Class 12 Notes Biology for Chapter 1 Sexual Reproduction in flowering Plants

Sexual reproduction in flowering plants (angiosperms) involves the development of male and female gametophytes, pollination, double fertilization, and the subsequent formation of seeds and fruits. The flower is the reproductive unit where the male androecium (stamens) produces pollen grains (male gametophytes) and the female gynoecium (pistils) contains ovules (housing the female gametophyte or embryo sac). Pollination facilitates the transfer of pollen to the stigma, followed by pollen-pistil interaction leading to the growth of the pollen tube and the release of male gametes into the embryo sac. Double fertilization, unique to angiosperms, involves syngamy (fusion of one male gamete with the egg cell to form a zygote) and triple fusion (fusion of the other male gamete with the polar nuclei to form the primary endosperm nucleus). The zygote develops into the embryo, the primary endosperm nucleus into the nutritive endosperm, and the ovule matures into a seed enclosed within the fruit developed from the ovary. Some plants exhibit apomixis (seed formation without fertilization) and polyembryony (more than one embryo per seed).

1. Flower - A Fascinating Organ of Angiosperms

• Flowers are the sites of sexual reproduction in angiosperms.
• They are also objects of aesthetic, ornamental, social, religious, and cultural value.
• Biologically, flowers are morphological and embryological marvels.
• A typical flower has various parts :
  • Calyx: Whorl of sepals (usually green and protective).
  • Corolla: Whorl of petals (often colorful to attract pollinators).
  • Androecium: Whorl of stamens (male reproductive organ).
  • Gynoecium (Pistil): Female reproductive organ.
• The androecium and gynoecium are the parts where the most important units of sexual reproduction develop.

2. Pre-fertilisation: Structures and Events

2.1. Male Reproductive Part - The Androecium

• The androecium consists of a whorl of stamens, representing the male reproductive organ.
• A typical stamen has two parts:
  • Filament: Long and slender stalk.
  • Anther: Terminal, generally bilobed structure.
  • The filament is attached to the thalamus or the petal.
• Anther Structure:
  • Typically bilobed (dithecous), with each lobe having two theca.
  • A longitudinal groove separates the theca.
  • A transverse section reveals a tetragonal structure with four microsporangia located at the corners (two in each lobe).
  • Microsporangia develop further into pollen sacs, packed with pollen grains.
• Microsporangium :
  • Near circular in outline in transverse section.
  • Surrounded by four wall layers:
    • Epidermis: Outermost protective layer.
    • Endothecium: Helps in anther dehiscence.
    • Middle layers: Provide nourishment.
    • Tapetum: Innermost layer, nourishes the developing pollen grains; has dense cytoplasm and often more than one nucleus.
  • The center of each microsporangium is occupied by sporogenous tissue (compactly arranged homogenous cells) when the anther is young.
• Microsporogenesis:
  • The process of formation of microspores from a pollen mother cell (PMC) or microspore mother cell through meiosis.
  • Each cell of the sporogenous tissue acts as a potential PMC.
  • Meiosis in PMC results in the formation of a microspore tetrad (a cluster of four haploid microspores).
• Pollen Grain (Male Gametophyte):
  • As the anthers mature and dehydrate, microspores dissociate and develop into pollen grains.
  • Pollen grains represent the male gametophytes.
  • Generally spherical, measuring about 25-50 micrometers in diameter.
  • Has a prominent two-layered wall:
    • Exine: Hard outer layer made of sporopollenin (one of the most resistant organic materials known, can withstand high temperatures, strong acids, and alkali; no known degrading enzyme; present with prominent apertures called germ pores where sporopollenin is absent). The exine exhibits fascinating patterns and designs. It provides protection. Germ pores are for the emergence of the pollen tube.
    • Intine: Thin and continuous inner layer made of cellulose and pectin.
  • The cytoplasm is surrounded by a plasma membrane.
  • At maturity, the pollen grain contains two cells in over 60% of angiosperms :
    • Vegetative cell: Bigger, has abundant food reserve and a large irregularly shaped nucleus.
    • Generative cell: Small, spindle-shaped with dense cytoplasm and a nucleus, floats in the cytoplasm of the vegetative cell.
  • In the remaining species, the generative cell divides mitotically to form two male gametes (3-celled stage) before pollen grains are shed.
• Pollen Viability:
  • The period for which pollen grains remain viable is highly variable and depends on temperature and humidity.
  • Examples: Rice and wheat (within 30 minutes), Rosaceae, Leguminoseae, Solanaceae (months).
  • Pollen grains can be stored for years in liquid nitrogen (-196°C) in pollen banks for crop breeding programmes.
• Pollen Products and Allergies:
  • Pollen grains are rich in nutrients and used as food supplements (pollen tablets, syrups).
  • Some claims suggest increased performance of athletes and race horses.
  • Pollen of many species can cause severe allergies and bronchial afflictions (asthma, bronchitis) in some people (e.g., Parthenium or carrot grass).

