Sexual Reproduction in Flowering Plants Chapter Notes | Biology Class 12 - NEET PDF Download

What is a Flower?

A flower is the reproductive structure of a flowering plant (angiosperm) that typically consists of various parts such as petals, sepals, stamens, and pistils. It is responsible for the plant's sexual reproduction, as it produces seeds through the process of pollination and fertilization. Flowers come in various shapes, sizes, and colors, and they often emit fragrances and nectar to attract pollinators like insects, birds, or the wind.

Parts of Flower

Structure of Flower

A flower comprises the following parts:

(A) Accessory whorls

(i) Calyx: It is the outermost whorl of a flower. It comprises units called sepals. In the bud stage, calyx encloses the rest of the flower. They usually exhibit green colouration, at some other instances, they may be a colour like petals. This state of Calyx is termed as petaloid. Calyx can either be prominent or absent.

(ii) Corolla: It consists of many numbers of petals and it is the second whorl of the flower. These petals are sometimes fragrant. They are coloured, thin and soft that would help in the process of pollination as they would attract animals and insects.

(B) Reproductive whorls

(i) Androecium:- male reproductive whorl , smallest unit:- stamen

(ii) Gynoecium:- female reproductive whorl, smallest unit:- carpel/ pistil

Pre-fertilisation: Structures and Events

Before the actual appearance of a flower on a plant, the decision for flowering has already been made. This process triggers various hormonal and structural changes that lead to the formation and development of the floral primordium. Inflorescences, which eventually bear floral buds and flowers, are produced. Within the flower, the male and female reproductive structures, known as the androecium and gynoecium, respectively, undergo differentiation and development. The androecium comprises a whorl of stamens, which constitute the male reproductive organs, while the gynoecium represents the female reproductive organs.

Stamen, Microsporangium and Pollen Grain

 Stamen:  These are the male part of a flower. Many stamens are collectively known as the androecium.   They are structurally divided into two parts:

Structure of Stamen

• Filament: the part that is long and slender and attached the anther to the flower. The base of the filament is connected to the thalamus or the petal of the flower. Different types of flowers have varying numbers and lengths of stamens.
• Anthers: It is the head of the stamen and is responsible for producing the pollen which is transferred to the pistil or female parts of the same or another flower to bring about fertilization.

The structure of anther: 

• A typical angiosperm anther is bilobed, with each lobe having two theca, making them dithecous.
• A longitudinal groove often runs lengthwise, separating the theca.
• The anther is a four-sided (tetragonal) structure consisting of four microsporangia located at the corners, two in each lobe.
• The microsporangia develop into pollen sacs that extend longitudinally through the length of an anther and are packed with pollen grains.

Microsporangium

Microsporangia consist of  a typical microsporangium has a near-circular outline. It is enclosed by four wall layers - epidermis, endothecium, middle layers, and the tapetum.

Enlarged view of one microsporangium

                                                                                                                   A mature dehisced anther

• The four wall layers perform different functions.
• Epidermis, endothecium and middle layers protect the microsporangium and aid in its dehiscence to release pollen.
• The tapetum is the innermost layer and nourishes the developing pollen grains.
• Tapetal cells have dense cytoplasm and usually more than one nucleus. They could become bi-nucleate in the following ways:
• Nuclear division without cytokinesis
• Fusion of two adjacent nuclei
• When the anther is young, the sporogenous tissue, a group of compactly arranged homogenous cells, occupies the centre of each microsporangium.

Microsporogenesis

• As the anther develops, cells of the sporogenous tissue undergo meiotic divisions to form microspore tetrads.
• Each cell of the sporogenous tissue is capable of giving rise to a microspore tetrad. 
• The process of forming microspores from a pollen mother cell through meiosis is microsporogenesis.
• As microspores are formed, they are arranged in a cluster of four cells - the microspore tetrad.
• As anthers mature and dehydrate, microspores dissociate and develop into pollen grains.
• Several thousand microspores or pollen grains are formed inside each microsporangium and released with anther dehiscence.

Pollen Grain

Pollen grains, also known as male gametophytes, play a crucial role in plant reproduction.

