Sexual Reproduction in Flowering Plants

FLOWER – A FASCINATING ORGAN OF ANGIOSPERMS

A **flower** is the reproductive organ of angiosperms (flowering plants) and serves as the site of sexual reproduction. Beyond aesthetic and cultural significance, flowers are morphological and embryological marvels displaying remarkable diversity in structure and adaptation for sexual reproduction. The flower produces both male and female reproductive structures—the **androecium** (male reproductive organ consisting of stamens) and the **gynoecium** (female reproductive organ consisting of pistils). Understanding flower structure is fundamental to comprehending sexual reproduction in flowering plants.

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PRE-FERTILISATION: STRUCTURES AND EVENTS

Pre-fertilisation events involve the differentiation, development, and maturation of male and female reproductive structures before actual pollination and fertilisation occur. These structures develop from floral primordia through hormonal and structural changes, leading to the formation of inflorescences bearing floral buds and mature flowers.

THE STAMEN, MICROSPORANGIUM, AND POLLEN GRAIN

**Stamen Structure:**

A typical **stamen** consists of two main parts:

**Filament**: A long, slender stalk that is typically attached at its proximal end to the thalamus or petal

**Anther**: A terminal, generally bilobed structure representing the pollen-producing organ

A typical angiosperm **anther** is **dithecous** (having two thecae per lobe), meaning each lobe contains two theca, resulting in a total of four microsporangia at the four corners of the anther. A longitudinal groove typically separates the two theca lengthwise.

**Microsporangium Structure:**

In transverse section, a typical **microsporangium** (pollen sac) appears nearly circular and is surrounded by four wall layers:

**Epidermis**: The outermost protective layer

**Endothecium**: Provides protection and assists in anther dehiscence

**Middle Layers**: Additional protective layers

**Tapetum**: The innermost nutritive layer consisting of cells with dense cytoplasm and multiple nuclei that nourish developing pollen grains

When the anther is young, a group of compactly arranged homogenous cells called the **sporogenous tissue** occupies the centre of each microsporangium.

**Microsporogenesis:**

**Microsporogenesis** is the process of formation of microspores from pollen mother cells (PMCs) through meiotic division. Each cell of the sporogenous tissue is a potential PMC capable of undergoing meiosis to produce four haploid microspores arranged in a **microspore tetrad**. As the anther matures and dehydrates, the microspores dissociate from each other and develop into pollen grains. Several thousand pollen grains are produced per microsporangium and released upon anther dehiscence. The ploidy of microspores is **haploid (n)**.

**Pollen Grain:**

**Pollen grains** represent the **male gametophyte** of flowering plants and are typically spherical, measuring 25–50 micrometres in diameter. They possess a distinctive two-layered wall:

**Exine**: The hard outer layer composed of **sporopollenin**, one of the most resistant organic materials known. Sporopollenin can withstand high temperatures, strong acids, and alkali, and no enzyme degrading it is known. This resistance allows pollen grains to fossilize well. The exine exhibits prominent **apertures** called **germ pores** where sporopollenin is absent. These pores serve as exit sites for the pollen tube during pollen germination.

**Intine**: The inner, thin, continuous layer made of cellulose and pectin

The interior cytoplasm is surrounded by a plasma membrane. A mature pollen grain contains:

**Vegetative Cell**: The larger cell with abundant food reserves and a large, irregularly shaped nucleus; will form the pollen tube

**Generative Cell**: The smaller cell with dense cytoplasm and a nucleus that will divide to form two male gametes

In over 60% of angiosperms, pollen grains are shed at the **2-celled stage**. In the remaining species, the generative cell divides mitotically before shedding, producing **3-celled pollen grains** with two male gametes already formed.

**Pollen Viability:**

Pollen grains must land on the stigma before losing viability. Pollen viability varies: in cereals like rice and wheat, viability is retained for only 30 minutes; in Rosaceae, Leguminosae, and Solanaceae, viability lasts for months. Pollen can be stored in liquid nitrogen (–196°C) for years, enabling the creation of **pollen banks** for crop breeding programmes, analogous to seed banks.

**Pollen Allergies:**

Pollen grains from many species cause severe allergies and bronchial afflictions, leading to chronic respiratory disorders such as asthma and bronchitis. *Parthenium* (carrot grass), an invasive species introduced to India as a wheat contaminant, has become ubiquitous and is a major cause of pollen allergy.