2.2. Female Reproductive Part – The Gynoecium

• The gynoecium represents the female reproductive part of the flower.
• It may consist of a single pistil (monocarpellary) or more than one pistil (multicarpellary).
• If multicarpellary, pistils may be:
  • Fused together (syncarpous).
  • Free (apocarpous).
• Each pistil has three parts:
  • Stigma: Serves as a landing platform for pollen grains.
  • Style: Elongated slender part beneath the stigma.
  • Ovary: Basal bulged part containing the ovarian cavity (locule).
  • Placenta: Located inside the ovarian cavity, giving rise to megasporangia (ovules).
  • The number of ovules per ovary varies (one in wheat, paddy, mango; many in papaya, watermelon, orchids).
• Megasporangium (Ovule):
  • Small structure attached to the placenta by a stalk called funicle.
  • The body of the ovule fuses with the funicle at the hilum (junction between ovule and funicle).
  • Has one or two protective envelopes called integuments, encircling the nucellus except at the micropyle (a small opening at the tip).
  • Opposite the micropylar end is the chalaza (basal part of the ovule).
  • Nucellus: Mass of cells enclosed within the integuments, with abundant reserve food materials.
  • Embryo sac (female gametophyte): Located within the nucellus, typically a single embryo sac per ovule formed from a megaspore.
• Megasporogenesis:
  • The process of formation of megaspores from the megaspore mother cell (MMC).
  • The MMC is usually a single, large cell with dense cytoplasm and a prominent nucleus, differentiated in the micropylar region of the nucellus.
  • The MMC undergoes meiotic division to produce four haploid megaspores.
• Female Gametophyte (Embryo Sac):
  • In most flowering plants, one of the megaspores is functional, while the other three degenerate.
  • The functional megaspore develops into the embryo sac (female gametophyte) through monosporic development.
  • The nucleus of the functional megaspore divides mitotically to form two nuclei that move to opposite poles (2-nucleate embryo sac).
  • Two more sequential mitotic nuclear divisions result in the formation of 4-nucleate and then 8-nucleate stages. These divisions are free nuclear (nuclear divisions not immediately followed by cell wall formation).
  • After the 8-nucleate stage, cell walls are laid down, leading to the organization of the 7-celled, 8-nucleate mature embryo sac.
  • Cellular organization:
    • Egg apparatus (at the micropylar end): Consists of two synergids and one egg cell. Synergids have filiform apparatus (cellular thickenings at the micropylar tip) that guides the pollen tube into the synergid.
    • Antipodals (at the chalazal end): Three cells.
    • Central cell: Large cell in the center with two polar nuclei (which may fuse to form a diploid secondary nucleus before fertilization).

3. Pollination

• The transfer of pollen grains (shed from the anther) to the stigma of a pistil.
• Essential for bringing together non-motile male and female gametes for fertilization.
• Flowering plants have evolved various adaptations and utilize external agents for pollination.

3.1. Kinds of Pollination (based on the source of pollen):

• Autogamy (Self-pollination): Pollination achieved within the same flower. Requires synchrony in pollen release and stigma receptivity, and close proximity of anthers and stigma.
  • Chasmogamous flowers: Open flowers with exposed anthers and stigma; autogamy is rare.
  • Cleistogamous flowers: Flowers that do not open at all; anthers and stigma lie close, ensuring autogamy and assured seed-set even without pollinators (e.g., Viola, Oxalis, Commelina). Cleistogamy is advantageous as it ensures seed production but disadvantageous as it limits genetic variation.
• Geitonogamy (Neighbour pollination): Transfer of pollen from the anther to the stigma of another flower of the same plant. Functionally cross-pollination (requires a pollinating agent) but genetically similar to autogamy (pollen from the same plant).
• Xenogamy (Cross-pollination) : Transfer of pollen from the anther to the stigma of a different plant. This is the only type that brings genetically different pollen to the stigma.

