Chapter 9: Sexual Reproduction in Flowering Plants

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Introduction

Sexual Reproduction in Angiosperms

• All flowering plants reproduce sexually via formation and fusion of gametes.


• Flowers are specialized organs for sexual reproduction in angiosperms.


• Inflorescences, flowers, and floral parts show diverse adaptations to ensure fruit and seed formation.

Memory Aids

• Flowers = Sexual reproduction organs in angiosperms.


• Fruits and seeds result from successful sexual reproduction.

Quick Checkpoints

• What is the role of flowers in angiosperms?


• How do floral adaptations aid reproduction?

Flower

Structure and Function

• Flower is a modified shoot with determinate growth and condensed internodes.


• Four whorls attached to the receptacle: calyx, corolla, androecium, gynoecium.


• Calyx: sepals, usually green, protect flower bud.


• Corolla: petals, often colorful, attract pollinators.


• Androecium: male part, consists of stamens (anther + filament).


• Gynoecium: female part, consists of carpels (stigma, style, ovary with ovules).


• Ovules develop into seeds after fertilization.


• Thalamus (receptacle) supports all floral parts.


• Floriculture: cultivation of ornamental flowers.

Memory Aids

• Flower = Modified shoot with 4 whorls: C-C-A-G (Calyx, Corolla, Androecium, Gynoecium).


• Stamen = Anther + Filament; Carpel = Stigma + Style + Ovary.


• Floriculture = Flower farming.

Quick Checkpoints

• What are the four whorls of a flower?


• Identify male and female floral parts.


• What is the function of petals and sepals?


• Define floriculture.

Pre-fertilisation: Structures and Events

Floral Development

• Floral primordium differentiates into inflorescences bearing floral buds and flowers.


• Flowers develop male (androecium) and female (gynoecium) reproductive organs.


• Androecium: whorl of stamens (male organ).


• Gynoecium: female reproductive organ.

Memory Aids

• Androecium = Male stamens; Gynoecium = Female pistils.

Quick Checkpoints

• What are the main floral reproductive structures?


• Define androecium and gynoecium.

Stamen, Microsporangium and Pollen Grain

Stamen Structure

• Stamen consists of filament and anther.


• Filament attaches proximally to thalamus or petal.


• Anther is bilobed, each lobe with two theca (dithecous).


• Longitudinal groove separates theca.


• Anther is tetragonal with four microsporangia (pollen sacs) at corners.


• Microsporangia contain pollen grains.

Memory Aids

• Anther = 4 pollen sacs (microsporangia) arranged in 2 lobes.

Quick Checkpoints

• Describe the structure of a typical angiosperm anther.


• What are microsporangia?

Structure of Anther

Anther Wall Layers

• Four layers: epidermis, endothecium, middle layers, tapetum.


• Epidermis: outer protective layer.


• Endothecium: has fibrous bands aiding anther dehiscence.


• Middle layers: degenerate as pollen matures.


• Tapetum: innermost layer, nourishes developing pollen.

Types of Tapetum

• Amoeboid (invasive) tapetum: found in primitive angiosperms; absorbs food from middle layers; e.g., Typha, Alisma, Tradescantia.


• Glandular (secretory) tapetum: stationary; secretes nutrients into microsporangia; common in most flowering plants; e.g., Capsella.

Memory Aids

• Tapetum nourishes pollen; two types: amoeboid (primitive), glandular (common).

Quick Checkpoints

• What are the functions of anther wall layers?


• Differentiate amoeboid and glandular tapetum.

Microsporogenesis

Process

• Sporogenous tissue cells undergo meiosis forming microspore tetrads.


• Each microspore separates and develops into a pollen grain.


• Microsporogenesis = formation of microspores from pollen mother cell by meiosis.


• As anther matures and dehydrates, microspores dissociate and mature.

Memory Aids

• Microsporogenesis = meiosis in anther producing microspores.

Quick Checkpoints

• Define microsporogenesis.


• What happens to microspore tetrads?

