Chapter 9: Sexual Reproduction in Flowering Plants

Introduction

Sexual Reproduction in Angiosperms

• All flowering plants reproduce sexually by forming and fusing gametes.
• Flowers are the specialized organs for sexual reproduction in angiosperms.
• Inflorescences, flowers, and floral parts show diverse structures adapted for fruit and seed formation.

Memory Aids

• Flowers = Sexual reproduction organs in angiosperms.
• Diversity ensures successful fruit and seed formation.

Quick Checkpoints

• What is the role of flowers in angiosperms?
• How do floral structures aid reproduction?

Flower

Structure and Function

• Flower is a modified shoot with determinate growth and condensed internodes.
• Four whorls on the receptacle: calyx, corolla, androecium, gynoecium.
• Calyx: outermost whorl, usually green sepals protecting the flower bud.
• Corolla: petals, often colorful to attract pollinators.
• Androecium: male reproductive part, consists of stamens (anther + filament).
• Gynoecium: female reproductive part, consists of carpels (stigma, style, ovary).
• Ovules inside ovary develop into seeds after fertilization.
• Thalamus (floral base) supports all floral parts.
• Floriculture is the cultivation of ornamental flowering plants.

Memory Aids

• Flower = Modified shoot with 4 whorls: C-C-A-G (Calyx, Corolla, Androecium, Gynoecium).
• Stamen = Anther + Filament (male).
• Carpel = Stigma + Style + Ovary (female).
• Floriculture = Flower cultivation.

Quick Checkpoints

• What are the four whorls of a flower?
• Which parts form the male and female reproductive organs?
• What is the function of the thalamus?
• Define floriculture.

Pre-fertilisation: Structures and Events

Overview

• Floral primordium differentiates under hormonal and structural changes.
• Inflorescences bear floral buds and flowers.
• Flower has 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 initiates floral primordium development?
• Define androecium and gynoecium.

Stamen, Microsporangium and Pollen Grain

Structure and Function

• 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 develop into pollen sacs filled with pollen grains.

Memory Aids

• Dithecous anther = 2 theca per lobe.
• Tetragonal anther = 4 microsporangia.

Quick Checkpoints

• What are the parts of a stamen?
• Describe the structure of an anther.

Structure of Anther

Layers and Functions

• Anther has 4 pollen sacs (microsporangia).
• Microsporangial wall has four layers: epidermis, endothecium, middle layers, tapetum.
• Outer three layers protect and aid anther dehiscence.
• Tapetum nourishes developing pollen.

Memory Aids

• Tapetum = innermost nourishing layer.
• Endothecium = fibrous bands for anther opening.

Quick Checkpoints

• List the layers of microsporangial wall.
• What is the role of tapetum?

Microsporogenesis

Process

• Sporogenous tissue cells undergo meiosis forming microspore tetrads.
• Each microspore separates and develops into pollen grain.
• Microsporogenesis = formation of microspores from pollen mother cell by meiosis.
• As anther matures and dehydrates, microspores dissociate.

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 liberation.
• Wall (sporoderm) has two layers: outer exine (sporopollenin) and inner intine.
• Exine is thick, sculptured or smooth, resistant to decay.
• Thin areas in exine are germ pores (circular) or germinal furrows (elongated).
• 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, floats in vegetative cytoplasm.
• 60% angiosperms shed pollen at 2-celled stage; generative cell divides later.
• Pollen causes 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 outer layer.
• Intine = inner elastic layer.
• Vegetative cell = tube cell; generative cell = male gamete precursor.

Quick Checkpoints

• What is sporoderm?
• Describe the two cells in mature pollen grain.
• Why is pollen viable for different durations?

Pistil, Megasporangium and Embryo Sac

Gynoecium Structure

• Gynoecium = female reproductive part; single (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).
• Ovary contains locules and placenta bearing ovules.
• Ovule number varies: one (wheat, mango) to many (papaya, orchids).

Memory Aids

• Syncarpous = fused pistils; Apocarpous = free pistils.
• Stigma = pollen receptor; Style = pollen tube path; Ovary = ovule container.

Quick Checkpoints

• Define syncarpous and apocarpous pistils.
• What are the parts of a pistil?

