1. SEXUAL REPRODUCTION IN FLOWERING PLANTS

SEXUAL REPRODUCTION IN FLOWERING PLANTS

Flower – A Fascinating Organ of Angiosperms

Introduction and Significance of Flowers

Pre-fertilisation: Structures and Events

Pre-fertilisation Events

Structure and Parts of a Typical Flower

A complete flower consists of four whorls divided into accessory and reproductive parts.

Essential / Reproductive Parts:

These are directly involved in sexual reproduction.

Androecium (Male Reproductive Part):

• It consists of a whorl of stamens.
• A typical stamen has two parts: a long and slender stalk called the filament, and a terminal, generally bilobed structure called the anther.
• The proximal end of the filament is attached to the thalamus or petal of the flower.

Gynoecium (Female Reproductive Part):

• It consists of a whorl of carpels (pistils).

Each pistil has three parts:

• the stigma – landing platform for pollen grains
• the style– elongated slender part beneath the stigma
• the ovary– the basal bulged part containing the ovarian cavity or locule where the placenta is located.

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Stamen, Microsporangium and Pollen Grain

Structure of a Typical Stamen and Anther Development

Stamen:
A typical stamen has two parts: a long, slender stalk called the filament, and a terminal, generally bilobed structure called the anther.

Anther:
An angiosperm anther is bilobed and dithecous (each lobe has two theca separated by a longitudinal groove). It is a four-sided (tetragonal) structure consisting of four microsporangia.

Early Development:
A young anther develops as a homogenous mass of cells surrounded by an epidermis. Hypodermal cells enlarge to form the archesporium, which divides to form primary parietal cells (outer) and primary sporogenous cells (inner).

Wall Layers of the Microsporangium (Anther Wall)

The mature anther wall consists of four distinct layers from the periphery to the centre:

Tapetum

The innermost wall layer that attains maximum development at the tetrad stage.

Origin:
It has a dual origin (derived partly from the peripheral wall layer and partly from the connective tissue).

Types:

It can be Secretory (parietal/glandular)– which retains its cellular integrity, or Invasive (periplasmodial/amoeboid)– where the cell walls dissolve and protoplasts coalesce.

Functions:
It nourishes the developing microspores, controls fertility/sterility, and contributes to the wall materials sporopollenin (via Ubisch bodies) and pollenkitt. It also provides exine proteins responsible for the “rejection reaction” on the stigma.

Anther Cavity

The anther cavity is filled with microspores in young stages or with pollen grains at maturity. The meiotic division of microspore mother cells gives rise to microspores which are haploid in nature.

Connective tissue

It is the column of sterile tissue surrounded by the anther lobe. It possesses vascular tissues. It also contributes to the inner tapetum.

Microsporogenesis

The process of the formation of microspores from a pollen mother cell (PMC) through meiosis is called microsporogenesis.

• As the anther develops, the cells of the sporogenous tissue undergo meiotic divisions to form a cluster of four cells known as a microspore tetrad.
• As the anthers mature and dehydrate, the microspores dissociate from each other and develop into pollen grains.

Pollen Grain (Male Gametophyte)

Pollen grains represent the male gametophyte and are generally spherical, measuring about 25-50 micrometers in diameter.

Wall Structure

It has a prominent two-layered wall.

Cellular Stages (Mature Pollen)

The cytoplasm of the pollen grain is surrounded by a plasma membrane. When mature, it contains two cells:

Shedding Stages

• In over 60 percent of angiosperms, pollen grains are shed at the 2-celled stage.
• In the remaining species, the generative cell divides mitotically to give rise to the two male gametes before pollen grains are shed (3-celled stage).

Pollen Allergy

Pollen grains of many species cause severe allergies and bronchial afflictions (like asthma and bronchitis). For example, Parthenium (carrot grass), which came into India as a contaminant with imported wheat, causes severe pollen allergy.

Pollen Viability

The period for which pollen grains remain viable varies greatly depending on temperature and humidity.

Cryopreservation (Pollen Banks)

Pollen grains can be stored for years in liquid nitrogen (-196°C) to be used as pollen banks in crop breeding programmes.

