Understanding Sexual Reproduction in Flowering Plants: Detailed Study Guide

This study guide is designed to help you review the key concepts related to sexual reproduction in flowering plants (angiosperms)

I. Fundamentals of Reproduction

• Definition of Biology and Life: Understand how biology is defined as the story of life, and the concept of species continuity despite individual mortality.
• Importance of Reproduction: Recognize reproduction as a vital process for species survival.
• Modes of Reproduction: Distinguish between asexual and sexual reproduction, noting the advantage of sexual reproduction in creating new variants for enhanced survival.
• Scope of the Unit: Identify the main topics covered in the unit: reproductive processes in flowering plants and humans, and human reproductive health.

II. Sexual Reproduction in Flowering Plants (Angiosperms) - Chapter 1

A. The Flower: A Fascinating Organ

• Nature of Flowers: Comprehend flowers as morphological and embryological marvels, serving as the sites of sexual reproduction.
• Human-Flower Relationship: Appreciate the aesthetic, ornamental, social, religious, and cultural significance of flowers.
• Basic Floral Anatomy (Recall from previous knowledge): Be able to recall and, if prompted, identify the main parts of a typical flower (e.g., sepals, petals, stamens, pistil).

B. Pre-fertilisation: Structures and Events

Floral Development:

• Initiation: Understand that the decision to flower involves hormonal and structural changes.
• Formation of Reproductive Structures: Identify the development of androecium (male) and gynoecium (female) from floral primordium.

Stamen, Microsporangium, and Pollen Grain (Male Reproductive Structures):

• Stamen Anatomy: Describe the two parts of a typical stamen: filament and anther. Note the variability in number and length.
• Anther Structure: Understand that a typical angiosperm anther is bilobed and dithecous (two theca per lobe), making it a four-sided (tetragonal) structure with four microsporangia.
• Microsporangium Development: Recognize that microsporangia develop into pollen sacs, filled with pollen grains.
• Microsporangium Wall Layers: List and describe the four wall layers:
  • Epidermis, Endothecium, Middle Layers: Functions in protection and dehiscence of the anther.
  • Tapetum: Innermost layer, nourishes developing pollen grains, typically has dense cytoplasm and multiple nuclei.
• Sporogenous Tissue: Identify this group of homogenous cells in the center of a young microsporangium.

Microsporogenesis:

• Definition: The process of microspore formation from a pollen mother cell (PMC) through meiosis.
• Ploidy: Understand that PMCs are diploid, and meiosis results in haploid microspore tetrads.
• Pollen Grain Development: Explain how microspores dissociate from the tetrad and develop into pollen grains as the anther matures and dehydrates.

Pollen Grain (Male Gametophyte):

• Description: Generally spherical, 25-50 micrometers in diameter, with a two-layered wall.
• Exine: Hard outer layer made of sporopollenin (most resistant organic material), which makes pollen grains good fossils and allows them to withstand harsh conditions. Note the presence of germ pores (where sporopollenin is absent).
• Intine: Inner, thin, continuous layer made of cellulose and pectin.
• Cytoplasm and Cells: Understand that mature pollen grains contain a vegetative cell (larger, food reserve, irregular nucleus) and a generative cell (smaller, spindle-shaped, dense cytoplasm, floats in vegetative cell cytoplasm).
• Shedding Stage: Differentiate between 2-celled (vegetative + generative) and 3-celled (vegetative + two male gametes) shedding stages.
• Allergies: Be aware that pollen grains from some species (e.g., Parthenium) can cause allergies and respiratory disorders.
• Viability and Storage: Understand the variable viability of pollen grains and the concept of pollen banks (storage in liquid nitrogen) for crop breeding.
• Pollen Products: Recognize pollen tablets/syrups as food supplements, claimed to enhance performance.

