Sexual Reproduction in Flowering Plants

Flower – A Fascinating Organ Of Angiosperms

Flowering plants, or angiosperms, exhibit a remarkable diversity of forms, shapes, and habitats. The flower is the characteristic feature of angiosperms and is the site of sexual reproduction.

Flowers are not just for aesthetic appeal; they are structures highly adapted for sexual reproduction, involving the production of male and female gametes, their fusion (fertilisation), and the subsequent development of seeds and fruits.

Introduction To The Flower

Morphologically, a flower is considered a modified shoot. The shoot apical meristem gets transformed into a floral meristem. The internodes do not elongate, and the axis becomes condensed. The apex produces different kinds of floral appendages laterally at successive nodes instead of leaves.

A typical flower is borne on a stalk called a pedicel (a sessile flower lacks a pedicel). The swollen end of the pedicel is the thalamus or receptacle, which is the base on which the four floral whorls are arranged.

Parts of a Typical Flower:

A typical bisexual flower has four whorls, arranged concentrically on the thalamus:

Calyx: The outermost whorl, consisting of sepals. Sepals are usually green and protective in the bud stage.
Corolla: The whorl inner to the calyx, consisting of petals. Petals are usually brightly coloured to attract pollinators.
Androecium: The male reproductive whorl, consisting of stamens. Each stamen has a filament and an anther (containing pollen grains).
Gynoecium (Pistil): The female reproductive whorl, consisting of one or more carpels. Each carpel typically has a stigma, style, and ovary (containing ovules).

Diagram showing the parts of a typical flower (pedicel, thalamus, sepals, petals, stamens, pistil)

*(Image shows a longitudinal section of a typical flower highlighting pedicel, thalamus, calyx (sepals), corolla (petals), androecium (stamens), and gynoecium (pistil/carpel with stigma, style, ovary, ovules))*

The flower facilitates the process of sexual reproduction, leading to the formation of seeds (which develop from ovules) and fruits (which develop from the ovary).

Pre-Fertilisation: Structures And Events

Pre-fertilisation events in sexual reproduction are those that occur before the fusion of gametes. In flowering plants, these events involve the development of the male and female reproductive structures and the transfer of male gametes (pollen) to the female reproductive part.

Stamen, Microsporangium And Pollen Grain

The stamen is the male reproductive organ of a flower. Each stamen consists of two parts:

Filament: A long, slender stalk.
Anther: The terminal, typically bilobed structure that contains the pollen sacs.

Microsporangium (Pollen Sac):

The anther is usually bilobed, and each lobe typically has two chambers, the pollen sacs. So, a typical anther is dithecous (two lobes) and has four pollen sacs (tetrasporangiate).

Pollen sacs are the structures where pollen grains are formed.

The wall of a microsporangium typically consists of four layers: epidermis, endothecium, middle layers, and tapetum.

The outer three layers are protective and help in the dehiscence (opening) of the anther to release pollen.

The innermost layer, the tapetum, is a nutritive tissue that nourishes the developing pollen grains.

Inside the pollen sac, sporogenous tissue is present, consisting of compactly arranged homogeneous cells. These cells undergo meiosis to form microspores.

Microsporogenesis:
The process of formation of microspores from a pollen mother cell (PMC) or microspore mother cell (MMC) through meiosis is called microsporogenesis.

Each PMC (diploid, 2n) undergoes meiosis to form a cluster of four haploid cells called a microspore tetrad.

As the anther matures and dehydrates, the microspores separate from the tetrad and develop into pollen grains.

(Image shows a cross-section of an anther, highlighting the four pollen sacs and wall layers. May also include a microspore mother cell undergoing meiosis to form a tetrad)

Pollen Grain (Male Gametophyte):

Pollen grains are the male gametophytes. They are generally spherical, with a size of about 25-50 micrometres in diameter.

Pollen grain has a prominent two-layered wall:

Exine: The outer layer. Made of sporopollenin, one of the most resistant organic materials known. Sporopollenin can withstand high temperatures and strong acids/alkalis, protecting the pollen grain. The exine has characteristic patterns and designs and may have pores called germ pores where sporopollenin is absent.

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

Inside the cell wall is the cytoplasm and nucleus. A mature pollen grain typically contains two cells:

Vegetative cell: Larger cell, contains abundant food reserve, and has a large, irregularly shaped nucleus.