Pollen Products (Nutritional Value of Pollen Grains)

Nutrient-Rich, Food Supplements, Tablet & Syrups, Performance Enhancers

• Pollen grains are extremely rich in nutrients.
• Because of this nutritional value, it has become a popular fashion in recent years to use pollen tablets as daily food supplements.
• In western countries, a large variety of pollen products are widely available in the market in the form of both tablets and syrups.
• The consumption of pollen products has been claimed to significantly increase the physical performance of athletes and racehorses.

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Also Read – EXCERCISE WITH SOLUTIONS ON SEXUAL REPRODUCTION IN FLOWERING PLANTS

The Pistil, Megasporangium (ovule) and Embryo sac

The Pistil (Gynoecium – Female Reproductive Part)

Composition:
The gynoecium represents the female reproductive organ.

• It may consist of a single pistil (monocarpellary) or more than one pistil (multicarpellary).
• In multicarpellary conditions, the pistils may be fused together (syncarpous) or remain free (apocarpous).

Parts of a Pistil

The Megasporangium (Ovule)

• The ovules arise from the placenta.
• The number of ovules in an ovary varies from one (e.g., wheat, paddy, mango) to many (e.g., papaya, water melon, orchids).

Structure of a Typical Anatropous Ovule/ Megasporangium

Megasporogenesis

The process of formation of megaspores from the megaspore mother cell (MMC) is called Megasporogenesis.

• Ovules generally differentiate a single MMC in the micropylar region of the nucellus. It is a large cell containing dense cytoplasm and a prominent nucleus.
• The MMC undergoes meiotic division, resulting in the production of four haploid megaspores.

Female Gametophyte (Embryo Sac) Development

The MMC undergoes meiotic division, resulting in the production of four haploid megaspores.

Degeneration:
In a majority of flowering plants, out of the four megaspores formed, three degenerate and only one megaspore remains functional.

Free-Nuclear Mitosis:
The nucleus of this functional megaspore undergoes strictly free-nuclear mitotic divisions (nuclear divisions are not immediately followed by cell wall formation).

Successive Stages:
It first divides to form a 2-nucleate embryo sac, followed by sequential divisions to form a 4-nucleate, and finally an 8-nucleate stage through mitotic division.

Cellularization:
After the 8-nucleate stage, cell walls are finally laid down, leading to the organization of the typical mature female gametophyte.

A typical mature angiosperm embryo sac is 8-nucleate but 7-celled.

Monosporic Development:
The method of embryo sac formation where it develops from a single functional megaspore is termed monosporic development.

Structure of the Mature Embryo Sac

The cells inside the embryo sac have a highly characteristic distribution. A typical mature angiosperm embryo sac is 8-nucleate but 7-celled.

The Egg Apparatus (Micropylar End)

• Three cells are grouped together at the micropylar end to constitute the egg apparatus.
• It consists of two synergids and one egg cell.

Filiform Apparatus:
The synergids possess special cellular thickenings at their micropylar tip called the filiform apparatus. It plays a crucial role in guiding the pollen tubes into the synergid during fertilization.

The Central Cell (Middle)

• This is a single large cell situated below the egg apparatus.
• It contains two polar nuclei.

The Antipodals (Chalazal End)

Three cells are grouped together at the basal (chalazal) end of the embryo sac, known as the antipodals.

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Ovule Types based on Nucellus	Tenuinucellate: Sporogenous cell is hypodermal with a single layer of nucellar tissue around it (nucellus is very small).Crassinucellate: Sporogenous cell is subhypodermal (fairly large nucellus).
Endothelium (Integumentary Tapetum)	In some unitegmic tenuinucellate species (like Asteraceae), the inner layer of the integument becomes specialized to perform a nutritive function for the embryo sac.
Special Structures	Raphe: In inverted (anatropous) ovules, the funicle is adnate (fused) to the body of the ovule, forming a ridge called the raphe.Hypostase & Epistase: A group of cells found at the base between the chalaza and embryo sac is the hypostase, while thick-walled cells above the micropylar end are called the epistase.
Classification of Ovules (6 main types)	Orthotropous: Micropyle, funicle, and chalaza are in one straight vertical line (e.g., Piperaceae).Anatropous: Completely inverted body; micropyle and funicle lie very close. Common in dicots and monocots.Hemianatropous: Body placed transversely at right angles to the funicle (e.g., Primulaceae).Campylotropous: Curved body, embryo sac slightly curved (e.g., Leguminosae).Amphitropous: Strong curvature leads to a horse-shoe shaped nucellus (e.g., Alismataceae).Circinotropous: Funiculus is very long and surrounds the ovule (e.g., Cactaceae).
Development Types	While Polygonum type is Monosporic, remember that development can also be Bisporic (e.g., Allium) or Tetrasporic (e.g., Peperomia) depending on how many megaspores participate.

