key Points

• Flowers are specialised organs for sexual reproduction in angiosperms.
• Stamen and pistil are the main reproductive parts.
• Microsporogenesis leads to formation of pollen grains.
• Pollen grains show structural adaptations for survival and fertilisation.
• Pollen has both biological importance and practical applications.

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

Gynoecium

• Definition:
  • The gynoecium is the female reproductive part of the flower.
• Number of Pistils:
  1. Monocarpellary – single pistil
  2. Multicarpellary – more than one pistil
• Fusion of Pistils:
  1. Syncarpous – pistils fused together
  2. Apocarpous – pistils free from each other

Parts of a Pistil

• Stigma:
  • Terminal part of the pistil.
  • Acts as the landing and receptive surface for pollen grains.
• Style:
  • Slender stalk-like part below the stigma.
  • Provides a passage for pollen tube growth.
• Ovary:
  • Basal, swollen part of the pistil.
  • Contains the ovarian cavity (locule) and ovules.

Ovule (Megasporangium)

• Definition:
  • Ovule is an integumented megasporangium present inside the ovary.
  • After fertilisation, it develops into a seed.
• Attachment:
  • Each ovule is attached to the placenta by a stalk called funicle.
• Hilum:
  • Point of attachment between the ovule and funicle.
• Integuments:
  • Protective envelopes surrounding the ovule.
  • They do not cover the tip completely, leaving an opening.
• Micropyle:
  • Small opening at the tip of the ovule.
  • Allows entry of pollen tube during fertilisation.
• Chalaza:
  • Region opposite to the micropyle.
  • Represents the basal part of the ovule.
• Nucellus:
  • Mass of diploid cells (2n) inside the integuments.
  • Contains abundant reserve food material.
• Embryo Sac:
  • Located within the nucellus.
  • Represents the female gametophyte of flowering plants.
  • Contains the egg cell and other nuclei essential for fertilisation and embryo development.

Megasporogenesis

• Definition:
  • Megasporogenesis is the process of formation of megaspores from the megaspore mother cell (MMC).
• Process:
  • A single megaspore mother cell differentiates in the nucellus.
  • MMC undergoes meiosis to form four haploid megaspores arranged in a tetrad.
• Functional Megaspore:
  • Usually, only one megaspore remains functional.
  • The remaining three megaspores degenerate.

Functional megaspore develops into the embryo sac.

Embryo Sac Formation (Female Gametophyte Development)

• Type of Development:
  • Embryo sac develops from a single functional megaspore.
  • This is called monosporic development.
• Mitotic Divisions:
  • Functional megaspore undergoes mitotic divisions.
  • Forms sequentially:
    • 2-nucleate stage
    • 4-nucleate stage
    • 8-nucleate stage
• Nature of Division:
  • Divisions are free nuclear.
  • Cell walls are not formed immediately after nuclear division.
• Cellular Organisation:
  • After the 8-nucleate stage, cell walls form.
  • A 7-celled embryo sac is produced.

Structure of a Mature Embryo Sac

• Egg Apparatus (Micropylar End): Consists of three cells:
  • One egg cell
  • Two synergids
• Synergids:
  • Contain filiform apparatus.
  • Help in guiding the pollen tube towards the egg cell.
• Central Cell:
  • Largest cell of the embryo sac.
  • Contains two polar nuclei, which may fuse to form a diploid secondary nucleus.
• Antipodal Cells (Chalazal End):
  • Three cells present at the chalazal end.
  • May play a nutritive role and usually degenerate later.

Summary of Embryo Sac:

• A mature angiosperm embryo sac is:
  • 8-nucleate
  • 7-celled

Key Points

• The pistil is the female reproductive organ of the flower.
• Ovule acts as the megasporangium and later forms the seed.
• Megasporogenesis leads to formation of haploid megaspores.
• Embryo sac is the female gametophyte and is 7-celled, 8-nucleate at maturity.

1.2.3 Pollination

Pollination

• Definition:
  • Pollination is the process of transfer of pollen grains from the anther (male reproductive part) to the stigma (female reproductive part).
• Biological Significance:
  • Pollination brings male and female gametes closer.
  • It is an essential step before fertilisation.
• Agents of Pollination:
  • Flowering plants use various external agents such as wind, water, insects, birds, and other animals to achieve pollination.

Pollination is the first external step towards fertilisation in flowering plants.

Types of Pollination

A. Self Pollination

• Definition:
  • Self pollination is the transfer of pollen grains from the anther to the stigma of the same plant.

Types of Self Pollination

1. Autogamy

• Definition:
  • Autogamy is the transfer of pollen grains from the anther to the stigma of the same flower.
• Occurrence:
  • Common in cleistogamous flowers (flowers that do not open).

• Cleistogamous Flowers:
  • Flowers remain closed, ensuring compulsory self pollination.
  • Examples: Viola, Oxalis, Commelina.
• Chasmogamous Flowers:
  • Open flowers where anthers and stigma are exposed.
  • Autogamy is rare in such flowers.

