Sexual Reproduction in Flowering Plants: NEET Revision Notes
1. Essential Keywords
- Microsporogenesis: The process of formation of haploid microspores from a pollen mother cell (PMC) through meiotic division.
- Tapetum: The innermost wall layer of the microsporangium, possessing dense cytoplasm and multiple nuclei (bi/multi-nucleate), which nourishes developing pollen grains.
- Sporopollenin: One of the most resistant organic materials known, forming the hard outer exine layer of pollen grains; it withstands high temperatures, strong acids/alkalis, and lacks any known degrading enzymes.
- Megasporogenesis: The process of formation of haploid megaspores from the megaspore mother cell (MMC) via meiosis.
- Anatropous Ovule: A typical inverted angiosperm ovule where the body is fused with the funicle, with the micropyle and hilum lying close to each other.
- Monosporic Development: The method of embryo sac formation from a single functional megaspore, while the other three degenerate.
- Autogamy: Transfer of pollen grains from the anther to the stigma of the same flower.
- Geitonogamy: Transfer of pollen grains from the anther to the stigma of another flower of the same plant (functionally cross-pollination, genetically similar to autogamy).
- Xenogamy: Transfer of pollen grains from the anther to the stigma of a different plant, bringing genetically different pollen grains.
- Cleistogamous Flowers: Flowers that do not open at all, ensuring assured seed-set and making them invariably autogamous.
- Double Fertilisation: An event unique to flowering plants where two types of fusions (syngamy and triple fusion) occur within the embryo sac.
- Syngamy: The fusion of one haploid male gamete (n) with the haploid egg cell (n) to form a diploid zygote (2n).
- Triple Fusion: The fusion of the second haploid male gamete (n) with the two haploid polar nuclei (2n combined) in the central cell to produce a triploid Primary Endosperm Nucleus (PEN, 3n).
- Apomixis: A special mechanism in some flowering plants to produce seeds without fertilisation, mimicking sexual reproduction (a form of asexual reproduction).
- Polyembryony: The occurrence of more than one embryo in a single seed.
2. Chapter Summary & Key Notes
A. Pre-fertilisation: Structures and Events
- Stamen & Anther: Consists of a filament and a bilobed, dithecous anther. Each lobe has two theca, making it a 4-sided (tetragonal) structure with four microsporangia (pollen sacs) at the corners.
- Microsporangium Wall Layers (4): Epidermis, endothecium, middle layers (protective, assist in dehiscence), and tapetum (innermost, nourishing, bi/multi-nucleate).
- Pollen Grain:
- Size: 25–50 micrometres, generally spherical.
- Wall: Two-layered. Outer exine (sporopollenin, has germ pores where sporopollenin is absent) and inner intine (thin, continuous layer of cellulose + pectin).
- Cells at shedding: 2-celled stage (vegetative + generative cell) in >60% of angiosperms; 3-celled stage (vegetative + 2 male gametes) in the rest.
- Viability: Highly variable. 30 minutes in cereals (rice, wheat); up to months in Rosaceae, Leguminosae, Solanaceae. Cryopreserved at -196°C in liquid nitrogen for pollen banks.
- Pistil & Ovule:
- Pistil parts: Stigma (landing platform), style (elongated), ovary (basal bulged part containing ovules on the placenta).
- Ovule parts: Funicle (stalk), hilum (junction), integuments (1 or 2 protective envelopes), micropyle (small opening), chalaza (basal part), nucellus (mass of cells with food reserves), and embryo sac.
- Embryo Sac Development:
- MMC undergoes meiosis → 4 megaspores. 1 functional, 3 degenerate.
- Functional megaspore undergoes 3 sequential mitotic divisions (free nuclear) → 8-nucleate stage → cell wall formation → 7-celled, 8-nucleate mature embryo sac.
- Distribution: 3 cells at micropylar end (Egg apparatus: 1 egg cell + 2 synergids; synergids have filiform apparatus to guide the pollen tube), 3 cells at chalazal end (antipodals), 1 large central cell with 2 polar nuclei.