Generative cell: Smaller cell, floats in the cytoplasm of the vegetative cell. It is spindle-shaped with dense cytoplasm and a nucleus. The generative cell divides by mitosis to form two male gametes.

In many species, pollen is shed at this 2-celled stage (vegetative cell + generative cell). In some species, the generative cell divides before shedding, and pollen is shed at the 3-celled stage (vegetative cell + two male gametes).

Pollen grains are resistant structures due to sporopollenin and can be preserved as fossils. They can cause severe allergies and bronchial afflictions in some people (e.g., pollen allergy).

(Image shows a diagram of a pollen grain highlighting its layers and cellular components)

The Pistil, Megasporangium (Ovule) And Embryo Sac

The pistil or carpel is the female reproductive organ of a flower (collectively called the Gynoecium). A pistil typically consists of three parts:

Stigma: The receptive surface for pollen grains, usually located at the tip of the pistil. It may be sticky or hairy to trap pollen.
Style: A slender stalk connecting the stigma to the ovary.
Ovary: The swollen basal part containing one or more ovules. The ovules are attached to a flattened cushion-like structure called the placenta.

The arrangement of ovules within the ovary is called placentation (e.g., marginal, axile, parietal, free central, basal - discussed in Plant Morphology).

If a gynoecium has a single pistil, it is called monocarpellary. If it has more than one pistil, it can be multicarpellary apocarpous (free pistils, e.g., Lotus, Rose) or multicarpellary syncarpous (fused pistils, e.g., Tomato, Mustard).

Megasporangium (Ovule):

The ovule is the structure that develops into a seed after fertilisation. It is attached to the placenta by a stalk called the funicle.

The point where the funicle attaches to the ovule is called the hilum.

Each ovule has one or two protective coverings called integuments, which surround the central mass of tissue called the nucellus.

A small opening at the apex of the ovule, where integuments are absent, is called the micropyle. This is the entry point for the pollen tube.

Opposite the micropyle is the chalaza, the basal part of the ovule, representing the base of the nucellus.

The nucellus is a mass of parenchymatous tissue rich in food reserves.

Located within the nucellus is the embryo sac (female gametophyte).

(Image shows a diagram of an inverted (anatropous) ovule highlighting its parts)

Megasporogenesis:
The process of formation of megaspores from a single megaspore mother cell (MMC) through meiosis is called megasporogenesis.

Usually, a single MMC (diploid, 2n) differentiates in the micropylar region of the nucellus.

The MMC undergoes meiosis to form four haploid (n) megaspores.

In most flowering plants, only one of the megaspores is functional (usually the one located towards the chalazal end), while the other three degenerate (usually towards the micropylar end).

Female Gametophyte (Embryo Sac):
The functional megaspore develops into the female gametophyte, which is called the embryo sac. This development involves mitotic divisions of the megaspore nucleus. In most angiosperms, the development of the embryo sac from a single megaspore is called monosporic development.

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 an 8-nucleate embryo sac.

Cell walls are then laid down, organising the nuclei into cells.

A mature embryo sac is typically an 8-nucleate, 7-celled structure:

At the micropylar end, there is the egg apparatus, consisting of one egg cell and two supporting cells called synergids. Synergids have special cellular thickenings at the micropylar tip called filiform apparatus, which guides the pollen tube entry.

At the chalazal end, there are three cells called antipodal cells. Their function is not fully understood, but they may provide nutrition.

In the centre, there is a large central cell with two polar nuclei (which often fuse before fertilisation to form a diploid secondary nucleus).

The embryo sac is ready for fertilisation.

(Image shows stages from MMC $\rightarrow$ meiosis I $\rightarrow$ two dyad cells $\rightarrow$ meiosis II $\rightarrow$ four megaspores (tetrad), showing three degenerating and one functional. Then shows mitotic divisions of functional megaspore nucleus (2-nucleate, 4-nucleate, 8-nucleate stages), and final organisation into a 7-celled, 8-nucleated embryo sac (egg apparatus, antipodals, central cell with polar nuclei))

Pollination

Pollination is the process of transfer of pollen grains from the anther to the stigma of a flower. This is a crucial step to bring the male gametes (contained in pollen) to the female gametophyte (embryo sac in the ovule) for fertilisation.

Types of Pollination:

Autogamy (Self-pollination): Transfer of pollen grains from the anther to the stigma of the same flower. Requires synchrony in pollen release and stigma receptivity, and anthers and stigma being close together.

