Sexual Reproduction In Flowering Plants Class 12 Biology Chapter 1 Notes

Sexual Reproduction in Flowering Plants Class 12 Biology Chapter 1 Notes

The Significance of Flowers in Human Culture and Biology

Flowers have played a vital role in human culture throughout history, serving aesthetic, ornamental, social, religious, and cultural purposes.
They are symbols for expressing emotions like love, affection, happiness, grief, and mourning.
Five commonly cultivated ornamental flowers:

Roses
Tulips
Daisies
Lilies
Sunflowers

Five flowers used in social and cultural celebrations:

Marigolds
Orchids
Chrysanthemums
Lotus
Jasmine

Floriculture refers to the commercial cultivation and management of flowers.
Biologically, flowers are marvels of morphology and embryology, serving as sites for sexual reproduction.
Key parts of a flower where the two most important units of sexual reproduction develop:

Stamen (androecium) – Male reproductive organ
Pistil (gynoecium) – Female reproductive organ

The Preparations for Flowering in Plants

The decision for a plant to flower occurs before the actual flower becomes visible.
This decision triggers hormonal and structural changes leading to floral primordium differentiation and development.
Inflorescences are formed to carry floral buds and eventually the flowers themselves.
Within the flower, the androecium (whorl of stamens) represents the male reproductive organ, while the gynoecium represents the female reproductive organ.
Androecium and gynoecium differentiate and develop as part of the flowering process.
This preparation is crucial for the plant’s reproductive cycle.

Structure and Function of Plant Anthers and Pollen Grains

Anther Structure:

Comprises two parts: the long filament and the terminal bilobed anther.
Filament attaches to the thalamus or petal of the flower.
Number and length of stamens vary among different flower species.
Variation in size and attachment of anthers among flowers.

Anther Anatomy:

A typical angiosperm anther is bilobed with two theca in each lobe (dithecous).
Longitudinal groove often separates the theca.
Anther has a tetragonal shape with four microsporangia, two in each lobe.

Microsporangium Structure:

Microsporangium is roughly circular in outline in transverse section.
Surrounded by four wall layers: epidermis, endothecium, middle layers, and tapetum.
Outer wall layers protect and assist in anther dehiscence; innermost layer (tapetum) nourishes developing pollen grains.
Tapetal cells are often bi-nucleate.
Sporogenous tissue, a group of compact cells, occupies the center.

Microsporogenesis:

As anther develops, sporogenous tissue cells undergo meiotic divisions to form microspore tetrads.
Microspore tetrad cells are haploid.
Each cell in sporogenous tissue can become a potential pollen or microspore mother cell.
Microsporogenesis is the process of forming microspores from a pollen mother cell (PMC) through meiosis.
Microspores arrange in a cluster of four cells called the microspore tetrad.

Pollen Grain Development:

Microspores mature and dehydrate as anthers dry, dissociating from each other to become pollen grains.
Each microsporangium produces numerous pollen grains released during anther dehiscence.

Pollen Grain Characteristics:

Pollen grains represent male gametophytes.
Varied in size, shape, color, and design across species.
Typically spherical, 25-50 micrometers in diameter.
Two-layered wall: outer layer (exine) made of sporopollenin, resistant to environmental factors, with germ pores; inner layer (intine) made of cellulose and pectin.
Mature pollen grain contains two cells: vegetative cell (larger with food reserve) and generative cell (smaller with dense cytoplasm).

Pollen Viability:

Viability duration varies with temperature and humidity.
Some species’ pollen grains lose viability within 30 minutes, while others maintain it for months.
Pollen grains can be stored in liquid nitrogen (-196°C) for many years and used in pollen banks for crop breeding programs.

Allergies and Nutritional Value:

Pollen grains from many species cause allergies and respiratory issues.
Pollen is rich in nutrients and used as food supplements.

Pollen in Fertilization:

Pollen grains must land on the stigma for fertilization before losing viability.
Viability period varies among species and depends on environmental conditions.
Pollen grains can be stored in liquid nitrogen for extended periods, similar to seed banks.

Structure and Development of the Gynoecium and Ovule in Angiosperms

The gynoecium is the female reproductive part of a flower, consisting of one or more pistils.
A pistil has three parts: stigma, style, and ovary.
The stigma receives pollen grains, the style is the slender part below the stigma, and the ovary is the basal bulged part.
Ovary contains the ovarian cavity (locule), placenta, and ovules.
Ovules attach to the placenta through a stalk called funicle, with the junction known as the hilum.
Integuments protect the nucellus, and a small opening called the micropyle is present at the tip.
The chalaza represents the basal part of the ovule.
The nucellus contains abundant reserve food materials and houses the embryo sac or female gametophyte.
Megasporogenesis is the formation of megaspores from the megaspore mother cell (MMC) through meiosis.
One functional megaspore forms the female gametophyte, while the others degenerate (monosporic development).
The embryo sac initially has two nuclei and undergoes mitotic divisions to reach the 8-nucleate stage.
The embryo sac consists of 7 cells at maturity:

Egg apparatus: two synergids and one egg cell.
Antipodals: three cells at the chalazal end.
Central cell with two polar nuclei.

