Class 11 Biology - Transport in Plants

Activity 11.1: Comparison of Stomatal Distribution on Upper and Lower Surfaces

Aim: To compare the distribution of stomata on the upper and lower surfaces of a leaf using Cobalt Chloride paper.

Materials Required:

Potted plant (e.g., Hibiscus or Nerium)
3% Cobalt Chloride solution ( $CoCl_2$ )
Filter paper strips (Whatman No. 1)
Petri dishes and forceps
Hot plate or Oven (set to 60°C)
Glass slides and clips
Desiccator (with anhydrous Calcium Chloride)
Stop clock

Procedure:

Preparation of $CoCl_2$ Paper: Soak filter paper strips in 3% $CoCl_2$ solution. Dry them in an oven at 60°C until they turn deep blue. Store them in a desiccator immediately.
Select a healthy, broad-leaved potted plant.
Remove two blue $CoCl_2$ paper strips from the desiccator using forceps.
Place one strip on the upper surface and one on the lower surface of the same leaf.
Immediately cover each strip with a glass slide and secure them with clips to make the setup airtight.
Start the stop clock.
Observe the time taken for each strip to turn from blue to pink.

Scientific Note Cobalt Chloride paper is a hygroscopic indicator. It turns from blue (anhydrous) to pink (hydrated) upon absorbing moisture transpired through stomata. In most dicots (dorsiventral leaves), stomatal density is significantly higher on the lower (abaxial) surface to minimize water loss from direct sunlight.

Observation: The $CoCl_2$ paper on the lower surface turns pink much faster (e.g., 2-5 mins) than the one on the upper surface (e.g., 15-30 mins).

Lab Best Practice Handle $CoCl_2$ paper only with dry forceps. Even slight atmospheric humidity or moisture from fingertips will turn the paper pink before the experiment begins.

Activity 11.2: Measurement of Transpiration Rate using Ganong’s Potometer

Aim: To measure the rate of transpiration in a leafy shoot under different environmental conditions.

Materials Required:

Ganong’s Potometer (capillary tube, reservoir, and graduated scale)
Freshly cut leafy shoot (e.g., Balsam or Coleus)
Beaker of water
Vaseline (Petroleum jelly)
Stop clock
Colored water (Eosin dye)

Procedure:

Fill the Ganong’s Potometer with water, ensuring no air bubbles are trapped in the system.
Cut the twig/shoot obliquely under water to prevent air embolism in the xylem vessels.
Fix the shoot into the potometer using a single-holed cork.
Apply Vaseline to all joints to make the apparatus completely airtight.
Dip the end of the capillary tube into a beaker containing colored water.
Lift the capillary tube for a second to introduce a small air bubble, then dip it back.
Record the distance moved by the air bubble on the graduated scale over a fixed time interval (e.g., 10 minutes).

Safety First Ensure the shoot is cut under water. If an air bubble enters the xylem (cavitation), it will block the water column, and the experiment will fail as transpiration pull cannot be transmitted.

Scientific Note Transpiration pull creates a negative pressure (tension) in the xylem, which is transmitted down to the potometer. The rate of bubble movement is a direct proxy for the rate of water absorption, which, in a steady state, is approximately equal to the rate of transpiration.

Observation: The air bubble moves towards the shoot. The rate of movement increases under high light intensity or wind (using a fan) and decreases in high humidity.

Lab Best Practice Ensure the reservoir tap is tightly closed during the experiment to prevent water from leaking and affecting the bubble's movement.

Class 11 Biology - Transport in Plants

Activity 11.1: Comparison of Stomatal Distribution on Upper and Lower Surfaces

Aim: To compare the distribution of stomata on the upper and lower surfaces of a leaf using Cobalt Chloride paper.

Materials Required:

Potted plant (e.g., Hibiscus or Nerium)
3% Cobalt Chloride solution ( $CoCl_2$ )
Filter paper strips (Whatman No. 1)
Petri dishes and forceps
Hot plate or Oven (set to 60°C)
Glass slides and clips
Desiccator (with anhydrous Calcium Chloride)
Stop clock

Procedure:

Preparation of $CoCl_2$ Paper: Soak filter paper strips in 3% $CoCl_2$ solution. Dry them in an oven at 60°C until they turn deep blue. Store them in a desiccator immediately.
Select a healthy, broad-leaved potted plant.
Remove two blue $CoCl_2$ paper strips from the desiccator using forceps.
Place one strip on the upper surface and one on the lower surface of the same leaf.
Immediately cover each strip with a glass slide and secure them with clips to make the setup airtight.
Start the stop clock.
Observe the time taken for each strip to turn from blue to pink.

Observation: The $CoCl_2$ paper on the lower surface turns pink much faster (e.g., 2-5 mins) than the one on the upper surface (e.g., 15-30 mins).

Lab Best Practice Handle $CoCl_2$ paper only with dry forceps. Even slight atmospheric humidity or moisture from fingertips will turn the paper pink before the experiment begins.

Activity 11.2: Measurement of Transpiration Rate using Ganong’s Potometer

Aim: To measure the rate of transpiration in a leafy shoot under different environmental conditions.

Materials Required:

Ganong’s Potometer (capillary tube, reservoir, and graduated scale)
Freshly cut leafy shoot (e.g., Balsam or Coleus)
Beaker of water
Vaseline (Petroleum jelly)
Stop clock
Colored water (Eosin dye)

Procedure:

Fill the Ganong’s Potometer with water, ensuring no air bubbles are trapped in the system.
Cut the twig/shoot obliquely under water to prevent air embolism in the xylem vessels.
Fix the shoot into the potometer using a single-holed cork.
Apply Vaseline to all joints to make the apparatus completely airtight.
Dip the end of the capillary tube into a beaker containing colored water.
Lift the capillary tube for a second to introduce a small air bubble, then dip it back.
Record the distance moved by the air bubble on the graduated scale over a fixed time interval (e.g., 10 minutes).

Observation: The air bubble moves towards the shoot. The rate of movement increases under high light intensity or wind (using a fan) and decreases in high humidity.

Lab Best Practice Ensure the reservoir tap is tightly closed during the experiment to prevent water from leaking and affecting the bubble's movement.

Class 11 Biology - Transport in Plants (Activities)

Class 11 Biology - Transport in Plants

Activity 11.1: Comparison of Stomatal Distribution on Upper and Lower Surfaces

Activity 11.2: Measurement of Transpiration Rate using Ganong’s Potometer

On this page

Class 11 Biology - Transport in Plants (Activities)

Class 11 Biology - Transport in Plants

Activity 11.1: Comparison of Stomatal Distribution on Upper and Lower Surfaces

Activity 11.2: Measurement of Transpiration Rate using Ganong’s Potometer

On this page