In some plants, flowers are closed (cleistogamous) and obligate autogamy occurs (e.g., Viola, Oxalis, Commelina). Cleistogamous flowers produce assured seed-set even in the absence of pollinators.

In chasmogamous flowers (open flowers), autogamy is possible.

Geitonogamy: Transfer of pollen grains from the anther to the stigma of a different flower of the same plant. Genetically, it is similar to autogamy as the pollen comes from the same plant (same genotype).

Xenogamy (Cross-pollination): Transfer of pollen grains from the anther to the stigma of a flower of a different plant of the same species. This brings about genetic variation.

(Image shows illustrations differentiating autogamy (pollen within same flower), geitonogamy (pollen to another flower on same plant), and xenogamy (pollen to flower on different plant))

Agents of Pollination:
Plants use various agents to achieve pollination:

Abiotic agents:

Wind pollination (Anemophily): Pollen is light, non-sticky, often produced in large quantities. Stigma is large, feathery to trap airborne pollen. Flowers are inconspicuous, without nectar or fragrance. Common in grasses, maize, wheat, palm.

Water pollination (Hydrophily): Less common. Occurs in about 30 genera, mostly monocots. Pollen may float on the surface (e.g., Vallisneria - female flower reaches surface) or remain submerged (e.g., Seagrasses - pollen long and ribbon-like). Flowers are small, inconspicuous.

Biotic agents:

Animal pollination (Zoophily): Most common type, involving insects (bees, butterflies, flies, beetles), birds (sunbirds, hummingbirds), bats, etc.

Flowers pollinated by animals are usually large, colourful, fragrant, and produce rewards like nectar and pollen to attract visitors. Specific adaptations exist for different pollinators (e.g., sticky pollen for insects, tube-shaped flowers for birds).

Example: Bees are the most common insect pollinators.

Outbreeding Devices:
Many flowering plants have developed mechanisms to discourage self-pollination and promote cross-pollination, leading to greater genetic diversity. These are called outbreeding devices.

Dichogamy (anther and stigma mature at different times).

Herkogamy (physical barrier between anther and stigma).

Self-incompatibility (genetic mechanism preventing self-pollen germination or pollen tube growth).

Unisexuality (flowers are either male or female).

Dioecy (male and female flowers on different plants).

Pollen-Pistil Interaction

Pollen-pistil interaction is the dialogue between pollen grain and the pistil, starting from the landing of pollen on the stigma until the pollen tube enters the ovule. It involves chemical recognition by the pistil to determine if the pollen is compatible (of the same species).

Steps:

Pollen Landing: Pollen grains land on the stigma.

Recognition: The pistil recognises compatible or incompatible pollen. If incompatible, the pistil rejects the pollen, preventing germination or pollen tube growth. This is a crucial mechanism to prevent hybridisation between different species and self-pollination in self-incompatible plants.

Pollen Germination: If compatible, the pollen grain absorbs moisture and nutrients from the stigma. The intine grows out through one of the germ pores to form a pollen tube. The vegetative nucleus moves into the pollen tube, followed by the generative cell.

Pollen Tube Growth: The pollen tube grows through the style towards the ovary. The generative cell, if it hasn't already, divides mitotically to form two male gametes as the pollen tube grows. The pollen tube contains the two male gametes and the tube nucleus.

Entry into Ovule: The pollen tube typically enters the ovule through the micropyle (porogamy). In some cases, it may enter through the chalaza (chalazogamy) or through the integuments (mesogamy).

Entry into Embryo Sac: After entering the ovule, the pollen tube usually enters the embryo sac through one of the synergids. The filiform apparatus in the synergids guides the entry of the pollen tube. After entering the synergid, the pollen tube releases the two male gametes into the cytoplasm of the synergid.

(Image shows a pistil with pollen grains on the stigma, pollen tube growing down the style, and entering an ovule, possibly showing the pollen tube releasing gametes into a synergid)

Artificial Hybridisation

Artificial hybridisation is a plant breeding technique used to create desired hybrids by controlling pollination. It involves selecting parents with desirable traits and performing controlled crosses.

Steps:

Emasculation: If the female parent plant has bisexual flowers, the anthers must be removed from the flower bud before they dehisce (release pollen) to prevent self-pollination. This process is called emasculation, usually done with forceps.

Bagging: The emasculated flower is immediately covered with a bag (usually made of butter paper) to prevent contamination by unwanted pollen. The stigma is allowed to become receptive.

