Chapter 1: Sexual Reproduction in Flowering Plants

Class 12 Biology | NCERT Textbook Based | RBSE CBSE Board 2025-26

1. Introduction

Reproduction is a fundamental biological process by which organisms produce new individuals of their own kind, ensuring the continuity of species. In the plant kingdom, reproduction occurs through two primary mechanisms: asexual reproduction and sexual reproduction.

Sexual reproduction brings about genetic variation through the combination of genetic material from two parents. This genetic diversity increases the ability of populations to adapt to changing environmental conditions and provides the raw material for evolution.

Flowering plants or angiosperms are the most advanced and successful group of plants in the plant kingdom. They comprise more than 2,50,000 species. Their success is largely attributed to their highly specialized reproductive structures and unique reproductive mechanisms.

Flowering plants exhibit an alternation of generations. The plant body we see represents the sporophyte, which is the dominant, diploid (2n) phase. The gametophyte phase is highly reduced and consists of only a few cells. The male gametophyte (pollen grain) and female gametophyte (embryo sac) are produced within the flower.

2. Structure of a Flower

The flower is the reproductive unit in angiosperms. It is a modified shoot with a condensed axis bearing whorls of floral appendages. A typical flower consists of four types of floral appendages borne on a swollen end of the stalk called thalamus or receptacle.

2.1 Parts of a Flower

The four whorls of a typical flower are arranged as follows:

Calyx: The outermost whorl of the flower consisting of sepals. Sepals are generally green and perform the function of protection for the flower in the bud stage. The calyx may be gamosepalous (sepals united) or polysepalous (sepals free).

Corolla: The whorl composed of petals. Petals are usually brightly coloured to attract insects for pollination. The corolla may be gamopetalous (petals united) or polypetalous (petals free). In some flowers, the calyx and corolla are not distinct and are termed perianth.

Androecium: The third whorl consisting of stamens. Each stamen consists of a slender filament with an anther at its tip. The anther is a bilobed, dithecous structure. Each lobe contains two pollen sacs (microsporangia), making the anther tetrasporangiate. The androecium represents the male reproductive part of the flower.

Gynoecium: The innermost whorl consisting of pistils or carpels, representing the female reproductive part. Each pistil has three parts: stigma (receptive surface for pollen), style (elongated tube), and ovary (swollen basal part containing ovules). The gynoecium may be monocarpellary (one carpel) or multicarpellary (many carpels). In multicarpellary gynoecium, carpels may be free (apocarpous) or united (syncarpous).

2.2 Types of Flowers

Based on the presence of reproductive whorls, flowers may be classified as:

Bisexual or Hermaphrodite: Flowers having both androecium and gynoecium. Examples include mustard, rose, Hibiscus.

Unisexual: Flowers having either androecium or gynoecium. Staminate flowers have only stamens, while pistillate flowers have only pistils. Plants bearing unisexual flowers may be monoecious (both types on the same plant, e.g., maize, coconut) or dioecious (male and female flowers on different plants, e.g., papaya, date palm).

3. Pre-fertilization: Structures and Events

Pre-fertilization events include the development and maturation of reproductive structures and the formation of male and female gametes.

3.1 The Stamen, Microsporangium and Male Gametophyte

A. Structure of Microsporangium

A transverse section of a young anther reveals a typical structure consisting of four microsporangia (pollen sacs) located at the corners of the anther. Each microsporangium is surrounded by four wall layers from the outside to the inside:

The center of the microsporangium contains the sporogenous tissue. Cells of the sporogenous tissue differentiate into microspore mother cells, which undergo meiotic divisions to form microspore tetrads.

B. Microsporogenesis

Microsporogenesis is the process of formation of microspores from a pollen mother cell (PMC) or microspore mother cell through meiosis.

Cells of the sporogenous tissue differentiate into microspore mother cells. Each microspore mother cell (2n) divides meiotically to produce four haploid (n) microspores. These four microspores initially remain together as a cluster called a microspore tetrad. As the anther matures and dehydrates, the microspores separate from one another and develop into pollen grains.