Pollination

Introduction to Pollination

Pollination is the mechanism of transferring pollen grains (shed from the anther) to the stigma of a pistil.

Significance:
Since male and female gametes in flowering plants are non-motile, pollination is a necessary pre-requisite to bring them together for fertilization to occur.

Kinds of Pollination

Depending on the source of the pollen, pollination is divided into three types:

Self-Pollination

Transfer of pollen grains from the anther to the stigma of the same flower or another flower on the same plant.

1. Autogamy (Self-Pollination)

Transfer of pollen grains from the anther to the stigma of the same flower on the same plant.

Chasmogamous flowers (Flowers with exposed anthers and stigma)

In open (chasmogamous) flowers, complete autogamy is rare and requires synchrony in pollen release and stigma receptivity.

Examples:
Hibiscus: ದಾಸವಾಳ, Sunflower: ಸೂರ್ಯಕಾಂತಿ.Rose: ಗುಲಾಬಿ

Cleistogamous flowers (Flowers which do not open at all)

Examples:
Peanuts / Groundnuts: ನೆಲಗಡಲೆ / ಶೇಂಗಾ. Peas: ಬಟಾಣಿ (Batani).

Cleistogamous flowers are invariably autogamous and produce assured seed-sets even in the absence of pollinators.

Plants like Viola (common pansy), Oxalis, and Commelina produce both types of flowers.

2. Geitonogamy

Transfer of pollen grains from the anther to the stigma of another flower of the same plant.

Though functionally cross-pollination (requires a pollinating agent), it is genetically similar to autogamy as the pollen comes from the same plant.

Examples:
Coconut, Cucumber, Maize / Corn

3. Xenogamy (Cross-Pollination)

Transfer of pollen grains from the anther to the stigma of a different plant.

This is the only type of pollination that brings genetically different types of pollen grains to the stigma.

Examples:
Papaya, Pumpkin, Onion

Agents of Pollination

Agents which carry pollen grains from anther to stigma of same/ different plant.

Plants use two abiotic agents (wind and water) and one biotic agent (animals) to achieve pollination.

Wind Pollination (Anemophily)

• The most common abiotic agent.
• Pollen grains are light and non-sticky, allowing easy transport in wind currents.
• Stamens are well-exposed, and the stigma is large and often feathery to trap air-borne pollen.
• Flowers often have a single ovule per ovary and are packed into an inflorescence (e.g., corn cob tassels, grasses).

Water Pollination (Hydrophily)

• Quite rare, limited to about 30 genera (mostly monocotyledons).
• In Vallisneria, the female flower reaches the water surface, and male flowers/pollen are released to float passively on the surface until they reach the stigma.
• In seagrasses like Zostera, female flowers remain submerged, and long, ribbon-like pollen grains are carried passively underwater.
• Pollen grains are protected from wetting by a mucilaginous covering.

Animal Pollination (Zoophily & Entomophily)

• Majority of plants use biotic agents (bees, butterflies, birds, bats). Bees are the dominant biotic pollinating agents.
• Insect-pollinated flowers are usually large, colourful, fragrant, and rich in nectar to attract animals.
• To sustain visits, flowers provide floral rewards like nectar, pollen, or safe places to lay eggs (e.g., the tallest flower Amorphophallus, and the Yucca plant which has an obligate mutualism with a specific moth).

