Sexual Reproduction in Flowering Plants

Biology of Reproduction: Flowering Plants and Humans

Study Guide

This study guide is designed to help you review the fundamental concepts of reproduction in flowering plants, as detailed in "lebo101.pdf." It covers the structures, processes, and related health aspects of sexual reproduction in these organisms.

I. Introduction to Reproduction

Life Cycle and Species Survival: Understand why reproduction is a vital process for the continuation of species, even though individual organisms eventually die.
Modes of Reproduction: Differentiate between asexual and sexual reproduction and the advantage of sexual reproduction (creation of new variants, enhanced survival advantage).
Scope of the Unit: Recognize that the unit focuses on reproductive processes in flowering plants and humans, including human reproductive health.

II. Panchanan Maheshwari (1904-1966)

Distinguished Botanist: Recall his nationality and significant contributions to botany globally.
Education and Inspiration: Note his higher education at Allahabad and the influence of Dr W. Dudgeon.
Key Research Areas: Understand his work on embryological aspects, popularizing embryological characters in taxonomy.
Establishment of Research Centre: Recognize his role in establishing the Department of Botany, University of Delhi, as a research hub.
Pioneering Techniques: Identify his work on artificial culture of immature embryos, test tube fertilisation, and intra-ovarian pollination.
Honours and Contributions to Education: Be aware of his fellowships and leadership in creating the first NCERT Biology textbooks for higher secondary schools.

III. Sexual Reproduction in Flowering Plants (Angiosperms)

A. Flower – A Fascinating Organ of Angiosperms

Aesthetic and Cultural Value: Understand the multifaceted human relationship with flowers (aesthetic, ornamental, social, religious, cultural).
Biological Significance: Recognize flowers as morphological and embryological marvels, serving as sites of sexual reproduction.
Floral Parts: Be able to recall and identify the various parts of a typical flower, especially the male (androecium/stamens) and female (gynoecium/pistil) reproductive structures.

B. Pre-fertilisation: Structures and Events

Floral Development: Understand the sequence from hormonal/structural changes to floral primordium, inflorescences, floral buds, and finally flowers.
Male Reproductive Organ (Androecium):Stamen Structure: Identify the filament and anther.
Anther Structure: Describe a typical angiosperm anther as bilobed and dithecous, containing four microsporangia (pollen sacs).
Microsporangium Wall Layers: Name and state the function of the four wall layers (epidermis, endothecium, middle layers, tapetum). Emphasize the nutritive role of the tapetum.
Sporogenous Tissue: Understand its location and role in microsporangium.
Microsporogenesis: Define this process (formation of microspores from pollen mother cell via meiosis). Understand the formation of microspore tetrads and their dissociation into pollen grains.
Pollen Grain (Male Gametophyte):Structure: Describe its general size, spherical shape, and two-layered wall.
Exine: Understand its composition (sporopollenin), resistance, and the presence of germ pores. Explain why sporopollenin makes pollen grains good fossils.
Intine: Describe its composition (cellulose and pectin) and continuity.
Pollen Grain Cells: Differentiate between the vegetative cell (bigger, food reserve, large nucleus) and the generative cell (smaller, spindle-shaped, dense cytoplasm, nucleus).
Shedding Stages: Note that pollen grains are shed at the 2-celled or 3-celled stage (after generative cell divides mitotically into two male gametes).
Pollen Allergies: Be aware of issues like asthma and bronchitis caused by certain pollens (e.g., Parthenium).
Pollen Viability: Understand the variability in viability duration and factors affecting it (temperature, humidity).
Pollen Banks: Recognize the concept of storing pollen in liquid nitrogen for crop breeding programmes.
Pollen Products: Note their use as food supplements for athletes/racehorses.
Female Reproductive Organ (Gynoecium):Pistil Structure: Differentiate between monocarpellary and multicarpellary (syncarpous or apocarpous) pistils.
Parts of a Pistil: Identify the stigma (landing platform for pollen), style (elongated part), and ovary (basal bulged part containing ovarian cavity/locule).
Placenta and Ovules: Understand the location of the placenta and the origin of ovules (megasporangia) from it. Note the variability in ovule number.
Megasporangium (Ovule) Structure:Funicle and Hilum: Understand their roles in attachment.
Integuments: Describe them as protective envelopes.
Micropyle: Identify it as a small opening at the tip.
Chalaza: Recognize it as the basal part, opposite the micropyle.
Nucellus: Understand it as a mass of cells with abundant reserve food, enclosing the embryo sac.
Megasporogenesis: Define this process (formation of megaspores from megaspore mother cell via meiosis). Understand the production of four megaspores.
Female Gametophyte (Embryo Sac):Monosporic Development: Explain how a single functional megaspore develops into the embryo sac (others degenerate).
Embryo Sac Formation: Describe the sequential mitotic divisions (free nuclear) leading to 2-nucleate, 4-nucleate, and 8-nucleate stages.
Mature Embryo Sac Structure: Understand its 7-celled, 8-nucleate nature.
Egg Apparatus: Location (micropylar end), components (two synergids, one egg cell), and function of filiform apparatus (guiding pollen tube).
Antipodals: Location (chalazal end), number (three).
Central Cell: Location (centre), components (two polar nuclei).

