Study of Distribution of Stomata in the Upper and Lower Surface of Leaves

We are well aware of the indispensable role of plants in the carbon cycle in the biosphere. During respiration, plants take in carbon dioxide and release oxygen, hence are valuable in maintaining the oxygen-carbon balance of life. To do this, plants “inhale” carbon dioxide and “exhale” oxygen. This gaseous exchange occurs through microscopic pores present on the epidermis of the leaves. These pores are the stomata, the breathing apparatus of the plants. Carry out this simple experiment to understand the anatomy and role of stomata in detail.

Table of Contents

• Aim

• Apparatus required

• Theory

• Procedure

• Observations

• Result

Aim

To study the distribution of stomata on the upper and lower surfaces of leaves 

Apparatus Required

Leaves from 4 o'clock plant (Mirabilis jalapa), distilled water, safranin solution, glycerine, forceps, dropper, blade, glass slide, coverslip, compound microscope, needle and brush, compound microscope 

Theory

• Stomata are the breathing apparatus of the leaves of a plant. This implies that the stomata are the media for exchanging gases, namely, carbon dioxide and oxygen. 

• The stomata are present on the epidermis of a leaf among the epidermal cells. 

• The stomatal apparatus comprises the pore where the gaseous exchange occurs, guard cells and subsidiary cells. The guard cells regulate the opening and closing of the stomatal pore.

• The stomata are also involved in the exchange of water vapour during transpiration. 

• Stomatal distribution on the leaf surface is often used to identify monocots and dicots. 

• Generally, monocots have an equal number of stomata on the leaves' adaxial (dorsal) and abaxial (ventral) surface. Such leaves are also known as isobilateral leaves

• In the case of dicots, the stomata are predominantly present on the underside (ventral surface) of the leaves

• To calculate the stomatal index of a leaf, the following formula is used 

\[{\rm{stomatal index  =  }}\dfrac{{{\rm{no}}{\rm{. of stomatal cells }}}}{{{\rm{no}}{\rm{. of stomatal cells  +  no}}{\rm{. of epidermal cells }}}} \times 100\]

Procedure

1. Set two watch glasses with a small amount of distilled water in each of them

2. Take a freshly plucked leaf from the 4 o’clock plant

3. Cut the leaf obliquely, such that you can easily remove the peel of the upper side and the underside of the leaf

4. On one of the watch glasses, place the peel of the upper side of the leaf

5. On the other watch glass, place the peel of the underside of the leaf 

6. Next, add a few drops of safranin onto the peels in each of the watch glass

7. Now set two glass slides, one for each side of the leaf peels 

8. Transfer the leaf peel from each watch glass onto its respective glass slide. Use the brush for this step

9. Next, cut a small piece of the leaf peel on each glass slide. 

10.  Add one drop of glycerine onto the piece of leaf peel on each of the glass slide

11.  Cover the specimen on each of the glass slides with a cover slip. Use a needle to do this

12. Now, observe the glass slides using a compound microscope. Count the number of stomata on both sides of the leaf peels and note down your observation

Anatomy of the stomata on the surface of leaves

Observations and Calculations 

The peel of the underside of the 4 o’clock leaf bears a greater number of stomata as compared to the peel of the upper side of the leaf

1. Number of epidermal cells in the microscopic field _________

2. Number of stomatal cells in microscopic field ___________

3. Stomatal index

\[{\rm{stomatal index  =  }}\dfrac{{{\rm{no}}{\rm{. of stomatal cells }}}}{{{\rm{no}}{\rm{. of stomatal cells  +  no}}{\rm{. of epidermal cells }}}} \times 100\]

Results

The upper side of the leaf bears a lesser number of stomata than the underside of the leaf

Precautions

1. Be careful while transferring the leaf peels from the watch glasses onto the glass slides- the peels must not overlap with themselves 

2. Care must be taken while handling the razor blade

3. Do not keep the peel out in the open for long, as the section may dry

4. The coverslip must be placed gently with the help of a needle to avoid air bubbles

Lab Manual Questions

1. What is the stomatal index? 

Ans. The stomatal index is the ratio of the number of stomatal cells to the total number of stomata and the epidermal cells on the surface of a leaf

2. What regulates the opening and closing of the stomatal pore?

Ans. The guard cells are present on both sides of the stomatal pore. They regulate the opening and closing of the stomata by either becoming turgid by taking in water or becoming flaccid when water is lost by these cells, respectively.  

3. Why are stomata absent in the roots?

Ans. Roots are the underground part of a plant that is not exposed to sunlight. Moreover, root cells acquire oxygen sufficiently via diffusion and do not require stomata. 

4. What are isobilateral leaves? 

Ans: Isobilateral leaves are leaves wherein both the abaxial and adaxial surfaces are similar, i.e. mesophyll is not differentiated into palisade and spongy tissues. They are generally vertically oriented, and hence both surfaces are equally exposed to the sun.

