Chapter 2: Sexual Reproduction in Flowering Plants

In an angiosperm flower, the development of male and female gametophytes takes place in the following parts:

1. Male Gametophyte (Pollen grain): The male gametophyte develops within the anther, specifically inside the microsporangia (pollen sacs) present in the anther.
2. Female Gametophyte (Embryo sac): The female gametophyte develops within the ovule, specifically inside the megasporangium (nucellus) which is enclosed by integuments and located within the ovary.

The differentiation between microsporogenesis and megasporogenesis is as follows:

The male gametophyte in angiosperms is represented by the pollen grain. Its development involves a series of mitotic divisions starting from a microspore. This process occurs within the microsporangium (pollen sac) of the anther.

1. Microspore Formation (Microsporogenesis):
* Inside the microsporangium, diploid (2n) microspore mother cells (MMCs) or pollen mother cells (PMCs) undergo meiosis.
* Meiosis results in the formation of four haploid (n) microspores, typically arranged in a tetrad.
* As the anther matures, these microspores dissociate from the tetrad and develop into pollen grains.

2. First Mitotic Division:
* Each microspore (which is the first cell of the male gametophyte) undergoes an unequal mitotic division.
* This division produces two cells: a larger vegetative cell and a smaller generative cell.
* The vegetative cell is rich in food reserves and has a large, irregularly shaped nucleus. It is responsible for forming the pollen tube.
* The generative cell is small, spindle-shaped, and floats in the cytoplasm of the vegetative cell. It will later divide to form male gametes.

3. Pollen Grain Maturation and Release:
* At this two-celled stage (vegetative cell + generative cell), pollen grains are typically shed from the anther in about 60% of angiosperms.
* The pollen grain is covered by a two-layered wall: the outer exine (made of sporopollenin) and the inner intine (made of pectin and cellulose).

4. Second Mitotic Division (After Pollination):
* In the remaining 40% of angiosperms, the generative cell undergoes a second mitotic division *before* the pollen grain is shed.
* This division produces two haploid male gametes.
* Thus, the pollen grain is shed at a three-celled stage (vegetative cell + two male gametes).
* In species where pollen is shed at the two-celled stage, the generative cell divides mitotically to form two male gametes *after* the pollen grain lands on the stigma and germinates, i.e., during pollen tube growth.

Mature Male Gametophyte:
The mature male gametophyte consists of a vegetative cell and two non-motile male gametes. It is responsible for carrying the male gametes to the embryo sac for fertilization.

The female gametophyte, also known as the embryo sac, is typically a 7-celled, 8-nucleate structure in angiosperms. It develops from a single functional megaspore (monosporic development) within the ovule.

1. Functional Megaspore: Out of the four megaspores formed during megasporogenesis, usually only one (the chalazal one) remains functional, while the other three degenerate.
2. First Mitotic Division: The nucleus of the functional megaspore undergoes a mitotic division to form two nuclei. These two nuclei move to opposite poles of the megaspore.
3. Second Mitotic Division: Both nuclei at the poles divide mitotically, resulting in four nuclei (two at each pole).
4. Third Mitotic Division: These four nuclei again undergo mitotic division, leading to the formation of eight nuclei (four at each pole).
5. Cellular Organization: After the 8-nucleate stage, cell walls are laid down, leading to the organization of the embryo sac into a 7-celled structure:
* Egg Apparatus (Micropylar End): Three cells are grouped together at the micropylar end. This group is called the egg apparatus and consists of:
* One Egg Cell: A large central cell, which is the female gamete and fuses with one male gamete during fertilization.
* Two Synergids: Two lateral cells flanking the egg cell. They have special cellular thickenings at the micropylar tip called the filiform apparatus, which guides the pollen tube into the synergid.
* Central Cell: The largest cell of the embryo sac, located in the center. It contains two polar nuclei which eventually fuse to form a diploid secondary nucleus (or definitive nucleus) before fertilization. This cell fuses with the second male gamete to form the primary endosperm nucleus.
* Antipodal Cells (Chalazal End): Three cells are located at the chalazal end. Their function is not clearly understood, but they are believed to provide nourishment to the embryo sac or degenerate before or after fertilization.

Thus, the mature embryo sac contains 7 cells (1 egg cell, 2 synergids, 1 central cell, 3 antipodal cells) but 8 nuclei (1 egg nucleus, 2 synergid nuclei, 2 polar nuclei, 3 antipodal nuclei) before the fusion of polar nuclei.

