Chapter 6: Sexual Reproduction in Flowering Plants

Learning Objectives

By the end of this chapter, you should be able to:

• Describe the structure and functions of different flower parts
• Explain the development of pollen grains and embryo sac
• Understand pollination mechanisms and types
• Analyze double fertilization and its significance
• Describe seed and fruit development processes
• Evaluate the importance of seeds for plant survival and dispersal

Overview

Sexual reproduction in flowering plants is a sophisticated process that ensures genetic diversity and species continuity. From the intricate structure of flowers to the complex mechanisms of pollination and fertilization, each step is finely adapted to maximize reproductive success. This chapter explores the complete journey from flower development to seed dispersal, highlighting the remarkable adaptations that have made flowering plants the dominant plant group on Earth.

Key Reproductive Statistics:

• Pollen Production: Up to 5 million grains per flower in some species
• Pollination Success: Typically 1-10% of pollen grains reach stigmas
• Double Fertilization: Unique to flowering plants, ensures efficient resource use
• Seed Viability: Can range from days to centuries depending on species

Flower Structure and Function

Basic Flower Anatomy

Flower Parts and Their Functions:

Pollination Efficiency Equation:

$$E = \frac{N_p \times P_s \times P_f}{N_t} \times 100\%$$

Where:

• $E$ = pollination efficiency
• $N_p$ = number of pollen grains produced
• $P_s$ = probability of pollen reaching stigma
• $P_f$ = probability of fertilization
• $N_t$ = number of seeds targeted

Complete vs. Incomplete Flowers

Complete Flowers:

• Contain all four whorls: sepals, petals, stamens, and carpels
• Example: Rose, hibiscus, lily

Incomplete Flowers:

• Lack one or more whorls
• Example: Grasses (lack petals and often sepals)

Perfect vs. Imperfect Flowers:

Pollen and Embryo Sac Development

Microsporogenesis (Pollen Development)

Process of Pollen Grain Formation:

1. Microsporocyte Development: Diploid cells in anther undergo meiosis
2. Microspore Formation: Meiosis produces four haploid microspores
3. Pollen Grain Development: Microspores develop into mature pollen grains
4. Maturity: Pollen grains contain generative cell and tube cell

Pollen Grain Structure:

Megasporogenesis (Embryo Sac Development)

Process of Embryo Sac Formation:

1. Megaspore Mother Cell: Diploid cell in ovule undergoes meiosis
2. Functional Megaspore: Four cells produced, only one survives
3. Mitotic Divisions: Functional megaspore undergoes three mitotic divisions
4. Mature Embryo Sac: Seven cells, eight nuclei typical structure

Embryo Sac Structure:

Pollination Mechanisms

Types of Pollination

Self-Pollination:

• Definition: Transfer of pollen from anther to stigma of same flower
• Advantages: Guaranteed fertilization, energy efficient, preserves parental traits
• Disadvantages: Reduced genetic diversity, inbreeding depression
• Examples: Peas, beans, wheat

Cross-Pollination:

• Definition: Transfer of pollen between different flowers on different plants
• Advantages: Increased genetic diversity, adaptation to environment
• Disadvantages: Energy intensive, depends on pollinators
• Examples: Most flowering plants, fruit trees

Pollination Agents

Wind Pollination (Anemophily):

Insect Pollination (Entomophily):

Bird Pollination (Ornithophily):

Animal Pollination (Other Agents):

Fertilization Process

Double Fertilization

Unique Process in Angiosperms:

Step-by-Step Process:

1. Pollen Germination: Pollen grain lands on stigma and germinates
2. Pollen Tube Growth: Tube cell grows down style toward ovule
3. Entry into Ovule: Pollen tube enters ovule through micropyle
4. Sperm Cell Release: Generative cell divides to form two sperm cells
5. First Fertilization: One sperm fertilizes egg cell to form zygote
6. Second Fertilization: Other sperm fertilizes central cell to form endosperm
7. Seed Development: Zygote becomes embryo, endosperm provides nutrition

Genetic Equations:

$$\text{Egg Cell} + \text{Sperm Cell} \rightarrow \text{Zygote (2n)}$$ $$\text{Polar Nuclei (2n)} + \text{Sperm Cell} \rightarrow \text{Endosperm (3n)}$$

Significance of Double Fertilization:

• Efficient Resource Use: Endosperm develops only after fertilization
• Nutritive Tissue: Triploid endosperm provides balanced nutrition
• Genetic Diversity: Two separate fertilization events increase genetic variation

Resource Investment Equation:

$$\text{Resource Efficiency} = \frac{\text{Endosperm Mass}}{\text{Total Ovule Mass}} \times \text{Fertilization Success Rate}$$

Post-Fertilization Changes

Ovule to Seed Transformation:

Seed and Fruit Development

Seed Development

Embryo Development Stages:

1. Zygote Stage: First cell after fertilization
2. Cleavage: Rapid cell divisions forming proembryo
3. Organogenesis: Formation of cotyledons, plumule, radicle
4. Maturation: Accumulation of stored food, desiccation tolerance

Seed Structure:

Seed Viability Factors:

Fruit Development

From Ovary to Fruit:

True Fruits: Develop only from ovary

• Simple Fruits: Single ovary (peach, cherry)
• Aggregate Fruits: Multiple ovaries from one flower (strawberry, raspberry)
• Multiple Fruits: Multiple ovaries from multiple flowers (pineapple, fig)

False Fruits: Include other floral parts

• Accessory Fruits: Include receptacle, sepals, or other parts (apple, pear)

Fruit Types:

Seed Dispersal Mechanisms

Dispersal Adaptations

Wind Dispersal:

Water Dispersal:

Animal Dispersal:

Mechanical Dispersal:

Seed Dormancy

Types of Dormancy:

Physical Dormancy:

• Cause: Hard, impermeable seed coat
• Breaking: Scarification by abrasion, freezing, fire
• Examples: Legumes, some desert plants

Physiological Dormancy:

• Cause: Internal physiological factors
• Breaking: Stratification, after-ripening
• Examples: Many temperate trees, some flowers

Morphological Dormancy:

• Cause: Underdeveloped embryo
• Breaking: Extended development period
• Examples: Some tropical plants, orchids

Combined Dormancy:

• Cause: Both physical and physiological barriers
• Breaking: Multiple treatments required
• Examples: Many weed species

Laboratory Investigations

Pollen Grain Study

Microscopic Examination:

• Slide Preparation: Stain pollen grains with basic fuchsin or safranin
• Observation: Study shape, size, and surface features
• Measurement: Use calibrated eyepiece graticule for size determination

Experiment: Comparing Pollen from Different Species

• Objective: Observe pollen diversity and adaptations
• Method: Prepare slides from various flower types
• Analysis: Correlate pollen features with pollination methods

Seed Germination Experiment

Germination Testing:

• Materials: Seeds, petri dishes, filter paper, different conditions
• Variables: Temperature, light, moisture, soil type
• Measurements: Germination percentage, rate, seedling growth

Dormancy Breaking Techniques:

• Scarification: Mechanical or chemical seed coat damage
• Stratification: Cold treatment for physiological dormancy
• Soaking: Water imbibition to break physical dormancy

Pollination Simulation

Wind Pollination Simulation:

• Setup: Artificial flowers with sticky stigmas, fan for wind
• Pollen Source: Quantified pollen powder
• Collection: Count pollen grains captured by stigmas
• Analysis: Efficiency of wind pollination vs. distance

Insect Pollination Study:

• Setup: Flowers with nectar rewards, insect visitors
• Observation: Record visitor type, behavior, pollen transfer
• Measurement: Pollen load on visitors and stigmas

Practice Tips for SPM Students

Key Concepts to Master

1. Flower structure and identification of reproductive parts
2. Pollination mechanisms and their adaptations
3. Double fertilization process and its significance
4. Seed development stages and structure
5. Fruit formation and dispersal adaptations

Experimental Skills

1. Identify flower parts from dissected specimens and diagrams
2. Prepare pollen slides and observe under microscope
3. Test seed viability using germination experiments
4. Analyze dispersal mechanisms in local plant species

Problem-Solving Strategies

1. Pollination efficiency: Adaptation analysis for different pollinators
2. Seed dormancy: Breaking techniques for different dormancy types
3. Dispersal prediction: Relate fruit characteristics to dispersal methods
4. Fertilization timing: Understanding developmental sequences

Environmental and Health Connections

Agricultural Applications

• Crop breeding: Understanding pollination for controlled crosses
• Seed production: Optimizing conditions for high seed yield
• Pollinator management: Supporting beneficial insects for pollination
• Fruit quality: Factors affecting fruit development and ripening

Conservation Significance

• Pollinator decline: Effects on plant reproduction and ecosystem health
• Seed banking: Conservation of endangered plant species
• Habitat restoration: Using native plants for ecosystem recovery
• Invasive species: Dispersal mechanisms and control strategies

Economic Importance

• Agriculture: Pollination services for crop production
• Horticulture: Flower and seed production for ornamental plants
• Pharmaceuticals: Plant-based medicines and active compounds
• Food security: Seed diversity and crop improvement programs

Summary

• Sexual reproduction in flowering plants involves complex structural and functional adaptations
• Flower structure varies with pollination method and environmental conditions
• Double fertilization is unique to angiosperms and ensures efficient resource use
• Seed dormancy and dispersal mechanisms enhance survival and colonization
• Understanding plant reproduction is crucial for agriculture, conservation, and ecosystem management

Related Topics

• Chapter 1: Organisation of Plant Tissues and Growth
• Chapter 2: Leaf Structure and Function
• Chapter 3: Nutrition in Plants
• Chapter 5: Response in Plants