Diagram and Types of Stomata, Practice Problems and FAQs

All the aerobic living organisms need oxygen to survive. What happens if we don't get enough oxygen? We tend to suffocate, right? Ever been in a situation where the air around you is heavily polluted with dust and smoke? What is the first thing that we tend to do involuntarily in such situations? We tend to cover our mouth and nostrils because these are the passages for the entry and exit of air in our body. Once the oxygen rich air reaches through the nose to our lungs, the alveoli in the lungs help in exchanges of the gases between the blood and the air in the alveoli. Oxygen needed for respiration diffuses into the blood and carbon dioxide released by cells due to respiration diffuses from the blood into the alveolar air so that it can be exhaled.

But, do you think the plants also have such an elaborate system for exchange of gases with the atmosphere?

Plants have a more simple mechanism for this. Just like the nostrils in animals, plants use openings on their body such as lenticels and stomata for the exchange of gases directly between the body part and the atmosphere. While lenticels are found on the woody stems of the plants, stomata are more numerous and found on all the leaves and young green parts of the plant.

Our focus of discussion in this article will be on the stomata. Did you know that the stomatal pores need to be regulated to control the exchange of the gases and also to control the loss of water vapour from the plant body? So there should be some structures supporting the functioning of the stomata. How is the stomata fixed on the surface of the plants? Is the distribution, shape and arrangement of stomata the same in every plant? To understand more about this we should know more about the structure of the stomata. So let us get on with that.

Table of contents:

• Stomata
• Diagram of stomata
• Types of stomata
• Practice Problems
• FAQs

Stomata

The epidermal tissue system is the outermost layer of a plant body, which is derived from the protoderm. Stomata is a part of the epidermal tissue system, along with the other components like epidermis and the epidermal appendages.

The tiny pores or interruptions in the epidermis of leaves and young shoots are known as stomata. If we look through the microscope we can see the stomata as tiny pores surrounded by two bean shaped cells placed close together and surrounded by other large cells. The number of stomata in a square millimetre varies in different plants. The major functions of the stomata are the transpiration and exchange of gases.

                                                       Fig: Stomata

Diagram of stomata

Stomata interrupts the continuity of the epidermis. Stomata has three major parts and together they form the stomatal apparatus. So the stomatal apparatus is composed of the following parts:

• Guard cells
• Stomatal aperture
• Subsidiary cells

                                           Fig: Stomatal apparatus

Guard cells

The two specialised cells limiting the intercellular spaces of stomata are called guard cells. The shape of guard cells varies in dicot and monocot plants. The bean shaped guard cells can be observed in the dicot plants and in the monocot plants the shape of guard cells are dumb-bell shaped. This difference is used for the identification and classification of dicot and monocot plants.

                                      Fig: Guard cells in dicots and monocots

In the bean shaped guard cells of dicot plants, the outer and inner walls of the guard cells are different in their thickness. The outer wall is thin and flexible and the inner wall is thick and rigid. The dumb-bell shaped guard cells have elongated cells with expanded ends with thin walls and a middle portion which has narrow lumen with thick walls.

Chloroplasts are present in the guard cells and they can perform photosynthesis. Apart from the chloroplast, other cell organelles like mitochondria, endoplasmic reticulum, Golgi bodies and vacuoles can be seen in a guard cell.

                                   Fig: Thick inner wall and thin outer wall of guard cells

Working of guard cells

The major function of guard cells is to open and close the stomata. Osmosis is the one process which helps the guard cells to function. Endosmosis helps in the opening of stomata and exosmosis helps in the closure.

The most accepted hypothesis that explains the mechanism of opening and closing of stomata is the K⁺ ion pump hypothesis. According to this hypothesis, the blue light during daytime is received by a phototropin receptor which passes the signal on to a protein phosphatase I. The latter activates the H⁺-ATPase pumps in the guard cell membrane. As the pumps transport H⁺ ions out of the guard cells, the pH within the guard cells increases and opens up the voltage gated K⁺ channels. K⁺ ions enter the guard cells through these channels. Some Cl^- ions may be internalised to balance the positive charge of the K⁺ ions.

Simultaneously, starch contained in the guard cell is hydrolysed due to increase in pH and organic acids such as phosphoenol pyruvate (PEP) are formed. PEP combines with CO₂ taken in during the day to form malic acid. The malic acid dissociates into H + ions, that are pumped out, and mallate ions, which along with chloride ions help to maintain the balance of positive and negative charges within the guard cell. The accumulation of K⁺ ions, Cl^- ions and malate ions increases the osmotic potential of the guard cells and hence water from the surrounding cells enters the guard cells by endosmosis.

