Sexual Reproduction in Flowering Plants Chapter Notes | Biology Class 12 - NEET PDF Download

What is a Flower?

A flower is the reproductive structure of an angiosperm (flowering plant). It produces the sexual gametes and organs necessary for seed formation through pollination and fertilisation. Flowers vary widely in size, shape, colour, presence of fragrance and nectar, and in their adaptations to different pollinators (insects, birds, mammals) or to abiotic agents (wind, water).

Parts of a Flower

Structure of Flower

A typical flower is organised into concentric whorls. These include accessory (sterile) whorls and reproductive whorls.

(i) Accessory whorls

• Calyx: outermost whorl composed of sepals. In bud stage it encloses other floral parts. Usually green but may be petaloid (coloured like petals) in some species. Calyx may be prominent or reduced/absent.
• Corolla: the second whorl composed of petals. Petals are often coloured and sometimes fragrant; they attract pollinators and guide them to nectar.
  

(ii) Reproductive whorls

• Androecium: male reproductive whorl; smallest unit is the stamen.
• Gynoecium: female reproductive whorl; smallest unit is a carpel (also called the pistil when a single or fused set of carpels forms the ovary, style and stigma).

Pre‑fertilisation: Structures and Events

The decision to flower involves complex physiological and hormonal regulation. Floral primordia develop into flowers that form in inflorescences. Within each flower the male and female organs (androecium and gynoecium) differentiate and develop before gamete formation and pollination.

Stamen, Microsporangium and Pollen Grain

Stamen — the male organ of a flower. A collection of stamens is called the androecium. Each stamen has:

• Filament: a slender stalk that supports the anther; attached to the thalamus (receptacle) or directly to other floral parts.
• Anther: the pollen‑producing head of the stamen. It contains microsporangia (pollen sacs) where pollen grains develop.

Structure of Stamen

Structure of an Anther

• Most angiosperm anthers are dithecous (two lobes) with each lobe containing two thecae (four pollen sacs in total).
• An anther is often tetragonal in cross‑section with four microsporangia arranged in the corners.
• A longitudinal groove (connective) may separate the two thecae in a lobe.
• Microsporangia (pollen sacs) contain the developing pollen grains (microspores).

Microsporangium

A typical microsporangium (pollen sac) has a near‑circular outline in transverse section and is enclosed by four wall layers:

• Epidermis
• Endothecium
• Middle layers
• Tapetum (innermost layer)

Enlarged view of one microsporangium 

A mature dehisced anther

• The epidermis, endothecium and middle layers provide protection and assist in anther dehiscence (opening) to release pollen.
• The tapetum nourishes developing pollen grains; tapetal cells are metabolically active and may be binucleate by nuclear division without cytokinesis or by nuclear fusion.
• The central mass of cells in a young microsporangium is the sporogenous tissue, which gives rise to pollen mother cells (microspore mother cells) that undergo meiosis.

Microsporogenesis

• Sporogenous cells divide to form pollen mother cells (PMCs or microspore mother cells).
• Each PMC undergoes meiosis (reduction division) to produce a tetrad of haploid microspores — this is microsporogenesis.
• Microspores arranged as a tetrad separate (dissociate) as the anther matures and dehydrates, each microspore developing into a pollen grain.
• Thousands of pollen grains are usually produced in each microsporangium; they are released when the anther dehisces.

Pollen Grain (Male Gametophyte)

Pollen grains are the male gametophytes of angiosperms and carry the male gametes (sperm cells). They are well adapted for survival and dispersal.

Structure of Pollen Grain

Structure of a Pollen Grain

• Size typically between 25–50 μm (varies widely among species).
• Outer wall: exine, composed of sporopollenin — an extremely durable, chemically inert biopolymer that resists heat, degradation and preserves pollen as fossils.
• Exine often shows sculpturing and has apertures/germ pores where sporopollenin is thin or absent; the pollen tube emerges here.
• Inner wall: intine, thin and continuous, made of cellulose and pectins.
• The cytoplasm is bounded by the plasma membrane; mature pollen usually contains two cells — a large vegetative (tube) cell and a small generative cell.
• In >60% of angiosperms pollen is shed in the 2‑celled stage (vegetative + generative). In other species the generative cell divides before shedding, producing 3‑celled pollen (two male gametes + vegetative cell).