The Megasporangium

Ovule Structure

• Ovule = integumented megasporangium developing into seed after fertilisation.
• Attached to placenta by funicle (stalk).
• Hilum = junction of ovule and funicle.
• Integuments (one or two) surround nucellus (food-rich tissue).
• Micropyle = small opening at tip of ovule.
• Chalaza = basal part opposite micropyle.
• Contains embryo sac (female gametophyte).

Memory Aids

• Unitegmic ovule = single integument (Gamopetalae).
• Bitegmic ovule = two integuments (Polypetalae, monocots).
• Ategmic ovule = no integuments (e.g., Santalum album).
• Tritegmic ovule = three integuments (rare, e.g., Asphodelus).

Quick Checkpoints

• What is funicle and hilum?
• Differentiate unitegmic and bitegmic ovules.

Megasporogenesis

Process

• Megaspore mother cell (MMC) in nucellus undergoes meiosis.
• Produces four haploid megaspores in linear tetrad.
• Upper three megaspores degenerate; lowermost becomes functional megaspore.
• Functional megaspore develops into embryo sac.

Memory Aids

• Megasporogenesis = meiosis in ovule forming megaspores.

Quick Checkpoints

• Define megasporogenesis.
• What happens to the four megaspores?

Female Gametophyte

Embryo Sac Formation

• Functional megaspore forms female gametophyte (embryo sac) by mitosis.
• Monosporic development: embryo sac from single megaspore.
• Nucleus divides to 2-, 4-, then 8-nucleate stages without cell walls (free nuclear).
• Two polar nuclei fuse to form diploid secondary nucleus.
• Chalazal end: 3 antipodal cells.
• Micropylar end: egg apparatus with 2 synergids and 1 egg cell (oosphere).
• Synergids have filiform apparatus guiding pollen tube.
• Egg fertilizes to form zygote; synergids and antipodals degenerate post-fertilisation.
• Mature embryo sac is 8-nucleate, 7-celled.

Memory Aids

• Embryo sac = female gametophyte, 8 nuclei, 7 cells.
• Filiform apparatus guides pollen tube.

Quick Checkpoints

• What is monosporic development?
• Name the cells of the embryo sac.

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 to stigma of same flower or genetically similar flower.
• Autogamy: pollen to stigma of same flower.
• Geitonogamy: pollen to stigma of different flower on same plant (ecologically cross-pollination).

Memory Aids

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

Quick Checkpoints

• Define autogamy and geitonogamy.
• What are contrivances for self-pollination?

Cross Pollination

Mechanisms

• Cross pollination (allogamy/xenogamy): pollen transfer between different plants of same species.
• Contrivances to prevent self-pollination:
• Dicliny (unisexual flowers): monoecious (same plant, different flowers) or dioecious (separate plants).
• Self-sterility: pollen cannot fertilize same flower stigma.
• Dichogamy: male and female organs mature at different times.
• Protandry: anthers mature first (e.g., sunflower).
• Protogyny: carpels mature first (e.g., mustard).
• Heterostyly: flowers with different anther and stigma lengths.
• Herkogamy: physical barrier between androecium and gynoecium (e.g., Calotropis).

Memory Aids

• Dicliny = unisexual flowers.
• Dichogamy = temporal separation of maturity.
• Herkogamy = physical barrier to self-pollination.

Quick Checkpoints

• What is dichogamy? Name its types.
• Explain heterostyly and herkogamy.

Types of Pollination

Abiotic Agents

• Anemophily: pollination by wind.
• Characteristics: non-showy flowers, no scent/nectar, exerted stamens and stigmas, dry small pollen, feathery stigmas.
• Examples: maize, grasses, mulberry, date palm.
• Hydrophily: pollination by water.
• Flowers are small, inconspicuous, odourless, nectarless, often waxy.
• Pollen is small, light, non-sticky, coated with wax.
• Stigma is sticky but unwettable.
• Types: Hypohydrophily (underwater, e.g., Ceratophyllum), Epihydrophily (surface, e.g., Vallisneria).

Memory Aids

• Anemophily = wind, dry pollen, feathery stigma.
• Hydrophily = water, waxy pollen, sticky stigma.

Quick Checkpoints

• Define anemophily and hydrophily.
• Differentiate hypohydrophily and epihydrophily.