Outbreeding Devices (Contrivances for Cross-Pollination)

Continued self-pollination results in inbreeding depression, so plants developed devices to discourage self-pollination:

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Pollen-pistil Interaction

Introduction and Recognition

All the events—from the deposition of pollen on the stigma until the pollen tube enters the ovule—are collectively referred to as Pollen-Pistil Interaction.

It is a dynamic process involving pollen recognition followed by either the promotion or inhibition of the pollen.

The pistil has the ability to recognise whether the pollen is of the right type (compatible) or the wrong type (incompatible). This continuous dialogue is mediated by chemical components of the pollen interacting with those of the pistil.

Post-Pollination Events (If Compatible)

Figure

(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

Artificial Hybridisation

Artificial hybridisation is one of the major approaches in crop improvement programmes.

Breeders cross different species or genera to combine desirable characters to produce commercially “superior” varieties.

Goal:
In these crossing experiments, it is crucial to make sure that only the desired pollen grains are used for pollination and the stigma is protected from contamination by unwanted pollen.

Technique 1: Emasculation

If the female parent bears bisexual flowers,

the anthers must be removed from the flower bud before they dehisce (burst open). This is done using a pair of forceps and the step is referred to as emasculation.

Technique 2: Bagging

Emasculated flowers have to be covered with a bag of a suitable size, generally made of butter paper. This prevents the contamination of its stigma with unwanted pollen and is called bagging.

Pollination and Rebagging

When the stigma of the bagged flower attains receptivity, mature pollen grains collected from the desired male parent are dusted on the stigma. The flowers are then rebagged, and the fruits are allowed to develop.

Unisexual Flowers

If the female parent produces unisexual flowers, there is no need for emasculation. The female flower buds are simply bagged before the flowers open, and pollination is carried out when the stigma becomes receptive.

Artificial Hybridisation Technique (Flow Chart)

A bisexual flower is taken

↓

Anthers are removed (emasculation)

↓

The stigma of emasculated flower is covered with butter paper (bagging)

↓

When the bagged stigma attains receptivity, the desired set of pollen grains is dusted (pollination) on the stigma of the bagged flower

↓

Rebagging is done.

↓

Fruits are allowed to develop.

↓

Desired seeds are obtained.

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Stigmatic Surface	Pollen germination is facilitated by the stigmatic fluid in a wet stigma and by a pellicle in a dry stigma.
Early Changes	The first visible change in the pollen grain soon after it lands on the stigma is hydration.
Cap Block	The extreme tip of the growing pollen tube appears hemispherical and transparent under a microscope, known as the cap block. As soon as the cap block disappears, the growth of the pollen tube stops.
Types of Style	Hollow/Open Style: Common in monocots; has a hollow canal lined by glandular canal cells (transmitting tissue) that secrete mucilaginous substances for nutrition and controlling incompatibility.Solid/Closed Style: Common in dicots; characterised by a solid central core of elongated, highly specialised transmitting tissue.
Types of Pollen Tube Entry into the Ovule	Porogamy: Entry through the micropyle (most common).Chalazogamy: Entry through the chalaza.Mesogamy: Entry through the integument.
Obturator	A special structure that directs or guides the pollen tube towards the micropyle of the ovule. It can originate from the placenta, funiculus, style, or ovary wall.

Double Fertilisation

Double fertilization is a unique process in angiosperms in which two male gametes participate in two fertilization events. One male gamete fuses with the egg cell to form a diploid zygote (syngamy), while the other male gamete fuses with the two polar nuclei to form a triploid primary endosperm nucleus (triple fusion). Thus, it involves syngamy and triple fusion.

Double fertilization = Syngamy + Triple fusion occurring simultaneously in the embryo sac of angiosperms.

Entry of Male Gametes

After the pollen tube reaches the embryo sac and enters one of the synergids, it releases two male gametes into the cytoplasm of the synergid.

First Fusion: Syngamy

One of the male gametes moves towards the egg cell and fuses with its nucleus. This fusion is called syngamy and it completes the fertilization of the egg.

The result of syngamy is the formation of a diploid cell called the zygote (2n). The zygote will later develop into the plant embryo.