Scanning electron micrographs of a few pollen grains

Variability and Viability

• Pollen viability depends on temperature and humidity.
• Some cereals (e.g., rice, wheat) have pollen that loses viability within ~30 minutes of release.
• Many families (Rosaceae, Fabaceae, Solanaceae) may have pollen viable for days to months.
• Pollen can be cryopreserved in liquid nitrogen for years for seed breeding programmes.

The Pistil, Megasporangium (Ovule) and Embryo Sac

Gynoecium — the female reproductive apparatus — typically consists of one or more fused carpels forming the pistil (stigma, style, ovary). Ovules develop internally in the ovary and produce megaspores by meiosis; these develop into the female gametophyte (embryo sac).

Structure of carpel

• Stigma: the receptive tip of the pistil; often sticky or feathery to trap pollen.
• Style: a stalk connecting stigma to ovary; guides pollen tube growth toward the ovary.
• Ovary: basal part enclosing one or more ovules.
• Ovules: attached to the placenta by the funicle; the point of attachment on the ovule is the hilum. Ovules have protective integuments with an opening called the micropyle and a basal region called the chalaza. Inside is the nucellus which contains nutritive reserves and where the embryo sac forms.

Gynoecium may be simple (single carpel), apocarpous (separate carpels) or syncarpous (fused carpels forming a compound pistil).

Types of Gynoecium

Megasporangium (Ovule)

An ovule is a megasporangium where megaspore mother cells (MMC) undergo meiosis to produce megaspores. Typically one functional megaspore gives rise to the embryo sac.

• Integuments surround the nucellus except at the micropyle.
• The nucellus contains the MMC and later the developing embryo sac.
• Most ovules contain a single embryo sac formed from one megaspore (but there are exceptions with multiple embryo sacs in certain species).

(a) A dissected flower of Hibiscus showing pistil (other floral parts have been removed);(b) Multicarpellary, syncarpous pistil of Papaver ; (c) A multicarpellary, apocarpousgynoecium of Michelia; (d) A diagrammatic view of a typical anatropous ovule

Megasporogenesis

• A single large cell (the megaspore mother cell, MMC) in the nucellus undergoes meiosis to form four haploid megaspores.
• Commonly, three megaspores degenerate and one becomes the functional megaspore.

Female Gametophyte (Embryo Sac)

(i) Megaspore Development:

• In most flowering plants, one of the four megaspores formed is functional, while the other three degenerate.
• The functional megaspore develops into the female gametophyte (embryo sac).

(ii) Monosporic Development: This process, where the embryo sac forms from a single megaspore, is called monosporic development.

(iii) Ploidy Levels:

• Nucellus Cells: Diploid (2n)
• Megaspore Mother Cell (MMC): Diploid (2n)
• Functional Megaspore: Haploid (n)
• Female Gametophyte (Embryo Sac): Haploid (n)

(iv) Embryo Sac Formation:

• Initial Division: The nucleus of the functional megaspore divides mitotically to form two nuclei, which migrate to opposite poles, creating a 2-nucleate embryo sac.
• Subsequent Divisions: Two more mitotic divisions occur, leading to the 4-nucleate and then the 8-nucleate stages.
• Free Nuclear Divisions: These mitotic divisions are free nuclear, meaning they are not followed immediately by cell wall formation.

(v) Mature Embryo Sac Structure: Cell Formation: After the 8-nucleate stage, cell walls are formed, organizing the embryo sac.

(vi) Cell Distribution:

• Egg Apparatus: Three cells at the micropylar end, consisting of two synergids and one egg cell. Synergids have filiform apparatus at the micropylar tip.
• Antipodals: Three cells at the chalazal end.
• Central Cell: Contains two polar nuclei.