The diagram should illustrate a mature embryo sac (female gametophyte) with the following labeled parts:

• Overall Structure: An oval-shaped sac within the ovule.
• Micropylar End: Top end, showing the egg apparatus.
• Chalazal End: Bottom end, showing the antipodal cells.
• Egg Apparatus: Clearly show:
• Egg cell: Central, larger cell of the apparatus.
• Synergids: Two cells flanking the egg cell, with finger-like projections (filiform apparatus) at their micropylar tips.
• Central Cell: The large, central cell occupying most of the embryo sac volume.
• Polar Nuclei: Two nuclei located within the central cell, typically shown near the center, before fusion.
• Antipodal Cells: Three cells located at the chalazal end.
• Ploidy: Indicate 'n' for all nuclei (egg, synergids, polar, antipodal) before polar nuclei fusion, and '2n' for the central cell after polar nuclei fusion (secondary nucleus).

The functions of the given structures are:

(c) Micropyle:
* The micropyle is a small pore or opening at the apex of the ovule (and later the seed).
* Functions:
1. It serves as the main entry point for the pollen tube into the ovule during fertilization (porogamy).
2. In the mature seed, it facilitates the absorption of water and exchange of gases (oxygen) required for seed germination.

Type of Cell Division
Both microsporogenesis and megasporogenesis involve meiotic cell division. This is a reductional division that reduces the chromosome number from diploid (2n) to haploid (n).

Structures Formed at the End of These Events
* At the end of microsporogenesis: Microspores (which mature into pollen grains).
* At the end of megasporogenesis: Megaspores (typically one functional megaspore that develops into the embryo sac).

A labelled diagram of a Longitudinal Section (L.S.) of a typical bisexual flower should clearly show the following parts:

1. Pedicel: The stalk of the flower.
2. Thalamus (Receptacle): The swollen tip of the pedicel on which floral parts are borne.
3. Sepals: The outermost whorl, usually green and leaf-like, protecting the bud (collectively called calyx).
4. Petals: The second whorl, often brightly coloured and scented to attract pollinators (collectively called corolla).
5. Stamen (Androecium): The male reproductive part, consisting of:
* Anther: The bilobed structure containing pollen sacs where pollen grains are produced.
* Filament: The stalk supporting the anther.
6. Pistil/Carpel (Gynoecium): The female reproductive part, consisting of:
* Stigma: The receptive tip for pollen grains, often sticky.
* Style: The slender stalk connecting the stigma to the ovary.
* Ovary: The swollen basal part containing one or more ovules.
* Ovule: The structure inside the ovary that contains the embryo sac and develops into a seed after fertilisation.

The diagram should show these parts in their correct anatomical positions, with clear lines pointing to each label.

Various Types of Pollination
Pollination is the transfer of pollen grains from the anther to the stigma of a flower. It can be broadly classified into two main types: self-pollination and cross-pollination.

A. Self-Pollination (Autogamy)
This type of pollination involves the transfer of pollen grains from the anther to the stigma of the same flower or another flower on the same plant. It is further divided into:

1. Autogamy: Transfer of pollen from the anther to the stigma of the *same flower*.
* Chasmogamous flowers: Flowers that open and expose their anthers and stigmas. Autogamy in such flowers requires synchrony in pollen release and stigma receptivity, and the anther and stigma should lie close to each other.
* Cleistogamous flowers: Flowers that do not open at all. In such flowers, the anthers and stigma lie close to each other, ensuring self-pollination even in the absence of pollinators. Cleistogamous flowers are invariably autogamous and produce assured seed-set.

1. Geitonogamy: Transfer of pollen grains from the anther of one flower to the stigma of *another flower on the same plant*. Genetically, it is similar to autogamy (as the pollen comes from the same plant), but ecologically, it is cross-pollination as it involves a pollinating agent.

B. Cross-Pollination (Allogamy / Xenogamy)
This involves the transfer of pollen grains from the anther of a flower on one plant to the stigma of a flower on a *different plant of the same species*. This type of pollination brings about genetic variation.

After pollination, a series of crucial events occur that ultimately lead to fertilisation in flowering plants. These events collectively constitute pollen-pistil interaction and are as follows:

1. ### Pollen Landing on Stigma
Once pollen grains are transferred to the stigma (pollination), the pistil has the ability to recognise whether the pollen is of the right type (compatible) or of the wrong type (incompatible). This recognition is mediated by chemical components exchanged between the pollen and the stigma.

2. ### Pollen Germination
If the pollen is compatible, the stigma provides nutrients (sugars, boron, inositol) that stimulate the pollen grain to germinate. The pollen grain absorbs moisture and nutrients, swells, and then produces a thin tube-like outgrowth called the pollen tube through one of its germ pores.