Biotic Agents

• Entomophily: pollination by insects.
• Flowers produce nectar, edible pollen, are fragrant and brightly coloured.
• Pollen grains are spiny, heavy, sticky (pollenkitt).
• Ornithophily: pollination by birds.
• Flowers are large, bright, odourless, with tubular or cup-shaped corollas, produce mucilaginous nectar.
• Chiropterophily: pollination by bats.
• Flowers are dull-coloured, strong fruity smell, abundant nectar and pollen.
• Other types: malacophily (snails), myrmecophily (ants).

Memory Aids

• Entomophily = insect pollination, nectar and scent.
• Ornithophily = bird pollination, bright and nectar-rich.
• Chiropterophily = bat pollination, dull and smelly.

Quick Checkpoints

• What are features of entomophilous flowers?
• Describe ornithophily and chiropterophily.

Pollen-Pistil Interaction

Process and Importance

• Pollination may deposit incompatible pollen.
• Stigma identifies compatible pollen via pollen-pistil interaction.
• Stigma surface has glandular papillae; types: dry (no exudate) and wet (with exudate).
• Exudates contain sugars, amino acids, proteins, mucopolysaccharides.
• Pollen tube secretes enzymes to digest stigma tissues.
• Pollen tube grows chemotropically guided by calcium-boron-inositol sugar complex.
• Tube enters embryo sac via synergid.
• Interaction regulates pollen acceptance or rejection, aiding breeding.
• Artificial hybridisation uses emasculation (removal of anthers) and bagging to prevent unwanted pollen.
• Unisexual flowers do not require emasculation; bagging before stigma receptivity is done.

Memory Aids

• Emasculation = anther removal to prevent self-pollination.
• Bagging = covering flowers to avoid contamination.
• Pollen-pistil interaction = compatibility check.

Quick Checkpoints

• What is emasculation and why is it done?
• How does pollen tube reach the embryo sac?

Double Fertilisation

Process and Types

• Fertilisation: fusion of male and female gametes.
• After pollination, pollen grain germinates on stigma, producing pollen tube via germ pore (siphonogamy).
• Pollen grains are usually monosiphonous (one tube per grain) but can be polysiphonous (multiple tubes per grain, e.g., Malvaceae).
• Pollen tube enters ovule through micropyle (porogamy, e.g., Lilium), chalaza (chalazogamy, e.g., Juglans, Casuarina), or integuments (mesogamy, e.g., Cucurbita, Populus).
• Regardless of entry, pollen tube enters embryo sac via micropylar region.
• Inside embryo sac, one male gamete fuses with egg forming zygote (2n) - syngamy or generative fertilisation.
• Second male gamete fuses with two polar nuclei forming triploid primary endosperm nucleus - triple fusion or vegetative fertilisation.
• Zygote develops into embryo; primary endosperm nucleus develops into endosperm.
• Syngamy and triple fusion together are called double fertilisation.
• Discovered by S.G. Nawaschin (1898) in Fritillaria and Lilium.

Memory Aids

• Siphonogamy = pollen tube formation.
• Monosiphonous = one tube; Polysiphonous = many tubes.
• Porogamy = micropyle entry; Chalazogamy = chalaza entry; Mesogamy = integument entry.
• Syngamy = zygote formation; Triple fusion = endosperm formation.
• Double fertilisation = syngamy + triple fusion.

Quick Checkpoints

• What is siphonogamy?
• Difference between monosiphonous and polysiphonous pollen grains?
• Routes of pollen tube entry into ovule?
• Define syngamy and triple fusion.
• What forms the primary endosperm nucleus?
• Who discovered double fertilisation and in which plants?

Embryo Development Stages

• Fertilized embryo sac contains zygote and primary endosperm nucleus.
• Zygote grows into globular embryo stage.
• Embryo develops into heart-shaped stage showing early organ formation.
• Mature embryo forms radicle (root), plumule (shoot), cotyledons (seed leaves), and suspensor (nourishment).
• Each stage marks key steps in seed plant development.

Memory Aids

• Globular stage = embryo shape formation.
• Heart-shaped stage = organ initiation.
• Radicle = future root; Plumule = future shoot.
• Suspensor = embryo nourishment.

Quick Checkpoints

• What does the primary endosperm nucleus develop into?
• Sequence of embryo development stages?
• Functions of radicle, plumule, and suspensor?

Post-Fertilisation Events

Overview

• Occurs after double fertilisation.
• Includes development of endosperm and embryo.
• Maturation of ovule into seed and ovary into fruit.

Memory Aids

• Post-fertilisation = seed + fruit formation.

