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Stomata are minute pores present on the surface of green plants that are involved in the exchange of water and carbon dioxide between the plants and its atmosphere. The stomatal structure can be easily seen under a microscope. A single pore is called the stoma (Greek origin which means mouth), which is found in the epidermis of leaves, stems, and other organs of the plant.
Thousands of stomata are present on the surface of the leaves. Thus, it help in the process of transpiration and photosynthesis which are the most essential process for the survival of a plant. Both abiotic and biotic interactions can be observed in stomatal cells. In this article we will learn about stomata, types, structure and its functions in more detail.
What are Stomata?
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Stomata are minute pores or openings in the epidermis layer of the leaves, young stems, and some other parts of green plants which form an important constituent of their functioning. The opening and closing of stomata helps in regulation of the process of photosynthesis in higher plants. The chloroplasts present in the stomatal pores contain chlorophyll which along with sunlight facilitate the process of photosynthesis. The major functions are carried through the mechanism of its opening and closing of stomata.
Location of Stomata
Stomata are tiny pore-like structures found in plant leaves and surrounded by a layer of specialized cells called guard cells. In most plants, stomata are located mainly on the lower surface of leaves to prevent water loss.
Role of Stomata
Stomatal pores help in gaseous exchanges between the plant and the atmosphere and maintain a healthy rate of transpiration in plants. The presence of stomata facilitate the intake of carbon dioxide and release of oxygen in plants. In this way, along with photosynthesis, stomata plays a significant role in respiration in plants.
Structure of Stomata
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The structure of the stomata comprises a kidney-shaped epidermal cell along with an opening in the center called a pore. The stomata are bordered by a pair of specialized parenchyma cells which are the guard cells. These are responsible for regulating the size of its opening, thereby saving the plant from water loss. Each Stoma pore comprises a similar structure. It might vary to some extent in the shape of the cells, but the mechanism and composition remain the same.
Structure of Stomata
The four vital parts of a stoma are:
Key Features of Structure of Stomata
The entry of carbon dioxide into plants is controlled by the opening and closing of stomata. As seen in the stomata diagram, the key features are –
Epidermal Cells
- These are the cells of the plant that provide physical and mechanical support to the plants.
- Epidermal cells are irregular in shape and form the outermost layer of plants.
- These cells are present as the building blocks of a plant.
- Since they are stiff, they provide scope for the stomatal pores to close again.
Subsidiary Cells
- These are present in parts throughout the plant, and especially around the stomata.
- Unlike the epidermal cells, subsidiary cells are soft and provide scope for the guard cells to expand and the pore to open.
- They essentially form a working space for the stoma.
- Without them, the pores won’t be efficient to work for a long period of time.
Stoma Pore
- The pore is the main opening through which all of the gaseous exchanges, vapor exchanges, and absorption from the atmosphere take place.
- Without the pores present, all of the functioning of Stomata would be completely useless.
Guard Cells
- These are the most important part of a Stomata.
- These are kidney-shaped cells, with a thick inner cell wall.
- Since all of the functioning of Stomata depends upon the opening and closing of it, and the Guard cells are the sole doer of it, these are pretty vital to the overall plant maintenance.
As the Guard cells absorb water, their outer cell walls being thin, expand. While doing so, they pull the inner walls along causing the stomatal pore to open. Hence, it can be said that guard cells expand when turgid and contract when flaccid. With proper Guard cells, there would be a healthy rate of transpiration, and gaseous exchanges, and hence, a healthier process of growth in plants. The total number of stomata present on the upper and lower surfaces of leaves of different plants is explained in the table below:
| Monocotyledon | Upper Surface | Lower surface |
|---|---|---|
| Wheat | 50 | 40 |
| Barley | 70 | 85 |
| Onion | 175 | 175 |
| Dicotyledon | Upper Surface | Lower surface |
| Alfalfa | 169 | 188 |
| Geranium | 29 | 179 |
| Sunflower | 120 | 175 |
Read more: Tendril
Types of Stomata
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The Stomata can be broadly classified into 5 types based on the subsidiary cells, their number and arrangement as follows:
- Anomocytic Stomata
- Anisocytic Stomata
- Paracytic Stomata
- Diacytic Stomata
- Gramineous Stomata
Anomocytic Stomata
The subsidiary cells in anomocytic stomata are irregularly shaped and mostly, difficult to distinguish. They are also known as ranunculaceous cells or irregular-celled-type stomata.
