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Sexual Reproduction in Flowering Plants | CBSE Biology Class XII Notes

Sexual Reproduction in flowering plants - Class-12 - cbsebiology4u

    FLOWER

       Reproductive unit of the flowering plants.

       Atypical bisexual flower consists of 4 different whorls – calyx, corolla, androecium and gynoecium.

       Calyx – outermost whorl consists of sepals.

       Corolla – composed of petals that are usually brightly colored.

       Androecium – represents the male reproductive organ.

o   Composed of stamens.

o   Each stamen is made up of Anther and a stalk / filament.

       Gynoecium – represents the female reproductive organ

o   Composed of carpels.

o   Carpels have 3 parts- stigma, style and ovary.

o   Ovary is the basal enlarged portion.

o   Style is the tubular structure that connects the stigma to the ovary.

o   Stigma is the tip of the style that typically acts as the receptive surface for the pollen grains.

o   Monocarpellary: gynoecium consisting of a single pistil.

o   Multicarpellary: gynoecium consisting of more than one pistil.

o   Syncarpous: When carpels are fused.

o   Apocarpous: When carpels are free

   PRE-FERTILIZATION: STRUCTURE AND EVENTS

   Stamens, Microsporangium and Pollen Grains

       Structure of stamen – Consists of 2 parts:

o   Filament- Long, slender stalk

o   Anther- Terminal, bilobed structure

       Proximal end of filament is attached to the base of the flower (thalamus).

       Structure of Anther – Bilobed (2 parts of anther) and dithecous (Each lobe has 2 theca)

o   A longitudinal groove separates the two theca.

o   Due to the bilobed & dithecous nature the anther is tetragonal (four sided) structure.

o   Each of the corners consists of one microsporangia, i.e. 2 microsporangia are present in each of the theca.

o   Microsporangia develops into pollen sacs that contain the pollen grains.

  Structure of Microsporangia:

       In young anthers, each of the microsporangium is composed of sporogenous tissue surrounded by a total of 4 layers.

       The outer 3 layers (epidermis, endothecium & middle layer) perform the function of protection.

       The innermost layer (Tapetum) helps in nourishing the developing pollen grains.

       Nature of the cells of Tapetum

o   Dense cytoplasm

o   Possess more than one nucleus.

   Microsporogenesis

       Each of the cells of the sporogenous tissue acts as the pollen mother cell (PMC) or microspore mother cell.

       The process of formation of microspore from PMC is called microsporogenesis.

       The PMC undergoes meiotic cell division to form microspore tetrads (haploid cells arranged in a cluster of 4 cells).

       The microspores dissociate from each other and give rise to the pollen grain as the anther matures and dehydrates.

   Male Gametophyte (Pollen Grains)

       It represents the male gametophyte.

       They have 2 layered walls.

o   Exine – outer layer, made up of sporopollenin.

    Sporopollenin is resistant to high temperatures and strong acids and alkali.

    They are also resistant to degradation by enzymes.

    Due to this the pollen grains are well preserved as fossils.

    Germ pore: Prominent aperture in the exine where the sporopollenin is absent.

    Germ pore is essential for the pollen germination.

o   Intine – inner thin layer, made up of cellulose and pectin.

       Cytoplasm of the pollen grain is surrounded by plasma membrane.

       Matured pollen grains have 2 cells

o   Vegetative cell –

    Bigger cell

    Abundant food reserve

    Large irregular shaped nucleus

o   Generative cell

    Smaller cell

    Floats in the cytoplasm of the vegetative cell

    Dense cytoplasm

       In 60% angiosperms the pollen grains are released in 2 celled stage, while in the remaining 40% it is released in a 3 celled condition (generative cell divides mitotically to give rise to the two male gametes).