The Pistil, Megasporangium (Ovule), and Embryo Sac (Female Reproductive Structures):

• Gynoecium: Represents the female reproductive part.
• Pistil Variations: Differentiate between monocarpellary, multicarpellary (syncarpous and apocarpous) pistils.
• Parts of a Pistil: Identify the stigma (landing platform), style (elongated part), and ovary (basal bulged part containing ovarian cavity/locule and placenta).

Ovule (Megasporangium):

• Attachment: Attached to the placenta by a funicle; hilum is the junction.
• Protective Layers: One or two integuments encircling the nucellus.
• Micropyle: Small opening at the tip of integuments.
• Chalaza: Basal part of the ovule, opposite the micropyle.
• Nucellus: Mass of cells enclosed by integuments, rich in reserve food materials. Contains the embryo sac.

Megasporogenesis:

• Definition: Formation of megaspores from a megaspore mother cell (MMC) through meiosis.
• Location: MMC differentiates in the micropylar region of the nucellus.
• Outcome: Meiosis results in four megaspores.

Female Gametophyte (Embryo Sac):

• Monosporic Development: In most flowering plants, only one megaspore is functional, developing into the embryo sac.
• Embryo Sac Formation: Detail the mitotic divisions of the functional megaspore nucleus (2-nucleate, 4-nucleate, 8-nucleate stages). Emphasize free nuclear divisions before cell wall formation.
• Mature Embryo Sac Structure: Describe its 7-celled, 8-nucleate nature:
  • Egg Apparatus: Three cells at the micropylar end: one egg cell and two synergids.
  • Filiform Apparatus: Special thickenings on synergids, guiding pollen tubes.
  • Antipodals: Three cells at the chalazal end.
  • Central Cell: Large cell with two polar nuclei.

C. Pollination

• Definition: Transfer of pollen grains from anther to stigma.
• Purpose: To bring non-motile male and female gametes together for fertilisation.
• Pollinating Agents: Distinguish between abiotic (wind, water) and biotic (animals) agents.
• Compensation for Uncertainty: Explain why flowers produce enormous amounts of pollen when relying on abiotic agents.

Kinds of Pollination:

• Autogamy: Pollination within the same flower. Rare in normal open flowers, requires synchrony and proximity of anther/stigma.
  • Chasmogamous Flowers: Open, exposed anthers and stigma.
  • Cleistogamous Flowers: Do not open, anthers and stigma lie close, invariably autogamous, assured seed-set (e.g., Viola, Oxalis, Commelina). Discuss advantages/disadvantages.
• Geitonogamy: Transfer of pollen between different flowers on the same plant. Functionally cross-pollination, but genetically similar to autogamy.
• Xenogamy: Transfer of pollen between anther and stigma of different plants. Only type that brings genetically different pollen to the stigma.

Agents of Pollination (Detailed):

• Wind Pollination: Common among abiotic agents. Characteristics: light, non-sticky pollen; well-exposed stamens; large, feathery stigma; single ovule per ovary; numerous flowers in an inflorescence (e.g., corn cob, grasses). Not colorful, no nectar.
• Water Pollination: Rare, mostly monocotyledons (e.g., Vallisneria, Hydrilla, Zostera). Not all aquatic plants use water. Explain variations like Vallisneria (female flowers reach surface, pollen floats) and seagrasses (flowers submerged, ribbon-like pollen). Pollen often protected by mucilaginous covering. Not colorful, no nectar.
• Animal Pollination: Majority of flowering plants. Common agents: bees, butterflies, flies, beetles, wasps, ants, moths, birds, bats. Even larger animals can be pollinators.
  • Adaptations: Flowers often large, colorful, fragrant, rich in nectar. Small flowers clustered into inflorescences. Foul odors for flies/beetles.
  • Floral Rewards: Nectar and pollen are common rewards.
  • Special Relationships: Examples like Amorphophallus (safe place to lay eggs) and Yucca-moth (mutualistic relationship).

Outbreeding Devices:

Mechanisms to discourage self-pollination and encourage cross-pollination, preventing inbreeding depression.