(vii) Final Configuration: The mature embryo sac is 7-celled but 8-nucleate.

(a) Parts of the ovule showing a large megaspore mother cell, a dyad and a tetrad of megaspores; (b) 2, 4, and 8-nucleate stages of embryo sac and a mature embryo sac; (c) A diagrammatic representation of the mature embryo sac.

Pollination

Pollination is the transfer of pollen from the anther to a compatible stigma. It is an ecological process essential for sexual reproduction in angiosperms. Pollination may be self or cross, and it may be effected by biotic (living) or abiotic (non‑living) agents.

Cross-Pollination

Kinds of Pollination

• Autogamy (self‑pollination within a single flower):
  • Pollen from anther to stigma of the same flower.
  • Requires synchrony of pollen release and stigma receptivity, and appropriate floral morphology.
  • Examples: some Viola, Oxalis, Commelina. Special forms: cleistogamous flowers (closed, ensure selfing) and chasmogamous flowers (open).
  • Advantage of cleistogamy: assured seed set without pollinators; disadvantage: reduced genetic variation.
• Geitonogamy:
  • Transfer of pollen between two different flowers of the same plant.
  • Functionally similar to cross‑pollination because a pollinating agent transports pollen, but genetically equivalent to selfing (same genotype).
• Xenogamy (cross‑pollination):
  • Pollen transfer between flowers of different plants of the same species.
  • Promotes genetic diversity and is often preferred in natural populations for evolutionary advantages.

Agents of Pollination

Pollination agents are abiotic (wind, water) or biotic (animals: insects, birds, bats, mammals). Biotic agents are responsible for the majority of angiosperm pollination.

Agents of pollination

1. Abiotic (Non‑living) Agents

(i) Wind Pollination (Anemophily)

• Common in grasses and many trees.
• Pollen grains are light, dry and non‑sticky; flowers often have exposed stamens and large feathery stigmas to intercept wind‑borne pollen.
• Often many small flowers in inflorescences and reduced perianth (not colourful, no nectar).
• Example group: cereals (grasses).

A wind-pollinated plant showing compact inflorecence and well-exposed stamens

(ii) Water Pollination (Hydrophily)

• Relatively rare; occurs in ~30 genera, mostly aquatic monocots.
• Male gamete transport by water is common in lower plant groups, but among angiosperms examples include freshwater generaVallisneria, Hydrilla, and marine seagrasses such as Zostera.
• Vallisneria: female flowers rise to surface; male pollen floats on the water surface and reaches female flowers. In some seagrasses pollen is carried submerged.

 Pollination by water in Vallisneria

2. Biotic (Living) Agents

Animal pollinators include insects (bees, butterflies, moths, flies, beetles), birds (hummingbirds, sunbirds), bats and some small mammals. Flowers pollinated by animals usually show adaptations to attract them.

• Insect‑pollinated flowers: colourful, fragrant, nectar‑producing, often with structural landing platforms or tubular corollas suited to specific insects.
• Bird‑pollinated flowers: often tubular, brightly coloured (reds/oranges), with nectar accessible to bird bills; pollen often placed on head/bill.
• Bat‑pollinated flowers: usually open at night, strong odour, often large and pale or dull coloured, produce copious nectar (examples: Kigelia africana, Anthocephalus cadamba, Bauhinia monandra).

Insect Pollination

Flower Adaptations and Rewards

• Many animal‑pollinated flowers provide nectar and pollen as rewards.
• Some attract pollinators through deception (mimicry of female insects, imitation of carrion odour to attract flies/beetles).
• Some have mutualistic specificity (e.g., yucca moth and Yucca plant) where pollinator and plant are tightly co‑adapted.
• Some animals act as pollen/nectar robbers (they take rewards without effecting pollination).