C. Pollination

Definition: Understand it as the transfer of pollen grains from anther to stigma.
Importance: Recognize its role in bringing non-motile male and female gametes together.
Agents of Pollination: Differentiate between abiotic (wind, water) and biotic (animals) agents.
Kinds of Pollination:Autogamy: Pollination within the same flower. Understand conditions for its occurrence (synchrony, proximity of anther/stigma).
Chasmogamous Flowers: Flowers with exposed anthers and stigma.
Cleistogamous Flowers: Flowers that do not open, ensuring autogamy and assured seed-set (discuss advantages/disadvantages).
Geitonogamy: Transfer of pollen between different flowers on the same plant. Recognize it as functionally cross-pollination but genetically similar to autogamy.
Xenogamy: Transfer of pollen between different plants. Understand it as the only type bringing genetically different pollen.
Adaptations for Pollination:Wind Pollination: Characteristics of pollen (light, non-sticky), stamens (well-exposed), stigma (large, feathery). Examples: corn cob, grasses.
Water Pollination: Rarity in flowering plants, common in lower plant groups. Examples: Vallisneria, Hydrilla, Zostera. Note exceptions (water hyacinth, water lily pollinated by insects/wind). Describe mechanisms in Vallisneria and sea-grasses. Understand protection of pollen by mucilaginous covering.
Animal Pollination: Majority of flowering plants. Common agents (bees, butterflies, birds, bats, etc.). Floral adaptations (large, colourful, fragrant, nectar-rich; clustered if small). Foul odours for flies/beetles. Floral rewards (nectar, pollen, safe places to lay eggs – Amorphophallus, Yucca). Understand how animals become coated with pollen and transfer it.
Pollen/Nectar Robbers: Identify visitors that consume rewards without pollinating.
Outbreeding Devices (to discourage self-pollination):Asynchrony: Pollen release and stigma receptivity not synchronized.
Anther/Stigma Position: Placed at different positions.
Self-incompatibility: Genetic mechanism preventing self-pollen germination/tube growth.
Unisexual Flowers:Monoecious Plants: Male and female flowers on same plant (prevents autogamy, not geitonogamy).
Dioecious Plants: Male and female flowers on different plants (prevents both autogamy and geitonogamy).

D. Pollen-Pistil Interaction

Recognition and Dialogue: Understand the pistil's ability to recognize compatible pollen through chemical interaction.
Post-pollination Events:Pollen Germination: Production of pollen tube through germ pore on the stigma.
Pollen Tube Growth: Through stigma and style, reaching the ovary.
Male Gamete Formation: Generative cell divides (if 2-celled stage) or male gametes already present (if 3-celled stage).
Entry into Ovule: Through micropyle, into one of the synergids (guided by filiform apparatus).
Significance: Understand this dynamic process for plant breeders to manipulate for desired hybrids.
Artificial Hybridisation:Emasculation: Removal of anthers from bisexual flowers before dehiscence.
Bagging: Covering emasculated or female unisexual flowers to prevent contamination.
Procedure: Pollination with desired pollen, rebagging, and fruit development.