Pollen Grain

Structure and Development

• First cell of male gametophyte; spherical, 25–50 µm diameter.


• Initially uninucleate; becomes 2–3 celled at release.


• Wall (sporoderm) has two layers: outer exine and inner intine.


• Exine made of sporopollenin; resistant to decay; has germ pores/furrows for pollen tube emergence.


• Intine is thin, elastic; forms pollen tube during germination.


• Nucleus divides mitotically into vegetative (tube) cell and generative cell.


• Vegetative cell: large, food-rich, irregular nucleus.


• Generative cell: small, spindle-shaped, produces two male gametes later.


• ~60% angiosperms shed pollen at 2-celled stage; generative cell divides in pollen tube.


• Pollen grains can cause allergies; viability varies by species and conditions.


• Pollen can be stored in liquid nitrogen for breeding (pollen banks).


• Used as nutrient supplements (pollen tablets).

Memory Aids

• Exine = sporopollenin, tough; Intine = pollen tube origin.


• Vegetative cell guides tube; generative cell forms gametes.

Quick Checkpoints

• What are the layers of pollen wall?


• Describe the cell types in mature pollen grain.


• How is pollen viability maintained?

Pistil, Megasporangium and Embryo Sac

Gynoecium Structure

• Gynoecium = female reproductive part; consists of one (monocarpellary) or multiple pistils (multicarpellary).


• Multiple pistils may be fused (syncarpous) or free (apocarpous).


• Each pistil has stigma (receptive), style (stalk), and ovary (swollen base with ovules).


• Ovary contains locules and placentae from which ovules arise.


• Ovule number varies: one (wheat, paddy, mango) to many (papaya, watermelon, orchids).

Memory Aids

• Syncarpous = fused pistils; Apocarpous = free pistils.


• Stigma receives pollen; style connects stigma to ovary.

Quick Checkpoints

• Define syncarpous and apocarpous pistils.


• What are the parts of a pistil?

The Megasporangium

Ovule Structure

• Ovule = integumented megasporangium; develops into seed after fertilization.


• Attached to placenta by funicle; junction called hilum.


• Has one or two integuments surrounding nucellus (food reserve tissue).


• Micropyle: small opening at integument tip for pollen tube entry.


• Chalaza: basal part opposite micropyle.


• Contains embryo sac (female gametophyte).

Ovule Types

• Unitegmic: single integument (common in Gamopetalae).


• Bitegmic: two integuments (Polypetalae, monocots).


• Ategmic: no integuments (e.g., Santalum album, Loranthus).


• Tritegmic: three integuments (rare, e.g., Asphodelus).

Memory Aids

• Hilum = ovule-funicle junction; micropyle = pollen tube entry.


• Unitegmic = 1 integument; Bitegmic = 2 integuments.

Quick Checkpoints

• What is the function of integuments?


• Define micropyle and chalaza.

Megasporogenesis

Process

• Megaspore mother cell (MMC) in nucellus undergoes meiosis forming 4 haploid megaspores.


• Usually, 3 megaspores degenerate; 1 functional megaspore enlarges.


• Megasporogenesis = formation of megaspores from MMC by meiosis.

Memory Aids

• Only one megaspore becomes functional; others degenerate.

Quick Checkpoints

• What is megasporogenesis?


• How many megaspores are functional?

Female Gametophyte

Embryo Sac Formation

• Functional megaspore develops into embryo sac (female gametophyte) by mitotic divisions.


• Monosporic development: embryo sac from single megaspore.


• Nucleus divides to 2, then 4, then 8 nuclei without cell walls (free nuclear divisions).


• Two polar nuclei fuse to form diploid secondary nucleus.


• Three chalazal nuclei form antipodal cells.


• Three micropylar nuclei form egg apparatus: 2 synergids + 1 egg cell (oosphere).


• Synergids have filiform apparatus guiding pollen tube.


• Egg cell fertilizes to form zygote; synergids and antipodals degenerate post-fertilization.


• Mature embryo sac is 8-nucleate, 7-celled.