Structure of Pollen Grain

Structure of pollen grains : 

Pollen grains are spherical in shape and measure between 25-50 micrometers in diameter. The outer layer of the pollen grain is called the exine, which is made up of a hard and resistant material called sporopollenin.

• Sporopollenin is one of the most resistant organic materials known.
• Sporopollenin can withstand high temperatures and strong acids and alkalis.
• No enzyme that degrades sporopollenin is currently known.
• The exine has prominent apertures called germ pores where sporopollenin is absent.
• The pollen grains are well-preserved as fossils because of the presence of sporopollenin.
• The inner wall of the pollen grain is called the intine, which is a thin and continuous layer made up of cellulose and pectin.
• The cytoplasm of pollen grain is surrounded by a plasma membrane.
• When the pollen grain is mature, it contains two cells - the vegetative cell and generative cell.
• The vegetative cell of the pollen grain is responsible for nourishing the generative cell and guiding it towards the female gametophyte.
• In over 60% 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).

Functions and Uses of Pollen Grains : 

• The pollen grains represent the male gametophytes and are responsible for fertilising the female gametophytes.
•  Pollen grains can be stored for years in liquid nitrogen (-1960C).
•  Such stored pollen can be used as pollen banks, similar to seed banks, in crop breeding programmes.
•  Pollen is rich in nutrients and is used as a food supplement in the form of tablets and syrups in many western countries.
• Some people claim that consuming pollen can increase athletic performance in humans and horses.

Variability and Viability of Pollen Grains :

• The period for which pollen grains remain viable is highly variable and depends on the prevailing temperature and humidity.
• In some cereals such as rice and wheat, pollen grains lose viability within 30 minutes of their release, while in some members of Rosaceae, Leguminoseae, and Solanaceae, they maintain viability for months.
• Pollen grains can be stored for years in liquid nitrogen and used in crop breeding programmes.

The Pistil, Megasporangium (ovule) and Embryo sac

Gynoecium is the female reproductive organ and the last whorl of the flower. It is composed of pistil and occupies the central position of the thalamus. The stigma, style, and ovary are the components of the pistil. The ovary produces ovules internally. Through meiosis, ovules produce megaspores which in turn develops into female gametophytes. As a result, egg cells are produced.

Structure of carpel

• Style - is a long slender stalk that holds the stigma. Once the pollen reaches the stigma, the style starts to become hollow and forms a tube called the pollen tube which takes the pollen to the ovaries to enable fertilization.

• Stigma– This is found at the tip of the style. It forms the head of the pistil. The stigma contains a sticky substance whose job is to catch pollen grains from different pollinators or those dispersed through the wind. They are responsible to begin the process of fertilization.
• Ovary – They form the base of the pistil. The ovary holds the ovules.
• Ovules– These are the egg cells of a flower. They are contained in the ovary. In the event of a favorable pollination where a compatible pollen reaches the stigma and eventually reaches the ovary to fuse with the ovules, this fertilized product forms the fruit and the ovules become the seeds of the fruit.

Gynoecium can be:

• Monocarpellary: The gynoecium consists of a single pistil. For eg., peas and beans.
• Multicarpellary: Here, gynoecium comprises more than one pistil.
• Syncarpous: It is the gynoecium with combined pistil. For eg., Tomato, cucumber.
• Apocarpous: It is the gynoecium with free pistil. For eg., Lotus Vinca.

(a) A dissected flower of Hibiscus showing pistil (other floral parts have been removed); (b) Multicarpellary, syncarpous pistil of Papaver ; (c) A multicarpellary, apocarpous gynoecium of Michelia; (d) A diagrammatic view of a typical anatropous ovule .

Megasporangium( Ovule) 

An ovule or megasporangia is a small structure attached to the placenta by means of a stalk called funicle. The body of the ovule fuses with funicle in the region called hilum. Each ovule has one or two protective envelopes called integuments.

• Integuments encircle the nucellus except at the tip where a small opening called the micropyle is organised. Opposite the micropylar end, is the chalaza, representing the basal part of the ovule. Enclosed within the integuments is a mass of cells called the nucellus. Cells of the nucellus have abundant reserve food materials.
• Located in the nucellus is the embryo sac or female gametophyte. An ovule generally has a single embryo sac formed from a megaspore.