B. Pollination
- Agents & Adaptations:
- Wind (Anemophily): Pollen is light, non-sticky; well-exposed stamens; large, feathery stigma; single ovule per ovary; numerous flowers packed in an inflorescence (e.g., grasses, corn cob).
- Water (Hydrophily): Rare (approx. 30 genera). Pollen protected by mucilaginous covering. Epihydrophily (Vallisneria - female flowers reach surface, male float) vs. Submerged (Zostera - marine, ribbon-like pollen; Hydrilla - freshwater). Note: Water hyacinth and water lily are insect/wind pollinated, NOT water-pollinated.
- Animals (Entomophily/Zoophily): Large, colourful, fragrant, nectar-rich flowers. Rewards include nectar, pollen, or safe egg-laying places (e.g., Amorphophallus provides egg-laying site, Yucca and moth mutualism).
- Outbreeding Devices (to prevent inbreeding depression):
- Asynchronous pollen release and stigma receptivity (Dichogamy).
- Different spatial positions of anther and stigma (Herkogamy).
- Self-incompatibility: Genetic mechanism preventing self-pollen germination or pollen tube growth in the pistil.
- Unisexual flowers: Monoecious (prevents autogamy, e.g., maize, castor) or Dioecious (prevents autogamy & geitonogamy, e.g., papaya).
- Pollen-Pistil Interaction & Artificial Hybridisation:
- A dynamic chemical dialogue. Compatible pollen germinates → pollen tube grows through stigma/style → enters ovule via micropyle → enters synergid via filiform apparatus.
- Artificial breeding uses emasculation (removal of anthers from bisexual buds before dehiscence) and bagging (covering with butter paper to prevent contamination).
C. Double Fertilisation
- Pollen tube discharges 2 male gametes into the synergid cytoplasm.
- Syngamy: 1 male gamete (n) + Egg cell (n) → Diploid Zygote (2n).
- Triple Fusion: 1 male gamete (n) + 2 Polar nuclei (n+n=2n) → Triploid Primary Endosperm Nucleus / PEN (3n).
D. Post-fertilisation: Structures and Events
- Endosperm: Develops before the embryo to provide assured nutrition.
- Development: PEN → free-nuclear divisions (e.g., tender coconut water) → cellularisation (e.g., coconut kernel).
- Fate: Completely consumed (pea, groundnut, beans) or persistent (castor, coconut, cereals).
- Embryo Development: Zygote → proembryo → globular → heart-shaped → mature embryo.
- Dicot: Embryonal axis, 2 cotyledons, epicotyl (terminates in plumule), hypocotyl (terminates in radicle + root cap).
- Monocot: 1 cotyledon (scutellum), coleorrhiza (radicle sheath), coleoptile (plumule sheath).
- Seed & Fruit:
- Non-albuminous: No residual endosperm (pea, groundnut).
- Albuminous: Endosperm persists (wheat, maize, barley, castor).
- Perisperm: Persistent residual nucellus (e.g., black pepper, beet).
- Seed viability records: Lupinus arcticus (10,000 years), Phoenix dactylifera (2,000 years).
- Fruit: Ovary → fruit (pericarp). False fruits involve the thalamus (apple, strawberry, cashew). Parthenocarpic fruits develop without fertilisation (banana).
E. Special Mechanisms
- Apomixis: Seed production without fertilisation. Occurs in Asteraceae and grasses. In Citrus and Mango, nucellar cells divide and protrude into the embryo sac to form embryos. Prevents segregation of hybrid characters, saving farmers the cost of buying new hybrid seeds yearly.
- Polyembryony: Presence of multiple embryos in a seed (e.g., orange seeds).