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THE PISTIL, MEGASPORANGIUM (OVULE), AND EMBRYO SAC

**Gynoecium Structure:**

The **gynoecium** (female reproductive part) may consist of a single **monocarpellary** pistil or multiple pistils. When multiple pistils are present, they may be **syncarpous** (fused together) or **apocarpous** (free). Each pistil has three parts:

**Stigma**: The terminal landing platform for pollen grains; often possesses receptive cells

**Style**: The elongated, slender portion beneath the stigma

**Ovary**: The basal, bulged portion containing the **ovarian cavity** (locule)

Inside the ovarian cavity lies the **placenta**, from which arise the **megasporangia** (ovules). Ovule number ranges from one (wheat, paddy, mango) to many (papaya, watermelon, orchids).

**Ovule (Megasporangium) Structure:**

A typical angiosperm ovule is a small structure with the following components:

**Funicle**: A stalk attaching the ovule to the placenta

**Hilum**: The junction region between ovule and funicle

**Integuments**: One or two protective envelopes encircling the nucellus, with an opening at the micropylar end

**Micropyle**: A small opening at the tip of the ovule serving as the entry point for the pollen tube

**Chalaza**: The basal part of the ovule opposite the micropylar end

**Nucellus**: A mass of cells within the integuments containing abundant reserve food materials and the embryo sac

**Embryo Sac**: The female gametophyte, typically a single structure formed from a megaspore

**Megasporogenesis:**

**Megasporogenesis** is the process of formation of megaspores from the **megaspore mother cell (MMC)**. A single MMC differentiates in the micropylar region of the nucellus. The MMC is a large cell with dense cytoplasm and a prominent nucleus. The MMC undergoes **meiotic division** producing four haploid megaspores. The importance of MMC undergoing meiosis is to produce haploid spores for sexual reproduction, maintaining genetic diversity through recombination.

**Female Gametophyte (Embryo Sac) Development:**

In most flowering plants, one megaspore is **functional** while the other three **degenerate**. This method of embryo sac formation from a single megaspore is termed **monosporic development**.

The functional megaspore undergoes sequential mitotic divisions (all **free nuclear**, i.e., nuclear divisions without immediate cell wall formation):

**2-nucleate stage**: The megaspore nucleus divides; the two nuclei migrate to opposite poles

**4-nucleate stage**: Two more mitotic divisions occur

**8-nucleate stage**: Additional mitotic divisions produce eight nuclei

After the 8-nucleate stage, cell walls are formed, organizing the mature embryo sac. The **ploidy of the functional megaspore and all embryo sac cells is haploid (n)**.

**Mature Embryo Sac Organization:**

Although 8-nucleate, the mature embryo sac is **7-celled** with characteristic distribution:

**Micropylar End**: The **egg apparatus** consisting of three cells:

**Egg Cell**: The female gamete

**Two Synergids**: Smaller cells flanking the egg cell; possess **filiform apparatus** (special cellular thickenings at the micropylar tip) that guide the pollen tube into the synergid

**Central Cell**: Contains two **polar nuclei** (nuclei without surrounding cell walls)

**Chalazal End**: Three **antipodal cells** (nutritive cells)

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POLLINATION

**Pollination** is the transfer of pollen grains from the anther to the stigma of a pistil. This mechanism is essential because both male and female gametes are non-motile. Flowering plants have evolved diverse adaptations to utilize external agents for pollination.

**Types of Pollination (Based on Pollen Source):**

**1. Autogamy (Self-pollination):**

Transfer of pollen from the anther to the stigma of the **same flower**. Complete autogamy is rare in flowers that open and expose anthers and stigma, as it requires:

Synchrony between pollen release and stigma receptivity

Anthers and stigma positioned close together

**Cleistogamous flowers** (non-opening flowers) facilitate autogamy:

Flowers do not open

Anthers and stigma lie very close together

When anthers dehisce in the flower bud, pollen contacts the stigma

Ensures seed-set even without pollinators

Examples: *Viola* (pansy), *Oxalis*, *Commelina*

**Chasmogamous flowers** (opening flowers) are typical flowers with exposed anthers and stigma.

**2. Geitonogamy:**

Transfer of pollen from the anther to the stigma of **another flower of the same plant**. Although functionally a cross-pollination involving pollinators, it is **genetically equivalent to autogamy** because pollen originates from the same plant.

**3. Xenogamy (Cross-pollination):**

Transfer of pollen from the anther to the stigma of a **different plant**. This is the **only type that introduces genetic diversity** through the combination of pollen from different plants.

**Agents of Pollination:**

**Abiotic agents**: Wind and water

**Biotic agents**: Animals (insects, birds, mammals)

Most flowering plants use biotic agents; only a small proportion depend on abiotic agents. Pollen grains must be structurally and biochemically adapted to their respective agents.

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POLLEN-PISTIL INTERACTION

Following pollen deposition on the stigma, a series of interactions occurs between the pollen grain and the pistil before pollen tube growth and eventual fertilisation.