C. The Pollen Grain

Pollen grains represent the highly reduced male gametophytes. Generally, pollen grains are spherical measuring about 25-50 micrometers in diameter.

Pollen Grain Wall: The pollen grain has a prominent two-layered wall. The hard outer layer is called the exine, which is made up of sporopollenin. Sporopollenin is one of the most resistant organic materials known and can withstand high temperatures, strong acids and alkali, and enzymatic degradation. The inner wall is called the intine, which is made up of cellulose and pectin. The exine has prominent apertures called germ pores where sporopollenin is absent. These germ pores are the sites where the pollen tube emerges during germination.

Pollen Grain Content: When a pollen grain is shed at the two-celled stage, it consists of two cells: a larger vegetative cell and a smaller generative cell. The vegetative cell has abundant food reserve and a large irregularly shaped nucleus. The generative cell is spindle-shaped with dense cytoplasm and a nucleus. In over 60 percent of angiosperms, pollen grains are shed at the two-celled stage. In the remaining species, the generative cell divides mitotically to give rise to two male gametes before pollen grains are shed (three-celled stage).

Pollen Viability and Storage

Pollen grains of different species exhibit considerable variation in viability. In some cereals such as rice and wheat, pollen grains lose viability within 30 minutes to a few hours of their release. In some members of Rosaceae, Leguminosae and Solanaceae, pollen grains remain viable for months. Pollen grains can be stored for years in liquid nitrogen. Such stored pollen can be used as pollen banks in crop breeding programmes.

Pollen Allergy and Pollen Products

Pollen grains of many species cause severe allergies and bronchial afflictions in some people, often leading to chronic respiratory disorders such as asthma and bronchitis. Parthenium or carrot grass, which came to India as a contaminant with imported wheat, is an example of such an allergen. It is a common weed that causes pollen allergy.

Pollen grains are rich in nutrients. It has become a fashion to use pollen tablets as food supplements. In Western countries, a large number of pollen products in the form of tablets and syrups are available in the market. Pollen consumption has been claimed to increase the performance of athletes and race horses.

D. Microgametogenesis

The process of development of the male gametophyte from a microspore is called microgametogenesis. The nucleus of the microspore divides mitotically to give rise to two nuclei: the generative nucleus and the vegetative nucleus. A thin wall is laid down separating the two nuclei, forming a larger vegetative cell and a smaller generative cell. This is the two-celled stage of the pollen grain. The generative cell subsequently divides mitotically to form two male gametes, either before pollen release or after pollen germination on the stigma.

3.2 The Pistil, Megasporangium and Female Gametophyte

A. Structure of an Ovule

The ovule is a small structure attached to the placenta by means of a stalk called funicle. The body of the ovule fuses with the funicle in the region called hilum. Thus, hilum represents the junction between ovule and funicle.

Each ovule has one or two protective envelopes called integuments. Integuments encircle the ovule except at the tip where a small opening called the micropyle is organized. The micropyle facilitates entry of the pollen tube into the ovule at the time of fertilization.

The main body of the ovule consists of parenchymatous cells called the nucellus. Cells of the nucellus provide nutrition to the developing structures. The chalaza is the basal part of the ovule where the nucellus and integuments are joined. Located in the nucellus is the embryo sac or female gametophyte.

Types of Ovules

Ovules can be classified based on the orientation of the micropyle with respect to the funicle and chalaza:

Orthotropous: In this type, the micropyle is in a straight line with the hilum and chalaza. The ovule is straight and erect. Example: Polygonum.

Anatropous: This is the most common type of ovule. Here, the ovule is inverted so that the micropyle comes to lie close to the hilum. Example: Capsella, Helianthus.

Hemianatropous: In this type, the ovule is placed transversely at right angles to the funicle. Example: Ranunculus.

B. Megasporogenesis

Megasporogenesis is the process of formation of megaspores from the megaspore mother cell (MMC). In a typical angiosperm ovule, a single megaspore mother cell is differentiated in the micropylar region of the nucellus. The megaspore mother cell is diploid (2n). It undergoes meiotic division to produce four haploid (n) megaspores.

