Sexual Reproduction in Flowering PlantsClass 12 Biology Notes

Flower - A Fascinating Organ of Angiosperms

Flowers are not just beautiful and fragrant; for a biologist, they are the primary sites of sexual reproduction in flowering plants, also known as angiosperms. The incredible diversity in the shape, size, color, and arrangement of flowers reflects the various adaptations plants have evolved to ensure the formation of fruits and seeds. The entire process of sexual reproduction in these plants revolves around the structures within the flower.

The main reproductive parts of a flower are:

• Androecium: The male reproductive part, consisting of a whorl of stamens.
• Gynoecium: The female reproductive part, consisting of one or more pistils (or carpels).

Pre-fertilisation: Structures and Events

Long before a flower blooms, the plant undergoes several hormonal and structural changes to initiate its development. A floral primordium (the earliest form of a flower) differentiates and grows. This leads to the development of the male (androecium) and female (gynoecium) reproductive structures.

Stamen, Microsporangium and Pollen Grain

The stamen is the male reproductive organ of a flower. A typical stamen has two main parts:

• Filament: A long, slender stalk that attaches to the base of the flower (thalamus) or a petal.
• Anther: A terminal, typically bilobed (two-lobed) structure at the tip of the filament. Each lobe contains two chambers called theca, making the anther dithecous.

Inside the anther, there are four microsporangia, located at the corners (two in each lobe). These microsporangia develop into pollen sacs, which are packed with pollen grains.

Structure of a Microsporangium A cross-section of a microsporangium reveals it is surrounded by four distinct wall layers:

1. Epidermis: The outermost protective layer.
2. Endothecium: The layer beneath the epidermis.
3. Middle layers: A few layers of cells beneath the endothecium.
4. Tapetum: The innermost layer. Its primary function is to provide nourishment to the developing pollen grains. Tapetum cells have dense cytoplasm and often contain more than one nucleus.

The outer three layers (epidermis, endothecium, and middle layers) are protective and help in dehiscence—the splitting of the anther to release the mature pollen grains.

At the center of a young microsporangium is a compact group of cells called the sporogenous tissue.

Microsporogenesis This is the process of forming microspores from a pollen mother cell (PMC) through meiosis.

1. As the anther develops, each cell of the sporogenous tissue becomes a potential PMC.
2. Each PMC undergoes meiosis (a type of reductional cell division) to produce a cluster of four haploid cells, called a microspore tetrad.
3. As the anther matures and dehydrates, the microspores separate from the tetrad and develop into individual pollen grains.

Thousands of pollen grains are formed inside each microsporangium.

Pollen Grain (Male Gametophyte) Pollen grains are generally spherical structures, about 25-50 micrometers in diameter. They represent the male gametophytes.

A pollen grain has a two-layered wall:

• Exine: The hard, outer layer made of sporopollenin, one of the most resistant organic materials known. Sporopollenin can withstand high temperatures, strong acids, and alkalis. No enzyme is known to degrade it. This resistance is why pollen grains are well-preserved as fossils. The exine has small openings called germ pores, where sporopollenin is absent. These pores are crucial for the emergence of the pollen tube during germination.
• Intine: The thin, inner wall made of cellulose and pectin.

Inside the intine, the cytoplasm contains two cells:

1. Vegetative Cell: This is the larger cell with abundant food reserves and a large, irregularly shaped nucleus.
2. Generative Cell: This is a smaller, spindle-shaped cell that floats in the cytoplasm of the vegetative cell.

In about 60% of angiosperms, pollen grains are shed at this 2-celled stage. In the remaining 40%, the generative cell divides by mitosis to form two male gametes before the pollen is shed (a 3-celled stage).

Pollen Grains and Humans

• Allergies: Pollen from many species (like Parthenium or carrot grass) can cause severe allergies and respiratory problems like asthma and bronchitis.
• Nutrients: Pollen grains are rich in nutrients and are used as food supplements in the form of tablets and syrups, claimed to enhance the performance of athletes and racehorses.

Pollen Viability The period for which pollen grains remain viable (able to germinate) varies greatly.