3. ### Growth of Pollen Tube Through the Style
The pollen tube grows downwards through the tissues of the stigma and style. The growth is guided by chemical signals released by the ovule (chemotropism). During its growth, the generative cell (if not already divided) divides to form two non-motile male gametes. The vegetative cell nucleus (tube nucleus) guides the pollen tube growth and eventually degenerates.

4. ### Entry of Pollen Tube into the Ovule
After reaching the ovary, the pollen tube enters the ovule. The entry can occur through different regions:
* Porogamy: Through the micropyle (most common).
* Chalazogamy: Through the chalazal end.
* Mesogamy: Through the integuments.

5. ### Entry of Pollen Tube into the Embryo Sac
Once inside the ovule, the pollen tube typically enters the embryo sac through the micropylar end. It usually penetrates one of the synergids. The synergids play a crucial role by secreting chemotropic substances that attract the pollen tube and by having filiform apparatus that guides the pollen tube into the embryo sac.

6. ### Discharge of Male Gametes
Upon entering the synergid, the tip of the pollen tube ruptures, and the two male gametes are released into the cytoplasm of the synergid. The synergid then degenerates.

7. ### Double Fertilisation
This is the most characteristic event in flowering plants and involves two fusions:
* Syngamy (Generative Fertilisation): One of the male gametes fuses with the egg cell (female gamete) to form a diploid zygote (2n). The zygote develops into the embryo.
* Triple Fusion (Vegetative Fertilisation): The other male gamete fuses with the diploid central cell (containing two polar nuclei) to form a triploid Primary Endosperm Nucleus (PEN) (3n). The PEN develops into the endosperm, which provides nourishment to the developing embryo.

These sequential events ensure the successful fusion of male and female gametes, leading to the formation of the embryo and endosperm, which are vital for seed development.

Apomixis is a form of asexual reproduction that mimics sexual reproduction, specifically seed formation, without fertilisation. In apomixis, seeds are produced without the fusion of gametes (syngamy) and meiosis. The resulting offspring are genetically identical to the parent plant, essentially clones.

Types of Apomixis (as per NCERT context):
1. Adventive Embryony: In this type, embryos develop directly from diploid sporophytic cells of the ovule, such as the nucellus or integuments, without undergoing meiosis or gamete formation. These embryos protrude into the embryo sac and develop into seeds. Examples include Citrus and Mango.
2. Recurrent Apomixis (Apogamy/Apospory):
* Apospory: A diploid sporophytic cell (e.g., nucellar cell) directly forms a diploid embryo sac without meiosis. The egg cell within this diploid embryo sac then develops into an embryo without fertilisation.
* Apogamy: The embryo develops from any diploid cell of the embryo sac other than the egg cell (e.g., synergids or antipodals) without fertilisation.

Scientists are actively trying to understand the genetics of apomixis and transfer apomictic genes into hybrid crops to make hybrid seed production more economical and accessible.

Polyembryony is the phenomenon of occurrence of more than one embryo in a single seed. This can lead to the development of multiple seedlings from a single seed.

Polyembryony is commonly observed in Citrus, Mango, Onion, and groundnut. It is of great significance in horticulture for raising genetically uniform seedlings from nucellar embryos, which are clones of the parent plant.

Seeds are the product of sexual reproduction in angiosperms and represent a crucial evolutionary adaptation that has contributed significantly to their dominance on land. The advantages of seeds to angiosperms are numerous:

1. Protection of the Embryo: The seed coat provides a tough, protective covering around the delicate embryo, safeguarding it from mechanical injury, desiccation, and microbial attack.
2. Nourishment for the Embryo: Seeds contain stored food reserves (in cotyledons or endosperm) that provide essential nutrients for the developing embryo during germination and for the young seedling until it can photosynthesize independently.
3. Dispersal Mechanisms: Seeds are equipped with various adaptations (e.g., wings, hooks, edible fruits) that facilitate their dispersal by wind, water, animals, or even humans. This allows angiosperms to colonize new habitats and reduce competition with parent plants.
4. Dormancy: Seeds often exhibit a period of dormancy, allowing them to survive unfavorable environmental conditions (e.g., extreme temperatures, drought) and germinate only when conditions are conducive for seedling establishment. This ensures higher survival rates.
5. Genetic Recombination and Variation: Seeds are typically the result of sexual reproduction (fusion of gametes), which involves meiosis and fertilisation. This process leads to genetic recombination, producing offspring with new combinations of traits, enhancing genetic diversity, and providing raw material for evolution and adaptation to changing environments.
6. Colonization of New Areas: Efficient dispersal mechanisms, combined with dormancy and stored food, enable seeds to travel long distances and establish new populations in diverse geographical regions, contributing to the widespread distribution of angiosperms.
7. Evolutionary Advantage: The development of seeds freed plants from the need for water for fertilisation (unlike lower plants), allowing them to thrive in drier terrestrial environments. The seed also represents a more efficient and protected propagule compared to spores.
8. Basis of Agriculture: Seeds are the foundation of human agriculture, providing food, feed, fiber, and fuel. Their ability to be stored for long periods without losing viability is critical for food security.