Comparison — Abiotic vs Biotic Pollinated Flowers

• Abiotic (wind/water) flowers: not colourful, no scent, usually no nectar, produce large quantities of small, light pollen.
• Biotic (animal) flowers: colourful, scented, produce nectar and sticky/coarse pollen adapted to adhere to pollinators; usually fewer pollen grains but more specialised transfer.

Outbreeding Devices

Most flowering plants produce hermaphrodite flowers, increasing the likelihood of self-pollination. However, continued self-pollination leads to inbreeding depression. 
To discourage self-pollination and encourage cross-pollination, plants have developed several mechanisms:

1. Asynchronous Pollen Release and Stigma Receptivity:

• In some species, pollen is released before the stigma becomes receptive or stigma becomes receptive before pollen is released.
• This prevents self-pollination (autogamy).

2. Different Placement of Anther and Stigma:

• In some species, the anther and stigma are positioned differently, preventing pollen from reaching the stigma of the same flower.
• This also discourages autogamy.

3. Self-Incompatibility (Genetic Mechanism):

• Prevents self-pollen (from the same flower or plant) from fertilizing the ovules.
• This happens by inhibiting pollen germination or pollen tube growth in the pistil.

4. Unisexual Flowers:

• Monoecious plants (e.g., castor and maize) have separate male and female flowers on the same plant, preventing autogamy but not geitonogamy (pollination between flowers of the same plant).
• Dioecious plants (e.g., papaya) have separate male and female plants, preventing both autogamy and geitonogamy.

Pollen-Pistil Interaction

The process from pollen deposition on the stigma to pollen tubes entering the ovule is known as pollen-pistil interaction.

Pollination does not always ensure the transfer of the right type of pollen. Sometimes, pollen from other species or self-incompatible pollen lands on the stigma. The pistil recognises the pollen and determines whether it is compatible or incompatible.

• If the pollen is compatible, the pistil accepts it and promotes post-pollination events leading to fertilisation.
• If the pollen is incompatible, the pistil rejects it by preventing pollen germination or blocking pollen tube growth in the style.
• This recognition and response involve a continuous dialogue between the pollen and pistil, mediated by their chemical components.

Pollen–Pistil Interaction

The sequence from pollen deposition on the stigma to pollen tube entry into the ovule is called the pollen–pistil interaction. The pistil discriminates between compatible and incompatible pollen and supports or inhibits pollen germination and tube growth accordingly.

• If pollen is compatible, the pistil promotes germination and provides chemical guidance for pollen tube growth toward the ovule.
• If pollen is incompatible, the pistil prevents germination or arrests pollen tube growth.
• This communication involves specific biochemical signalling between pollen and pistil tissues.

(a) Pollen grains germinating on the stigma; (b) Pollen tubes growing through the style; (c) L.S. of pistil showing path of pollen tube growth; (d) enlarged view of an egg apparatus showing entry of pollen tube into a synergid; (e) Discharge of male gametes into a synergid and the movements of the sperms, one into the egg and the other into the central cell

Pollen Germination and Pollen Tube Growth

• On a compatible stigma, a pollen grain germinates and produces a pollen tube from a germ pore.
• The pollen tube grows through the stigma and style, carrying the male gametes (either produced later by mitosis of the generative cell, or already present as two male gametes in 3‑celled pollen).
• The pollen tube reaches the ovule through the micropyle and enters one of the synergids via the filiform apparatus that guides the tube.
• The timing of generative cell division varies: in 2‑celled pollen the generative cell divides inside the pollen tube; in 3‑celled pollen two male gametes are already formed before germination.

Significance of Pollen-Pistil Interaction

• Ensures successful fertilisation by allowing only compatible pollen to proceed.
• Helps in plant breeding by enabling scientists to manipulate pollen-pistil interactions for hybrid production.
• The process of pollen germination can be studied using flowers like pea, chickpea, Crotalaria, balsam, and Vinca by observing pollen grains in a sugar solution under a microscope.

