Theory & Procedure, Study of distribution of stomata | Additional Study Material for NEET PDF Download

Objective

Our objective is to study the stomatal distribution on the upper and lower leaf surfaces and to calculate the stomatal index.

Theory

What is a Stomata?

Stomata are minute pores found on the epidermis of leaves and young shoots of plants that are used to control exchange of gases. The pore is surrounded by a pair of specialised cells called the guard cells that are responsible in regulating the size of the opening.

Water is released through the stomata into the atmosphere in the form of water vapour through the process called transpiration. Besides this, the exchange of oxygen and carbon dioxide in the leaf also occurs through the stomata.

Distribution of Stomata

Distribution of stomata varies between monocots and dicots, between plant species, and between the underside and top side of the leaves on a plant. 

Stomata are found more on plant surfaces thriving under higher light, lower atmospheric carbon dioxide concentrations and in moist environments.

Usually the lower surface of a dicot leaf has a greater number of stomata while in a monocot leaf they are more or less equal on both surfaces. In most of the floating plants, stomata are found only on the upper epidermis.

Calculation of Stomatal Index

The distribution of stomata on the upper and lower surfaces of the leaf can be studied by removing the peels of the leaf from the upper and lower surfaces and observing the same under a microscope.

The count of the the number of stomata and epidermal cells in the microscopic field is taken  and the stomatal index of each surface of the leaf can be calculated using the following formula:

Learning Outcomes

• Students get to know what a Stomata is.
• Students understand the distribution of stomata on different plants.
• Students learn how to calculate the stomatal index.
• Students will be able to do the experiment more accurately in the real lab once they understand the steps through the animation and simulation.

Materials Required

Real Lab Procedure

• Pluck one fresh leaf of a four-o’clock plant.
• Take two watch glasses and pour some distilled water into the both watch glasses.
• Split the leaf from the four-o’clock plant obliquely.
• Take the peel from the upper surface of the leaf using the forceps.
• Place the peel into a watch glass containing water.
• Take another peel from the lower surface of the leaf using the forceps.
• Place the peel into the other watch glass containing water.
• Using a dropper, take few drops of Safranin solution and put it into the two watch glasses.
• Take two clean glass slides and place the leaf peel on the slides one by one, using a brush.
• Take a blade and cut a small rectangle or square piece from each peel.
• Take some glycerine using a dropper and put one drop of glycerine on both slides.
• Take a cover slip and place it gently on the peel with the help of a needle.
• Take the glass slide and place it under compound microscope.
• Observe under the microscope.
• Count the number of stomata in the peels of both upper and lower epidermis of the leaf appearing in the microscopic field.

Simulator Procedure (as performed through the Online Labs)

• You can select the type of view from the ‘Select view’ drop down list.  To view the leaf structure, select  the 'Binocular view' .
• You can select the slide containing the leaf from the ‘Select sample’ drop down list.
• The power of the lens can be changed by selecting the required lens power from the drop down list ‘Select objective lens’.
• To coarse focus on the tissue, you can move the tube up and down by clicking on the ‘Coarse adjustment’ right and left arrow.
• To achieve a fine and sharp focus, you can move the tube very slowly by clicking on the ‘Fine adjustment’ right and left arrow.
• You can move the slide to observe each part of the sample by clicking on the directional arrow of the ‘Slide adjustment’.
• Count the number of stomata and epidermal cells in the specimen and enter the values into the corresponding text boxes on the left pane.
• It will calculate the stomatal index and show into ‘Stomatal Index’ textbox.
• To repeat the experiment, click the ‘Reset’ button.

Observations

• The number of stomata is greater in the lower epidermis, and fewer are present in the upper epidermis of the leaf taken from a four-o’clock plant.

Precautions

• The curling of the peel should be avoided.
• Always use a brush to transfer the peel from watch glass to the slide