• Asynchrony: Pollen release and stigma receptivity are not synchronized.
• Positioning: Anther and stigma placed at different positions.
• Self-incompatibility: Genetic mechanism preventing self-pollen germination or tube growth.
• Unisexual Flowers:
  • Monoecious: Male and female flowers on the same plant (prevents autogamy, not geitonogamy, e.g., castor, maize).
  • Dioecious: Male and female flowers on different plants (prevents both autogamy and geitonogamy, e.g., papaya).

Pollen-Pistil Interaction:

• Recognition: Pistil’s ability to distinguish compatible from incompatible pollen, mediated by chemical dialogue.
• Acceptance/Rejection: Promotion of post-pollination events for compatible pollen; inhibition for incompatible pollen.
• Pollen Tube Growth: Germination of pollen grain on stigma, formation of pollen tube through germ pore. Pollen tube grows through stigma and style to reach ovary.
• Gamete Delivery: Generative cell divides (if not already) to form two male gametes. Pollen tube enters ovule via micropyle, then into a synergid (guided by filiform apparatus).

Artificial Hybridisation:

• Purpose: To combine desirable characters for superior varieties.
• Techniques:
  • Emasculation: Removal of anthers from bisexual flower buds before dehiscence to prevent self-pollination.
  • Bagging: Covering emasculated flowers (or female unisexual flowers) with a bag to prevent unwanted pollen contamination. Rebagging after desired pollination.

D. Post-fertilisation: Structures and Events

Double Fertilisation: Unique to flowering plants.

• Syngamy: Fusion of one male gamete with the egg cell, forming a diploid zygote.
• Triple Fusion: Fusion of the other male gamete with the two polar nuclei in the central cell, forming a triploid primary endosperm nucleus (PEN).
• Products: Zygote develops into embryo; primary endosperm cell (PEC) (formed from central cell after triple fusion) develops into endosperm.

Endosperm Development:

• Precedes Embryo Development: Provides nutrition.
• Formation: PEN divides repeatedly to form triploid endosperm tissue.
• Types:
  • Free-nuclear endosperm: PEN undergoes successive nuclear divisions without cell wall formation (e.g., coconut water).
  • Cellular endosperm: Cell wall formation occurs after free nuclear stage (e.g., coconut kernel).
• Persistence: May be completely consumed by embryo (non-albuminous seeds, e.g., pea, groundnut, beans) or persist in mature seed (albuminous seeds, e.g., castor, coconut, wheat, rice, maize).

Embryo Development:

• Location: Micropylar end of embryo sac.
• Dormancy: Zygote typically divides after endosperm formation (assured nutrition).
• Stages (Dicotyledonous Embryo): Zygote -> proembryo -> globular -> heart-shaped -> mature embryo.
• Typical Dicot Embryo: Embryonal axis and two cotyledons.
  • Epicotyl: Above cotyledons, terminates in plumule (stem tip).
  • Hypocotyl: Below cotyledons, terminates in radicle (root tip), covered by root cap.
• Monocotyledonous Embryo: Only one cotyledon (scutellum).
  • Scutellum: Lateral to embryonal axis.
  • Coleorrhiza: Undifferentiated sheath enclosing radicle and root cap.
  • Coleoptile: Hollow foliar structure enclosing shoot apex and leaf primordia above epicotyl.

Seed: Final product of sexual reproduction, a fertilised ovule.

• Structure: Seed coat(s), cotyledon(s), embryo axis.
• Food Reserves: Cotyledons may be thick/swollen due to stored food.
• Types:
  • Non-albuminous/Ex-albuminous: No residual endosperm (e.g., pea, groundnut).
  • Albuminous: Retain part of endosperm (e.g., wheat, maize, castor).
• Perisperm: Remnants of nucellus in some seeds (e.g., black pepper, beet).
• Seed Coat: Hardened integuments, protective.
• Micropyle: Small pore for O2 and H2O entry during germination.
• Dormancy: Reduced water content, slowed metabolic activity, embryo enters inactivity state. Germinates under favorable conditions.
• Viability: Variable, from months to hundreds/thousands of years (e.g., Lupinus arcticus, Phoenix dactylifera).