The guard cells are surrounded by a mix of the subsidiary and epidermal cells. Such stomata and their pores are generally difficult to be located and specifically structured. However, both the subsidiary and epidermal cells are present in adequate numbers.
Anisocytic Stomata
This type of stomata has three subsidiary cells, unequal in size, where one cell is smaller than the other two. These subsidiary cells do not have a specific layout of presence, but the number is fixed. They are also known as the unequal-celled type or cruciferous stomata.
Paracytic Stomata
Paracytic Stomata have two specific subsidiary cells, one on the outer side of each guard cell. There may be more than one on either of the sides, but the arrangement remains parallel to the guard cells. Such arrangement provides great scope for the guard cells to expand and contract. These are also known as the parallel-celled type or rubiaceous stomata.
Diacytic Stomata
Just like the Paracytic stomata, these kinds also have two subsidiary cells. The difference between them lies in their positioning of the same. While in the previous type, the subsidiary cells were present and parallel to the guard cells, in the Diacytic stomata, the cells are perpendicular to the guard cells. That is, the subsidiary cells lie at right angles to the guard cells.
They are also known as caryophyllaceous or cross-walled stomata.
Gramineous Stomata
The last type consists of two dumbbell-shaped guard cells and two corresponding subsidiary cells. The guard cells are narrow in the middle, for the most part, and are bulky or wider at the ends. The walls of these cells are thicker in the middle and thinner at the ends. The subsidiary cells are present next to each guard cell and are exactly parallel to them.
Functions of Stomata
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Stomata has the following functions –
- Facilitate Exchange of Gases: This important function of Stomata involves the absorption of carbon dioxide, which the plants break into carbon and oxygen. The carbon is essential for its growth, while oxygen combines with hydrogen and forms the water required.
- Adequate Transpiration Rate: The stomatal pore openings help in the evaporation of excess water present in the plant. This maintains a healthy water level within the plant.
- Healthy Water Ratio: The stomata close during the night which reduces the loss of water through evaporation and hence keeps the plants from drying up.
- Assisting Photosynthesis: The exchange of gases and facilitation of water evaporation greatly facilitates the process of photosynthesis where the plant makes food with the help of sunlight.
Opening and Closing of Stomata
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The mechanism of opening and closing of stomata depends on the turgor pressure, which is caused by the osmotic flow of water in the guard cells. The guard cells expand, when they are turgid, resulting in the opening of stomata. And they become flaccid leading to stomatal closure when the guard cells lose water. Generally, when the light strikes the leaf stomata open and close during the night.
Opening & Closing of Stomata
Things to Remember
- Stomata are minute openings in the epidermis layer of the leaves, stems, and other parts of a green plant.
- The stomatal pores contain chloroplasts which have chlorophyll and stored energy in them.
- Stomata aid in the process of photosynthesis and transpiration in plants.
- The four vital parts of the stomata are the pores, guard cells, epidermal layer, and subsidiary cells.
- Stomata can be classified into 5 types based on the subsidiary cells: Anomocytic Stomata, Anisocytic Stomata, Paracytic Stomata, Diacytic Stomata, and Gramineous Stomata.
- Adequate exchange of gases, and maintaining a healthy water ratio are important functions of the stomata.
- The opening and closing of stomata are dependent on the turgor pressure caused by the guard cells.