Artificial Hybridisation for Crop Improvement

Artificial hybridisation is widely used to combine desirable characters from different parents and produce superior crop varieties. Careful control of pollination is required to ensure desired crosses.

• Emasculation: removal of anthers from the flower bud of the chosen female parent before anther dehiscence to prevent self‑pollination (necessary in bisexual flowers).
• Bagging: covering the emasculated flower to prevent contamination by unwanted pollen.
• When the stigma becomes receptive, pollen from the selected male parent is applied and the flower is re‑bagged until fruit set.
• In unisexual female flowers (female parent with separate female flowers), emasculation is unnecessary; bagging before anthesis is sufficient.

Fertilisation

Following pollination, the pollen grains travel to the ovary via the pollen tube. Upon reaching the ovary, one male gamete fertilizes the ovule, forming a zygote, which develops into an embryo. Meanwhile, the other male gamete fuses with the polar nuclei, leading to the formation of the endosperm nucleus. This endosperm provides nourishment to the developing embryo.

Fertilization transforms the ovules into seeds, while the ovary develops into the fruit.

Fertilization in Plants

Double Fertilisation

• The pollen tube releases two male gametes into the embryo sac.
• One male gamete fuses with the egg cell to form a diploid zygote (2n) — this develops into the embryo.
• The other male gamete fuses with the two polar nuclei (or with the fused polar nucleus) to form a triploid primary endosperm nucleus (PEN) — this develops into the endosperm that nourishes the embryo.
• Because two fertilisation events occur within one embryo sac, the phenomenon is called double fertilisation — it is characteristic of angiosperms.

• After triple fusion the central cell (primary endosperm cell, PEC) initiates endosperm development while the zygote undergoes embryogeny.

Post‑fertilisation: Structure and Events

Following fertilisation, coordinated development of the endosperm and embryo occurs; ovules mature into seeds and the ovary transforms into a fruit.

1. Endosperm development
2. Embryo development (embryogeny)
3. Maturation of ovule(s) into seed(s)
4. Ovary development into fruit

(a) Fertilised embryo sac showing zygote and Primary Endosperm Nucleus (PEN); (b) Stages in embryo development in a dicot [shown in reduced size as compared to (a)] 

1. Endosperm

• Timing: endosperm formation (from the PEN  usually starts before substantial embryo development; it supplies nutrition to the growing embryo.
• Initial stage: free nuclear endosperm — repeated nuclear divisions without immediate cytokinesis produce a coenocytic tissue containing many free nuclei.
• Later stage: cellularisation — cell walls form around nuclei to produce cellular endosperm.
• Examples: In coconut the liquid "coconut water" is free nuclear endosperm; the white kernel is cellular endosperm. In some species the endosperm persists in mature seeds (albuminous seeds), in others it is consumed by the embryo (non‑albuminous seeds).

2. Embryo

The zygote at the micropylar end of the embryo sac develops into the embryo. Early embryo growth is similar in monocots and dicots though final organisation differs.

Stages of Embryogeny

• Proembryo formation
• Globular stage
• Heart‑shaped stage (in dicots)
• Mature embryo formation

Structure of a Typical Dicot Embryo

• Embryonal axis with two cotyledons.
• Epicotyl (above cotyledons) ending in the plumule (shoot apex).
• Hypocotyl (below cotyledons) terminating in the radicle (root tip) with root cap.

Structure of a Typical Monocot Embryo

• Single cotyledon called the scutellum (in grasses) attached laterally to the embryonal axis.
• Radicle with root cap at the lower end enclosed by a sheath called the coleorhiza.
• Shoot apex and leaf primordia protected by the coleoptile.

(a) A typical dicot embryo; (b) L.S. of an embryo of grass 

3. Seed

A seed is a fertilised ovule that contains the embryo, stored food and protective seed coat (testa), formed from integuments.