Fruit: Developed from ovary after fertilisation.

• Pericarp: Wall of ovary develops into fruit wall.
• Types: Fleshy (guava, mango), dry (groundnut, mustard).
• True Fruits: Develop only from the ovary.
• False Fruits: Other floral parts (e.g., thalamus) also contribute (e.g., apple, strawberry, cashew).
• Parthenocarpic Fruits: Develop without fertilisation, typically seedless (e.g., banana). Can be induced by growth hormones.
• Advantages of Seeds: Independent of water for reproduction, better dispersal strategies, food reserves for seedling, hard seed coat for protection, genetic variations from sexual reproduction. Crucial for agriculture (storage).

E. Apomixis and Polyembryony

Apomixis:

• Definition: A form of asexual reproduction that mimics sexual reproduction, producing seeds without fertilisation (e.g., some Asteraceae, grasses, Citrus, Mango).
• Mechanism: Diploid egg cell forms without reduction division and develops into embryo without fertilisation; or nucellar cells protrude into embryo sac and develop into embryos.
• Genetic Nature: Apomictic embryos are genetically identical to the parent (clones).
• Importance: Allows farmers to continuously use hybrid seeds without losing desired characters due to segregation, reducing cost. Active research to transfer apomictic genes into hybrids.

Polyembryony:

• Definition: Occurrence of more than one embryo in a seed (e.g., Citrus, Mango). Often results from apomixis.

Quiz: Sexual Reproduction in Flowering Plants

Instructions: Answer each question in 2-3 sentences.

1. Define biology in the context of life on Earth. Why is reproduction considered vital for species?
2. Describe the structure and primary function of the tapetum layer within a microsporangium.
3. What is sporopollenin, and what is its significance in the context of pollen grains?
4. Distinguish between a 2-celled and a 3-celled pollen grain, indicating what each cell represents.
5. Explain the concept of monosporic development of the female gametophyte in angiosperms.
6. Name the three parts of a pistil and briefly state the function of the stigma.
7. What is the difference between autogamy and geitonogamy? In what way is geitonogamy genetically similar to autogamy?
8. Describe two structural adaptations observed in wind-pollinated flowers that aid in their pollination.
9. Explain the phenomenon of double fertilisation, naming the two fusion events and their respective products.
10. Define apomixis. How does this process offer an advantage in the context of hybrid crop cultivation?

Answer Key for Quiz

1. Biology is fundamentally the story of life on Earth, encompassing the processes that allow species to persist over millions of years despite individual death. Reproduction is vital because it ensures the continuity of species by producing new progeny, preventing extinction.
2. The tapetum is the innermost wall layer of the microsporangium. Its primary function is to nourish the developing pollen grains, and its cells typically possess dense cytoplasm and more than one nucleus.
3. Sporopollenin is one of the most resistant organic materials known, forming the hard outer layer (exine) of pollen grains. Its presence allows pollen grains to be well-preserved as fossils and withstand high temperatures, strong acids, and alkali, protecting the genetic material inside.
4. A 2-celled pollen grain contains a large vegetative cell with abundant food reserves and an irregular nucleus, and a smaller generative cell floating within it. A 3-celled pollen grain results when the generative cell divides mitotically to form two male gametes before pollen shedding.
5. Monosporic development refers to the formation of the female gametophyte (embryo sac) from a single functional megaspore, while the other three megaspores formed during meiosis degenerate. This is the most common method of embryo sac formation in flowering plants.
6. The three parts of a pistil are the stigma, style, and ovary. The stigma serves as the receptive landing platform for pollen grains, facilitating their capture during pollination.
7. Autogamy is self-pollination occurring within the same flower, where pollen transfers from anther to its own stigma. Geitonogamy is the transfer of pollen between different flowers on the same plant. Genetically, geitonogamy is similar to autogamy because the pollen grains come from the same plant, leading to progeny that are genetically identical to the parent.
8. Wind-pollinated flowers often possess well-exposed stamens, allowing easy dispersal of pollen into wind currents. They also typically have large, often feathery stigmas to effectively trap airborne pollen grains.
9. Double fertilisation is a unique event in flowering plants involving two fusion events. Syngamy is the fusion of one male gamete with the egg cell to form a diploid zygote. Triple fusion is the fusion of the other male gamete with the two polar nuclei in the central cell, forming a triploid primary endosperm nucleus (PEN).
10. Apomixis is a special mechanism in some flowering plants that produces seeds without fertilisation, mimicking sexual reproduction. In hybrid crop cultivation, apomixis is advantageous because it ensures that hybrid characters are maintained in subsequent generations, allowing farmers to reuse hybrid seeds year after year without segregation of desirable traits.