       Pollen grains can cause allergies and bronchial afflictions (infections), leading to asthma, bronchitis, etc. E.g. Parthenium or carrot grass

       Pollen products: pollen grains are rich in nutrients

o   Available in the form of tablets & syrups.

o   It can increase the performance of athletes & race horses

       Pollen Viability:

o   30 Minutes: Cereals like Rice & Wheat.

o   Months: Members of Solanaceae, Rosaceae, Leguminoseae.

o   Years: Artificial means- Liquid Nitrogen (-1960C)

   The Pistil, Megasporangium (ovule) and Embryo sac

       Each pistil consists of the stigma, style and ovary.

       The ovary contains the ovarian cavity (locule).

       The megasporangia (ovules) are located inside the ovarian cavity connected by placenta.

       The number of ovules depends on the plant species:

o   One: wheat, paddy, mango

o   Many: Papaya, water-melon, orchids

   Structure of Megasporangium (Ovule)

       Ovule connects with the placenta via a stalk called funicle.

       Hilum is the junction funicle and ovule.

       The protective layers of the ovule are called the Integuments (1 or 2 layers).

       Inner to the integuments are the nucellus.

       Integuments are present all around the nucellus except at a small opening called the micropyle.

       Chalaza end represents the basal part of the ovule, and is opposite to the micropylar end.

       The female gametophyte (embryo sac) is located inside the nucellus.

       Generally, the ovules have a single embryo sac.

       The embryo sac develops into the megaspore.

       Nucellar cells are rich in reserve food materials.

       The nucellus provides nutrition to the developing embryo sac.

   Megasporogenesis

       The process of formation of the megaspore from the megaspore mother cell is called megasporogenesis.

       A single megaspore mother cell (MMC) is formed in the micropylar region of the nucellus of the megasporangium (ovule).

       The MMC undergoes meiotic cell division resulting in 4 megaspores.

   Female Gametophyte (Embryo sac)

       The development of the embryo sac in the majority of the flowering plants follows the monosporic development process.

       Out of the 4 megaspores only one remains functional while the remaining three degenerates.

       Only the functional megaspore develops into female gametophyte.

       The functional megaspore undergoes free nuclear cell division.

       Three mitotic nuclear divisions of the functional megaspore nucleus give rise to a 8 nucleate stage of embryo sac.

       After this stage the six of the eight nuclei are surrounded by cell walls and are organized into six different cells.

o   3 of these cells move towards the chalazal end are called the antipodals.

o   The remaining 3 cells move to the micropylar end and form the egg apparatus.

    Egg apparatus consists of the egg cell and two synergids.

    The synergids have special cellular thickenings called the filiform apparatus.

       The remaining two nuclei (polar nuclei) are placed in the large central cell.

       As a result of this organization, a typical angiospermic embryo sac, at maturity is 7-celled but 8-nucleate.

   Pollination

       In the case of plants both gametes are non-motile.

       The transfer of pollen grains to the stigma of the pistil of the same flower or another flower is called pollination.

       This process occurs with the help of external agents – pollinating agents.

Pollination Types: Based on the source of pollination.

       Autogamy : (Self-pollination)

o   Involvement of a single flower.

o   Pollen grains are transferred from the anther to the stigma of the same flower.

o   Complete autogamy is rare in a flower remains open with exposed anther and stigma.

o   Criteria for autogamy.

    Synchrony in pollen release and stigma receptivity.

    Close proximity of the anther and stigma.

o   Plants with 2 types of flower – Viola (common pansy), Oxalis, and Commelina

    Chasmogamous Flower: (Regular flower with exposed anther & stigma)

    Cleistogamous Flower: (Flowers do not open at all)

    Anther and stigma lie close to each other.

    Pollen falls directly on the stigma upon pollen dispersal.

    No need of the pollinating agents

    No chance of cross pollination.

    Advantage of Cleistogamous flower:

o   Production of assured seed-set even in the absence of pollinators.

    Disadvantage of Cleistogamous flower:

o   Due to self-pollination variation may not be created.

o   Prevent the evolution of genetically superior variety.