As a result the guard cells become turgid and expand. In the bean shaped guard cells, the rigid inner wall and flexible outer wall the guard cells bulges outwards and this helps in the opening of the stomatal aperture. When turgor pressure increases in the dumb bell shaped guard cells, the expanded ends will swell and push apart the middle elongated portions of the cells. As a result the size of the aperture between the guard cells increases.

                                     Fig: Mechanism of opening of stomatal pore

At evening, the phototropin receptor is desensitised, which causes inactivation of the H⁺ ATPase pumps. Thus, H⁺ ions are not pumped out and pH of the guard cells decreases. Anion channels open to pass out Cl^- ions and the K⁺ efflux channels open up. Due to decrease in pH, malic acid is slowly converted back to starch. Loss of ions reduces the solute concentration of the guard cells. When the guard cells have less solute concentration, the water from the guard cells moves out by exosmosis. This will result in the collapse of guard cells and it becomes flaccid. As a result stomatal aperture closes.

            GIF: Closing and opening of stomata

Stomatal aperture

The pore present between the guard cells is called stomatal aperture. The exchange of gases takes place through the stomatal aperture. The guard cells together with the opening (stomatal aperture) are called stoma.

                                                          Fig: Stomatal aperture

Subsidiary cells

The specialised cells that are bordering the guard cells are called subsidiary cells. These cells differ from epidermal cells in their morphology. Subsidiary cells usually develop from the protodermal cells (outermost cells of apical meristem) that are seen adjacent to the guard mother cell. The self-renewing cells called meristemoid cells differentiate to form the guard mother cells which further divide and differentiate to form the guard cells. Subsidiary cells help the guard cells to function either by providing a mechanical support that helps in the turgor movements, and/or by serving as a storehouse for water and ions. In some xerophytic plants like cacti, the subsidiary cells lie above the guard cells and such stomata are called sunken stomata. This stomata helps the xerophytic plants from excess water loss through transpiration.

                                            Fig: Subsidiary cells

Types of stomata

There are different types of stomata and we classify them on the basis of ontogeny and based on the subsidiary cells.

Based on the ontogeny

Based on the ontogenic relationship between the guard cells and subsidiary cells, there are three types of stomata which are as follows:

• Mesogenous stomata
• Perigenous stomata
• Mesoperigenous stomata

Mesogenous stomata

If the subsidiary cells and guard cells share a common origin then the stomata are called mesogenous stomata.

Perigenous stomata

If the subsidiary cells develop from the protodermal cells adjacent to the stomatal mother cell, then the stomata are called perigenous stomata.

Mesoperigenous stomata

If the cells surrounding the stoma have a dual origin, then it is called a mesoperigenous stoma. Here one or more subsidiary cells share a common origin with guard cells and others have independent origin.

Based on the subsidiary cells

In 1950 Metcalfe and Chalk classified the stomata into different types on the basis of the type of subsidiary cells. Types of stomata seen in both dicot and monocot differ.

Types of stomata in dicots

There are six different type of stomata seen in dicot plants and they are as follows:

• Actinocytic stomata
• Anomocytic stomata
• Anisocytic stomata
• Paracytic stomata
• Hemi paracytic stomata
• Diacytic stomata

Actinocytic stomata

If the guard cells are surrounded by at least five radiating cells forming a star-like pattern, then it is called an actinocytic stomata. This type of stomata can be seen in the family Ebenaceae.

                               Fig: Actinocytic stomata

Anomocytic stomata

If the subsidiary cells are indistinguishable from other epidermal cells, then it is called anomocytic stomata. It is also called irregular celled stomata or ranunculaceous type stomata. This type of stomata are seen in plants such as Tridax, Ranunculus, Citrullus etc.

                       Fig: Anomocytic stomata

Anisocytic stomata

If there are three subsidiary cells and among these, one is small and the other two are relatively larger, then it is called anisocytic stomata. It is also called unequal-celled stomata or cruciferous type stomata. This type of stomata are seen in plants such as Sedum, Brassica, Solanum etc.

                        Fig: Anisocytic stomata

Paracytic stomata

If the stoma is flanked on either side by one or more subsidiary cells which lie parallel to the long axis of the pore and guard cells, then it is called paracytic stomata. It is also called parallel-celled stomata or rubiaceous type stomata. This type of stomata are seen in plants such as Magnolia, Ixora, Mimosa etc.

                           Fig: Paracytic stomata

Hemi paracytic stomata

If there is only one subsidiary cell whose length is parallel to the stomatal opening and differs from the surrounding epidermal cells, then it is called hemi paracytic stomata. Such stomata are found in the families Molluginaceae and Aizoaceae.