Essay Format Questions

1. Discuss the evolutionary significance of sexual reproduction in flowering plants, particularly focusing on the advantages it confers compared to asexual reproduction. Provide examples of adaptations that ensure successful sexual reproduction in angiosperms.
2. Compare and contrast the processes of microsporogenesis and megasporogenesis in flowering plants. Include details on the cell types involved, the type of cell division, and the resulting structures.
3. Elaborate on the diverse strategies that flowering plants have evolved to achieve pollination. Discuss the roles of both abiotic and biotic agents, providing specific examples and the floral adaptations associated with each.
4. Describe the sequence of events from compatible pollen landing on the stigma to the formation of the zygote and primary endosperm nucleus. Explain the significance of “pollen-pistil interaction” and “double fertilisation” in this process.
5. Analyze the developmental fate of the various parts of an ovule and ovary after fertilisation, leading to the formation of a mature seed and fruit. Discuss the different types of seeds and fruits based on their post-fertilisation characteristics, and highlight the benefits of seed formation to angiosperms and human agriculture.

Glossary of Key Terms

• Albuminous seeds: Seeds that retain a part of the endosperm as it is not completely used up during embryo development (e.g., castor, coconut).
• Androecium: The male reproductive organ in a flower, consisting of a whorl of stamens.
• Anther: The terminal, generally bilobed structure of a stamen that contains the pollen.
• Antipodals: Three cells located at the chalazal end of the embryo sac.
• Apocarpous: Refers to a multicarpellary pistil where the pistils are free.
• Apomixis: A form of asexual reproduction in flowering plants that mimics sexual reproduction, producing seeds without fertilisation.
• Artificial hybridisation: A plant breeding technique involving crossing different species or genera to combine desirable characters and produce commercially superior varieties.
• Autogamy: A type of self-pollination where the transfer of pollen grains occurs from the anther to the stigma of the same flower.
• Bagging: The process of covering emasculated flowers or female unisexual flowers with a bag (usually butter paper) to prevent contamination by unwanted pollen.
• Chalaza: The basal part of the ovule, opposite the micropyle.
• Chasmogamous flowers: Flowers that open and expose their anthers and stigma.
• Cleistogamous flowers: Flowers that do not open at all, ensuring self-pollination (invariably autogamous) and assured seed-set even in the absence of pollinators.
• Coleoptile: A hollow foliar structure that encloses the shoot apex and a few leaf primordia in monocotyledonous embryos.
• Coleorrhiza: An undifferentiated sheath that encloses the radicle and root cap at the lower end of the embryonal axis in monocotyledonous embryos.
• Cotyledons: Seed leaves of the embryo, often thick and swollen due to food reserves.
• Dehiscence: The splitting open of the anther to release pollen grains.
• Dithecous: Refers to an anther having two theca (lobes).
• Dioecy: A condition where male and female flowers are present on different plants (each plant is either male or female).
• Diploid: Having two sets of chromosomes ().
• Dormancy: A state of inactivity of the embryo within a mature seed, where metabolic activity is slowed down.
• Double fertilisation: A unique event in flowering plants involving two fusion events: syngamy and triple fusion.
• Egg apparatus: A group of three cells at the micropylar end of the embryo sac, consisting of one egg cell and two synergids.
• Egg cell: The female gamete within the embryo sac.
• Embryo: The rudimentary plant contained within the seed, developed from the zygote.
• Embryo sac: The female gametophyte in angiosperms, typically 7-celled and 8-nucleate, located within the nucellus of the ovule.