Memory Aids

• Embryo sac = 8 nuclei, 7 cells; egg apparatus = egg + 2 synergids.


• Filiform apparatus guides pollen tube.

Quick Checkpoints

• Describe the structure of a mature embryo sac.


• What is the role of synergids?

Pollination

Definition and Types

• Pollination: transfer of pollen from anther to stigma of same or different flower of same species.


• Two types: Self-pollination and Cross-pollination.

Self-pollination

• Pollen transferred within same flower or genetically similar flower.


• Types:
  
  
  
  • Autogamy: pollen to stigma of same flower.
  
  
  • Geitonogamy: pollen to stigma of different flower on same plant.
  
  
  
  


• Contrivances:
  
  
  
  • Cleistogamy: flowers never open; obligate self-pollination (e.g., Commelina benghalensis).
  
  
  • Physical contact: bending filaments or style to bring anther and stigma together (e.g., Mirabilis jalapa, Solanum tuberosum).
  
  
  
  

Cross-pollination (Allogamy)

• Pollen transfer between flowers of genetically different plants of same species.


• Also called xenogamy.


• Contrivances:
  
  
  
  • Dicliny (unisexual flowers): monoecious (male and female flowers on same plant, e.g., maize) or dioecious (male and female flowers on separate plants, e.g., Morus alba).
  
  
  • Self-sterility: pollen cannot fertilize same flower (e.g., Camellia sinensis).
  
  
  • Dichogamy: temporal separation of sex organ maturity.
    
    
    
    • Protandry: anthers mature first (e.g., sunflower).
    
    
    • Protogyny: carpels mature first (e.g., mustard).
    
    
    
    
  
  
  • Heterostyly: flowers with different style and anther lengths.
  
  
  • Herkogamy: physical barrier between anther and stigma (e.g., Calotropis).
  
  
  
  

Memory Aids

• Autogamy = same flower; Geitonogamy = same plant, different flower.


• Protandry = anther first; Protogyny = carpel first.


• Dicliny = unisexual flowers; Herkogamy = physical barrier.

Quick Checkpoints

• Differentiate autogamy and geitonogamy.


• What is dichogamy? Types?


• Explain contrivances for cross-pollination.

Types of Pollination

Abiotic Agents

• Anemophily (wind pollination):
  
  
  
  • Dry, small, non-sticky pollen; exposed stamens and feathery stigmas.
  
  
  • Flowers lack scent and nectar; e.g., maize, grasses, mulberry.
  
  
  
  


• Hydrophily (water pollination):
  
  
  
  • Small, light, wax-coated pollen; flowers inconspicuous, odourless.
  
  
  • Two types: hypohydrophily (underwater, e.g., Ceratophyllum), epihydrophily (surface, e.g., Vallisneria).
  
  
  
  

Biotic Agents

• Entomophily (insect pollination):
  
  
  
  • Flowers produce nectar, scent, bright colors; pollen is sticky with pollenkitt.
  
  
  
  


• Ornithophily (bird pollination):
  
  
  
  • Large, bright, tubular flowers; odourless; abundant nectar; e.g., Strelitzia reginae.
  
  
  
  


• Chiropterophily (bat pollination):
  
  
  
  • Dull-colored, strong fruity smell, abundant nectar; e.g., Kigelia, Adansonia.
  
  
  
  


• Others: malacophily (snails), myrmecophily (ants).

Memory Aids

• Anemophily = wind; Hydrophily = water; Entomophily = insects.


• Ornithophily = birds; Chiropterophily = bats.

Quick Checkpoints

• List features of wind-pollinated flowers.


• What is hydrophily? Types?


• Describe adaptations of insect-pollinated flowers.

Pollen-Pistil Interaction

Process and Significance

• Stigma recognizes compatible pollen via pollen-pistil interaction.


• Stigma surface has glandular cells with papillae; two types: dry (no exudate) and wet (covered with exudate).


• Exudates contain sugars, amino acids, proteins, mucopolysaccharides.


• Pollen tube secretes enzymes to digest stigma tissues.