Structure of Embryo Sac

Megasporogenesis

Megasporogenesis is the process of forming megaspores from the megaspore mother cell. Here are some key points about this process:

• Megaspores are formed in ovules from a single megaspore mother cell (MMC) in the micropylar region of the nucellus.
• The MMC is a large cell that contains dense cytoplasm and a prominent nucleus.
• The MMC undergoes meiotic division, which is important for the production of four megaspores.

Female gametophyte

(i) Megaspore Development:

• In most flowering plants, one of the four megaspores formed is functional, while the other three degenerate.
• The functional megaspore develops into the female gametophyte (embryo sac).

(ii) Monosporic Development: This process, where the embryo sac forms from a single megaspore, is called monosporic development.

(iii) Ploidy Levels:

• Nucellus Cells: Diploid (2n)
• Megaspore Mother Cell (MMC): Diploid (2n)
• Functional Megaspore: Haploid (n)
• Female Gametophyte (Embryo Sac): Haploid (n)

(iv) Embryo Sac Formation:

• Initial Division: The nucleus of the functional megaspore divides mitotically to form two nuclei, which migrate to opposite poles, creating a 2-nucleate embryo sac.
• Subsequent Divisions: Two more mitotic divisions occur, leading to the 4-nucleate and then the 8-nucleate stages.
• Free Nuclear Divisions: These mitotic divisions are free nuclear, meaning they are not followed immediately by cell wall formation.

(v) Mature Embryo Sac Structure: Cell Formation: After the 8-nucleate stage, cell walls are formed, organizing the embryo sac.

(vi) Cell Distribution:

• Egg Apparatus: Three cells at the micropylar end, consisting of two synergids and one egg cell. Synergids have filiform apparatus at the micropylar tip.
• Antipodals: Three cells at the chalazal end.
• Central Cell: Contains two polar nuclei.

(vii) Final Configuration: The mature embryo sac is 7-celled but 8-nucleate.

(a) Parts of the ovule showing a large megaspore mother cell, a dyad and a tetrad of megaspores; (b) 2, 4, and 8-nucleate stages of embryo sac and a mature embryo sac; (c) A diagrammatic representation of the mature embryo sac.

Pollination

Pollination is an ecological process carried out by all flowering plants. In this process, the matured pollen grains are transferred from the anther to the stigma for the purpose of sexual reproduction in flowering plants.

Cross-Pollination

Kinds of pollination

(i)Autogamy:

• Pollination occurs within the same flower.
• Pollen grains transfer from the anther to the stigma of the same flower.
• Complete autogamy is rare in flowers that open and expose their anthers and stigma.
• Requires synchronization of pollen release and stigma receptivity, and proximity of anthers and stigma.
• Examples include Viola (common pansy), Oxalis, and Commelina:
• Chasmogamous Flowers: Open flowers with exposed anthers and stigma.
• Cleistogamous Flowers: Closed flowers with anthers and stigma close together, allowing self-pollination.
• Advantage of Cleistogamy: Provides assured seed-set without the need for pollinators.
• Disadvantage of Cleistogamy: Limits genetic diversity since there is no chance for cross-pollination.

(ii) Geitonogamy:

• Transfer of pollen grains from the anther to the stigma of another flower on the same plant.
• Functionally similar to cross-pollination because it involves a pollinating agent.
• Genetically similar to autogamy since the pollen comes from the same plant.

(iii) Xenogamy:

• Transfer of pollen grains from the anther to the stigma of a different plant.
• Involves genetically different pollen grains, promoting genetic diversity.
  

Agents of Pollination

Plants use different agents for pollination, which can be biotic or abiotic. Biotic agents are more commonly used for pollination. However, only a small proportion of plants use abiotic agents. Abiotic pollination can occur by wind and water.

Agents of pollination

1: Abiotic or Non-living 

(i) Wind Pollination:

Characteristics:

• Common among abiotic pollinations.
• Pollen grains are light and non-sticky.
• Flowers have well-exposed stamens and large, feathery stigmas to catch airborne pollen.
• Often have a single ovule per ovary and many flowers clustered in an inflorescence.
• Example: Corn cob (tassels are stigmas and styles that capture pollen).
• Common in: Grasses.