3.2. Agents of Pollination:

• Abiotic Agents:
  • Wind (Anemophily): Pollen grains are light, non-sticky, often produced in large quantities. Flowers often have well-exposed stamens and large, often feathery stigmas to trap air-borne pollen. Common in grasses; often have a single ovule in each ovary and numerous flowers in an inflorescence (e.g., corn cob tassels). Wind-pollinated flowers are generally not colorful and do not produce nectar.
  • Water (Hydrophily) : Rare in flowering plants (limited to about 30 genera, mostly monocots). Examples: Vallisneria, Hydrilla (freshwater), Zostera (marine seagrasses). Some aquatic plants (water hyacinth, water lily) have flowers emerging above water and are pollinated by insects or wind. In Vallisneria, female flowers reach the surface, and male flowers/pollen are released on the surface and carried by water currents. In seagrasses, female flowers remain submerged, and pollen grains (often long and ribbon-like) are released into the water. Pollen grains are often protected from wetting by a mucilaginous covering. Water-pollinated flowers are not colorful and do not produce nectar.
• Biotic Agents (Zoophily) : Majority of flowering plants use animals. Common agents: bees, butterflies, flies, beetles, wasps, ants, moths, birds (sunbirds, hummingbirds), and bats. Insects, particularly bees, are dominant. Even larger animals like primates, rodents, and reptiles can be pollinators.
  • Flowers of animal-pollinated plants are often specifically adapted for a particular species of animal.
  • Majority of insect-pollinated flowers are large, colorful, fragrant, and rich in nectar. Small flowers are often clustered into inflorescences.
  • Flowers pollinated by flies and beetles may secrete foul odors.
  • Flowers provide floral rewards (nectar, pollen grains) to sustain animal visits. Pollen grains in animal-pollinated flowers are generally sticky.
  • Some provide safe places to lay eggs (e.g., Amorphophallus, Yucca and moth).
  • Some floral visitors (pollen/nectar robbers) may consume pollen or nectar without bringing about pollination.

3.3. Outbreeding Devices (to prevent self-pollination and encourage cross-pollination):

• Non-synchronization of pollen release and stigma receptivity (dichogamy).
• Different positions of anther and stigma (herkogamy).
• Self-incompatibility: Genetic mechanism preventing self-pollen from fertilizing the ovules by inhibiting pollen germination or pollen tube growth.
• Production of unisexual flowers:
  • Monoecious plants (male and female flowers on the same plant, e.g., castor, maize): Prevents autogamy but not geitonogamy.
  • Dioecious plants (male and female flowers on different plants, e.g., papaya): Prevents both autogamy and geitonogamy.

3.4. Pollen-Pistil Interaction:

• The ability of the pistil to recognize compatible pollen (of the same species) and promote post-pollination events leading to fertilization, and to reject incompatible pollen (from other species or self-incompatible pollen).
• A continuous dialogue mediated by chemical components of the pollen and pistil.
• Following compatible pollination, the pollen grain germinates on the stigma to produce a pollen tube through one of the germ pores.
• The contents of the pollen grain move into the pollen tube.
• The pollen tube grows through the tissues of the stigma and style and reaches the ovary.
• In 2-celled pollen stage (at shedding), the generative cell divides in the pollen tube to form two male gametes. In 3-celled pollen stage, pollen tube carries 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.
• Filiform apparatus guides the entry of the pollen tube.
• All events from pollen deposition to pollen tube entry into the ovule constitute pollen-pistil interaction.
• Knowledge of pollen-pistil interaction is useful for plant breeders to achieve desired hybrids even in incompatible pollinations through artificial hybridization.
• Emasculation and Bagging Techniques:
  • Emasculation: Removal of anthers from the bisexual flower bud before dehiscence (using forceps) when the female parent bears bisexual flowers.
  • Bagging: Covering the emasculated flower with a bag (butter paper) to prevent contamination of the stigma with unwanted pollen.
  • When the stigma becomes receptive, mature pollen grains from the desired male parent are dusted on the stigma, and the flower is rebagged.
  • For unisexual female flowers, emasculation is not needed; female buds are bagged before opening, and pollination is done at receptivity.

4. Double Fertilisation

• After entering a synergid, the pollen tube releases the two male gametes into the cytoplasm of the synergid.
• Syngamy: One male gamete moves towards the egg cell and fuses with its nucleus, forming a diploid zygote.
• Triple Fusion: The other male gamete moves towards the central cell and fuses with the two polar nuclei (either fused or separate), forming a triploid primary endosperm nucleus (PEN).
• The central cell after triple fusion becomes the primary endosperm cell (PEC).
• The phenomenon involving both syngamy and triple fusion is called double fertilisation, unique to flowering plants.

5. Post-fertilisation: Structures and Events

• Events following double fertilization: endosperm and embryo development, maturation of ovule(s) into seed(s), and ovary into fruit.

5.1. Endosperm:

• Endosperm development generally precedes embryo development to provide nutrition to the developing embryo.
• The primary endosperm cell (PEC) divides repeatedly and forms a triploid endosperm tissue filled with reserve food materials.
• Types of endosperm development:
  • Free-nuclear endosperm: PEN undergoes successive nuclear divisions to form free nuclei before cell wall formation (e.g., coconut water).
  • Cellular endosperm: Cell wall formation occurs after nuclear divisions (e.g., white kernel of coconut).
• Endosperm fate:
  • Completely consumed by the developing embryo before seed maturation (non-albuminous or ex-albuminous seeds, e.g., pea, groundnut, beans).
  • Persists in the mature seed and is used up during seed germination (albuminous or endospermous seeds, e.g., castor, coconut, wheat, maize, barley).