• Embryonal axis: The central axis of a dicotyledonous embryo, bearing the epicotyl and hypocotyl.
• Endosperm: A nutritive tissue that develops from the primary endosperm cell after triple fusion, providing nourishment to the developing embryo.
• Endothecium: One of the wall layers surrounding the microsporangium, contributing to protection and dehiscence.
• Epicotyl: The portion of the embryonal axis above the level of cotyledons, terminating with the plumule (stem tip).
• Epidermis: The outermost wall layer of the microsporangium.
• Exine: The hard outer layer of the pollen grain wall, made up of sporopollenin.
• False fruits: Fruits in which other floral parts, such as the thalamus, also contribute to fruit formation (e.g., apple, strawberry).
• Filament: The long and slender stalk of a stamen.
• Filiform apparatus: Special cellular thickenings at the micropylar tip of synergids, which guide the pollen tubes into the synergid.
• Floral primordium: The early developmental stage of a flower, from which floral parts differentiate.
• Free-nuclear endosperm: A stage of endosperm development where the PEN undergoes successive nuclear divisions without immediate cell wall formation (e.g., coconut water).
• Funicle: The stalk by which the ovule is attached to the placenta.
• Generative cell: The smaller cell in a pollen grain that divides to form two male gametes.
• Germ pores: Prominent apertures on the pollen grain exine where sporopollenin is absent, allowing for the emergence of the pollen tube.
• Geitonogamy: Transfer of pollen grains from the anther to the stigma of another flower on the same plant.
• Gynoecium: The female reproductive part of the flower, consisting of one or more pistils.
• Hilum: The region where the body of the ovule fuses with the funicle, representing their junction.
• Hypocotyl: The cylindrical portion of the embryonal axis below the level of cotyledons, terminating at its lower end in the radicle (root tip).
• Integuments: Protective envelopes surrounding the nucellus of the ovule.
• Intine: The inner wall layer of the pollen grain, thin and continuous, made up of cellulose and pectin.
• Locule: The ovarian cavity inside the ovary.
• Megaspore mother cell (MMC): A large cell in the micropylar region of the nucellus that undergoes meiosis to form megaspores.
• Megasporangium: The ovule, containing the megaspore mother cell and later the embryo sac.
• Megasporogenesis: The process of formation of megaspores from the megaspore mother cell through meiosis.
• Microsporangia: Four-sided structures located at the corners of a typical anther, which develop into pollen sacs.
• Microsporogenesis: The process of formation of microspores from a pollen mother cell (PMC) through meiosis.
• Microspore tetrad: A cluster of four microspores formed after meiosis of a pollen mother cell.
• Micropyle: A small opening at the tip of the integuments in the ovule, through which the pollen tube typically enters.
• Middle layers: Wall layers of the microsporangium located between the endothecium and tapetum, aiding in protection and dehiscence.
• Monoecious: A condition where both male and female flowers are present on the same plant.
• Monocarpellary: Refers to a gynoecium consisting of a single pistil.
• Multicarpellary: Refers to a gynoecium having more than one pistil.
• Non-albuminous seeds (Ex-albuminous seeds): Seeds that have no residual endosperm as it is completely consumed during embryo development (e.g., pea, groundnut).
• Nucellus: A mass of cells enclosed within the integuments of the ovule, rich in reserve food materials, and containing the embryo sac.
• Ovary: The basal, bulged part of the pistil containing the ovarian cavity and ovules.
• Ovule: The megasporangium, which develops into a seed after fertilisation.
• Parthenocarpic fruits: Fruits that develop without fertilisation, typically seedless (e.g., banana).
• Pericarp: The wall of the fruit, developed from the ovary wall.