Pollination: When the stigma of the bagged flower becomes receptive, mature pollen grains collected from the anthers of the desired male parent are dusted onto the stigma.

Rebagging: The flower is rebagged, and the fruits are allowed to develop.

If the female parent has unisexual flowers, emasculation is not necessary. The female flower is simply bagged before the stigma becomes receptive, and then pollination is done with desirable pollen when the stigma is ready.

This technique is widely used in crop improvement programs (e.g., wheat, rice, maize, cotton).

Double Fertilisation

Double fertilisation is a unique event characteristic of flowering plants (angiosperms). It occurs after the pollen tube releases the two male gametes into the embryo sac. It involves two separate fusion events within the embryo sac.

Syngamy And Triple Fusion

The two male gametes released from the pollen tube are involved in two different fertilisation events:

Syngamy: One of the male gametes fuses with the egg cell to form a diploid cell called the zygote (2n). This is the true fertilisation, which will develop into the embryo.
$ \text{Male gamete (n)} + \text{Egg cell (n)} \rightarrow \text{Zygote (2n)} $
Triple Fusion: The other male gamete fuses with the two polar nuclei (usually located in the central cell). Since the polar nuclei are typically haploid (n+n), their fusion with the haploid male gamete (n) results in a triploid primary endosperm nucleus (PEN).
$ \text{Male gamete (n)} + \text{Polar nuclei (n+n)} \rightarrow \text{Primary Endosperm Nucleus (PEN) (3n)} $

If the polar nuclei fuse to form a diploid secondary nucleus (2n) before fusion with the male gamete, the reaction is:

$ \text{Male gamete (n)} + \text{Secondary nucleus (2n)} \rightarrow \text{Primary Endosperm Nucleus (PEN) (3n)} $

Diagram illustrating double fertilisation within the embryo sac (showing syngamy and triple fusion events)

*(Image shows an embryo sac with the pollen tube entering, releasing two male gametes, one fusing with the egg, the other fusing with the polar nuclei in the central cell)*

Since two fusions (syngamy and triple fusion) occur in the embryo sac, the phenomenon is called double fertilisation. This event is unique to angiosperms.

The zygote develops into the embryo, and the PEN develops into the endosperm.

All the nuclei in the embryo sac before double fertilisation are haploid (egg cell, synergids, antipodal cells, polar nuclei). After double fertilisation, the zygote is diploid, and the PEN is triploid. The synergids and antipodal cells usually degenerate after fertilisation.

Post-Fertilisation: Structures And Events

Post-fertilisation events are those that occur in sexual reproduction after the formation of the zygote and PEN. These involve the development of the embryo, endosperm, seed, and fruit.

Following double fertilisation, several changes occur in the flower:

The zygote (2n) develops into the embryo.
The Primary Endosperm Nucleus (PEN) (3n) develops into the endosperm.
The ovule develops into the seed.
The ovary develops into the fruit.
Other floral parts (sepals, petals, stamens, style, stigma) generally wither and fall off. In some cases, the calyx may persist (e.g., in tomato, brinjal).

Endosperm

The endosperm is the tissue that provides nourishment to the developing embryo. It develops from the triploid Primary Endosperm Nucleus (PEN).

Endosperm Development:

Endosperm development precedes embryo development. The PEN undergoes repeated nuclear divisions to form free nuclei. This stage is called free-nuclear endosperm.

Subsequently, cell walls are formed, and the endosperm becomes cellular. This is called cellular endosperm.

Sometimes, both free-nuclear and cellular endosperm development occur (helobial endosperm, intermediate type).

The liquid endosperm in coconut is free-nuclear endosperm, while the white kernel is cellular endosperm.

Function of Endosperm:

Provides nutrition to the developing embryo.

In some seeds (e.g., pea, groundnut, beans), the endosperm is completely consumed by the embryo during seed development. Such seeds are called non-albuminous or exalbuminous seeds. Food is stored in cotyledons.

In other seeds (e.g., wheat, maize, castor, coconut), the endosperm persists in the mature seed and is used during seed germination. Such seeds are called albuminous seeds.

Embryo

The embryo is the rudimentary plant contained within the seed. It develops from the diploid zygote.

Embryo Development (Embryogeny):

Embryo development occurs at the micropylar end of the embryo sac where the zygote is located.

The zygote divides by mitosis to form a proembryo, and subsequently, the globular, heart-shaped, and mature embryo stages.