Synergids have filiform apparatus for guiding pollen tubes, aiding fertilization.

Pollination and Mechanisms to Prevent Self-Pollination in Angiosperms

Pollination is the transfer of pollen grains from the anther to the stigma, facilitating fertilization in flowering plants.
Pollination can be achieved by external agents such as wind, water, or animals.
Types of pollination include autogamy (self-pollination within the same flower), geitonogamy (cross-pollination within the same plant), and xenogamy (cross-pollination between different plants).
External agents like bees, butterflies, flies, beetles, wasps, ants, moths, birds, and bats serve as common pollinators.
Some plants use abiotic agents for pollination, such as wind and water.
Strategies to prevent self-pollination and encourage cross-pollination include:

Synchronizing pollen release and stigma receptivity.
Separating anthers and stigmas spatially.
Self-incompatibility mechanisms that inhibit self-pollen fertilization.
Producing unisexual flowers.

Pollen-pistil interactions involve the recognition and acceptance or rejection of pollen, mediated by chemical components.
Following compatible pollination, the pollen grain germinates on the stigma, forms a pollen tube, and delivers the male gametes to the ovule.
Hybridization in plant breeding often involves emasculation (removal of anthers) and bagging to ensure controlled pollination.
Emasculation is necessary for female parent bisexual flowers, while bagging prevents contamination.
In unisexual female flowers, bagging is sufficient for controlled pollination.
These techniques help breeders create desired hybrids and improve crop varieties.

Double Fertilization in Angiosperms

In angiosperms, double fertilization is a unique reproductive process where two fertilization events occur within a single embryo sac:
Syngamy: After a pollen tube enters one of the synergids in the embryo sac, one of the two male gametes fuses with the egg cell’s nucleus. This fusion results in the formation of a diploid cell called the zygote. The zygote will eventually develop into the embryo.
Triple Fusion: The second male gamete, within the same embryo sac, moves towards the two polar nuclei located in the central cell of the embryo sac. It then fuses with these polar nuclei, resulting in a triploid cell called the primary endosperm nucleus (PEN). This event is referred to as triple fusion.
The significance of double fertilization is that it initiates the development of two critical structures:
The zygote, which develops into the embryo.
The primary endosperm nucleus (PEN), which gives rise to the endosperm.
The endosperm is a nutrient-rich tissue that nourishes the developing embryo as it grows into a seed. This unique reproductive strategy contributes to the success of angiosperms by providing essential nutrients for the developing plant embryo.

Endosperm Development in Angiosperms

In angiosperms, the development of the endosperm is a crucial step in the process of seed formation. The endosperm is a specialized tissue that plays a vital role in providing nourishment to the developing embryo. Here’s an overview of endosperm development and its significance:

Preceding Embryo Development: Endosperm development occurs before embryo development. This sequential timing is important because the endosperm’s primary function is to serve as a source of nutrients for the growing embryo.
Formation of Triploid Endosperm: The endosperm begins its development from the triploid primary endosperm nucleus (PEN), which is the result of the fusion of two polar nuclei and one male gamete (from the pollen tube).
Formation of Free-Nuclear Endosperm: In the most common type of endosperm development, the PEN undergoes successive nuclear divisions without cell wall formation. This results in the formation of numerous free nuclei within a shared cytoplasm. This stage is referred to as “free-nuclear endosperm.”
Transition to Cellular Endosperm: Subsequently, the free nuclei present in the endosperm begin to get enclosed by cell walls. This transition from free nuclei to individual cells marks the development of “cellular endosperm.”
Accumulation of Reserve Food Materials: Throughout endosperm development, the cells of the endosperm accumulate reserve food materials, primarily starch and proteins. These reserves are stored within the endosperm cells and will later serve as a source of nourishment for the growing embryo.
Variability in the Number of Free Nuclei: The number of free nuclei formed in the endosperm before cellularization can vary significantly among different plant species. Some plants may have endosperms with a large number of free nuclei.
Fate of Endosperm: The fate of the endosperm varies among plant species:

In some plants like peas, groundnuts, and beans, the endosperm is entirely consumed by the developing embryo before seed maturation.
In other cases like castor and coconut, the endosperm persists in the mature seed and is used as a nutrient reserve during seed germination.