• In cereals like rice and wheat, pollen loses viability within 30 minutes.
• In some plant families like Rosaceae, Leguminosae, and Solanaceae, it can last for months.
• Pollen can be stored for years in liquid nitrogen at $-196^{\circ} \mathrm{C}$ in pollen banks for use in crop breeding programs.

The Pistil, Megasporangium (ovule) and Embryo sac

The gynoecium is the female reproductive part of the flower. It can be:

• Monocarpellary: Composed of a single pistil.
• Multicarpellary: Composed of more than one pistil.
  • Syncarpous: Pistils are fused together (e.g., Papaver).
  • Apocarpous: Pistils are free (e.g., Michelia).

Each pistil has three parts:

• Stigma: The tip of the pistil, which serves as a landing platform for pollen grains.
• Style: The elongated, slender part connecting the stigma to the ovary.
• Ovary: The basal, bulged part containing one or more cavities called locules. The placenta is located inside the locule, and attached to it are the megasporangia, commonly known as ovules.

The number of ovules can vary from one (e.g., wheat, mango) to many (e.g., papaya, watermelon).

The Megasporangium (Ovule) A typical angiosperm ovule has the following parts:

• Funicle: The stalk that attaches the ovule to the placenta.
• Hilum: The junction where the body of the ovule fuses with the funicle.
• Integuments: One or two protective outer layers that encircle the ovule.
• Micropyle: A small opening at the tip of the integuments.
• Chalaza: The basal part of the ovule, opposite the micropyle.
• Nucellus: A mass of cells enclosed within the integuments. It contains abundant reserve food materials.
• Embryo Sac (Female Gametophyte): Located within the nucellus. An ovule generally has a single embryo sac formed from a megaspore.

Megasporogenesis This is the process of forming megaspores from the megaspore mother cell (MMC).

1. A single MMC differentiates from the nucellus, typically in the micropylar region. It is a large cell with dense cytoplasm and a prominent nucleus.
2. The MMC undergoes meiosis to produce four haploid megaspores.

Female Gametophyte (Embryo Sac)

• In most flowering plants, only one of the four megaspores remains functional, while the other three degenerate.
• The embryo sac develops from this single functional megaspore. This is called monosporic development.
• The nucleus of the functional megaspore divides by mitosis to form two nuclei, which move to opposite poles.
• Two more sequential mitotic divisions result in an 8-nucleate stage. These divisions are free nuclear, meaning nuclear division is not immediately followed by cell wall formation.
• After the 8-nucleate stage, cell walls are formed, organizing the cells within the embryo sac.

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

• Egg Apparatus: Located at the micropylar end, it consists of:
  • One egg cell.
  • Two synergids. The synergids have special cellular thickenings called the filiform apparatus, which guides the pollen tube into the embryo sac.
• Antipodals: Three cells located at the chalazal end.
• Central Cell: A large central cell containing two nuclei, called polar nuclei.

Pollination

Since male and female gametes in flowering plants are non-motile, they must be brought together for fertilization. Pollination is the mechanism that achieves this. It is defined as the transfer of pollen grains from the anther to the stigma of a pistil.

Kinds of Pollination

1. Autogamy (Self-Pollination): Pollen is transferred from the anther to the stigma of the same flower. For this to occur, pollen release and stigma receptivity must be synchronized, and the anther and stigma must be close to each other.

   • Chasmogamous flowers: Flowers that open, exposing their anthers and stigma. Complete autogamy is rare in these.
   • Cleistogamous flowers: Flowers that never open (e.g., Viola, Oxalis). Autogamy is certain in these flowers, as the anther and stigma lie close and pollinate in the bud. They produce an assured seed-set even without pollinators.

2. Geitonogamy: Pollen is transferred from the anther of one flower to the stigma of another flower on the same plant. Functionally, it is cross-pollination because it involves a pollinating agent, but genetically it is similar to autogamy since the pollen comes from the same plant.

3. Xenogamy (Cross-Pollination): Pollen is transferred from the anther of a flower on one plant to the stigma of a flower on a different plant. This is the only type of pollination that brings genetically different pollen to the stigma.

Agents of Pollination Plants use external agents for pollination, which can be:

• Abiotic Agents (non-living): Wind and water.
• Biotic Agents (living): Animals.