Viva Questions

1. What do you know about the stomatal distribution of the leaves of aquatic plants?

Ans. The aquatic plants are often either fully or partially submerged or floating in the water. In the case of floating or partially submerged plants, the stomata are only present on the upper surface of the leaves, while in the case of fully submerged plants, stomata are absent altogether. 

2. What is the difference between the leaves of monocots and dicots?

Ans.  The leaves of monocots have parallel venation and are described as isobilateral, owing to the equal number of stomata on both leaf surfaces. On the other hand, the dicot leaf has reticulate venation and is termed dorsiventral. The upper surface of the dicot leaf has a lower number of stomata than the ventral surface. 

3. What do you understand by the term stomatal index?

Ans. The stomatal index is the ratio of the stomatal cells to the total number of epidermal and stomatal cells on the surface of a leaf. This ratio helps determine the stomatal distribution on the surface of the leaf. 

4. What is the difference between the leaves of an aquatic plant and a terrestrial plant?

Ans. The leaves of terrestrial plants have an abundance of stomata on their leaves. The stomata are found on both the upper and lower leaf surfaces. However, aquatic plants have fewer stomata, and the stomatal pores are situated on the upper surface of the leaf instead. 

5. What is the difference between the guard cells of the monocots and dicots?

Ans. The guard cells of the dicots are kidney-shaped, while those of the monocots have a dumb-bell-shaped appearance.

6. How do you think the stomatal opening and closing are regulated?

Ans. The stomatal opening and closing depending on the guard cells' turgidity. The stomatal pore opens when guard cells take in water and become turgid. However, when the guard cells lose water, they become flaccid, causing the pore to close. 

7. What are paracytic stomata?

Ans. Paracytic refers to those stomata in which the subsidiary cells are present and situated parallel to the long axis of the two guard cells, i.e., those stomata which possess a pair of lateral subsidiary cells. Anomocytic stomata lack subsidiary cells altogether. 

8. In the case of cactus, the leaves are reduced to spines. Where do the stomata occur on such plants?

Ans. In such plants where the leaves are reduced to spines, the stomata are found on the surface of the stems, which also perform photosynthesis.

9. Give functions of Stomata.

Ans: The main function of stomata is an exchange of carbon dioxide, oxygen and water vapour from the plant. 

10. How are stomata in isobilateral leaves?

Ans: Stomata on dorsal and ventral epidermises are distributed equally in an isobilateral leaf.

Practical Based Questions

1. In the case of monocots

1. More stomata are present on the ventral surface 

2. More stomata are present on the dorsal surface 

3. An equal number of stomata are present on both surfaces 

4. Stomata are present only on the dorsal surface

Ans. C) Equal number of stomata are present on both surfaces

2. Which of the following plants bear more stomata on their ventral surface?

1. Petunia 

2. Water lily

3. Grass

4. Orchid 

Ans. A) Petunia

3. In the case of aquatic plants, 

1. Stomata are present only on the upper leaf surface 

2. Stomata are present on the petals 

3. Stomata are absent; breathing occurs through general surface 

4. Stomata are present as seen in terrestrial plants 

Ans. A) Stomata are present only on the upper leaf surface

4. The stomatal pore opens when

1. Guard cells become flaccid

2. Guard cells become turgid

3. Guard cells plasmolyse

4. Subsidiary cells become turgid 

Ans. B) Guard cells become turgid

5. Sunken stomata are observed in

1. Aquatic plants

2. Desert plants 

3. Mesophytes 

4. Terrestrial plants 

Ans. B) Desert plants 

6. Dumb-bell shaped guard cells are found in the leaves of 

1. Maize

2. Petunia

3. 4 o’clock plant

4. Sunflower 

Ans. A) Maize 

7. The stomata occur in the

1. Epidermal tissue

2. Vascular tissue

3. Parenchyma

4. Sclerenchyma 

Ans. A)  Epidermal tissue 

8. The dye used to visualise the cells in this experiment is

1. Safranin

2. Methylene blue

3. Glycerine 

4. Phenolphthalein

Ans. A) Safranin 

9. The shape of the cells in an anomocytic stomata is ____which are similar to____

1. irregular, epidermal cells

2. epidermal cells, irregular

3. circular, palisade cells

4. narrow, subsidiary cells

Ans: irregular, epidermal cells

10. Which of the following does not have a stomata?

1. Mangifera indica

2. Magnolia champaca

3. Psidium guajava

4. Stylites andicola

Ans: Stylites andicola

Conclusion

From the above article, we understand that the epidermis of the leaves of the dicot-4 o’clock plant has a greater number of stomata on the ventral surface of the leaf than the dorsal surface. To prevent excessive water loss from stomata, this difference in the number of stomata between two surfaces is seen.