       Geitonogamy : (Cross  pollination involving one plant)

o   Pollen grains are transferred from the anther of one flower to the stigma of another flower but from the same plant.

o   It is functionally cross pollination.

o   Genetically it is similar to autogamy as pollen grain is from one plant.

       Xenogamy : (Cross  pollination involving different plant)

o   Pollen grain is transferred from anther of one flower to the stigma of another flower of a different flower.

o   Unites genetically different types of pollen grains.

Agents of Pollination

       Biotic pollinating agents – living organisms: [used by majority of plants]

       Abiotic pollinating agents – Wind and Water

       Why do plants produce a large amount of pollen grains compared to the numbers of ovules available for pollination??

o   Ans- As the event of pollen grain coming in contact with the stigma is a chance factor, to compensate this uncertainty and the probable loss of pollen large amounts of pollen grain is released.

Pollination by wind – Anemophily

     Common in grasses.

     Nature of Flower:

o   Pollen grains are lightweight.

o   The pollen is non-sticky.

o   Well exposed stamen.

o   Large feathery stigma.

o   Inflorescence consists of numerous flowers.

    Example- corn cob

Pollination by Water – Hydrophily

       Rare – limited only to 30 plant genera (mostly monocotyledons)

       Example- Vallisneria and Hydrilla (Fresh water), Zostera (marine sea-grass)

       Pollination in Vallisneria :

o   Female flower reaches the top of the water surface

o   Male flower/pollen grains released on the water surface

o   The pollen grains reach the stigma via passive water current.

       Pollination in Seagrasses :

o   The female flower remains submerged in the water.

o   Pollen grains (long & ribbon shaped) are released inside the water.

o   Pollen grains reach the stigma via passive water current and achieve pollination.

       Property of pollen grain in water pollinated plant species

o   Presence of protective mucilaginous covering that prevents them from wetting.

       Aquatic plants like water hyacinth and water lily – follow pollination by insects or wind, as the flower reaches the water surface.

Pollination by Biotic organisms

       Pollinating agents include- Bees, butterflies, flies, beetles, wasps, ants, moths, birds (sunbirds and hummingbirds) and bats.

       Among the animals, insects, particularly bees are the dominant biotic pollinating agents.

       Other organisms – primates (lemurs), arboreal (tree-dwelling) rodents, or even reptiles (gecko lizard and garden lizard).

       Nature of flower :

o   Large flowers

o   Colorful

o   Fragrant and rich in nectar

o   Pollen grains are sticky

o   In the case of small flowers- many are clustered into an inflorescence.

       Animals are attracted towards flowers due to the foul odours.

       Floral rewards are provided to sustain the animal visit.

o   Pollen and nectar are floral rewards

       When the pollinator visits the flower to harvest the floral reward the body of the pollinator gets a coating of the pollen.

       When these animals come in contact with stigma it brings about the pollination.

       Floral rewards as a safe place to lay egg

o   E.g.-Amorphophallus – tallest flower

o   E.g.- Yucca plant and a moth species.

    They can’t complete their life cycle without each other

    Moth deposits the egg in the locule of ovary

    The flower in turn gets pollinated by the moth

    The moth larva comes out of the eggs as the seeds start developing.

   Outbreeding Devices:

       Continued self-pollination result in inbreeding depression.

       As majority of the flowers are bisexual, there is a need for the plants to develop methods by which it can prevents self-pollination and promote cross pollination.

       The outbreeding devices enables them to achieve it.

o   Pollen release and stigma receptivity are not synchronized,

o   Different position of the stigma and the anther so that the pollen grains do not come in contact with the stigma

o   Self-incompatibility: genetic mechanism that prevents the self-pollen from pollen germination or pollen tube growth.

o   Production of unisexual flowers.

       In case of monecious plants (maize, castor) where both the male and female flowers are present on the same plant – it prevents autogamy but not geitonogamy.