In most plants, only one of the four megaspores is functional while the other three degenerate. The functional megaspore develops into the female gametophyte (embryo sac). This method of embryo sac formation from a single megaspore is termed monosporic development. The pattern of embryo sac development described here is termed the Polygonum type and is exhibited by nearly 70 percent of angiosperms.

C. Female Gametophyte (Embryo Sac)

The development of the female gametophyte or embryo sac from the functional megaspore is called megagametogenesis. The nucleus of the functional megaspore divides mitotically to form two nuclei which move to opposite poles, forming a 2-nucleate embryo sac. Two more sequential mitotic nuclear divisions result in the formation of a 4-nucleate and later an 8-nucleate stage of the embryo sac.

After the 8-nucleate stage, cell walls are laid down leading to the organization of the typical female gametophyte or embryo sac. Six of the eight nuclei are enclosed in cells while the remaining two nuclei remain free. A typical mature embryo sac of an angiosperm consists of:

At the micropylar end: Three cells constitute the egg apparatus. The egg apparatus consists of two synergids (each with filiform apparatus) and one egg cell. The synergids have special cellular thickenings at the micropylar tip called filiform apparatus, which play an important role in guiding the pollen tube into the embryo sac.

At the chalazal end: Three cells called the antipodals are present. The antipodals are generally ephemeral.

In the center: There is a large central cell which has two polar nuclei. These two polar nuclei may fuse to form a secondary nucleus (diploid, 2n).

Thus, a typical embryo sac at maturity is 7-celled (though it has 8 nuclei).

     Epidermis
     ↓
   ┌─────────────────┐
   │▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓│ ← Single layer
   ├─────────────────┤
   │▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒│ ← Endothecium (fibrous)
   ├─────────────────┤
   │░░░░░░░░░░░░░░░░░│ ← Middle layers (2-3)
   ├─────────────────┤
   │█████████████████│ ← Tapetum (nutritive)
   │                 │
   │   Pollen Sacs   │ ← Microsporangia
   │   (4 in total)  │    with sporogenous
   │                 │    tissue
   └─────────────────┘
   
Note: Anther is tetrasporangiate (4 pollen sacs)
Connective tissue joins the two lobes

        MICROPYLAR END
             ↓
    ┌─────────────────┐
    │   Synergid (n)  │ ← With filiform
    │  [Filiform app.]│    apparatus
    ├─────────────────┤
    │   Egg cell (n)  │ ← Female gamete
    ├─────────────────┤  } EGG APPARATUS
    │   Synergid (n)  │ ← With filiform    (3 cells)
    │  [Filiform app.]│    apparatus
    └─────────────────┘
    
    ╔═════════════════╗
    ║  CENTRAL CELL   ║
    ║                 ║
    ║   ●  ←  →  ●    ║ ← 2 Polar nuclei (n+n)
    ║   Polar nuclei  ║    May fuse → 2n
    ║                 ║
    ╚═════════════════╝
    
    ┌─────┬─────┬─────┐
    │ AC  │ AC  │ AC  │ ← Antipodal cells
    │ (n) │ (n) │ (n) │    (3 cells)
    └─────┴─────┴─────┘
             ↑
        CHALAZAL END

Total: 7 cells, 8 nuclei
All nuclei are haploid (n)

4. Pollination

Pollination is the transfer of pollen grains from the anther to the stigma of a flower. Based on the source of pollen, pollination can be divided into three types:

4.1 Types of Pollination

Autogamy: Transfer of pollen grains from the anther to the stigma of the same flower. Autogamy is a form of self-pollination and can occur only in bisexual flowers. In some plants like Viola (common pansy), Oxalis, and Commelina, two types of flowers are produced: chasmogamous flowers which are similar to flowers of other species with exposed anthers and stigma, and cleistogamous flowers that do not open at all. In such flowers, the anthers and stigma lie close to each other. When anthers dehisce in the flower buds, pollen grains come in contact with the stigma to effect pollination. Thus, cleistogamous flowers are invariably autogamous as there is no chance of cross-pollen landing on the stigma. Cleistogamous flowers produce assured seed-set even in the absence of pollinators.