Quick Checkpoints

• What events follow double fertilisation?
• Which structures mature into seeds and fruits?

Endosperm

Development and Types

• Develops before embryo.
• Primary endosperm cell divides to form triploid tissue.
• Cells store food for embryo nutrition.
• Free-nuclear stage: multiple nuclei without cell walls.
• Cellular stage: cell walls form around nuclei.
• Example: Coconut water is free-nuclear endosperm; white kernel is cellular endosperm.
• Endosperm may be consumed by embryo (non-endospermic seeds) or persist in mature seed (endospermic seeds).

Memory Aids

• Endosperm = embryo food reserve.
• Free-nuclear = many nuclei, no walls.
• Cellular = 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

Structure and Development

• Develops at micropylar end of embryo sac.
• Zygote divides after some endosperm forms for nutrition.
• Stages: proembryo → globular → heart-shaped → mature embryo.
• Dicot embryo: embryonal axis + two cotyledons.
• Embryonal axis parts: epicotyl (above cotyledons), hypocotyl (below cotyledons), radicle (root tip with root cap).
• Monocot embryo: one cotyledon called scutellum.
• Monocot embryonal axis: radicle with root cap in coleorhiza; epicotyl with shoot apex and leaf primordia in coleoptile.

Memory Aids

• Dicot = 2 cotyledons; monocot = 1 cotyledon (scutellum).
• Epicotyl = shoot tip; hypocotyl = stem base; radicle = root tip.
• Coleoptile and coleorhiza protect shoot and root in monocots.

Quick Checkpoints

• Where does embryo develop in the ovule?
• What are the stages of embryo development?
• Differences between dicot and monocot embryos?

Seed

Structure and Features

• Seed = fertilised ovule; final product of sexual reproduction in angiosperms.
• Components: seed coat, cotyledons, embryo axis.
• Cotyledons store food reserves; seeds may be albuminous or non-albuminous.
• Perisperm: persistent nucellus in some seeds (e.g., black pepper).
• Seed coat formed from integuments; micropyle allows water and oxygen entry.
• Seed water content reduces at maturity; embryo enters dormancy or germinates if conditions are favorable.
• Viability: ability to germinate over time; affected by storage conditions.
• Loss of viability due to food exhaustion, embryo damage, or enzyme denaturation.

Memory Aids

• Seed = fertilised ovule with embryo and food reserve.
• Dormancy = embryo inactivity until favorable conditions.
• Viability depends on storage and seed condition.

Quick Checkpoints

• What are the main parts of a seed?
• Difference between albuminous and non-albuminous seeds?
• Factors causing loss of seed viability?

Fruit

Definition and Types

• Fruit develops from fertilised ovary and surrounding tissues.
• False (pseudocarpic) fruit: flesh from ovary and other parts (e.g., apple, strawberry).
• True (eucarpic) fruit: develops only from ovary (e.g., mango).
• Pericarp: fruit wall with epicarp (skin), mesocarp (fleshy part), endocarp (inner layer around seeds).
• Fruit types: simple, aggregate, composite (multiple).
• Simple fruit: from monocarpellary or syncarpous ovary.
• Aggregate fruit: from apocarpous ovary.
• Composite fruit: from inflorescence.
• Parthenocarpy: fruit formation without fertilisation; seedless fruits (e.g., banana); induced by hormones.

Memory Aids

• False fruit = ovary + other flower parts.
• True fruit = ovary only.
• Parthenocarpy = seedless fruit formation.

Quick Checkpoints

• What is the difference between true and false fruits?
• What are the layers of pericarp?
• Define parthenocarpy and give examples.

Apomixis and Polyembryony

Apomixis

• Formation of new individuals without gamete formation or fusion.
• Asexual reproduction mimicking sexual reproduction.

Polyembryony

• Presence of more than one embryo in a seed.
• Examples: Citrus, onion, groundnut.
• Simple polyembryony: multiple embryos from fertilisation of more than one egg.
• Adventive polyembryony: extra embryos formed by sporophytic budding.

Memory Aids

• Apomixis = Asexual reproduction without gametes.
• Polyembryony = Multiple embryos in one seed.
• Simple polyembryony = Multiple eggs fertilised.
• Adventive polyembryony = Embryos from sporophyte cells.

Quick Checkpoints

• What defines apomixis?
• Which plants show polyembryony?
• Difference between simple and adventive polyembryony?
• How does adventive polyembryony occur?