       In case of dioecious plants (papaya) where both the male and female flowers are on different plants – it prevents both autogamy and geitonogamy.

   Pollen-pistil interaction

       All the events–from pollen deposition on the stigma until pollen tubes enter the ovule–are together referred to as pollen-pistil interaction.

       Pollination might lead to the deposition of pollen grains of various plant species.

       The process of pollination does not guarantee fertilization.

       Only if the right type of pollen (compatible pollen grain of the same species) is landing on the stigma, it might lead to fertilization.

       If the pollen grain is the right type (compatible) then the post-pollination events continue leading to fertilization.

       If the pollen grain is the wrong type (incompatible) the pistil rejects it.

       An incompatible pollen is rejected by:

o   Prevention of pollen germination

o   Prevention of pollen tube growth

       The decision of compatible and non-compatible pollen is due to the continuous chemical talk between the pollen grain and the pistil.

       Pollen germination:

o   Compatible pollen grain germinates to form pollen tubes through germ pore.

o   The content of the pollen grain moves into the pollen tube.

       Pollen tube travels through the style and reaches the ovary.

       It enters the ovule through the micropyle and then enters one of the synergids through the filiform apparatus.

       The filiform apparatus guides the entry of the pollen tube.

   Artificial Hybridization:

       These refer to the crossing experiments in plants where only the desired pollen grains are used for pollination and the stigma is protected from contamination of unwanted pollen grains.

       It is one of the major approaches in the crop improvement program.

       Steps:

o   Emasculation: Removal of anthers (in case of bisexual flower) before the dehiscence of anther.

    In case of unisexual flower this step is not necessary.

o   Bagging: Covering of the emasculated flower with a bag (butter paper) of suitable size to prevent contamination of stigma by unwanted pollens.

o   Controlled pollination: When the stigma matures, the matured pollen from a desired male parent is dusted on it and the flower is rebagged and further development is allowed.

   DOUBLE FERTILIZATION

       The pollen tube releases two male gametes to the cytoplasm of the synergids.

       One male gamete fuses with the nucleus of the egg forming a diploid cell called zygote.

o   This fertilization event is called syngamy.

       The remaining male gamete fuses with the two polar nuclei of the central cell and produces primary endosperm nucleus (PEN) that is triploid in nature.

o   This fertilization event is called triple fusion as it involves the fusion of 3 haploid nuclei.

       As there are two fertilization events taking place at the same time in the embryo sac, this phenomenon is called double fertilization.

o   This event is unique to the angiospermic plants.

       Fate of double fertilization:

o   The central cell after triple fusion becomes the Primary endosperm cell and develops into Endosperm.

o   The zygote divides and develops into the embryo.

   POST FERTILIZATION: STRUCTURE AND EVENTS

       This phase involves the following:

o   Endosperm development

o   Embryo development

o   Maturation of ovules into seeds

o   Maturation of ovary into fruits

  Endosperm

       Development of the endosperm starts prior to the embryo development.

o   The PEN divides and forms the endosperm tissue.

o   The cells are filled with reserve food materials.

o   They provide nutrition to the developing embryo.

       Endosperm development:

o   PEN undergoes repeated division to give rise to the free nuclei (free nuclear endosperm)

o   The free nuclear endosperm forms the cellular endosperm when they undergo cellularization.

       Example – Coconut

o   Free nuclear endosperm: tender coconut water

o   Cellular endosperm: white kernel (edible part)

   Embryo

       The embryo develops from the zygote in the micropylar region of the embryo sac.

       Stages of embryogeny (embryo development):

o   Proembryo

o   Globular stage

o   Heart-shaped stage

o   Mature embryo

       Component of dicot embryo:

o   an embryonal axis and two cotyledons

o   Epicotyl: portion of embryonal axis above the level of cotyledons.