Geitonogamy: Transfer of pollen grains from the anther to the stigma of another flower of the same plant. Although geitonogamy is functionally cross-pollination, genetically it is similar to autogamy since the pollen grains come from the same plant.

Xenogamy: Transfer of pollen grains from anther to the stigma of a different plant. This is the only type of pollination which brings genetically different types of pollen grains to the stigma during pollination.

4.2 Agents of Pollination

Various agents such as wind, water, and animals (especially insects) help in the transfer of pollen grains.

Anemophily (Wind Pollination): Pollination by wind is more common amongst abiotic agents of pollination. Wind pollinated flowers are characterized by production of enormous amounts of pollen grains compared to the number of ovules available for pollination. Pollen grains are light and non-sticky so that they can be transported in wind currents. They often possess well-developed wings. The stigmas are large, feathery and are exposed to wind currents. Wind pollinated flowers often have a single ovule in each ovary and numerous flowers packed into an inflorescence. Examples include maize and grasses.

Hydrophily (Water Pollination): Pollination by water is quite rare in flowering plants and is limited to about 30 genera, mostly monocotyledons. It is important to note that not all aquatic plants use water for pollination. In most aquatic plants such as water hyacinth and water lily, the flowers emerge above the level of water and are pollinated by insects or wind as in most of the land plants. However, in some plants like Vallisneria, the female flower reaches the surface of water by a long stalk and the male flowers or pollen grains are released on to the surface of water. They are carried passively by water currents. Some of them reach the female flowers and the stigma. In another group of water plants, pollination occurs inside the water and the pollen grains are released inside the water. They are carried by water currents to the female flower and the stigma. Example: Zostera (sea grass). In Zostera, the pollen grains are long, ribbon-like and they are carried passively inside the water.

Entomophily and Zoophily (Pollination by Animals): Insects, particularly bees, butterflies, flies, beetles and wasps are the dominant biotic pollinating agents. Even larger animals such as some birds and bats are also pollinators. The animal-pollinated flowers are characterized by coloured petals, presence of nectar and fragrance. They are large sized, produce scent and nectar. Flowers pollinated by flies and beetles secrete foul odours to attract these animals.

5. Outbreeding Devices

Continued self-pollination results in inbreeding depression. Flowering plants have developed many devices to discourage self-pollination and to encourage cross-pollination. Some important devices that facilitate outcrossing or cross-pollination are:

Pollen Release and Stigma Receptivity are not Synchronized: In some species, pollen release and stigma receptivity are not synchronized. If the pollen is shed before the stigma becomes receptive, the condition is called protandry. This is found in many species. If the stigma becomes receptive before pollen is released, the condition is called protogyny.

Anther and Stigma Placed at Different Positions: In some plants, the anthers and the stigma are placed at different positions so that the pollen cannot come in contact with the stigma of the same flower. This promotes cross-pollination.

Self-Incompatibility: This is a genetic mechanism which prevents self-pollen (from the same flower or other flowers of the same plant) from fertilizing the ovules by inhibiting pollen germination or pollen tube growth in the pistil.

Production of Unisexual Flowers: In several species, production of unisexual flowers either staminate (male) or pistillate (female) prevents autogamy. In monoecious plants where both male and female flowers are present on the same plant, such as in maize and castor, autogamy is prevented but geitonogamy is possible. In dioecious plants, where male and female flowers are present on different individual plants, both autogamy and geitonogamy are prevented, ensuring xenogamy only.

6. Pollen-Pistil Interaction

In majority of flowering plants, the pollen grains are released at the two-celled stage. After dispersal, pollen grains have to land on the stigma before they lose viability if they have to bring about fertilization.

It is the stigma which must recognize the pollen, whether it is of the right type (compatible) or of the wrong type (incompatible). The ability of the pistil to recognize the pollen followed by its acceptance or rejection is the result of a continuous dialogue between pollen grain and the pistil. This dialogue is mediated by chemical components of the pollen interacting with those of the pistil.