5.2. Embryo:

• Develops at the micropylar end of the embryo sac where the zygote is situated.
• Most zygotes divide only after some endosperm is formed.
• Early stages of embryo development (embryogeny) are similar in monocots and dicots.
• Dicot embryo:  
  • Embryonal axis:
    • Epicotyl: Portion above cotyledons, terminates with plumule (stem tip).
    • Hypocotyl: Cylindrical portion below cotyledons, terminates at the lower end in the radicle (root tip) covered by root cap.
  • Two cotyledons: Store food reserves.
• Monocot embryo (e.g., grass family):
  • Single cotyledon (scutellum): Situated laterally to the embryonal axis.
  • Embryonal axis:
    • Radicle and root cap: Enclosed in an undifferentiated sheath called coleorrhiza.
    • Epicotyl: Above the attachment of the scutellum, has a shoot apex and a few leaf primordia enclosed in a hollow foliar structure, the coleoptile.

5.3. Seed

• The final product of sexual reproduction; a fertilized ovule.
• Formed inside fruits.
• Typically consists of seed coat(s), cotyledon(s), and an embryo axis.
• Mature seeds:
  • Non-albuminous (ex-albuminous): No residual endosperm (e.g., pea, groundnut).
  • Albuminous (endospermous): Retain a part of the endosperm (e.g., wheat, maize, barley, castor).
  • Perisperm: Persistent remnants of the nucellus (e.g., black pepper, beet).
• Seed coat: Hardened integuments of the ovule, providing protection.
• Micropyle: Remains as a small pore in the seed coat, facilitating entry of oxygen and water during germination.
• Dormancy: As the seed matures, water content reduces, metabolic activity slows down, and the embryo may enter a state of inactivity (dormancy) or germinate under favorable conditions.
• Viability: Seeds remain alive for varying periods (few months to hundreds or thousands of years). Oldest viable seeds: Lupinus arcticus (10,000 years), Phoenix dactylifera (2000 years).
• Seed is the basis of agriculture, crucial for storage and raising crops in the next season.

5.4. Fruit

• Develops from the ovary after fertilization.
• Transformation of ovules into seeds and ovary into fruit proceeds simultaneously.
• The wall of the ovary develops into the fruit wall (pericarp).
• Fruits can be fleshy (guava, orange, mango) or dry (groundnut, mustard).
• Many fruits have evolved mechanisms for seed dispersal.
• Relationship between the number of ovules and seeds in a fruit.
• True fruits: Develop only from the ovary.
• False fruits: Thalamus also contributes to fruit formation (e.g., apple, strawberry, cashew).
• Parthenocarpic fruits: Develop without fertilization (e.g., banana); can be induced by growth hormones and are seedless.

6. Apomixis and Polyembryony

• Apomixis: A form of asexual reproduction that mimics sexual reproduction, producing seeds without fertilization (e.g., some species of Asteraceae and grasses).
  • Diploid egg cell formed without reduction division develops into the embryo without fertilization.
  • Nucellar cells surrounding the embryo sac start dividing and develop into embryos (common in Citrus and Mango varieties).
  • Apomictic embryos are genetically identical to the mother plant (clones).
  • Important in hybrid seed industry as it prevents segregation of characters in hybrid progeny, allowing farmers to reuse hybrid seeds. Active research is ongoing to understand and transfer apomictic genes into hybrid varieties.
• Polyembryony: Occurrence of more than one embryo in a seed (e.g., orange, mango). Can result from multiple embryos sacs in an ovule, or more commonly, from nucellar embryos.

Differences between:

Frequently Asked Questions:

1. Where do male and female gametophytes develop in an angiosperm flower?
   • The male gametophyte (pollen grain) develops within the microsporangia located in the anther of the stamen.
   • The female gametophyte (embryo sac) develops within the megasporangium (ovule), which is located inside the ovary of the pistil.
2. Explain the process of double fertilization in flowering plants.
   • Double fertilization occurs after the pollen tube enters the embryo sac and releases two male gametes.
   • One male gamete fuses with the egg cell (syngamy) to form a diploid zygote.
   • The other male gamete fuses with the central cell containing the two polar nuclei (triple fusion) to form a triploid primary endosperm nucleus (PEN).
   • This simultaneous occurrence of syngamy and triple fusion is unique to angiosperms.
3. What are the advantages of apomixis in agriculture?
   • Apomixis leads to the production of seeds without fertilization, resulting in offspring that are genetically identical to the mother plant.
   • In hybrid varieties, apomixis prevents the segregation of desired hybrid characters in the progeny.
   • This allows farmers to reuse hybrid seeds year after year without losing the beneficial traits, reducing the cost of purchasing new hybrid seeds every season.
   • It simplifies the process of maintaining and propagating superior hybrid varieties.