Previous Year Questions
- In which of the following plant sunken stomata are found? … (NEET 2001)
- Stomata of CAM plants … (NEET 2003)
- Stomata in grass leaf are … (NEET 2018)
- Stomata of a plant open due to … (NEET 2003)
- Which one of the following plants belongs to the Family Apocynaceae?….[COMEDK UGET 2009]
- Madhunashini is the popular name for…..[COMEDK UGET 2006]
- The roots that originate from the base of the stem are… [ NEET 2020]
- Bicarpellary gynoecium and oblique ovary occurs in...[ NEET 2001]
- Tetradynamous conditions occur in….[ NEET 2001]
- Coconut fruit is a….[ NEET 2017]
- Commercial cork is obtained from… [NEET 1991]
- Botanical name of cauliflower is…. [NEET 1991]
- Banana is...[JKCET 2007]
- Tricarpellary syncarpous superior ovary is….[JKCET 2007]
- The edible part in hesperidium fruit is...[JKCET 2007]
- The aerial vertical stout part of the banana is….[JIPMER 1999]
- The closing and opening of the leaves of...[NEET 1999]
- The closure of lid of pitcher in pitcher plant, is due to...[NEET 1995]
- The leaves are modified into tendrils, hooks, pitcher...[KEAM]
- The type of leaf in Daucus carora is… [ AMUEEE 2005]
Sample Question
Ques 1. Write the mechanism of opening and closing of stomatal pores. (2 marks)
Ans. The opening and closing of stomatal pores are controlled by the turgidity of guard cells. The mechanism is as follows:
- When guard cells uptake water from surrounding cells, they swell to become a turgid body, which enlarges the pore in between (Stomatal Opening).
- While, when water is released, they become flaccid shrinking to close the pore (Stomatal Closing).
Ques 2. What is the Significance of Photosynthesis? (2 marks)
Ans. Photosynthesis is the main way through which solar energy is made available for different living beings including plants. However, the stomatal pores in plants are storehouses of choloroplasts that have chlorophyll in them and take in the sunlight for the preparation of food. The salient features of photosynthesis are:
- Green plants are the main producers of food in the ecosystem. All other organisms directly or indirectly depend on green plants for food.
- The process of photosynthesis also helps in maintaining the balance of carbon dioxide and oxygen in the air.
Ques 3. What are the functions of stomata? (2 marks)
Ans. The key functions of stomata are:
- Exchange of gases, O2, CO2. and other gases with the atmosphere
- Facilitates transpiration by losing water as water vapor
- Maintains moisture balance by opening or closing of the stomatal pore
- Intake of carbon dioxide and elimination of oxygen to facilitate photosynthesis
- Prevention of escape of water by keeping pores closed at night
Ques 4. Explain the process of Photosynthesis. (2 marks)
Ans. The process by which green plants prepare food is called photosynthesis. The solar energy is converted into chemical energy, and carbohydrates are formed. Green leaves are the main site of photosynthesis. The green portion of the plant contains a pigment chloroplast, chlorophyll (green pigment).
Ques 5. What are Anisocytic Stomata? (2 marks)
Ans. Anisocytic stomata have three subsidiary cells that are unequal in size such that one cell is smaller than the other two. These subsidiary cells do not have a specific layout of presence, but the number is fixed. They are also known as the unequal-celled type or cruciferous stomata.
Ques 6. What are Gramineous Stomata? (2 marks)
Ans. Gramineous stomata are dumbbell-shaped guard cells with two corresponding subsidiary cells. The guard cells are narrow in the middle, for the most part, and are bulky or wider at the ends. The walls of these cells are thicker in the middle and thinner at the ends. The subsidiary cells are present next to each guard cell and are exactly parallel to them.
Ques 7. Explain the structure of stomata. (2 marks)
Ans. Each stomatal pore has a similar structure. However, to a certain extent, the shape of the cells may vary but, the mechanism and composition remain the same.
Stomata consist of the following four vital parts:
- Pore
- Guard cells
- Subsidiary cells
- Epidermal cell
Ques 8. What is Anomocytic Stomata? (2 marks)
Ans. In Anomocytic stomata, the subsidiary cells are irregularly shaped and difficult to differentiate. They are also known as ranunculaceous cells or irregular-celled-type stomata.