**Pollen-Stigma Interaction:**

When a pollen grain lands on a receptive stigma, it adheres due to:

**Stigmatic secretions**: Proteins, lipids, and carbohydrates on the stigma surface

**Pollen wall chemistry**: Compatibility between pollen exine and stigma surface

**Pollen Germination:**

A compatible pollen grain germinates by:

Absorbing moisture from the stigma

The **generative cell** divides mitotically to form **two sperm cells** (male gametes) if not already formed (3-celled pollen stage)

The **vegetative cell** develops into the **pollen tube**

The pollen tube grows through the stigma and style toward the ovule, guided by:

Chemotropic signals from the ovule

Physical rupture through stigmatic and stylar tissue

High osmotic potential maintaining turgor pressure

**Pollen Tube Growth:**

Growth is **apical** (from the tip)

The tip is bounded by a **plasma membrane** and **cell wall**

Contains the two sperm cells in the cytoplasm

Growth rate varies: hours to days depending on species

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DOUBLE FERTILISATION

**Double fertilisation** is the characteristic reproductive process of flowering plants, occurring only in angiosperms. It involves two simultaneous fusion events within the embryo sac.

**Events of Double Fertilisation:**

**1. Syngamy (Fusion of Egg and One Sperm):**

The pollen tube penetrates the ovule through the micropyle

The tube ruptures in the synergid region

The two sperm cells are released

**One sperm nucleus fuses with the egg nucleus (female gamete)** forming the **diploid (2n) zygote**

The zygote develops into the embryo

**2. Triple Fusion (Fusion of Second Sperm with Polar Nuclei):**

**The second sperm nucleus fuses with the two polar nuclei** of the central cell

This produces a **triploid (3n) nucleus** called the **polar fusion nucleus**

This nucleus undergoes mitotic divisions to form the **endosperm**, the nutritive tissue of the seed

**Significance of Double Fertilisation:**

**Endosperm is triploid (3n)**, while the embryo is diploid (2n)

Endosperm is haploid with respect to maternal genes, ensuring no nutritional drain on maternal tissue

Prevents parthenogenesis (development of unfertilized ovules)

Ensures rapid seed development

The **filiform apparatus** of synergids plays a crucial role by guiding the pollen tube to the micropylar synergid.

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POST-FERTILISATION: STRUCTURES AND EVENTS

Following double fertilisation, dramatic changes occur in the flower, ovule, and ovary, transforming them into the fruit and seed.

**Development of the Zygote (Embryogenesis):**

The zygote undergoes **mitotic divisions** to form the embryo:

**First Division**: Transverse division producing a **basal cell** and an **apical cell**

**Basal Cell**: Develops into the **suspensor** (a filament-like structure anchoring the developing embryo and transferring nutrients)

**Apical Cell**: Undergoes repeated divisions forming the **embryo proper**

The embryo develops cotyledons (seed leaves), epicotyl, hypocotyl, and radicle

**Dicots**: Two cotyledons; **Monocots**: One cotyledon

**Development of Endosperm:**

The polar fusion nucleus (3n) undergoes rapid **free nuclear mitotic divisions**, producing multiple nuclei in the central cell without immediate cell wall formation. Subsequently, cell walls form, creating the **endosperm tissue**, which:

Stores nutrients (starch, proteins, oils)

Gets consumed or persists in the mature seed

In **albuminous seeds**, endosperm is present at maturity (*Castor bean, Wheat*)

In **non-albuminous seeds**, endosperm is consumed by the embryo (*Pea, Bean*)

**Seed Maturation:**

As the seed develops:

The integuments develop into the **seed coat (testa)**

The nucellus may persist as the **perisperm** (in some seeds)

The funicle may persist, sometimes developing into a **caruncle** (an outgrowth assisting in water absorption) or **aril** (a seed covering)

Food reserves accumulate in the cotyledons or endosperm

The seed coat becomes hard and protective

Seed enters a state of **dormancy** (quiescence)

**Fruit Development:**

The **ovary wall** develops into the **fruit wall** (pericarp)

The pericarp may be **dry** (*Mustard, Pea*) or **fleshy** (*Mango, Guava*)

The **receptacle** and other floral parts may become incorporated into the fruit

The fruit serves for **seed protection** and **seed dispersal**

**Changes in the Flower:**

**Sepals, petals, stamens, and style** typically wither and are shed

**Ovary** enlarges as the fruit develops

**Receptacle** may contribute to fruit formation

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APOMIXIS AND POLYEMBRYONY

**Apomixis:**

**Apomixis** is asexual reproduction through seeds, bypassing meiosis and fertilisation. The seed develops from a somatic cell of the ovule without the formation of gametes.