Endosperm in Cereals: In cereals such as wheat, rice, and maize, the endosperm is typically retained in the mature seed. It is a significant component of these seeds and serves as a major source of nutrition for both the embryo and the developing seedling during germination.
Understanding the development and function of the endosperm is crucial for plant growth and seed germination. The endosperm’s role as a nutrient storehouse ensures that the embryo has the necessary resources to initiate growth and develop into a healthy seedling when conditions are favorable for germination.

Embryo Development in Angiosperms:

Embryo development is a crucial phase in the formation of seeds in angiosperms. It takes place within the embryo sac, typically at the micropylar end, and involves a series of developmental stages. Here’s an overview of embryo development:

Zygote Formation: After double fertilization, the zygote is formed by the fusion of one male gamete with the egg cell. The zygote represents the initial stage of embryo development and is diploid (2n).
Proembryo Stage: The zygote divides and gives rise to the proembryo. During this stage, the embryo undergoes several rounds of cell division, but the cells are not yet organized into distinct structures. The proembryo is still enclosed within the embryo sac.
Embryonic Development: As embryo development progresses, it goes through several distinct stages:
1. Globular Embryo: In this stage, the embryo forms a spherical or globular structure, and the cells begin to differentiate into different regions of the embryo.
2. Heart-Shaped Embryo: The globular embryo undergoes further differentiation, and the embryo takes on a heart-shaped appearance.
3. Mature Embryo: The mature embryo has well-defined regions, including the embryonal axis, cotyledons (seed leaves), epicotyl (portion above cotyledons), hypocotyl (portion below cotyledons), and the radicle (embryonic root tip). The radicle is covered by a protective root cap.
Dicotyledonous vs. Monocotyledonous Embryos:

Dicotyledonous Embryo: Dicotyledonous embryos typically have two cotyledons (seed leaves). The embryonal axis consists of the epicotyl (above cotyledons), hypocotyl (below cotyledons), and radicle (root tip).
Monocotyledonous Embryo: Monocotyledonous embryos, as seen in grasses, have only one cotyledon, which is called the scutellum. The embryonal axis includes the epicotyl (above attachment of scutellum), hypocotyl (below attachment of scutellum), and radicle. Monocotyledonous embryos also have a protective structure called the coleoptile that encloses the epicotyl.

The timing of zygote division and embryo development can vary among plant species. In many cases, the zygote does not divide immediately after fertilization but waits until a certain amount of endosperm is formed. This ensures that the developing embryo has a source of nutrients readily available.
To observe the different parts of an embryo, you can perform a simple experiment with seeds from various plant species. Here’s how:

Soak Seeds: Take seeds from different plants (e.g., wheat, maize, peas, chickpeas, groundnut) and soak them in water overnight. This will soften the seed coat and make it easier to dissect the embryo.
Split the Seeds: After soaking, carefully split the seeds using a scalpel or a knife. Be gentle to avoid damaging the embryo.
Observe the Parts: Examine the dissected seeds to identify and observe the various parts of the embryo, such as the cotyledons, epicotyl, hypocotyl, radicle, and any other structures specific to the plant species.

This experiment will help you visualize the embryo’s structure and better understand the early stages of embryo development in different plant types.

Seed Structure and Development in Angiosperms:

In angiosperms (flowering plants), seeds are the mature, fertilized ovules, and they serve as the final product of sexual reproduction. Here’s an overview of seed structure and development in angiosperms:

Components of a Seed: A typical seed consists of the following components:

Seed Coat: The seed is enclosed by one or more seed coats, which are derived from the integuments of the ovule. These seed coats are protective layers that shield the inner embryo and endosperm from external damage.
Cotyledons: Cotyledons are the seed leaves of the embryo. In dicotyledonous plants, seeds typically have two cotyledons, while monocotyledonous seeds have only one. Cotyledons may store food reserves that nourish the developing embryo during germination.
Embryo Axis: The embryo axis includes the embryonal axis, which gives rise to the shoot and root systems of the future plant. The embryonal axis consists of the epicotyl (above cotyledons in dicots), hypocotyl (below cotyledons), and radicle (embryonic root tip).
Endosperm (in some seeds): In certain seeds, especially those categorized as albuminous seeds, a portion of the endosperm may persist in the mature seed. Endosperm serves as a source of nutrients for the developing embryo.

Types of Seeds: Seeds can be categorized into two main types based on the presence or absence of endosperm:

Non-Albuminous Seeds: These seeds have no residual endosperm, as it is completely consumed during embryo development. Examples include pea and groundnut.
Albuminous Seeds: These seeds retain a part of the endosperm, as it is not completely used up during embryo development. Examples include wheat, maize, barley, and castor.