Following compatible pollination, the pollen grain germinates on the stigma to produce a pollen tube through one of the germ pores. The contents of the pollen grain move into the pollen tube. If the pollen is at the two-celled stage, the generative cell divides and forms the two male gametes during the growth of the pollen tube in the stigma. The pollen tube grows through the tissues of the stigma and style and reaches the ovary. After entering the ovary, it enters the ovule through the micropyle and then enters one of the synergids through the filiform apparatus.

7. Artificial Hybridisation

In plant breeding programmes, it is necessary to make crosses between plants having desirable characters for crop improvement. For such crosses, it is essential to make use of pollen grains of the male parent and prevent self-pollen from contaminating the stigma. Artificial hybridization is one of the major approaches of crop improvement programmes.

7.1 Emasculation

In bisexual flowers, removal of anthers from the flower bud before the anther dehisces using a pair of forceps is referred to as emasculation. Emasculated flowers have to be covered with a bag, usually a butter paper bag, to prevent contamination of its stigma with unwanted pollen. This process is called bagging. When the stigma of bagged flower attains receptivity, mature pollen grains collected from anthers of the male parent are dusted on the stigma, and the flowers are rebagged. Fruits developed from such manipulated flowers are collected and the seeds are separated. These seeds on germination produce hybrid plants with desired characters.

7.2 Technique for Unisexual Flowers

In unisexual flowers, there is no need for emasculation. The female flower buds are bagged before the flowers open. When the stigma becomes receptive, pollination is carried out using the desired pollen and the flower rebagged.

8. Double Fertilisation

Fertilization in angiosperms is a unique phenomenon because it involves two fusion events. This is called double fertilisation. Double fertilisation is a characteristic feature of angiosperms and was discovered by Sergei Nawaschin in 1898.

After reaching the ovule through the micropyle, the pollen tube enters one of the synergids through the filiform apparatus. The filiform apparatus present at the micropylar part of the synergid guides the entry of the pollen tube. The pollen tube releases the two male gametes into the cytoplasm of the synergid.

One of the male gametes moves towards the egg cell and fuses with its nucleus, thus completing the syngamy. This results in the formation of a diploid cell, the zygote. The other male gamete moves towards the two polar nuclei located in the central cell and fuses with them to produce a triploid primary endosperm nucleus (PEN). Since this process involves the fusion of three haploid nuclei, it is termed triple fusion.

Thus, the two events of fusion (syngamy and triple fusion) in an embryo sac constitute the phenomenon of double fertilisation which is unique to flowering plants.

9. Post-fertilisation: Structures and Events

After fertilisation, the zygote develops into the embryo and the primary endosperm nucleus forms the endosperm. The ovules develop into seeds and the ovary matures into a fruit.

9.1 Endosperm Development

The primary endosperm cell divides repeatedly and forms a triploid endosperm tissue. The cells of the endosperm tissue are filled with reserve food materials and are used for the nutrition of the developing embryo. In the most common type of endosperm development (called nuclear type), the primary endosperm nucleus undergoes successive nuclear divisions without cell wall formation, thus forming a large number of free nuclei. Subsequently, cell wall formation occurs and the tissue becomes cellular. The mature seeds of coconut, castor and many cereals have endosperm and are called albuminous or endospermic seeds.

9.2 Embryo Development

The zygote is the first cell of the sporophyte. It divides to form a proembryo and later a globular embryo. The embryo develops at the micropylar end of the embryo sac. The parts of a mature embryo are the embryonal axis and one or two cotyledons.

Dicot Embryo: A typical dicot embryo consists of an embryonal axis and two cotyledons. The portion of the embryonal axis above the level of cotyledons is called epicotyl, which terminates with the plumule or stem tip. The cylindrical portion below the level of cotyledons is called hypocotyl, which terminates with the radicle or root tip. The root tip is covered with a root cap.

