Class 11 Biology Notes Chapter 8 (Chapter 8) – Lab Manual (English) Book

Alright students, let's focus on Chapter 8 from your Class 11 Biology Lab Manual, which deals with the 'Study of Distribution of Stomata in the Upper and Lower Surfaces of Leaves'. This is an important practical topic that often forms the basis for conceptual questions in various government exams.

Chapter 8: Study of Distribution of Stomata in the Upper and Lower Surfaces of Leaves

1. Introduction to Stomata

• Definition: Stomata (singular: stoma) are tiny pores or openings found primarily on the epidermis (outer layer) of leaves, stems, and other plant organs.
• Structure: Each stoma is surrounded by two specialized cells called guard cells. These cells regulate the opening and closing of the stomatal pore (also called stomatal aperture). The guard cells may themselves be surrounded by subsidiary cells (or accessory cells), which differ morphologically from other epidermal cells. The stomatal pore, guard cells, and surrounding subsidiary cells together constitute the stomatal apparatus.
  • Guard Cell Shape: In dicotyledonous plants, guard cells are typically kidney-bean shaped. In monocotyledonous plants (like grasses), they are usually dumbbell-shaped.
• Function: Stomata play a crucial role in:
  • Gaseous Exchange: Facilitating the uptake of Carbon Dioxide (CO2) required for photosynthesis and the release of Oxygen (O2) produced during photosynthesis.
  • Transpiration: The release of water vapour from the plant surface into the atmosphere. This process helps in cooling the plant and driving the ascent of sap.

2. Objective of the Experiment

To study and compare the number and distribution pattern of stomata on the upper (adaxial) and lower (abaxial) surfaces of different types of leaves (typically dicot and monocot).

3. Principle

The epidermis of a leaf can be peeled off as a thin layer. Staining this peel with a suitable stain (like safranin) makes the cell walls and nuclei prominent, allowing the stomata and surrounding epidermal cells to be clearly observed under a microscope. By observing peels from both surfaces and counting stomata within a defined field of view, their relative density can be compared.

4. Key Steps & Materials (Conceptual Understanding)

• Materials: Fresh leaves (e.g., Petunia/Sunflower for dicot, Grass/Maize for monocot), microscope, glass slides, coverslips, forceps, needle, brush, watch glasses, water, safranin stain (stains cell walls/nuclei red), glycerine (mounting medium to prevent drying).
• Procedure Outline:
  1. Obtain epidermal peels from both the upper (adaxial) and lower (abaxial) surfaces of the leaf. This requires carefully tearing or scraping the leaf to get a thin, transparent layer.
  2. Stain the peel with safranin for a short duration.
  3. Wash off excess stain.
  4. Mount the peel on a slide in a drop of glycerine and cover with a coverslip, avoiding air bubbles.
  5. Observe under low power and then high power of the microscope.
  6. Count the number of stomata visible in several different fields of view for both upper and lower epidermal peels.

5. Observations and Stomatal Distribution Patterns

• Dicot Leaf (e.g., Sunflower, Mango, Petunia - typically Dorsiventral):
  • Stomata are generally more numerous on the lower (abaxial) epidermis than on the upper (adaxial) epidermis. Sometimes, the upper epidermis may have very few or no stomata.
  • Reason: This reduces water loss due to direct sunlight exposure on the upper surface while still allowing sufficient gas exchange.
  • Guard cells are kidney-bean shaped.
  • Arrangement is usually random.
• Monocot Leaf (e.g., Grass, Maize, Wheat - typically Isobilateral):
  • Stomata are often distributed almost equally on both the upper (adaxial) and lower (abaxial) surfaces. This is characteristic of isobilateral leaves which receive sunlight more evenly on both sides.
  • Guard cells are dumbbell-shaped.
  • Arrangement is usually in parallel rows, aligned with the leaf veins.
• Variations based on Habitat:
  • Mesophytes: Plants growing in moderate conditions (like most common dicots and monocots discussed above).
  • Xerophytes: Plants adapted to dry conditions (e.g., Nerium, Opuntia). Stomata are often sunken in pits (stomatal crypts), usually more on the lower surface, and may be covered by hairs (trichomes). This is an adaptation to minimize water loss.
  • Hydrophytes (Aquatic Plants):
    • Floating leaves (e.g., Nymphaea - water lily): Stomata are present only on the upper (adaxial) epidermis which is exposed to air.
    • Submerged leaves (e.g., Hydrilla): Stomata are generally absent as gas exchange occurs directly through the thin cell walls with the surrounding water.