Structure of a Dicot Embryo (e.g., Bean, Pea, Gram):

Consists of an embryonal axis and two cotyledons.

The portion of the embryonal axis above the level of cotyledons is the epicotyl, which terminates with the plumule (gives rise to the shoot).

The portion below the level of cotyledons is the hypocotyl, which terminates at its lower end with the radicle (gives rise to the root).

The root tip is covered by a root cap.

In non-albuminous dicot seeds, the cotyledons are fleshy and store food.

(Image shows a dicot embryo with embryonal axis, cotyledons, plumule, epicotyl, radicle, hypocotyl)

Structure of a Monocot Embryo (e.g., Maize, Wheat):

Consists of an embryonal axis and a single cotyledon.

In grass embryos, the single cotyledon is called scutellum and is located towards one side of the embryonal axis.

At the lower end, the embryonal axis has the radicle and root cap enclosed in an undifferentiated sheath called coleorhiza.

At the upper end, the portion of the axis above the attachment of scutellum is the epicotyl. Epicotyl has a shoot apex (plumule) and a few leaf primordia enclosed in a hollow, folded structure called the coleoptile.

(Image shows a monocot embryo within the grain, highlighting the scutellum, embryonal axis, radicle, coleorhiza, plumule, coleoptile)

Seed

The seed is the mature ovule developed after fertilisation. It is the basis of agriculture.

Parts of a Seed:

Seed coat: Develops from the integuments of the ovule. The outer integument forms the outer seed coat (testa), and the inner integument forms the inner seed coat (tegmen).

Embryo: Developed from the zygote.

Endosperm: Present (in albuminous seeds) or absent (in non-albuminous seeds).

The hilum is a scar on the seed coat indicating where the seed was attached to the fruit. A small pore (micropyle) may be visible near the hilum.

Advantages of seeds: Provide nourishment to the young embryo, offer protection, are units of dispersal, and provide genetic variation (in sexual reproduction).

Fruit

The fruit is the mature or ripened ovary, developed after fertilisation.

Pericarp (Fruit Wall):
The ovary wall develops into the pericarp, which can be dry or fleshy. In fleshy fruits, it differentiates into three layers:

Epicarp: Outermost layer (skin).

Mesocarp: Middle layer (often fleshy and edible, e.g., mango pulp).

Endocarp: Innermost layer (may be hard and stony, e.g., mango seed cover; or membranous).

Types of Fruits:

True Fruit: Develops only from the ovary (e.g., Mango, Tomato).

False Fruit: Develops from the ovary along with other floral parts like the thalamus (e.g., Apple, Strawberry, Cashew).

Parthenocarpic Fruit: Develops without fertilisation. Such fruits are usually seedless (e.g., Banana). Parthenocarpy can be induced by the application of auxins.

Fruits protect the seed and aid in seed dispersal.

Apomixis And Polyembryony

Apomixis and Polyembryony are special phenomena related to seed formation that deviate from the typical sexual reproduction process.

Apomixis

Apomixis is a mode of asexual reproduction that mimics sexual reproduction. Seeds are produced without fertilisation.

Mechanism:

Apomictic seeds can develop in various ways:

In some species, the diploid egg cell is formed without undergoing meiosis and develops into an embryo without fertilisation (e.g., some species of Asteraceae and grasses).

In some Citrus and Mango varieties, some cells of the nucellus (diploid, 2n) surrounding the embryo sac start dividing and protrude into the embryo sac, developing into embryos. These are adventitious embryos.

Significance of Apomixis:

It produces seeds that are genetically identical to the parent plant. This means desirable traits in a hybrid parent can be maintained in the offspring.

If apomixis is introduced into hybrid varieties, farmers can save hybrid seeds from one year and sow them in the next season, without losing the hybrid vigour (superior traits) or having to buy expensive hybrid seeds each year. This is seen as a potential solution in agriculture.

Polyembryony

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

Causes:

Can arise from various sources within the ovule:

More than one egg cell in an embryo sac getting fertilised.

Presence of more than one embryo sac within a single ovule.

Cleavage of a single zygote/proembryo into multiple embryos.

Development of adventitious embryos from nucellar cells or integuments (as in Citrus and Mango, which are also apomictic). These are usually the most common source of extra embryos.

Examples:

Citrus (Orange, Lemon), Mango, Onion, Groundnut.

Both apomixis and polyembryony are fascinating deviations from standard sexual reproduction, with implications for plant breeding and understanding plant development.