2. Geitonogamy

• Definition:
  • Transfer of pollen grains from the anther of one flower to the stigma of another flower on the same plant.
• Genetic Nature:
  • Genetically similar to self pollination.
• Functional Nature:
  • Functionally similar to cross pollination as pollen transfer requires an external agent.

B. Cross Pollination

• Definition:
  • Transfer of pollen grains from the anther of one plant to the stigma of a flower on a different plant of the same species.

Types of Cross Pollination

1. Xenogamy

• Definition:
  • Transfer of pollen grains between flowers of two genetically different plants of the same species.
• Significance:
  • Introduces genetic variation.
  • Brings genetically different pollen to the stigma.

Agents of Pollination

Abiotic Pollination (Non-living Agents)

Wind Pollination (Anemophily)

• Characteristics of Wind-pollinated Flowers:
  • Flowers are small, inconspicuous, colourless, odourless, and nectarless.
  • Anthers are well exposed to release pollen easily.
  • Stigma is large, feathery or branched to trap pollen.
  • Pollen grains are light, dry, non-sticky, unwettable, and produced in large numbers.
• Examples:
  • Maize (corn), grasses, cotton, mulberry, date palm, Cannabis.

Water Pollination (Hydrophily)

• Definition:
  • Pollination occurring with the help of water.
• Characteristics:
  • Flowers are small, inconspicuous, colourless, odourless, and nectarless.
  • Pollen grains are light, non-sticky, and often coated with wax.
  • Stigma is unwettable but sticky.

Types of Hydrophily:

1. Hypohydrophily:
   • Pollination occurs below the water surface.
   • Examples: Ceratophyllum, Najas, Zostera.
2. Epihydrophily:
   • Pollination occurs on the water surface.
   • Example: Vallisneria.

Note: Water pollination is rare in seed plants but common in lower plants like algae, bryophytes, and pteridophytes.

Biotic Pollination (Living Agents)

Animal Pollination (Zoophily)

• General Features:
  • Flowers are large, colourful, fragrant, and nectar-rich.
  • Pollen grains are sticky or spiny to adhere to animal bodies.

Insect Pollination (Entomophily)

• Pollination by insects such as bees and butterflies.
• Pollen grains are heavy, sticky, and covered with pollenkitt.

Bird Pollination (Ornithophily)

• Flowers are tubular or cup-shaped, brightly coloured, usually odourless.
• Produce large quantities of nectar.
• Examples: Strelitzia, Callistemon, Nicotiana.

Bat Pollination (Chiropterophily)

• Flowers are dull coloured with strong fermenting or fruity smell.
• Produce abundant nectar and pollen.
• Examples: Kigelia, Adansonia, Bauhinia.

Other Biotic Agents

• Malacophily – pollination by snails (e.g., Arisaema).
• Myrmecophily – pollination by ants (e.g., some members of Rubiaceae).

• 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)

• Definition:
  • Outbreeding devices are mechanisms that prevent self pollination and promote cross pollination.

Major Outbreeding Devices

• Dichogamy:
  • Anther and stigma mature at different times.
    1. Protandry – anthers mature earlier (e.g., sunflower).
    2. Protogyny – stigma matures earlier (e.g., mustard).
• Herkogamy:
  • Physical or spatial separation between anther and stigma.
• Heterostyly:
  • Different lengths of stamens and styles in flowers.
• Self-incompatibility:
  • Genetic mechanism that prevents self pollen from fertilising the ovule.
• Unisexuality:
  • Presence of only male or female flowers.
  • Dioecy ensures compulsory cross pollination.

Pollen-Pistil Interaction

• Definition:
  • Pollen–pistil interaction is the ability of the pistil to recognise compatible pollen.
• Compatible Pollen:
  • Germinates on stigma.
  • Forms pollen tube and grows through style to reach the ovule.
• Incompatible Pollen:
  • Rejected by the stigma and does not germinate.
• Significance:
  • Ensures fertilisation by the correct pollen.
  • Important in plant breeding and hybridisation.

Artificial Hybridisation

• Definition:
  • A technique used in crop improvement to produce superior varieties by controlled pollination.
• Emasculation:
  • Removal of anthers from bisexual flower buds before dehiscence to prevent self pollination.
• Bagging:
  • Covering emasculated or female flowers with bags to prevent contamination by unwanted pollen.

• Procedure:
  • Desired pollen is dusted on receptive stigma.
  • Flower is rebagged until fruit development.

• Special Case:
  • If the female parent bears unisexual flowers, emasculation is not required; only bagging is done.

Key Points

• Self pollination maintains genetic purity.
• Cross pollination introduces genetic variation.
• Wind and water are abiotic agents; animals are biotic agents.
• Outbreeding devices promote genetic diversity.
• Artificial hybridisation is crucial in crop improvement programmes.

1.5 APOMIXIS AND POLYEMBRYONY

What is Apomixis?

• Apomixis is the formation of seeds without fertilisation.
• It is a form of asexual reproduction that mimics sexual reproduction.

Mechanisms of Apomixis

1. Diploid Egg Formation:
   • Diploid egg cell is formed without meiosis and develops into embryo without fertilisation.
2. Adventive Embryony:
   • Embryos arise from cells of nucellus or integuments surrounding the embryo sac.
   • Examples: Citrus, mango.