A wind-pollinated plant showing compact inflorecence and well exposed stamens 

(ii) Water Pollination:

Characteristics:

• Rare and limited to about 30 genera, mostly monocotyledons.
• Water is a regular transport medium for male gametes in lower plant groups (e.g., algae, bryophytes).
• Examples of Water-Pollinated Plants:
• Freshwater: Vallisneria and Hydrilla.
• Marine: Seagrasses like Zostera.

Pollination Process:

• Vallisneria: Female flowers rise to the water surface; male pollen is released into water and carried to female flowers.
• Seagrasses: Female flowers stay submerged; pollen grains are released into water and carried passively.

2: Biotic or Living Agents:

Biotic agents involve living organisms that contribute to the pollination process. Flowers adapted for biotic pollination are often colorful, attractive, and emit sweet scents to lure insects, birds, and other living creatures.

(a) Entomophily, or pollination by insects, is common in angiosperms. Bees, butterflies, beetles, and flies are common pollinators. These flowers typically have limited pollen and rough-textured grains that easily adhere to insect limbs.

(b) Ornithophily, or pollination by birds, involves birds like hummingbirds and honey thrushes that feed on flower nectar. These flowers have fewer pollen grains that attach to the birds' beaks and mouths, transferring to other flowers.

(c) Chiropterophily, or pollination by bats, is observed in specific trees like Kigelia Africana, Anthocephalus cadamba, and Bauhinia Monandra.

(i) Flower Adaptations for Animal Pollination:

• Insect-Pollinated Flowers:
• Typically large, colorful, fragrant, and rich in nectar.
• Small flowers are often clustered in inflorescences to increase visibility.
• Flowers attract animals through color and fragrance.

(ii) Flies and Beetles:

Attract these animals with foul odors.

(iii) Floral Rewards:

Provide nectar and pollen as rewards.

Some flowers offer safe places for laying eggs (e.g., Amorphophallus) or engage in mutualistic relationships (e.g., Yucca and moth).

(iv) Pollination Process:

Animals collect nectar and pollen, which are sticky in animal-pollinated flowers.

As animals move from flower to flower, they transfer pollen from anthers to stigmas, facilitating pollination.

(v) Pollen/Nectar Robbers:

Some insects may consume nectar or pollen without pollinating the flower. These are known as pollen or nectar robbers.

Comparison of Abiotic and Biotic Pollinated Flowers

• Both wind and water-pollinated flowers are not very colorful and do not produce nectar. This is because they rely on abiotic agents for pollination and do not need to attract animals. 
• In contrast, animal-pollinated flowers are more colourful, fragrant, and rich in nectar. They need to attract animals for pollination and provide them with rewards.

Outbreeding Devices

• Flowering plants have developed several mechanisms to discourage self-pollination and to encourage cross-pollination. 
• Majority of flowering plants produce hermaphrodite flowers which lead to self-pollination resulting in inbreeding depression. 
• To prevent autogamy, some species release pollen and stigma at different times, while others place anther and stigma at different positions. 
• Self-incompatibility is a genetic mechanism that prevents self-pollen from fertilizing ovules by inhibiting pollen germination or pollen tube growth in the pistil. 
• Production of unisexual flowers also prevents self-pollination. 
• In species like papaya, male and female flowers are present on different plants, known as dioecy, which prevents both autogamy and geitonogamy.

Pollen-Pistil Interaction

The process from pollen deposition on the stigma to pollen tubes entering the ovule is known as pollen-pistil interaction.

• Pollination doesn't always result in compatible pollen transfer.
• The pistil can recognize and accept or reject pollen based on its compatibility.
• The recognition process involves chemical interactions between the pollen and pistil components.
• Following compatible pollination, the pollen grain germinates on the stigma, producing a pollen tube that grows through the tissues of the stigma and style.
• In plants that shed pollen in the two-celled condition, the generative cell divides and forms two male gametes during the growth of the pollen tube in the stigma.
• In plants that shed pollen in the three-celled condition, pollen tubes carry the two male gametes from the beginning.
• The pollen tube enters the ovule through the micropyle and then enters one of the synergids through the filiform apparatus.