**Types of Apomixis:**

**1. Gametophytic Apomixis:**

The **female gametophyte** develops without meiosis

The **megaspore mother cell** develops into the embryo sac through **mitotic divisions** instead of meiosis

All cells of the embryo sac are genetically identical to the parent plant

Example: *Dandelion*, *Hawkweed*

**2. Sporophytic Apomixis:**

The embryo develops from **somatic cells** of the nucellus or integument (**nucellar embryony** or **integumentary embryony**)

The parent plant's genetic material is directly transferred to the seed

Example: *Mango*, *Citrus*

**Advantages of Apomixis:**

Produces genetically identical offspring (clones)

Seeds develop even without pollination

Ensures uniform quality and characteristics in crop varieties

Saves time compared to sexual reproduction

**Disadvantages:**

Reduces genetic variation, reducing adaptability to environmental changes

No opportunity for genetic recombination and natural selection

**Polyembryony:**

**Polyembryony** is the development of **multiple embryos within a single ovule**. This results in seeds containing more than one embryo.

**Types of Polyembryony:**

**1. True Polyembryony:**

Results from the **division of the zygote** (cleavage polyembryony)

All embryos are genetically identical to each other

Example: *Citrus*, *Mango*

**2. False Polyembryony:**

Results from **apomixis** (sporophytic apomixis)

Additional embryos develop from nucellar or integumentary tissue

The apomictic embryo is identical to the mother plant

The sexual embryo (from zygote) is genetically different

Example: *Mango*, *Citrus*, *Guava*

**Significance of Polyembryony:**

Increases the number of seeds per ovule

Ensures reproductive success if some embryos are damaged

In crops like citrus, allows propagation of superior varieties

Provides backup embryos enhancing germination rates

**Parthenocarpy:**

**Parthenocarpy** is the development of **fruits without fertilisation**, resulting in **seedless fruits**. Parthenocarpic fruits may develop from:

Unfertilized ovules

Without pollination or fertilisation

Response to specific hormones or environmental conditions

**Examples:**

**Naturally parthenocarpic**: Banana, Pineapple

**Induced parthenocarpy**: Application of auxins or gibberellins to flowers (e.g., seedless grapes)

**Advantages:**

Seedless fruits are commercially valuable

Improved consumer preference (e.g., seedless grapes, watermelons)

Predictable fruit quality

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SIGNIFICANCE OF SEED DISPERSAL

**Seed Dispersal** is the mechanism by which seeds are transported away from the parent plant to new locations. This is crucial for plant survival and distribution.

**Mechanisms of Seed Dispersal:**

**1. Wind Dispersal (Anemochory):**

Seeds possess **wings** or **hair-like structures** reducing density

Examples: *Maple* (winged seeds), *Dandelion* (plumed seeds), *Thistle* (seeds with parachute-like structures)

Allows colonization of distant areas

Requires light seeds

**2. Water Dispersal (Hydrophily):**

Seeds are waterproof and buoyant

Dispersal through water bodies

Examples: *Coconut*, *Water lily*, *Mangrove* seeds

Allows colonization across water barriers

**3. Animal Dispersal (Zoochory):**

**Endozoochory**: Animals ingest seeds; seeds pass through digestive system and are excreted in feces

Examples: Fleshy fruits like *Mango*, *Guava*, *Berries*

Seed coat is resistant to digestive acids

**Epizoochory**: Seeds attach to animal fur/feathers via hooks, spines, or adhesive surfaces

Examples: *Bur*, *Prickly seeds* of *Xanthium*, *Bidens*

**Synzoochory**: Animals intentionally collect and transport seeds (squirrels, birds storing nuts)

**4. Mechanical Dispersal (Autochory):**

**Dehiscence**: Fruits forcefully eject seeds due to turgor pressure or desiccation

Examples: *Legumes* (*Pea*, *Bean*), *Ricinus* (castor bean)

Allows immediate spreading around the plant

**Significance of Seed Dispersal:**

**Reduces Intraspecific Competition**: Offspring are not in direct competition with parents for light, water, and nutrients

**Colonization**: Enables plants to colonize new habitats and geographic areas

**Genetic Diversity**: Allows gene flow among populations

**Reduces Pathogen Concentration**: Dispersed seeds avoid concentrated pathogen populations around parent plants

**Evolutionary Advantage**: Increases survival probability of offspring in new environments

**Ecosystem Distribution**: Maintains genetic diversity within plant populations and ecosystems

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This comprehensive coverage of sexual reproduction in flowering plants provides complete preparation for board examination questions on flower structure, gametogenesis, pollination, fertilisation, fruit and seed development, apomixis, polyembryony, and seed dispersal.