• Perisperm: Remnants of the nucellus that persist in some mature seeds (e.g., black pepper, beet).
• Pistil: The female reproductive organ of a flower, consisting of the stigma, style, and ovary.
• Placenta: Tissue located inside the ovarian cavity from which the ovules arise.
• Plumule: The embryonic shoot, or stem tip, located at the epicotyl end of the embryonal axis.
• Polar nuclei: Two nuclei located in the large central cell of the embryo sac, which fuse with one male gamete during triple fusion.
• Pollen grain: The male gametophyte, containing the male gametes, released from the anther.
• Pollen mother cell (PMC): A cell within the sporogenous tissue that undergoes meiosis to form microspores.
• Pollen sacs: The mature microsporangia, filled with pollen grains.
• Pollen tube: A tube that grows from the pollen grain after germination on the stigma, carrying the male gametes to the ovule.
• Pollen-pistil interaction: The dynamic process involving the recognition, acceptance, or rejection of pollen by the pistil, mediated by chemical components.
• Pollination: The transfer of pollen grains from the anther to the stigma of a pistil.
• Pollinators: Agents (abiotic or biotic) that facilitate the transfer of pollen.
• Polyembryony: The occurrence of more than one embryo in a seed.
• Primary endosperm cell (PEC): The central cell after triple fusion, which develops into the endosperm.
• Primary endosperm nucleus (PEN): The triploid nucleus formed by the fusion of one male gamete and the two polar nuclei.
• Proembryo: An early stage in embryo development, formed from the zygote.
• Radicle: The embryonic root, or root tip, located at the hypocotyl end of the embryonal axis.
• Scutellum: The single cotyledon in monocotyledonous embryos, situated laterally to the embryonal axis.
• Seed: The final product of sexual reproduction in angiosperms, a fertilised ovule, containing an embryo, food reserves, and protective seed coats.
• Seed coat(s): Tough, protective outer layers of the seed, developed from the integuments of the ovule.
• Self-incompatibility: A genetic mechanism that prevents self-pollen from fertilising the ovules by inhibiting pollen germination or pollen tube growth.
• Sporogenous tissue: A group of compactly arranged homogenous cells that occupies the center of each young microsporangium, from which pollen mother cells develop.
• Stamen: The male reproductive organ of a flower, consisting of a filament and an anther.
• Stigma: The receptive landing platform for pollen grains, located at the tip of the pistil.
• Style: The elongated slender part of the pistil, located beneath the stigma, through which the pollen tube grows.
• Synergids: Two cells located in the egg apparatus of the embryo sac, flanking the egg cell, and possessing filiform apparatus.
• Syngamy: The fusion of one male gamete with the egg cell to form a diploid zygote.
• Syncarpous: Refers to a multicarpellary pistil where the pistils are fused together.
• Tapetum: The innermost wall layer of the microsporangium, which nourishes the developing pollen grains.
• Testa: Outer seed coat. (Note: The provided text refers to “seed coat(s)” which are derived from integuments. “Testa” is a specific term for the outer seed coat.)
• Theca: A lobe of the anther, containing two microsporangia.
• Triple fusion: The fusion of one male gamete with the two polar nuclei in the central cell of the embryo sac, forming a triploid primary endosperm nucleus (PEN).
• True fruits: Fruits that develop only from the ovary.
• Vegetative cell: The larger cell in a pollen grain, which has abundant food reserves and a large, irregularly shaped nucleus, and helps in pollen tube growth.
• Xenogamy: Cross-pollination involving the transfer of pollen grains from the anther of one plant to the stigma of a genetically different plant.
• Zygote: The diploid cell formed by the fusion of the male gamete and the egg cell (syngamy), which develops into the embryo.