Artificial Hybridization for Crop Improvement

Artificial hybridization is a major approach in crop improvement programs to produce commercially superior varieties by combining desirable characteristics of different species and genera.

• To ensure only desired pollen grains are used for pollination and the stigma is protected from contamination, emasculation and bagging techniques are used.
• In flowers with bisexual flowers, emasculation is necessary by removing anthers from the flower bud before the anther dehisces
• The emasculated flowers have to be covered with a suitable bag to prevent contamination of the stigma with unwanted pollen, which is called bagging.
• When the stigma of bagged flower attains receptivity, mature pollen grains collected from anthers of the male parent are dusted on the stigma, and the flowers are rebagged, and the fruits allowed to develop.
• In female parents with unisexual flowers, emasculation is not required. The female flower buds are bagged before the flowers open, and pollination is carried out using the desired pollen when the stigma becomes receptive.

Fertilization

Following pollination, the pollen grains travel to the ovary via the pollen tube. Upon reaching the ovary, one male gamete fertilizes the ovule, forming a zygote, which develops into an embryo. Meanwhile, the other male gamete fuses with the polar nuclei, leading to the formation of the endosperm nucleus. This endosperm provides nourishment to the developing embryo.

Fertilization transforms the ovules into seeds, while the ovary develops into the fruit.

Fertilization in Plants

Double Fertilisation

When pollen grains land on the stigma of a flower, they produce a pollen tube that travels down to the ovary, where the embryo sacs containing the female gametes are located. The process of double fertilisation takes place when the male gametes released by the pollen tube enter the embryo sac.The Process of Double Fertilisation

During double fertilisation, two types of fusion take place:

• The male gamete fuses with the egg cell to form a diploid zygote
• The other male gamete fuses with the two polar nuclei to produce a triploid primary endosperm nucleus (PEN)

This phenomenon is called double fertilisation because two types of fertilisation take place in one embryo sac.

Formation of Endosperm and Embryo

• After triple fusion, the central cell becomes the primary endosperm cell (PEC), which develops into the endosperm. Meanwhile, the zygote develops into an embryo.
• Double fertilisation is a unique event to flowering plants and ensures the proper development of the endosperm, which provides nutrients to the developing embryo.

(a) Fertilised embryo sac showing zygote and Primary Endosperm Nucleus (PEN); (b) Stages in embryo development in a dicot [shown in reduced size as compared to (a)] 

Post Fertilisation : Structure and Events

After double fertilisation, the following events occur:

1. Endosperm and Embryo Development
2. Maturation of Ovule(s) into Seed(s)
3. Ovary Development into Fruit

Endosperm

Timing of Development:

• Endosperm development occurs before embryo development.
• The primary endosperm nucleus (PEN) divides repeatedly, forming a triploid endosperm tissue.

Endosperm Formation:

• Initial Stage: PEN undergoes multiple nuclear divisions, creating free nuclei. This stage is known as free-nuclear endosperm.
• Subsequent Stage: Cell walls form around these nuclei, resulting in cellular endosperm.

Examples:

• Coconut: Tender coconut water consists of free-nuclear endosperm (many nuclei), while the white kernel is cellular endosperm.

Endosperm Fate:

• Consumed Before Seed Maturation: In some seeds (e.g., pea, groundnut, beans), the endosperm is completely consumed by the developing embryo before seed maturation.
• Persistent Endosperm: In other seeds (e.g., castor, coconut), endosperm persists in the mature seed and is used during seed germination.

Embryo

• Embryo Development:

  • The embryo develops at the micropylar end of the embryo sac, where the zygote is located.
  • Typically, zygotes divide only after a certain amount of endosperm has formed, ensuring sufficient nutrition for the developing embryo.

  Early Stages of Embryo Development:

  • The early stages of embryo development (embryogeny) are similar in both monocotyledons and dicotyledons.
  • Dicotyledonous Embryo Development: The zygote forms the proembryo, which then develops into the globular, heart-shaped, and mature embryo.