Pollination by Wind (Anemophily)

• Common in grasses.
• Pollen grains are light, non-sticky, and produced in enormous quantities.
• Flowers often have well-exposed stamens for easy dispersal of pollen and large, feathery stigmas to trap airborne pollen.
• Flowers are usually not colorful and do not produce nectar.

Pollination by Water (Hydrophily)

• Rare in flowering plants (found in about 30 genera, mostly monocots like Vallisneria and Zostera).
• In Vallisneria, the female flower reaches the water surface on a long stalk, and male flowers release pollen onto the surface, which is carried by water currents.
• In seagrasses, flowers remain submerged, and long, ribbon-like pollen is released inside the water.
• Pollen grains are often protected from wetting by a mucilaginous covering.

Pollination by Animals (Zoophily)

• The majority of flowering plants use animals like bees, butterflies, birds, bats, wasps, ants, and even some reptiles and primates as pollinators.
• Adaptations of Animal-Pollinated Flowers:
  • Large, colorful, and fragrant to attract pollinators.
  • Rich in nectar and pollen, which serve as floral rewards for the animals.
  • Flowers pollinated by flies and beetles may secrete foul odors.
  • Pollen grains are often sticky to adhere to the animal's body.
• Co-evolution: Some plant-animal relationships are highly specific. For example, the Yucca plant and a species of moth cannot complete their life cycles without each other. The moth lays its eggs in the ovary of the flower, and in the process, pollinates it.

Outbreeding Devices Most flowering plants have bisexual flowers, which can lead to self-pollination. Continued self-pollination can cause inbreeding depression (reduced biological fitness). To discourage self-pollination and encourage cross-pollination, plants have evolved several mechanisms:

• Non-synchronization: Pollen is released before the stigma is receptive, or vice-versa.
• Different Positions: Anther and stigma are placed at different positions so that pollen cannot easily land on the stigma of the same flower.
• Self-incompatibility: A genetic mechanism that prevents self-pollen from fertilizing the ovules by inhibiting pollen germination or pollen tube growth.
• Production of Unisexual Flowers:
  • Monoecious plants (e.g., castor, maize) have both male and female flowers on the same plant. This prevents autogamy but not geitonogamy.
  • Dioecious plants (e.g., papaya) have male and female flowers on different plants. This prevents both autogamy and geitonogamy.

Pollen-Pistil Interaction This refers to all events from the moment pollen lands on the stigma until the pollen tube enters the ovule.

1. Recognition: The pistil can recognize whether the pollen is compatible (right type) or incompatible (wrong type). This recognition is mediated by a chemical dialogue between the pollen and the pistil.
2. Acceptance/Rejection: If the pollen is compatible, the pistil accepts it and promotes post-pollination events. If it is incompatible, the pistil rejects it by preventing germination or inhibiting pollen tube growth.
3. Pollen Tube Growth: A compatible pollen grain germinates on the stigma, producing a pollen tube that grows down through the style and reaches the ovary.
4. Entry into Ovule: The pollen tube enters the ovule through the micropyle and then enters one of the synergids, guided by the filiform apparatus. The two male gametes are then released into the synergid.

Artificial Hybridisation This is a crop improvement technique used by plant breeders to cross different species or varieties to combine desirable characteristics. It involves:

• Emasculation: If the female parent has bisexual flowers, the anthers are removed from the flower bud before they mature. This prevents self-pollination.
• Bagging: The emasculated flower is covered with a bag (usually butter paper) to prevent contamination from unwanted pollen.
• Pollination: When the stigma becomes receptive, desired pollen is dusted onto it.
• Rebagging: The flower is bagged again to allow the fruit and seeds to develop without contamination.

Double Fertilisation

This is a unique event that occurs only in flowering plants. After the pollen tube releases the two male gametes into the synergid of the embryo sac, two fusion events take place:

1. Syngamy: One of the male gametes fuses with the egg cell nucleus to form a diploid cell called the zygote.
2. Triple Fusion: The other male gamete moves towards the two polar nuclei in the central cell and fuses with them to produce a triploid primary endosperm nucleus (PEN).

Since two types of fusions (syngamy and triple fusion) occur in the embryo sac, this phenomenon is called double fertilisation.

• The zygote develops into the embryo.
• The central cell, now called the primary endosperm cell (PEC), develops into the endosperm.