• Pollen tube grows chemotropically and intercellularly guided by calcium-boron-inositol sugar complex gradient.


• Tube enters embryo sac via synergid at micropyle.


• Interaction controls promotion or inhibition of pollen; important for breeding and hybridization.

Artificial Hybridisation

• Emasculation: removal of anthers from bisexual flower buds before dehiscence to prevent self-pollination.


• Bagging: covering emasculated flowers to prevent unwanted pollen contamination.


• Pollination: dusting desired pollen on receptive stigma; rebagging to allow fruit development.


• Unisexual flowers do not require emasculation; bagging before flower opening suffices.

Memory Aids

• Emasculation + bagging = controlled pollination for hybridization.

Quick Checkpoints

• What is emasculation and why is it done?


• Explain the role of bagging in artificial hybridisation.

Double Fertilisation

Process and Mechanism

• Fertilisation: Fusion of male and female gametes.


• After pollination, pollen grain germinates on stigma, producing a pollen tube via a germ pore; this is siphonogamy.


• Pollen grains are usually monosiphonous (one tube per grain); some are polysiphonous (multiple tubes), e.g., Malvaceae.


• Pollen tube enters ovule through:
  
  
  
  • Micropyle (porogamy, e.g., lily)
  
  
  • Chalaza (chalazogamy, e.g., Juglans, Casuarina)
  
  
  • Integuments (mesogamy, e.g., Cucurbita, Populus)
  
  
  
  Regardless, it enters embryo sac via micropylar region.


• Inside embryo sac:
  
  
  
  • One male gamete fuses with egg → zygote (2n) (syngamy or generative fertilisation).
  
  
  • Second male gamete fuses with two polar nuclei → triploid primary endosperm nucleus (triple fusion or vegetative fertilisation).
  
  
  • Zygote develops into embryo; primary endosperm nucleus develops into endosperm.
  
  
  • Simultaneous occurrence of syngamy and triple fusion is double fertilisation.
  
  
  • Discovered by S.G. Nawaschin (1898) in Fritillaria and Lilium.
  
  
  
  

Memory Aids

• Siphonogamy: Pollen tube formation for fertilisation.


• Mono vs Poly: One tube = monosiphonous; multiple tubes = polysiphonous.


• Entry routes: Porogamy (micropyle), Chalazogamy (chalaza), Mesogamy (integuments).


• Double fertilisation: Syngamy + triple fusion together.


• Discoverer: Nawaschin in 1898.

Quick Checkpoints

• What is siphonogamy?


• Difference between monosiphonous and polysiphonous pollen grains?


• Three routes of pollen tube entry into ovule?


• Define syngamy and triple fusion.


• What constitutes double fertilisation?


• Who discovered double fertilisation and in which plants?

Post-Fertilisation: Structures and Events

Overview

• Post-fertilisation events include development of endosperm and embryo, and maturation of ovule into seed and ovary into fruit.


• These processes follow double fertilisation in angiosperms.

Memory Aids

• Post-fertilisation = Seed + Fruit formation steps.

Quick Checkpoints

• What are the main events after fertilisation?


• Which structures develop into seed and fruit?

Endosperm

Development and Types

• Endosperm develops before the embryo from the primary endosperm nucleus (PEN).


• PEN divides repeatedly to form triploid endosperm tissue storing food for embryo.


• Initial stage: free-nuclear endosperm with many free nuclei.


• Later, cell walls form, making cellular endosperm.


• Example: Coconut water is free-nuclear endosperm; white kernel is cellular endosperm.


• Endosperm may be consumed by embryo before seed matures (non-endospermic seeds, e.g., pea) or persist in mature seed (endospermic seeds, e.g., coconut).

Memory Aids

• Endosperm = Embryo's food reserve.


• Free-nuclear stage = many nuclei, no walls.


• Cellular stage = nuclei enclosed in cells.

Quick Checkpoints

• What is the ploidy of endosperm?


• Difference between free-nuclear and cellular endosperm?