Following successful pollination and fertilisation, a series of profound changes occur within the flower, leading to the development of the fruit and seeds. These changes involve the transformation of various floral parts:

Here are the differentiations between the given pairs:

Seeds are the final product of sexual reproduction in angiosperms and offer several significant advantages that have contributed to their evolutionary success and dominance in terrestrial environments. These advantages include:

1. Protection of Embryo: The hard seed coat provides crucial protection to the delicate young embryo inside from mechanical injury, desiccation, and adverse environmental conditions.

1. Nourishment: Seeds contain stored food reserves (in the endosperm or cotyledons) that provide essential nutrition to the developing embryo during its early growth and to the young seedling until it can photosynthesize independently. This ensures a strong start for the new plant.

1. Dispersal: Seeds are equipped with various adaptations for dispersal (e.g., wings, hooks, fleshy fruits, buoyancy) by wind, water, animals, or even humans. This allows the plant to colonize new habitats, reduce competition with parent plants, and expand its geographical range.

1. Dormancy: Seeds often exhibit a period of dormancy, which is a state of suspended growth. This allows them to survive unfavorable conditions (e.g., extreme temperatures, drought) and germinate only when conditions are optimal for seedling establishment, thus increasing the chances of survival.

1. Genetic Recombination and Variation: Seeds are products of sexual reproduction, involving the fusion of male and female gametes. This process leads to genetic recombination and the creation of new genetic variations among offspring, which is vital for adaptation to changing environments and evolutionary success.

1. Storage and Viability: Seeds can remain viable for varying periods, sometimes for many years, allowing for storage and future use. This property is fundamental to agriculture, enabling farmers to store seeds for planting in the next season and for maintaining genetic diversity in seed banks.

1. Basis of Agriculture: Seeds form the very foundation of agriculture. Most of our food crops (cereals, pulses, oilseeds) are cultivated for their seeds, which are a primary source of nutrition for humans and animals.

1. Better Adaptive Strategies: Overall, seeds represent a more efficient and robust reproductive strategy compared to spores, providing better adaptive strategies for the survival and propagation of the species in diverse terrestrial environments.

An artificial seed, also known as a synthetic seed or synseed, is a somatic embryo (or other plant propagule like an axillary bud, shoot tip, or callus) encapsulated in a protective coating, usually a hydrogel, to mimic the structure and function of a natural seed. It is a biotechnological tool used for plant propagation.

Formation of an Artificial Seed
The formation of an artificial seed typically involves two main steps:

1. Somatic Embryogenesis: This is the process where somatic cells (non-sexual cells) of a plant are induced to form embryos *in vitro* (in a test tube or culture dish). Explants (small pieces of plant tissue like leaves, stems, roots, or even single cells) are cultured on a suitable nutrient medium containing specific plant growth regulators. Under appropriate conditions, these cells dedifferentiate and then redifferentiate to form somatic embryos (embryoids). These embryoids are bipolar structures with a radicle and a plumule, resembling zygotic embryos, but they originate from somatic cells rather than from a zygote.

2. Encapsulation: Once somatic embryos are formed, they are individually encapsulated in a protective matrix. The most common encapsulating material is sodium alginate, a polysaccharide derived from brown algae. The process is as follows:
* Somatic embryos are mixed with a sterile solution of sodium alginate (typically 2-5%).
* This mixture is then dropped into a sterile solution of calcium chloride (CaCl₂). The calcium ions react with the alginate to form a calcium alginate gel, which solidifies around each somatic embryo, creating a bead-like structure. This process is called ionotropic gelation.
* The resulting beads are firm, transparent, and resemble natural seeds. The encapsulating matrix can also be supplemented with nutrients, growth regulators, fungicides, antibiotics, or even symbiotic microorganisms to enhance the survival and growth of the encapsulated embryo.