E. Double Fertilisation

Process:One male gamete fuses with the egg cell nucleus (syngamy) to form a diploid zygote.
The other male gamete fuses with the two polar nuclei in the central cell (triple fusion) to form a triploid primary endosperm nucleus (PEN).
Uniqueness: Recognize this phenomenon as unique to flowering plants.
Products: Diploid zygote and triploid primary endosperm cell (PEC).

F. Post-fertilisation: Structures and Events

Collective Term: Understand post-fertilisation events include endosperm/embryo development, ovule maturation into seed, and ovary maturation into fruit.
Endosperm:Development: Precedes embryo development (provides nutrition).
Tissue Formation: PEN repeatedly divides to form triploid endosperm tissue, filled with food reserves.
Free-nuclear Endosperm: Initial stage (e.g., coconut water).
Cellular Endosperm: Follows cell wall formation (e.g., coconut kernel).
Consumption: May be completely consumed by embryo (non-albuminous seeds like pea, groundnut) or persist (albuminous seeds like castor, coconut, wheat, rice, maize).
Embryo:Development: Zygote develops at micropylar end, usually after some endosperm formation.
Embryogeny: Similar early stages in monocots and dicots. Sequence: zygote -> proembryo -> globular -> heart-shaped -> mature embryo.
Dicotyledonous Embryo:Components: Embryonal axis and two cotyledons.
Epicotyl: Above cotyledons, terminates in plumule (stem tip).
Hypocotyl: Below cotyledons, terminates in radicle (root tip).
Root Cap: Covers root tip.
Monocotyledonous Embryo:Single cotyledon: Scutellum (in grasses, lateral).
Coleorrhiza: Undifferentiated sheath enclosing radicle and root cap.
Epicotyl: Above scutellum, contains shoot apex and leaf primordia in coleoptile (hollow foliar structure).
Seed:Definition: Fertilised ovule, final product of sexual reproduction.
Components: Seed coat(s), cotyledon(s), embryo axis.
Types:Non-albuminous/Ex-albuminous: No residual endosperm (consumed by embryo).
Albuminous: Retains part of endosperm.
Perisperm: Remnants of nucellus in some seeds (e.g., black pepper, beet).
Seed Coat: Formed from integuments, tough and protective. Micropyle remains as pore for water/oxygen entry.
Dormancy: Reduced water content, slowed metabolic activity, state of inactivity until favourable conditions for germination.
Viability: Highly variable (few months to hundreds/thousands of years, e.g., Lupinus arcticus, Phoenix dactylifera).
Advantages of Seeds: Independence from water for reproduction, better dispersal strategies, food reserves for seedling, protection by seed coat, genetic variation from sexual reproduction.
Importance in Agriculture: Dehydration and dormancy crucial for storage and year-round food supply.
Fruit:Development: Ovary develops into fruit simultaneously with ovule maturation into seeds.
Pericarp: Fruit wall developed from ovary wall.
Types: Fleshy (guava, orange, mango) or dry (groundnut, mustard).
True Fruits: Develop only from the ovary.
False Fruits: Other floral parts (e.g., thalamus in apple, strawberry, cashew) contribute to fruit formation.
Parthenocarpic Fruits: Develop without fertilisation (e.g., banana); seedless; can be induced by growth hormones.

IV. Apomixis and Polyembryony

Apomixis:Definition: Special mechanism in some flowering plants (e.g., Asteraceae, grasses) to produce seeds without fertilisation.
Nature: A form of asexual reproduction that mimics sexual reproduction.
Mechanisms: Diploid egg cell formed without reduction division develops into embryo without fertilisation; or nucellar cells surrounding embryo sac protrude and develop into embryos (e.g., Citrus, Mango).
Genetic Nature: Apomictic embryos are genetically identical to the parent plant (clones).
Importance in Hybrid Seed Industry: Overcomes the problem of character segregation in hybrid progeny, allowing farmers to reuse seeds year after year, reducing costs.
Polyembryony:Definition: Occurrence of more than one embryo in a seed.
Example: Common in Citrus and Mango varieties due to apomixis.