Monocot Embryo: The structure of a monocot embryo is different from that of a dicot. In the grass family, the cotyledon is called scutellum which is situated towards one side (lateral) of the embryonal axis. At its lower end, the embryonal axis has the radicle and root cap enclosed in an undifferentiated sheath called coleorhiza. The portion of the embryonal axis above the level of attachment of scutellum is called epicotyl. The epicotyl has a shoot apex and a few leaf primordia enclosed in a hollow, foliar structure called the coleoptile.

DICOT EMBRYO (Bean)          MONOCOT EMBRYO (Grass)

    ┌─ Plumule                    ┌─ Coleoptile
    │  (Shoot tip)                │  (Protective sheath
    ├─ Epicotyl                   │   around shoot)
    │                             ├─ Shoot apex
  ┌─┴─┐ Cotyledons               │
  │   │ (2 in dicots)             ├─ Epicotyl
  │   │                           │
  └─┬─┘                           ├─ Scutellum
    ├─ Hypocotyl                  │  (Single cotyledon)
    │                             │
    ├─ Radicle                    ├─ Radicle
    │  (Root tip)                 │
    └─ Root cap                   └─ Coleorhiza
                                     (Root sheath)

9.3 Seed Development

The ovules after fertilisation develop into seeds. A seed is made up of a seed coat and an embryo. The embryo consists of cotyledons, an embryonal axis and sometimes the endosperm.

Seeds are of two types based on the presence or absence of endosperm in mature seeds:

Albuminous or Endospermic Seeds: In these seeds, the endosperm persists and stores food that is used during germination of seeds. Examples include wheat, maize, barley, castor, and coconut.

Non-albuminous or Non-endospermic Seeds: In these seeds, the endosperm is completely consumed during embryo development, and the embryo stores food in its cotyledons. Examples include pea, groundnut, bean.

Perisperm

In some seeds such as black pepper, beet, and Canna, remnants of the nucellus persist. This residual, persistent nucellus is the perisperm. The perisperm also stores nutrients.

Integuments and Seed Coat

The integuments of the ovule become the seed coat after fertilisation. The seed coat has two layers: the outer testa and the inner tegmen. The hilum is a scar on the seed coat through which the developing seeds were attached to the fruit. Above the hilum is a small pore called the micropyle.

9.4 Fruit Development

The fruit is a mature or ripened ovary, developed after fertilisation. During fruit formation, the wall of the ovary develops into the wall of the fruit called pericarp.

True Fruits: Fruits that develop from the ovary are called true fruits. The pericarp may be dry or fleshy.

False Fruits: In some fruits, besides the ovary wall, the thalamus also contributes to fruit formation. Such fruits are called false fruits. Examples include apple, strawberry, and cashew. In apple, the fleshy receptacle forms the edible part.

Parthenocarpy

In some plants, fruits are formed without fertilisation. This phenomenon is called parthenocarpy, and such fruits are seedless. Parthenocarpic fruits can be induced by the application of auxins (growth hormones such as IAA and NAA). Examples of parthenocarpic fruits include banana, grapes, and pineapple.

10. Apomixis and Polyembryony

10.1 Apomixis

In some species, seeds are formed without fertilisation and such a type of reproduction is called apomixis. In apomixis, the diploid egg cell is formed without reduction division and develops into an embryo without fertilisation. Apomixis is a form of asexual reproduction that mimics sexual reproduction. Since apomictic seeds are not products of genetic recombination, the plants produced from such seeds are genetically similar to the parent plant.

The phenomenon of apomixis is of great importance to the plant breeders because if they are able to introduce this property in hybrid plants, they could maintain hybrid vigor in successive generations. However, apomixis has not been successfully exploited commercially to date because the genetic mechanism of apomixis is complex and incompletely understood.

10.2 Polyembryony

The occurrence of more than one embryo in a seed is called polyembryony. Polyembryony is common in some varieties of Citrus and in mango. In such cases, besides the embryo resulting from the fertilized egg, additional embryos develop from the synergids, antipodals, integuments or nucellus. These additional embryos arise from cells other than the fertilized egg, representing a form of asexual reproduction within the seed.

11. Practice Questions

Multiple Choice Questions

Very Short Answer Questions (1 mark)

Short Answer Questions (2-3 marks)

Long Answer Questions (5 marks)