    Terminates with Plumule (future shoot)

o   Hypocotyl: Cylindrical portion below the level of cotyledons.

    Terminates with radicle (future root)

    The root tip (radicle) is covered with root cap.

       Monocot Embryo:

o   Consists of only one cotyledon

o   Scutellum: Cotyledons of grass family

o   Coleorrhiza:  Sheath of the radical and root cap

o   Coleoptile: Hollow foliar structure that encloses the shoot apex and few leaf primordia.

   Seed

       Developed from the fertilised ovule.

       Components of seed:

o   seed coat(s)

o   cotyledon(s)

o   an embryo axis.

       Non-albuminous seeds:

o   Matured seeds with no residual endosperm

o   E.g.- Pea, Groundnut

       Albuminous seeds:

o   Matured seeds with residual endosperm

o   E.g.- wheat, maize, barley, castor

       Perisperm: The residual, persistent nucellus in seeds.

o   E.g.- black pepper, beet

       Seed Development:

o   Seed coat develops as the integuments hardens.

o   Micropyle remains as a small pore in the seed coat.

    Essential to facilitate the entry of oxygen and water for seed germination.

o   When the seed matures, it becomes dry & the metabolic rate of the embryo slows down.

o   At this stage the embryo enters the dormancy stage.

   Fruit

       It is developed from the ovary.

       The ovarian walls develop into the fruit wall.

       Types of fruit

o   Fleshy fruit : guava, orange, mango, etc.,

o   Dry fruit : groundnut, and mustard, etc.

o   True fruit: when fruit is developed from ovary

    E.g.- Mango, Pea, etc.,

o   False fruit: fruit developed from parts other than ovary like thalamus

    E.g.- apple, strawberry, cashew, etc.,

  PARTHENOCARPY

       Development of fruit without fertilization.

       The fruit developed by this process is called parthenocarpic fruit.

       They are generally seedless.

       E.g.- pineapple, banana, cucumber, grape, orange, etc.

       Can be induced with the help of hormones.

  Advantage of seeds:

       Pollination and fertilization are independent of water, seed formation is more dependable.

       seeds have better adaptive strategies for dispersal to new habitats.

       They can nourish the young seedlings as they are rich in reserve food materials.

       The Hard seed coat provides protection to the embryo.

       Produce new genetic combinations leading to variations, as they are the product of sexual mode of reproduction.

   Seed Viability:

       Ability of seed to remain alive after their dispersal.

       It can be a few months to several years.

       Oldest recorded Viable Seed: Lupine(Lupinus arcticus) excavated from Arctic Tundra.

o   10000 years of dormancy.

       Date Palm (Phoenix dactylifer) : 2000 years old viable seed

  APOMIXIS & POLYEMBRYONY

   Apomixis

       It is a form of asexual reproduction that mimics sexual reproduction.

       Process of production of seeds without fertilization.

       Example- Some species of Asteraceae and grasses.

       Method-1:

o   Diploid egg cell is produced without reduction division and it develops onto embryo without fertilization.

       Method-2:

o   Nucellar cells surrounding the embryo sac starts dividing, protrudes into the embryo sac and develop into embryo. (Citrus, Mango)

o   Such cases each ovule have more than one embryo – POLYEMBRYONY

  Hybrid Seeds and Apomixis

       Cultivation of hybrids has tremendously increased productivity

       Problems with Hybrid Seeds:

o   Hybrids seeds has to be produced every year.

o   Seeds obtained from hybrid plants when grown, tend to segregate and loose the hybrid traits.

o   Cost factor

       Solution to this problem

o   Apomictic Hybrid Seeds

o   As there is no fertilization, there will be no segregation in the hybrid progeny.

o   The farmer can use the apomictic hybrid seeds for many years to raise the crop and need not buy the seed every year.

       Because of its importance there is active research going on to understand the genetics of apomixis and to transfer apomictic genes into hybrid varieties.

 

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