Quiz

Instructions: Answer each question in 2-3 sentences.

What is the primary biological advantage of sexual reproduction over asexual reproduction for a species?
Name the four wall layers of a microsporangium and briefly state the function of the innermost layer.
Describe the composition and significance of the exine layer of a pollen grain.
What is the filiform apparatus, and what is its role in the process of fertilisation?
Explain the difference between chasmogamous and cleistogamous flowers. Which type ensures seed-set even without pollinators?
How does geitonogamy differ from autogamy in terms of the pollinating agent and genetic outcome?
List two distinct adaptations found in wind-pollinated flowers.
Define self-incompatibility. How does it prevent self-pollination from leading to seed formation?
Briefly explain the two fusion events that constitute 'double fertilisation' in angiosperms.
Differentiate between a true fruit and a false fruit, providing an example of each.

Answer Key

The primary advantage of sexual reproduction is the creation of new genetic variants. This enhanced variation provides a survival advantage, allowing species to adapt better to changing environments and increasing their resilience.
The four wall layers of a microsporangium are the epidermis, endothecium, middle layers, and tapetum. The innermost layer, the tapetum, is crucial for nourishing the developing pollen grains, possessing dense cytoplasm and often multiple nuclei.
The exine is the hard outer layer of a pollen grain, made up of sporopollenin, which is one of the most resistant organic materials known. Its resistance to high temperatures, acids, and alkalis allows pollen grains to be well-preserved as fossils and protects the male gametophyte.
The filiform apparatus consists of special cellular thickenings at the micropylar tip of the synergids within the embryo sac. Its crucial role is to guide the growing pollen tube into one of the synergids, facilitating the discharge of male gametes for fertilisation.
Chasmogamous flowers are normal flowers that open and expose their anthers and stigma, while cleistogamous flowers do not open at all, keeping anthers and stigma close. Cleistogamous flowers invariably ensure assured seed-set because self-pollination occurs within the closed flower, making them independent of pollinators.
Geitonogamy is the transfer of pollen from the anther of one flower to the stigma of another flower on the same plant, functionally involving a pollinating agent. Genetically, however, it is similar to autogamy because the pollen grains originate from the same parent plant, thus not introducing new genetic combinations.
Wind-pollinated flowers often possess light and non-sticky pollen grains, enabling easy transport by wind currents. Additionally, they typically have well-exposed stamens for efficient pollen dispersal and large, often feathery stigmas to effectively trap airborne pollen.
Self-incompatibility is a genetic mechanism that prevents self-pollen (from the same flower or plant) from fertilising the ovules. It does so by inhibiting pollen germination on the stigma or preventing the growth of the pollen tube in the style, thus promoting cross-pollination.
Double fertilisation involves two fusion events: syngamy, where one male gamete fuses with the egg cell to form a diploid zygote, and triple fusion, where the other male gamete fuses with the two polar nuclei in the central cell to form a triploid primary endosperm nucleus.
A true fruit develops exclusively from the ovary of a flower, such as a mango or a pea pod. In contrast, a false fruit is formed when other floral parts, like the thalamus, also contribute to the fruit's formation, with examples including apples and strawberries.

Essay Questions

Discuss the evolutionary significance of the various adaptations seen in flowering plants for different agents of pollination (wind, water, and animals). Consider both the floral characteristics and the potential costs/benefits to the plant.
Describe the entire process of male gametophyte development (microsporogenesis and pollen grain maturation) and female gametophyte development (megasporogenesis and embryo sac formation) in angiosperms. Explain how these processes contribute to the 7-celled, 8-nucleate structure of the mature embryo sac.
Explain the concept of pollen-pistil interaction, detailing the "dialogue" between pollen and pistil. How is this interaction crucial for successful fertilisation, and what practical applications does this understanding have in plant breeding?
Trace the post-fertilisation events in an angiosperm, starting from the products of double fertilisation. Describe the development of the endosperm, embryo, seed, and fruit, highlighting the interconnectedness of these processes.
Analyse the biological implications of apomixis and polyembryony. How do these phenomena differ from typical sexual reproduction, and what are their significant advantages, particularly in agricultural practices?