• Examples of endospermic and non-endospermic seeds?

Embryo

Development and Structure

• Embryo develops at micropylar end of embryo sac from zygote.


• Zygote divides after some endosperm forms to ensure nutrition.


• Stages: proembryo → globular → heart-shaped → mature embryo.


• Dicot embryo: embryonal axis + two cotyledons.


• Embryonal axis parts: epicotyl (above cotyledons, ends in plumule), hypocotyl (below cotyledons), radicle (root tip with root cap).


• Monocot embryo (e.g., grasses): one cotyledon called scutellum.


• Radicle and root cap enclosed in coleorhiza; epicotyl has shoot apex and leaf primordia inside coleoptile.

Memory Aids

• Dicot: Two cotyledons; Monocot: One cotyledon (scutellum).


• Epicotyl → shoot; Hypocotyl → stem; Radicle → root.


• Coleoptile protects shoot; Coleorhiza protects root in monocots.

Quick Checkpoints

• Where does embryo develop in the ovule?


• Stages of embryo development?


• Differences between dicot and monocot embryos?

Seed

Structure and Types

• Seed = fertilised ovule; contains embryo, cotyledons, and seed coat.


• Cotyledons store food; seeds may be albuminous (with endosperm) or non-albuminous (without endosperm).


• Perisperm = persistent nucellus in some seeds (e.g., black pepper).


• Seed coat develops from ovule integuments; micropyle remains as pore for water and oxygen entry.


• Seed water content reduces at maturity; embryo enters dormancy or germinates under favorable conditions.


• Seed viability = ability to germinate over time; influenced by storage conditions.


• Viability loss due to food exhaustion, embryo damage, or enzyme denaturation.

Memory Aids

• Seed = Embryo + Food + Protection.


• Micropyle = entry point for germination.


• Dormancy = embryo inactivity until conditions are right.

Quick Checkpoints

• What are the main parts of a seed?


• Difference between albuminous and non-albuminous seeds?


• Factors affecting seed viability?

Fruit

Definition and Classification

• Fruit develops from fertilised ovary and surrounding tissues.


• True (eucarpic) fruit develops only from ovary (e.g., mango).


• False (pseudocarpic) fruit develops from ovary plus other parts like thalamus (e.g., apple, strawberry).


• Fruit wall = pericarp, with epicarp (skin), mesocarp (fleshy part), and endocarp (inner layer around seeds).


• Fruit types: simple (from single ovary), aggregate (from multiple free carpels), composite/multiple (from inflorescence).


• Parthenocarpy = fruit formation without fertilisation; seedless fruits (e.g., banana); can be hormone-induced.


• Dehiscent fruits release seeds at maturity (e.g., pea); indehiscent fruits do not (e.g., cashew).

Memory Aids

• True fruit = ovary only; False fruit = ovary + other parts.


• Pericarp layers: Epicarp (skin), Mesocarp (flesh), Endocarp (seed cover).


• Parthenocarpy = seedless fruit formation.

Quick Checkpoints

• What is the difference between true and false fruits?


• What are the three layers of pericarp?


• Define parthenocarpy and give examples.


• Difference between dehiscent and indehiscent fruits?

Apomixis and Polyembryony

Apomixis

• Asexual reproduction forming new individuals without gamete formation or fusion.


• Mimics sexual reproduction but involves no fertilisation.

Polyembryony

• Presence of more than one embryo in a single seed.


• Examples: Citrus, onion, groundnut.


• Simple polyembryony: multiple embryos from fertilisation of more than one egg.


• Adventive polyembryony: extra embryos form by sporophytic budding.

Memory Aids

• Apomixis = Asexual reproduction mimicking sexual process.


• Polyembryony = \"Poly\" means many embryos in one seed.


• Simple polyembryony = multiple fertilised eggs.


• Adventive polyembryony = embryos from sporophyte, not gametes.

Quick Checkpoints

• What defines apomixis in plants?


• How does polyembryony differ from apomixis?


• What causes simple polyembryony?


• What is adventive polyembryony?

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