• Seed parts: protective seed coat(s), one or more cotyledons (food storage), and the embryo axis.
• Types by endosperm:
  • Non‑albuminous (exalbuminous) seeds: endosperm consumed by embryo during development (e.g., pea, groundnut).
  • Albuminous (endospermic) seeds: endosperm persists in mature seed (e.g., wheat, maize, castor).
  • Perisperm: in some seeds remnants of nucellus persist (e.g., black pepper, beet).
• Seed coat: hardened integuments protect the embryo; the micropyle remains as a pore for water and oxygen entry during germination.
• During maturation, seed water content falls (typically to ~10–15%) and metabolic activity slows; seeds may enter dormancy until favourable germination conditions occur (moisture, oxygen, temperature).

4. Fruit Formation

• As ovules develop into seeds, the ovary usually develops into a fruit — a mature ovary containing seeds.
• The ovary wall differentiates into the pericarp (fruit wall).
• Fruits may be fleshy (e.g., mango, orange, guava) or dry (e.g., groundnut, mustard).
• True fruits: develop solely from the ovary.
• False (accessory) fruits: develop from other floral parts in addition to the ovary (e.g., apple, strawberry, cashew).
• Parthenocarpic fruits: develop without fertilisation and are seedless (e.g., cultivated banana); parthenocarpy can be induced by hormonal treatments.

Advantages of Seeds in Angiosperms

• Fertilisation is independent of water, enabling reproduction in diverse terrestrial habitats.
• Seeds possess adaptations for dispersal (wind, water, animals) aiding colonisation.
• Stored food reserves nourish the young seedling until it can photosynthesise.
• Hard seed coats protect embryos from physical damage and desiccation.
• Being products of sexual reproduction, seeds promote genetic variation in progeny.

Role of Seeds in Agriculture

• Seed dehydration and dormancy permit long‑term storage of viable seeds for sowing and food reserves.
• Seed longevity varies widely; some seeds lose viability in months, while others remain viable for many years or even centuries under suitable conditions.
• If seeds germinated immediately after formation agricultural use would be impractical; dormancy enables seasonal sowing and storage.

Longevity of seeds

• The oldest recorded viable seed germinated was of Lupinus arcticus (reported germination after ~10,000 years of dormancy in permafrost conditions).
• A ~2,000‑year‑old Phoenix dactylifera (date palm) seed from King Herod’s palace near the Dead Sea was reported to have germinated successfully.

Reproductive Capacity of Flowering Plants

• Each embryo sac contains one egg; many ovules may be present in an ovary, and many flowers per plant increase reproductive output.
• Some fruits (e.g., orchids) produce thousands of tiny seeds; parasitic plants (Orobanche, Striga) and species like Ficus produce enormous numbers of seeds over time.

Apomixis

• Apomixis is a type of asexual reproduction in plants that involves the formation of a seed without the fertilization of the egg cell by a sperm cell. 
• The resulting offspring are genetically identical to the parent plant. 
• In apomixis, the embryo sac develops into an embryo without being fertilized, and the resulting seed carries the same genetic information as the parent plant. 
• This process can occur naturally in some plant species or can be induced artificially in others.

Polyembryony

• Polyembryony is the occurrence of more than one embryo in a single seed.
• In some species, a diploid egg cell forms without reduction division and develops into an embryo without fertilisation.
• In other species, such as Citrus and Mango varieties, nucellar cells surrounding the embryo sac start dividing, protrude into the embryo sac, and develop into multiple embryos within a single ovule.

Apomixis and Hybrid Varieties

• Apomixis produces genetically identical embryos and can be considered a type of cloning
• Hybrid varieties are being extensively cultivated to increase productivity
• However, hybrid seeds need to be produced every year, which is costly and makes them expensive for farmers
• If hybrid varieties are made into apomicts, there is no segregation of characters in the hybrid progeny
• This means farmers can keep using the hybrid seeds to raise new crops year after year without buying new hybrid seeds
• Active research is ongoing in many laboratories worldwide to understand the genetics of apomixis and transfer apomictic genes into hybrid varieties.