Post-fertilisation: Structures and Events

After double fertilization, a series of changes occur, leading to the development of the endosperm, embryo, seed, and fruit.

Endosperm

Endosperm development begins before embryo development. The primary endosperm cell (PEC) divides repeatedly to form a triploid endosperm tissue. This tissue is filled with reserve food materials and provides nutrition to the developing embryo.

• Free-Nuclear Endosperm: In the most common type of development, the PEN undergoes repeated nuclear divisions without immediate cell wall formation, creating a large number of free nuclei. The liquid endosperm of tender coconut (coconut water) is an example.
• Cellular Endosperm: Later, cell walls form, and the endosperm becomes cellular. The white kernel of coconut is the cellular endosperm.

Based on its persistence in the mature seed, seeds can be:

• Non-albuminous (or ex-albuminous): The endosperm is completely consumed by the developing embryo (e.g., pea, groundnut).
• Albuminous: The endosperm persists in the mature seed and is used during germination (e.g., wheat, maize, castor).

Embryo

The embryo develops from the zygote at the micropylar end of the embryo sac. The zygote usually divides only after some endosperm has formed, ensuring a supply of nutrition. The early stages of embryo development, or embryogeny, are similar in both monocots and dicots, progressing from a proembryo to globular, heart-shaped, and finally mature stages.

Dicotyledonous Embryo A typical dicot embryo consists of:

• An embryonal axis.
• Two cotyledons (seed leaves).
• Epicotyl: The portion of the axis above the cotyledons, which terminates in the plumule (future shoot).
• Hypocotyl: The cylindrical portion below the cotyledons, which terminates in the radicle (future root). The radicle is covered by a root cap.

Monocotyledonous Embryo A monocot embryo has only one cotyledon. In the grass family, this cotyledon is called the scutellum.

• Coleorrhiza: An undifferentiated sheath that encloses the radicle and root cap.
• Coleoptile: A hollow, foliar structure that encloses the shoot apex and a few leaf primordia.

Seed

The seed is the final product of sexual reproduction in angiosperms. It is essentially a fertilized ovule.

• Seed Coat: The integuments of the ovule harden to form a tough, protective outer covering. The micropyle remains as a small pore, allowing entry of oxygen and water during germination.
• Cotyledons: These are often thick and swollen due to stored food reserves.
• Embryo Axis: The main axis of the embryo.

As the seed matures, its water content is reduced, and it becomes dry (10-15% moisture). The embryo's metabolic activity slows down, and it may enter a state of inactivity called dormancy. Under favorable conditions (moisture, oxygen, suitable temperature), the seed germinates.

Fruit Simultaneously, as the ovule matures into a seed, the ovary develops into a fruit. The wall of the ovary develops into the pericarp, the wall of the fruit.

• True Fruits: Develop only from the ovary (e.g., mango, orange).
• False Fruits: Other floral parts, such as the thalamus, also contribute to fruit formation (e.g., apple, strawberry).
• Parthenocarpic Fruits: Fruits that develop without fertilization (e.g., banana). They are seedless and can be induced by applying growth hormones.

Advantages of Seeds

• Provide protection to the young embryo via the hard seed coat.
• Contain food reserves to nourish seedlings until they can photosynthesize.
• Are products of sexual reproduction, leading to genetic variation.
• Have adaptive strategies for dispersal to new habitats.
• Can be stored for long periods due to dehydration and dormancy, forming the basis of agriculture.

Apomixis and Polyembryony

Apomixis While most seeds are products of fertilization, some plants, like certain species of Asteraceae and grasses, can produce seeds without fertilization. This phenomenon is called apomixis. It is a form of asexual reproduction that mimics sexual reproduction.

In some species, a diploid egg cell is formed without meiosis and develops into an embryo without fertilization. Apomixis is important in the hybrid seed industry. If a hybrid plant is made apomictic, its desirable characters will not segregate in the progeny, allowing farmers to reuse the seeds year after year.

Polyembryony This is the occurrence of more than one embryo in a single seed. In some Citrus and Mango varieties, some of the nucellar cells surrounding the embryo sac start dividing and develop into embryos. These nucellar embryos are genetically identical to the parent plant, making them clones.