Importance of Artificial Seeds
Artificial seeds offer several significant advantages and have various applications in agriculture and horticulture:

1. Mass Propagation of Elite Varieties: They enable rapid, large-scale, and clonal propagation of genetically uniform plants from desirable genotypes (e.g., high-yielding, disease-resistant varieties) that are difficult to propagate through conventional methods or produce few seeds.

1. Propagation of Recalcitrant Seeds: Useful for plants that produce recalcitrant seeds (seeds that cannot tolerate desiccation and thus cannot be stored for long periods) or plants that do not produce viable seeds at all.

1. Storage and Transport: Artificial seeds are easier to store and transport compared to tissue cultures or whole plants, reducing costs and logistical challenges.

1. Production of Disease-Free Plants: Somatic embryos can be derived from disease-free explants (e.g., meristematic tissues), ensuring the production of pathogen-free plants.

1. Genetic Conservation: They can be used for the conservation of endangered plant species or valuable germplasm by storing somatic embryos.

1. Direct Sowing: Artificial seeds can be directly sown in the field or nursery, similar to natural seeds, simplifying planting procedures and potentially reducing labor costs.

1. Delivery of Biocontrol Agents: The encapsulating matrix can be used to deliver beneficial microbes (e.g., nitrogen-fixing bacteria, mycorrhizal fungi) or pesticides along with the embryo, providing an integrated delivery system.

What is Apomixis?
Apomixis is a form of asexual reproduction that mimics sexual reproduction, specifically in the formation of seeds without fertilization. In apomixis, the embryo develops directly from an unfertilized egg cell or from other diploid cells of the ovule (like nucellus or integuments) without meiosis and syngamy (fusion of gametes). This results in the production of seeds that contain embryos genetically identical to the parent plant. It is essentially asexual reproduction through seed formation.

Apomixis is common in some species of Asteraceae and grasses, and also found in plants like Citrus and Mango.

Types of Apomixis (relevant to seed formation):
1. Agamospermy: This is the most common type of apomixis where seeds are formed without fertilization.
* Adventive Embryony (Sporophytic Apomixis): Embryos develop directly from diploid sporophytic cells of the ovule, such as nucellar cells or integument cells, which grow into the embryo sac. The zygotic embryo may or may not be present. Examples: Citrus, Mango.
* Recurrent Apomixis (Gameto-sporophytic Apomixis): A diploid embryo sac is formed without meiosis. The egg cell within this diploid embryo sac then develops into an embryo parthenogenetically (without fertilization).
* Diplospory: The megaspore mother cell directly develops into a diploid embryo sac without undergoing meiosis. Example: *Taraxacum* (dandelion).
* Apospory: A diploid nucellar cell (or other sporophytic cell) develops into a diploid embryo sac, bypassing the megaspore mother cell. Example: Many grasses, Asteraceae.
2. Non-recurrent Apomixis: In this rare type, meiosis occurs, producing a haploid egg cell. However, this haploid egg cell develops into an embryo parthenogenetically without fertilization. The resulting plant is haploid and usually sterile. This type is generally not considered true apomixis in the context of producing viable, fertile seeds for propagation.

Importance of Apomixis
Apomixis holds significant importance, particularly in plant breeding and agriculture:

1. Maintenance of Hybrid Vigor (Heterosis): Hybrid varieties often exhibit superior characteristics (hybrid vigor or heterosis) compared to their parental lines. However, in sexually reproducing plants, hybrid vigor is lost in subsequent generations due to segregation of genes during meiosis. Apomixis allows for the indefinite propagation of these desirable hybrid characteristics generation after generation, as the offspring are genetically identical clones of the parent hybrid.

1. Cost-Effective Propagation: Farmers typically have to purchase new hybrid seeds every year because the progeny from hybrid seeds lose their vigor. If these hybrids can be made apomictic, farmers can use the same hybrid seeds year after year without having to buy new expensive seeds, significantly reducing cultivation costs.

1. Avoids Segregation: Since apomictic reproduction bypasses meiosis and fertilization, there is no genetic segregation. This ensures that the progeny are genetically uniform and identical to the parent plant, preserving specific desirable traits without variation.

1. Rapid Multiplication: Apomixis can facilitate the rapid and large-scale multiplication of superior genotypes, which is beneficial for commercial agriculture.

1. Disease Resistance: It can be used to propagate disease-resistant varieties consistently without the risk of losing these traits through sexual recombination.

In essence, apomixis offers a powerful tool for maintaining genetic purity and exploiting hybrid vigor in crop plants, making it a highly desirable trait for plant breeders.