3. Ploidy Levels Table (Must Memorize)
| Structure / Cell | Ploidy Level | Notes |
|---|---|---|
| Microspore Mother Cell (PMC) / Megaspore Mother Cell (MMC) | 2n | Undergoes meiosis |
| Microspores / Megaspores (Functional) / Pollen Grain | n | Products of meiosis / Male gametophyte |
| Tapetum | Polyploid / Multinucleate | Nourishing layer |
| Synergids / Antipodals / Egg Cell | n | Part of female gametophyte |
| Polar Nuclei (individual) | n each | Two present in central cell |
| Central Cell (before fusion) / Secondary Nucleus | 2n | Contains two 'n' nuclei |
| Primary Endosperm Nucleus (PEN) | 3n | Result of triple fusion |
| Endosperm Tissue / Aleurone Layer | 3n | Triploid nutritive tissue |
| Zygote / Embryo | 2n | Result of syngamy |
| Nucellus / Integuments / Perisperm | 2n | Maternal sporophytic tissue |
Crucial NCERT Examples (Direct Match-the-Following Targets)
Important for NEET
- Cleistogamous & Chasmogamous Flowers: Viola (common pansy), Oxalis, Commelina.
- Water Pollinated Plants: Vallisneria, Hydrilla (freshwater); Zostera (marine seagrass).
- Aquatic Plants NOT pollinated by water: Water hyacinth, Water lily (pollinated by insects/wind).
- Wind-pollinated: Grasses, corn cob.
- Mutualistic pollination: Yucca plant and moth; Amorphophallus (tallest flower, provides egg-laying site).
- Viability Retention for Months: Rosaceae, Leguminosae, Solanaceae.
- Pollen Allergy: Parthenium (Carrot grass) imported with wheat.
- Persistent Nucellus (Perisperm): Black pepper, Beet.
- False Fruits: Apple, Strawberry, Cashew.
- Parthenocarpic Fruit: Banana.
- Apomixis Examples: Asteraceae, grasses, Citrus, Mango.
- Polyembryony Example: Orange.
4. High-Yield Points for Board Exams (Diagrams & Long Answers)
Essential Diagrams for 3-Mark & 5-Mark Questions
- T.S. of Young Anther / Microsporangium: Must clearly label all 4 wall layers (epidermis, endothecium, middle layers, tapetum) and sporogenous tissue.
- Structure of a Pollen Grain: Show exine, intine, germ pore, vegetative cell, and generative cell.
- L.S. of Anatropous Ovule: Label funicle, hilum, integuments, micropyle, chalaza, nucellus, and embryo sac.
- Mature Embryo Sac (7-celled, 8-nucleate): Clearly show the egg apparatus (with filiform apparatus), central cell with 2 polar nuclei, and antipodals.
- Double Fertilisation: Pollen tube entering the synergid, showing syngamy (male gamete + egg) and triple fusion (male gamete + polar nuclei).
- Mature Dicot vs. Monocot Embryo: Show embryonal axis, cotyledons, epicotyl, hypocotyl, plumule, radicle, scutellum, coleoptile, and coleorhiza.
Recurring Board-Style Long-Answer Questions (5 Marks)
Practice These Prompts
- Female Gametophyte Development: Explain the steps involved in megasporogenesis and the formation of a 7-celled, 8-nucleate female gametophyte from a functional megaspore. Support your answer with a neat, labelled diagram.
- Double Fertilisation & Post-fertilisation: Describe the process of double fertilisation in angiosperms. Name the structures formed as a result of syngamy and triple fusion, state their ploidy levels, and explain the subsequent post-fertilisation changes in the ovule and ovary.
- Outbreeding Devices & Pollen-Pistil Interaction: What is inbreeding depression? Describe four outbreeding mechanisms evolved by flowering plants to prevent self-pollination (autogamy) and encourage cross-pollination.
- Microsporogenesis & Pollen Structure: Describe the structure of a microsporangium with a labelled diagram. Explain the structure of a mature pollen grain including its wall layers, viability, and cellular components.
- Seed & Fruit Formation: Trace the post-fertilisation changes in a flower that lead to the formation of a seed and a fruit. Differentiate between albuminous and non-albuminous seeds with examples.