Second Fusion: Triple Fusion

The second male gamete moves towards the large central cell where the two polar nuclei are located. It fuses with these two polar nuclei (or their fusion product, the secondary nucleus) to produce a triploid Primary Endosperm Nucleus (PEN) (3n).

Because this process involves the fusion of three haploid nuclei (one male gamete + two polar nuclei), it is termed triple fusion.

Double Fertilisation

Since two distinct types of fusions—syngamy and triple fusion—take place simultaneously within the same embryo sac, the entire phenomenon is termed double fertilisation.

This is an event highly unique to flowering plants (angiosperms).

Post-Fertilisation Fate

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Post-Fertilisation: Structures and Events, Endosperm, and Embryo

Introduction to Post-Fertilisation Events

The series of events following double fertilisation include endosperm and embryo development.

After fertilisation, several floral parts undergo changes: the sepals, petals, and stamens usually wither and fall off.

Endosperm Development

The endosperm develops before the embryo because the cells of the endosperm provide nutrition to the developing embryo.

The primary endosperm cell divides repeatedly and forms a triploid (3n) endosperm tissue. Its cells are filled with reserve food materials used for the nutrition of the embryo.

In the most common type of endosperm development, the PEN undergoes successive nuclear divisions to give rise to free nuclei without immediate cell wall formation known as Free-Nuclear Endosperm. A classic example is the coconut water from tender coconut, which is made up of thousands of free nuclei.

Subsequently, cell wall formation occurs, and the endosperm becomes cellular. The white kernel surrounding the coconut water is the cellular endosperm.

Embryo Development (Embryogeny)

The embryo develops at the micropylar end of the embryo sac where the zygote is situated.

Though seeds differ greatly, the early stages of embryo development (embryogeny) are similar in both monocotyledons and dicotyledons.

The zygote gives rise to the proembryo, followed by the globular, heart-shaped, and finally the mature embryo.

Structure of a Dicotyledonous Embryo

A typical dicot embryo consists of an embryonal axis and two cotyledons.

Figure – Dicot (left) and Monocot (right) embryo

Structure of a Monocotyledonous Embryo

Monocot embryos possess only one cotyledon.

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Types of Endosperm (Advanced)	Nuclear Endosperm: Primary endosperm nucleus (PEN) divides without wall formation (e.g., Coccinia, Capsella, Arachis).Cellular Endosperm: PEN division is immediately followed by wall formation (e.g., Adoxa, Helianthus, Scoparia).Helobial Endosperm: PEN moves to the base and divides into a large micropylar chamber and a small chalazal chamber. The micropylar chamber undergoes free nuclear division (e.g., Hydrilla, Vallisneria).
Ruminate Endosperm	The endosperm has an irregular and uneven surface due to the activity of the seed coat or endosperm itself (e.g., Areca catechu, Passiflora, Myristica).
Aleurone Layer	Highly specialised cells of one or few layers found around the endosperm of cereals (barley, maize). They contain sphaerosomes and secrete hydrolytic enzymes (amylases, proteases) during seed germination to digest reserve food.
Endosperm Haustoria	In cellular and nuclear endosperms, special lateral, micropylar, or chalazal structures are produced that absorb nutrients from outer tissues to supply the growing embryo.
Coconut Milk Tissue Culture	Coconut milk (water) is a free-nuclear endosperm that acts as a basic nutrient medium to induce differentiation of embryos (embryoids) from plant tissues in vitro.
Dicot Embryo Development (Onagrad/ Crucifer type)	In Capsella bursa-pastoris, the zygote divides transversely into a terminal and basal cell. The basal cell forms a 6 to 10-celled suspensor which pushes the embryo into the endosperm.The lowest suspensor cell forms the hypophysis (gives rise to root cap/epidermis).The terminal cell forms a quadrant, then an octant (epibasal and hypobasal tiers), eventually forming the dermatogen, periblem, and pleurome.

Post-Fertilisation – Seed, Fruit, Apomixis and Polyembryony

Seed

The seed is the final product of sexual reproduction in angiosperms, often described as a fertilised ovule.

Location

Seeds are formed inside fruits.

Structure

A typical seed consists of seed coat(s), cotyledon(s), and an embryo axis.