6. Inference

The distribution and density of stomata vary significantly between the upper and lower surfaces of leaves and among different plant groups. This distribution pattern is an adaptation related to the plant's habitat (water availability, light exposure) and physiology (balancing the need for CO2 uptake for photosynthesis against the risk of excessive water loss through transpiration). Dicot leaves are typically dorsiventral (different upper and lower surfaces), while monocot leaves are often isobilateral (similar surfaces).

7. Significance for Exams

• Understand the structure and function of stomata.
• Know the difference in guard cell shape between dicots and monocots.
• Remember the typical distribution patterns in dicots (more on lower) and monocots (equal on both).
• Relate stomatal distribution to habitat adaptations (xerophytes, hydrophytes).
• Know the terms adaxial (upper) and abaxial (lower).
• Understand the role of safranin (staining) and glycerine (mounting).

Multiple Choice Questions (MCQs)

1. The primary function of stomata in plant leaves is:
   a) Absorption of water
   b) Providing structural support
   c) Gaseous exchange and transpiration
   d) Storage of food

2. In typical dicotyledonous plants, the guard cells surrounding the stomatal pore are:
   a) Dumbbell-shaped
   b) Kidney-bean shaped
   c) Spherical
   d) Irregularly shaped

3. In most common dicot leaves (dorsiventral), stomata are usually:
   a) More numerous on the upper (adaxial) surface
   b) More numerous on the lower (abaxial) surface
   c) Equally distributed on both surfaces
   d) Completely absent

4. The characteristic shape of guard cells in monocotyledonous plants like grass is:
   a) Kidney-bean shaped
   b) Oval shaped
   c) Dumbbell-shaped
   d) Triangular shaped

5. In isobilateral leaves, typical of many monocots, stomata are generally found:
   a) Only on the upper surface
   b) Only on the lower surface
   c) In sunken pits
   d) Approximately equally on both surfaces

6. In the experiment to study stomatal distribution, Safranin is used primarily to:
   a) Prevent the peel from drying
   b) Make the stomata open wider
   c) Stain the cell walls and nuclei for better visibility
   d) Dissolve the epidermal cells

7. Glycerine is used while mounting the epidermal peel on the slide because it:
   a) Stains the stomata specifically
   b) Prevents the material from drying out
   c) Helps in separating the epidermal layer
   d) Magnifies the image

8. Plants adapted to dry environments (xerophytes) often have stomata that are:
   a) Larger in size and always open
   b) Present only on the upper surface
   c) Sunken in pits or grooves
   d) Absent from the leaves

9. The lower surface of a leaf is technically referred to as the:
   a) Adaxial surface
   b) Abaxial surface
   c) Ventral surface
   d) Proximal surface

10. In floating aquatic plants like water lily (Nymphaea), stomata are typically located:
    a) On both surfaces equally
    b) Only on the submerged lower surface
    c) Only on the exposed upper surface
    d) They lack stomata completely

Answer Key:

1. c) Gaseous exchange and transpiration
2. b) Kidney-bean shaped
3. b) More numerous on the lower (abaxial) surface
4. c) Dumbbell-shaped
5. d) Approximately equally on both surfaces
6. c) Stain the cell walls and nuclei for better visibility
7. b) Prevents the material from drying out
8. c) Sunken in pits or grooves
9. b) Abaxial surface
10. c) Only on the exposed upper surface

Study these notes carefully, focusing on the structure, function, distribution patterns, and adaptations related to stomata. Good luck with your preparation!