Sexual Reproduction in Flowering Plants | CBSE Biology Class XII Notes
➢ 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.
********************************************************