                     Fig: Hemi paracytic stomata

Diacytic stomata

If the stoma is enclosed by a pair of subsidiary cells whose common wall is at right angles to the long axis of the guard cells, then it is called diacytic stomata. It is also called cross-celled stomata or caryophyllaceous type stomata. This type of stomata are seen in plants such as Dianthus, Arenaria, Caryophyllum etc.

                       Fig: Diacytic stomata

Types of stomata in monocots

The three major types of stomata seen in monocot plants are as follows:

• Gramineous or graminoid stomata
• Hexacytic stomata
• Tetracytic stomata

Gramineous or graminoid stomata

If the guard cells of the stomata are surrounded by two lens shaped subsidiary cells, then it is called gramineous stomata. Here the guard cells are thicker and narrower in the middle and bulbous at the ends. The stomatal opening and the axis of the subsidiary cells are parallel to each other. Examples are the plants included in the families such as Poaceae and Cyperaceae.

                            Fig: Gramineous stomata

Hexacytic stomata

If the stomata has six subsidiary cells around the guard cells then it is called a hexacytic stomata. Among the six subsidiary cells, one is at either end of the opening of stomata, one is adjoining each guard cell and one between the last subsidiary cell and the standard epidermis cells.

                         Fig: Hexacytic stomata

Tetracytic stomata

If the stomata has four subsidiary cells around the guard cells then it is called a tetracytic stomata. Among the four subsidiary cells, the one is on either end of the opening and one next to each guard cell. This can also be found in some dicots.

                          Fig: Tetracytic stomata

Distribution of stomata

The stomatal distribution is different in different types of plants. The main modes of stomatal distribution are as follows:

• Hypostomatous distribution
• Epistomatous distribution
• Amphistomatous distribution
• Astomatous condition

Hypostomatous distribution

If the stomata is present only on the lower surface of the leaf, then the distribution is called hypostomatous distribution. Examples are: Oxalis, apple, mulberry.

                          Fig: Plants with hypostomatous distribution of stomata

Epistomatous distribution

If the stomata is present only on the upper surface of the leaf, then the distribution is called epistomatous distribution. Examples are: water lily.

                                                         Fig: Water lily

Amphistomatous distribution

If the stomata is present on both the surfaces of the leaf, then it is called amphistomatous distribution. In monocot plants like maize and oats, the number of stomata on the upper and lower surfaces are equal. Such distribution is known as isostomatous distribution.

                                                   Fig: Maize and Oats

In the floating plants, the number of stomata is more on the upper leaf surface than the lower surface.

If we observe the leaves of cabbage, potato, beans etc. the number of stomata is more on the lower leaf surface than on the upper surface. This is known as heterostomatous distribution and is the case with most dicot leaves.

                                               Fig: Cabbage, potato and beans

Astomatous condition

The condition in which the stomata is vestigial is called astomatous condition. An example is Potamogeton.

                                      Fig: Potamogeton

Significance of stomata

Following are the significance of the stomata:

• Plays a vital role in the exchange of gases with the atmosphere. Facilitates the uptake of CO₂ and release of O₂ during photosynthesis in the day.
• Controls the process of transpiration and helps in the removal of excess water in the form of water vapour.
• Maintains the plant stress free, which will help to increase the productivity of the plant.
• Maintains the moisture in the plants according to the weather by closing and opening.

Practice Problems

1. Which of the following parts of the stomata is described below?

1. The two specialised cells limiting the stomatal pores
2. It has outer thin and flexible wall and inner thick and rigid wall
3. Chloroplast, mitochondria, endoplasmic reticulum, Golgi bodies are present

1. Stomatal aperture
2. Guard cells
3. Subsidiary cell
4. Stoma

Solution: The tiny pores or interruptions in the epidermis of leaves and young shoots are known as stomata.. Stomata has three major parts and they together are called stomatal apparatus. So the stomatal apparatus is composed of the parts like guard cells, stomatal aperture and subsidiary cells. The two specialised cells limiting the stomatal pore or aperture are called guard cells. The outer and inner walls of the guard cells are different in their size. The outer wall is thin and flexible and the inner wall is thick and rigid. Chloroplasts are present in the guard cells and they can perform the photosynthesis. Apart from the chloroplast, other cell organelles like mitochondria, endoplasmic reticulum, Golgi bodies and vacuoles can be seen in a guard cell. The pore present between the guard cells is called stomatal aperture. The exchange of gases takes place through the stomatal aperture. The guard cells together with the opening (stomatal aperture) are called stoma. The specialised cells that border the guard cells are called subsidiary cells. Hence the correct option is b.