The guard cells are surrounded by a mix of the subsidiary and epidermal cells. Both the subsidiary and epidermal cells are present in adequate numbers.
Ques 9. What is Paracytic Stomata? (2 marks)
Ans. Paracytic Stomata have two specific subsidiary cells on each outer side. This arrangement remains parallel to the guard cells. Such arrangement facilitates the expansion and contraction of guard cells.
These are also known as the parallel-celled type or rubiaceous stomata.
Ques 10. What are the raw materials needed for photosynthesis? (2 marks)
Ans. The raw materials needed for photosynthesis are as follows:
- Sunlight
- Chlorophyll: Sunlight absorbed by chloroplast
- CO2: Enters through stomata, and oxygen (O2) is released as a byproduct through stomata on the leaf.
- Water: Water + dissolved minerals like nitrogen, phosphorous, etc. are taken up by the roots from the soil.
Ques 11. State the types of stomata? (3 marks)
Ans. There are different types of stomata and are classified based on various criteria:
Based on the structure:
- Paracytic
- Tetracytic
- Diacytic or Caryophyllaceous
- Anomocytic or Ranunculaceous
- Actinocytic
- Graminaceous
- Anisocytic or Cruciferous
Based on Plant Development:
- Mesoperigynous Type
- Perigynous Type
- Misogynous Type
Based on their distribution or placement of plant leaves:
- Oat Type
- Water Lily Type
- Apple or Mulberry Type
- Potato Type
- Potamogeton Type
Ques 12. What are the 3 things that pass in and out of the stomata? (3 marks)
Ans. The 3 things that pass through stomata are –
- Water
- Carbon Dioxide
- Oxygen
Ques 13. What makes stomata open? (3 marks)
Ans. Stomata are formed of two guard cells. These cells have walls thicker on the inner side than the outer one. This unequal thickening of the guard cells causes stomata to open when they take in water and close when they lose water.
Ques 14. Why do stomata close during night? (2 marks)
Ans. Stomata in the leaves typically open during the day in order to favor the diffusion of CO2 when light is available for the process of photosynthesis, and then close at night to save water and limit transpiration.
Ques 15. Where are stomata located? (1 mark)
Ans. The stomata are most commonly found on green aerial parts of plants, especially the leaves. They can be found on stems, but less commonly than they were on leaves.
Ques 16. What opens and closes the stomata? (2 marks)
Ans. Opening and closing of the stomata occurs due to the turgor changes that occur in its guard cells. When the guard cells are turgid, the stomatal pore is open while in soft conditions, the stomatal aperture closes.
Ques 17. The leaves of a healthy plant are coated with Vaseline to block stomata. Can the plant remain healthy for long? Why? (3 marks)
Ans. The leaves of a healthy plant that is coated with vaseline will not remain healthy as the coating of the vaseline will block the stomata. Therefore:
- The plant won’t get oxygen for respiration.
- It won’t get carbon dioxide to carry out the process of photosynthesis.
- The upward movement of minerals and water would be hampered due to the lack of transpiration.
Ques 18. Which plant hormone promotes the opening of the stomata? (1 mark)
Ans. Cytokinins in low physiological concentrations promotes the opening of stomata.
Ques 19. How plants in desert area use CO2 during photosynthesis? (2 marks)
Ans. Desert plants have a process known as Crassulacean acid metabolism, or CAM pathway as they are in arid conditions. At night time, the desert plants take in CO2 and convert it into malic acid which is further broken down again into CO2 during the daytime for photosynthesis.
Ques 20. What are the events that take place during photosynthesis? (3 marks)
Ans. The events during photosynthesis are –
- Absorption of energy from the sun through the chlorophyll
- Conversion of light energy into chemical energy. The splitting of water into hydrogen and oxygen using the light energy
- Reduction of carbon dioxide through hydrogen in order to form carbohydrate such as glucose by using the chemical energy
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