Micropyle: The micropyle is a small pore in the seed coat that remains open. It facilitates the entry of oxygen and water into the seed during germination.
Perisperm: In some seeds, such as black pepper and beet, remnants of the nucellus may persist. This residual, persistent nucellus is known as the perisperm.
Seed Characteristics: As seeds mature, their water content is reduced, making them relatively dry, typically containing 10-15% moisture by mass. Mature seeds often enter a state of dormancy, where their metabolic activity slows down, allowing them to survive adverse conditions until germination is favorable.
Fruit Formation: The development of seeds from ovules and the transformation of the ovary into a fruit occur simultaneously. The wall of the ovary develops into the fruit’s wall, known as the pericarp. Fruits may be fleshy (e.g., guava, orange, mango) or dry (e.g., groundnut, mustard). Some fruits have evolved mechanisms for seed dispersal, aiding the spread of the species.
True vs. False Fruits: In most plants, the fruit develops solely from the ovary. These are called true fruits. However, in a few species like apple and strawberry, the thalamus (receptacle) also contributes to fruit formation. These are known as false fruits.
Parthenocarpic Fruits: In certain cases, fruits can develop without fertilization, resulting in seedless fruits. Banana is an example of a parthenocarpic fruit. Growth hormones can induce parthenocarpy.

Advantages of Seeds to Angiosperms:

They provide a reliable means of reproduction independent of water.
Seeds have adaptive strategies for dispersal to new habitats.
Seeds contain food reserves that nourish young seedlings until they can photosynthesize.
The hard seed coat provides protection to the embryo.
Seeds generate genetic diversity through sexual reproduction.

Longevity of Seeds:

The period of seed viability varies among species. Some seeds lose viability within a few months, while others can remain alive for several years. There are records of seeds remaining viable for hundreds or even thousands of years, under suitable conditions.

Remarkable Seed Examples:

Orchid fruits contain thousands of tiny seeds.
Some parasitic species like Orobanche and Striga produce fruits with numerous tiny seeds.
Seeds of Ficus are tiny, yet Ficus trees can grow into large biomass.
Some seeds have survived for thousands of years, such as Lupinus arcticus (10,000 years) and Phoenix dactylifera (2,000 years).

The remarkable diversity and adaptability of seeds contribute to the success of angiosperms in different ecosystems and their importance in agriculture. Seeds are the foundation of agriculture, and their dehydration and dormancy allow for storage and future crop cultivation.

Fruit Production Without Fertilization:

Fruit production without fertilization is called “parthenocarpy.” Parthenocarpy is a phenomenon where fruits develop without the occurrence of fertilization, resulting in seedless fruits. In parthenocarpic fruits, the development of the fruit occurs without the formation of seeds.

Apomixis and Its Reproductive Mechanism:

Apomixis is a form of asexual reproduction that mimics sexual reproduction. It is a process by which plants produce seeds without the involvement of fertilization. In apomixis, the seeds are formed asexually, and the resulting plants are essentially clones of the parent plant.

Development of Apomictic Seeds:

There are several ways in which apomictic seeds can develop:

Diploid Egg Cell Formation: In some species, the diploid egg cell is formed without undergoing the usual reduction division (meiosis). This diploid egg cell then develops into an embryo without fertilization.
Nucellar Embryo Formation: In many species, especially in some varieties of Citrus and Mango, nucellar cells (cells surrounding the embryo sac) start dividing, protrude into the embryo sac, and develop into embryos. Each ovule can contain multiple embryos in such species. This phenomenon is referred to as polyembryony.

Genetic Nature of Apomictic Embryos:

Apomictic embryos are typically genetically identical to the parent plant from which they originate. They are essentially clones of the parent plant because they are produced asexually, without the genetic variation introduced by fertilization.

Applications of Apomixis in Agriculture:

Apomixis has significant applications in agriculture, particularly in the context of hybrid seed production. Hybrid varieties of many food and vegetable crops are widely cultivated because they often exhibit desirable traits such as high productivity. However, there are challenges associated with hybrid seed production:

Hybrid seeds have to be produced every year because if seeds collected from hybrids are sown, the resulting plants do not maintain hybrid characteristics and segregate.
Producing hybrid seeds is costly, making hybrid seeds expensive for farmers.
Apomixis provides a solution to these challenges. If hybrid plants can be converted into apomicts, there is no segregation of traits in the hybrid progeny. This means that farmers can use the seeds produced by apomictic hybrids to raise new crops year after year without the need to purchase hybrid seeds annually. This can significantly reduce the cost of seed acquisition for farmers.
Due to the importance of apomixis in the hybrid seed industry, research is actively conducted in many laboratories worldwide to understand the genetics of apomixis and to transfer apomictic genes into hybrid varieties. This research aims to enable the production of more cost-effective and sustainable hybrid crops.