  Structure of a Dicotyledonous Embryo

• Embryonal Axis: Includes the epicotyl (above the cotyledons) and the hypocotyl (below the cotyledons).

  • Epicotyl: Ends with the plumule (shoot tip).
  • Hypocotyl: Ends with the radicle (root tip), which is covered by a root cap.
  • Cotyledons: Two cotyledons are present.

• Structure of a Monocotyledonous Embryo:

  • Cotyledon: One cotyledon, known as the scutellum, located laterally on the embryonal axis.
  • Coleorrhiza: An undifferentiated sheath enclosing the radicle and root cap.
  • Epicotyl: Portion above the scutellum, with a shoot apex and leaf primordia enclosed in the coleoptile (a protective sheath).

• (a) A typical dicot embryo; (b) L.S. of an embryo of grass 

  
  

Seed

Seeds are the end result of sexual reproduction in flowering plants. They develop from fertilized ovules inside fruits. A typical seed consists of a protective outer coat, one or more cotyledons (which store food), and an embryo axis.

• Cotyledons are usually thick and swollen in seeds like legumes due to stored food. Seeds can be either non-albuminous, without leftover endosperm, or albuminous, retaining part of the endosperm. Some seeds, like black pepper and beet, have a persistent nucellus called perisperm.
• The ovule's integuments harden to form tough seed coats. A small pore called the micropyle in the seed coat allows oxygen and water to enter during germination. As seeds mature, they become drier.
• Seeds may enter a dormant state until conditions are suitable for germination. Meanwhile, the ovary develops into a fruit, and ovules mature into seeds simultaneously. The fruit's ovary wall becomes the pericarp. Fruits can be fleshy or dry and often have ways to disperse seeds.
• False fruits like apples and strawberries form from the ovary and thalamus. True fruits, however, develop only from the ovary. Parthenocarpic fruits, such as bananas, develop without fertilization and can be induced by growth hormones.
• Seeds provide several benefits, including reliable reproduction, better dispersal to new environments, protection for seedlings with food reserves, and creating genetic diversity through variations.

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

Seed Storage and Viability

• Dehydration and dormancy of mature seeds are crucial for storage.
• Seeds can be used as food throughout the year and for raising crops in the next season.
• Seeds are the basis of agriculture.
• Seeds vary in their viability period, from a few months to hundreds of years.
• Some seeds can remain alive for hundreds or thousands of years, such as the Lupinus arcticus and Phoenix dactylifera.
• The oldest viable seed on record is the Lupinus arcticus, estimated to have remained dormant for 10,000 years.
• A recent record of 2000 years old viable seed is the Phoenix dactylifera discovered during the archeological excavation at King Herod’s palace near the Dead Sea.

Apomixis

• Apomixis is a type of asexual reproduction in plants that involves the formation of a seed without the fertilization of the egg cell by a sperm cell. 
• The resulting offspring are genetically identical to the parent plant. 
• In apomixis, the embryo sac develops into an embryo without being fertilized, and the resulting seed carries the same genetic information as the parent plant. 
• This process can occur naturally in some plant species or can be induced artificially in others.

Polyembryony

• Apomixis can develop in various ways, depending on the plant species.
• In some species, a diploid egg cell forms without reduction division and develops into an embryo without fertilisation.
• In other species, such as Citrus and Mango varieties, nucellar cells surrounding the embryo sac start dividing, protrude into the embryo sac, and develop into multiple embryos within a single ovule.
• The occurrence of more than one embryo in a seed is called polyembryony.

Apomixis and Hybrid Varieties

• Apomixis produces genetically identical embryos and can be considered a type of cloning
• Hybrid varieties are being extensively cultivated to increase productivity
• However, hybrid seeds need to be produced every year, which is costly and makes them expensive for farmers
• If hybrid varieties are made into apomicts, there is no segregation of characters in the hybrid progeny
• This means farmers can keep using the hybrid seeds to raise new crops year after year without buying new hybrid seeds
• Active research is ongoing in many laboratories worldwide to understand the genetics of apomixis and transfer apomictic genes into hybrid varieties