Importance of Apomixis

• Fixation of Hybrid Characters:
  • Hybrid traits are preserved generation after generation.
• Agricultural Advantage:
  • Farmers can reuse hybrid seeds without loss of vigour.
  • Reduces cost of seed production.
• Research Significance
  • Apomixis is being studied for developing cost-effective and high-yield crops.

POLYEMBRYONY

What is Polyembryony?

• Polyembryony is the occurrence of more than one embryo in a single seed.

Types of Polyembryony

1. Simple Polyembryony:
   • More than one embryo formed due to fertilisation of multiple eggs.
2. Adventive Polyembryony:
   • Extra embryos arise from nucellus or integument tissues.

Examples of Polyembryony

1. Citrus (orange)
2. Onion
3. Groundnut

Key Point – In citrus seeds, multiple embryos can be observed within a single seed.

Key Points to Remember

• Seed is a fertilised ovule containing embryo, cotyledons and seed coat.
• Seeds may be albuminous or non-albuminous.
• Fruit develops from ovary and may be true or false.
• Parthenocarpy produces seedless fruits.
• Apomixis ensures seed formation without fertilisation.
• Polyembryony results in more than one embryo per seed.

Chapter Summary

Flowers as Reproductive Structures

• Flowers are the seat of sexual reproduction in angiosperms.
• They contain specialized reproductive organs arranged in distinct whorls.

Male Reproductive Organ: Androecium

• Androecium consists of stamens, which represent the male reproductive organs.
• Each stamen has a filament and an anther.
• A typical anther is bilobed, dithecous, and tetrasporangiate.
• Each anther contains four microsporangia where pollen grains develop.

Structure and Development of Microsporangium

• Each microsporangium has four wall layers:
  1. Epidermis
  2. Endothecium
  3. Middle layers
  4. Tapetum (nutritive layer for developing pollen)

• The central sporogenous tissue undergoes meiosis.
• This process, called microsporogenesis, produces microspore tetrads.
• Microspores mature into pollen grains.

Pollen Grain: Male Gametophyte

• Pollen grains represent the male gametophytic generation.
• They are surrounded by a two-layered wall:
• Exine: Outer, thick, made of sporopollenin, resistant and sculptured
• Intine: Inner, thin, made of cellulose and pectin

• Exine has germ pores for pollen tube emergence.

• Pollen grains may be:
  • Two-celled: Vegetative cell + generative cell
  • Three-celled: Vegetative cell + two male gametes

Female Reproductive Organ: Gynoecium

• Gynoecium consists of one or more pistils.
• Each pistil has three parts:
  1. Stigma: Receives pollen
  2. Style: Connects stigma to ovary
  3. Ovary: Contains ovules

Structure of Ovule (Megasporangium)

• Ovules are attached to placenta by a stalk called funicle.
• Ovule structure includes:
  • Integument(s): Protective covering
  • Micropyle: Opening at the tip
  • Nucellus: Central tissue with food reserve

Megasporogenesis and Embryo Sac Formation

• Within the nucellus, a megaspore mother cell undergoes meiosis.
• Four megaspores are formed; usually only one is functional.
• The functional megaspore develops into the embryo sac.

• The mature embryo sac is 8-nucleate but 7-celled.

Structure of embryo sac:

• Egg apparatus at micropylar end (1 egg cell + 2 synergids)
• Three antipodal cells at chalazal end
• Central cell with two polar nuclei

Pollination

• Pollination is the transfer of pollen grains from anther to stigma.
• Types of pollination agents:
  1. Abiotic: Wind, water
  2. Biotic: Insects, birds, animals

Pollen–Pistil Interaction

• Stigma recognizes compatible pollen.
• Compatible pollen germinates and forms a pollen tube.
• The pollen tube grows through the style into the ovule.
• It enters the embryo sac through one synergid and releases two male gametes.

Double Fertilisation

• Double fertilisation is a unique feature of angiosperms.
• It involves two fusion events:

1. Syngamy
   • One male gamete fuses with the egg cell.
   • Forms a diploid zygote (2n).
2. Triple Fusion
   • The second male gamete fuses with two polar nuclei.
   • Forms a triploid primary endosperm nucleus (3n).

Post-Fertilisation Events

• Zygote develops into the embryo.
• Primary endosperm cell develops into endosperm tissue.
• Endosperm development occurs before embryo development.

Embryo Development

• Embryo development stages:
  1. Proembryo
  2. Globular stage
  3. Heart-shaped stage
  4. Mature embryo

• Dicot Embryo
  • Has two cotyledons.
  • Embryonal axis includes epicotyl and hypocotyl.
• Monocot Embryo
  • Has one cotyledon called scutellum.

Seed and Fruit Formation

• Ovules develop into seeds.
• Ovary develops into fruit.

Apomixis

• Apomixis is seed formation without fertilisation.
• It mimics sexual reproduction but is asexual in nature.
• Common in some angiosperms, especially grasses.
• Useful in agriculture and horticulture for maintaining hybrid traits.

Polyembryony

• Polyembryony is the formation of more than one embryo in a single seed.
• Seen in plants like Citrus.