The cotyledons are generally thick and swollen due to the storage of food reserves.

Seed Coats

The integuments of ovules harden to become tough, protective seed coats.

The micropyle remains as a small pore to facilitate the entry of oxygen and water during germination.

Dormancy and Germination

• As the seed matures, its water content is reduced (10-15% moisture by mass), and its general metabolic activity slows down.
• The embryo may enter a state of inactivity called dormancy.
• If favourable conditions (adequate moisture, oxygen, and suitable temperature) are available, they germinate.

Types of Seeds Based on Endosperm

Advantages of Seeds to Angiosperms

Seed Viability

Seed viability (capacity) varies greatly.

Some seeds lose viability within a few months, while others live for several years.

The oldest viable seed is that of a lupine (Lupinus arcticus) excavated from the Arctic Tundra, which germinated and flowered after an estimated 10,000 years of dormancy.

A 2,000-year-old viable seed of the date palm (Phoenix dactylifera) was discovered during an archeological excavation at King Herod’s palace near the Dead Sea.

The Fruit

Figure- a) Structure of some seeds. (b) False fruits of apple and strawberry

Development

As ovules mature into seeds, the ovary simultaneously develops into a fruit. The wall of the ovary develops into the wall of the fruit called the pericarp.

Types of Fruits

Apomixis and Polyembryony

Apomixis

• A special mechanism evolved by a few flowering plants (like some species of Asteraceae and grasses) to produce seeds without fertilisation.
• It is a form of asexual reproduction that mimics sexual reproduction.
• In some cases, the diploid egg cell is formed without reduction division and develops into the embryo without fertilisation.

Polyembryony

• The occurrence of more than one embryo in a seed.
• In many Citrus and Mango varieties, nucellar cells surrounding the embryo sac start dividing, protrude into the embryo sac, and develop into multiple embryos.

Figure- Polyembryony – Embryo sac of Ulmus glabra showing zygotic and antipodal embryo

Significance in Agriculture

• Cultivation of hybrids requires buying costly seeds every year because hybrid characters segregate in the progeny.
• If hybrids are made into apomicts, there is no segregation of characters, and farmers can keep using the hybrid seeds year after year.

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Sexual Reproduction in Flowering Plants Summary

Chapter Summary Notes

Pre-Fertilization

Flowers are the seat of sexual reproduction in angiosperms. The androecium (stamens) represents the male reproductive organs, and the gynoecium (pistils) represents the female reproductive organs.

Male Gametophyte

A typical anther is bilobed, dithecous, and tetrasporangiate. Cells of the sporogenous tissue undergo meiosis (microsporogenesis) to form microspore tetrads, which mature into pollen grains.

Pollen grains have a two-layered wall: the outer exine (made of sporopollenin with germ pores) and inner intine.

They are shed at a two-celled or three-celled stage.

Female Gametophyte

The pistil consists of a stigma, style, and ovary.

Inside the ovary are ovules (megasporangia).

The megaspore mother cell divides meiotically, and one functional megaspore forms the 7-celled, 8-nucleate embryo sac through monosporic development.

The mature embryo sac has an egg apparatus (two synergids, one egg cell), three antipodals, and a central cell with two polar nuclei.

Pollination and Interaction

Pollination is the transfer of pollen to the stigma, facilitated by abiotic (wind, water) or biotic (animals) agents.

Pollen-pistil interaction involves pollen recognition, germination, and the growth of the pollen tube through the style to discharge male gametes into a synergid.

Double Fertilization

Angiosperms exhibit double fertilization: syngamy (fusion of a male gamete with the egg to form a diploid zygote) and triple fusion (fusion of a male gamete with two polar nuclei to form a triploid primary endosperm nucleus).

Post-Fertilization

Formation of the endosperm precedes embryo development to assure nutrition.

The zygote develops into the embryo (passing through proembryo, globular, and heart-shaped stages), the ovules develop into seeds, and the ovary develops into a fruit.

Special Mechanisms

Apomixis is the formation of seeds without fertilization, and polyembryony is the occurrence of more than one embryo in a seed.

reference/ source book: NCERT CLASS 12 BIOLOGY