Study the distribution of stomata on the upper and lower surfaces of leaves

Introduction

Stomata are microscopic pores found on the epidermis of plant leaves that play a crucial role in gas exchange and transpiration. Understanding the distribution of stomata on upper and lower leaf surfaces is essential for comprehending plant physiology and adaptation mechanisms. This laboratory experiment provides hands-on experience in examining stomatal patterns across different leaf surfaces.

Aim of the Experiment

To study and compare the distribution of stomata on the upper and lower surfaces of selected leaves and understand their structural adaptations for optimal gas exchange.

Apparatus Required

• Fresh leaves from different plants (dicot and monocot)
• Compound microscope
• Permanent slides of leaf sections
• Cover slips
• Methylene blue solution (stain)
• Glycerine
• Filter paper
• Brush
• Microscope slides
• Dropper
• Razor blade or microtome (for section preparation)

Theory

What are Stomata?

Stomata (singular: stoma) are microscopic pores surrounded by two specialized cells called guard cells. These pores facilitate:

• Carbon dioxide intake for photosynthesis
• Oxygen release as a byproduct
• Water vapor loss during transpiration

Distribution Patterns

Dicotyledonous Plants: Typically show more stomata on the lower surface (hypostomatic) to reduce water loss while maintaining gas exchange efficiency.

Monocotyledonous Plants: Often exhibit uniform distribution on both surfaces or may be hypostomatic.

Why Study Distribution?

Understanding stomatal distribution helps explain:

• Plant adaptation to environmental conditions
• Water conservation strategies
• Photosynthetic efficiency
• Classification of plant types

Procedure

1. Sample Preparation:

   • Collect fresh leaves from different plants
   • Make thin sections using a razor blade
   • Prepare temporary mounts using glycerine and methylene blue stain

2. Microscopic Observation:

   • Place the slide under the compound microscope
   • Start with low power (10x) objective for initial observation
   • Switch to high power (40x) for detailed examination
   • Focus on both upper and lower epidermis separately

3. Counting Method:

   • Select 3-4 microscopic fields for each surface
   • Count the number of stomata in each field
   • Record observations systematically

4. Documentation:

   • Draw labeled diagrams of observed sections
   • Note the shape and arrangement of guard cells
   • Record environmental conditions during observation

Observation Table

Result

The experiment demonstrates that:

1. Dicot plants (Hibiscus, Mango) show significantly higher stomatal density on the lower surface
2. Monocot plants (Lily) exhibit more uniform distribution between upper and lower surfaces
3. Average stomatal density: Lower surface > Upper surface in most dicot plants
4. This distribution pattern represents an evolutionary adaptation for water conservation

Precautions

1. Handle microscope slides carefully to avoid breakage
2. Use fresh plant material for accurate observations
3. Apply appropriate staining to enhance visibility of stomata
4. Clean microscope lenses before and after use
5. Focus slowly to prevent damage to the slide or objective lens
6. Maintain consistent magnification for all counts
7. Use proper lighting for clear microscopic visualization
8. Handle razor blades with care during section preparation

Viva Questions and Answers

Q1: Why are stomata mainly present on the lower surface of dicot leaves?

A: To reduce water loss through transpiration while maintaining gas exchange efficiency. The lower surface receives less direct sunlight, minimizing water loss.

Q2: What is the function of guard cells?

A: Guard cells regulate the opening and closing of stomatal pores, controlling gas exchange and water loss.

Q3: Why do monocots show uniform stomatal distribution?

A: Many monocots have vertical leaf orientation or specialized anatomy that allows efficient gas exchange from both surfaces.

Q4: How does methylene blue help in this experiment?

A: Methylene blue stains the cell walls and nuclei of guard cells, making them more visible under the microscope.

Q5: What adaptation prevents excessive water loss in desert plants?

A: Desert plants often have sunken stomata or fewer stomata on upper surfaces to minimize water loss.

Q6: Why count multiple fields for accurate results?

A: Multiple counts provide statistical reliability and account for natural variations across the leaf surface.

Q7: What is the significance of stomatal index?

A: Stomatal index helps in plant identification and understanding adaptation mechanisms to environmental conditions.

Q8: How do aquatic plants differ in stomatal distribution?

A: Aquatic plants may have stomata only on upper surfaces or may lack stomata entirely, depending on their habitat.

Conclusion

This experiment successfully demonstrates the varied distribution of stomata between upper and lower leaf surfaces, highlighting evolutionary adaptations for optimal gas exchange and water conservation. The procedure provides valuable insights into plant physiology and morphological adaptations, making it a fundamental experiment in plant biology studies.

Understanding stomatal distribution is not just academically important but also has practical applications in agriculture, ecology, and environmental science. The hands-on experience gained through this experiment enhances comprehension of plant-water relationships and photosynthetic processes.

Related Topics: Photosynthesis process, transpiration in plants, plant anatomy and morphology, microscopic techniques in biology, environmental adaptation in plants