2. Match the following:

1. A - ii, B - iii, C - i, D - iv
2. A - i, B - ii, C - iii, D - iv
3. A - i, B - iii, C - iv, D - ii
4. A - iii, B - i, C - ii, D - iv

Solution: In 1950 Metcalfe and Chalk classified the stomata into four types on the basis of the type of subsidiary cells.

If the subsidiary cells are indistinguishable from other epidermal cells, then it is called anomocytic stomata. It is also called irregular celled stomata or ranunculaceous type stomata. If there are three subsidiary cells and among these, one is small and the other two are relatively larger, then it is called anisocytic stomata. It is also called unequal-celled stomata or cruciferous type stomata. If the stoma is flanked on either side by one or more subsidiary cells which lie parallel to the long axis of the pore and guard cells, then it is called paracytic stomata. It is also called parallel-celled stomata or rubiaceous type stomata. If the stoma is enclosed by a pair of subsidiary cells whose common wall is at right angles to the long axis of the guard cells, then it is called diacytic stomata. It is also called cross-celled stomata or caryophyllaceous type stomata. Hence the correct option is a.

3. Which of the following statements are correct about the distribution of stomata?

A) If the stomata is present only on the lower surface of the leaf, then the distribution is called hypostomatous distribution.
B) Water lily is an example for epistomatous distribution.
C) In the floating plants, the number of stomata is less on the upper leaf surface than the lower surface.
D) The condition in which the stomata is vestigial is called astomatous condition.

1. A, B and C
2. A, B and D
3. A, C and D
4. All the above

Solution: The stomatal distribution is different in different types of plants. The different types of stomatal distribution are as hypostomatous distribution, epistomatous distribution, amphistomatous distribution and astomatous condition. If the stomata is present only on the lower surface of the leaf, then the distribution is called hypostomatous distribution. Examples are: Oxalis, apple, mulberry. If the stomata is present only on the upper surface of the leaf, then the distribution is called epistomatous distribution. Examples are: water lily. If the stomata is present on both the surfaces of the leaf, then it is called amphistomatous distribution. In monocot plants like maize and oats, the number of stomata on the upper and lower surfaces are equal. In the floating plants, the number of stomata is more on the upper leaf surface than the lower surface. If we observe the leaves of cabbage, potato, beans etc. the number of stomata is more on the lower leaf surface than on the upper surface. The condition in which the stomata is vestigial is called astomatous condition. An example is Potamogeton. Hence the correct option is b.

4. Assertion: Shape of the guard cells varies in dicot and monocot plants.

Reason: The dumb-bell shaped guard cells have elongated cells with expanded ends with thin walls.

1. Both assertion and reason are true and reason is the correct explanation of assertion.
2. Both assertion and reason are true but reason is not the correct explanation of assertion.
3. The assertion is true, but the reason is false.
4. Both assertion and reason are false.

Solution: The shape of guard cells varies in dicot and monocot plants. The bean shaped guard cells can be observed in the dicot plants and in the monocot plants the shape of guard cells resemble that of dumb-bells. The dumb-bell shaped guard cells have elongated cells with expanded ends with thin walls and a middle portion which has narrow lumen with thick walls. When turgor pressure increases in these kinds of cells, the expanded ends will swell and push apart the middle elongated portions of the cells. As a result the size of the aperture between the guard cells changes. This difference is used for the identification and classification of dicot and monocot plants. Hence the correct option is b.

FAQs

1. Which is the hormone that controls the opening and closing of stomata?
Answer: The low physiological concentrations of cytokinins and auxins promote the opening of stomata and their high concentration can inhibit the process. Ethylene is another hormone that stimulates the closing and opening of the stomata. Ethylene can inhibit the ABA induced stomatal closure. ABA helps in the closure of stomata to reduce the loss of water through transpiration.

2. What are stomatal crypts?
Answer: The sunken areas of the leaf epidermis that form a chamber like structure are called stomatal crypts. These chambers may contain one or more stomata or wax accumulations or trichomes. These are an adaptation to the drought conditions. The plants in which stomatal crypts are observed are Nerium oleander, conifers and Drimys winteri.

3. Do stomata close at high temperatures?
Answer: Stomata closes on a hot day to conserve water. During a hot day the water will evaporate through the stomata. This will cause the plant to lose the water and it will reduce the water potential inside the leaf. This may lead to the wilting of the plant. So through closing the stomata, the water loss is prevented.

4. Which light affects the functioning of the stomata?
Answer: Red light affects the functioning of the stomata. Red light activates the PM H + ATPase by the photosynthesis-dependent phosphorylation in the leaves. This provides insight into the photosynthetic regulation of stomatal opening.