What are Stomata and its Function?
Stomata are tiny openings usually found in the epidermis of leaves, but they can also appear on stems and other plant parts. They play a major role in controlling the exchange of gases and water vapour between a plant’s internal tissues and the external environment. Stomata help plants obtain carbon dioxide (CO₂) for photosynthesis and release oxygen (O₂) back into the air. They also regulate water loss through transpiration.
Structure of Stomata
A single opening is called a stoma, while multiple openings are referred to as stomata. Each stoma has two specialised guard cells that open and close the pore. In many plants, there are also subsidiary cells, which support the guard cells. Here are the main parts you should know:
Epidermal Cells: These form the outermost layer of the leaf or stem. They protect the plant and often have a waxy covering.
Guard Cells: These are kidney-shaped or dumbbell-shaped cells that flank each stomatal pore. They become turgid (swollen) to open the pore and flaccid (relaxed) to close it.
Stomatal Pore: This is the actual opening through which gases (CO₂, O₂) and water vapour move in and out.
Subsidiary Cells: These are specialised cells adjacent to the guard cells. They support the guard cells in their opening and closing activities.
Types of Stomata with Examples
What are stomata and its function? Before we look at different types of stomata in plants, remember that their main function is gaseous exchange and regulation of water loss. Different plant families show variations in how the surrounding cells are arranged. Here are some commonly recognised types of stomata:
Anomocytic Stomata
The subsidiary cells are not distinct from other epidermal cells.
Example: Found in many Ranunculaceae family plants.
Anisocytic Stomata
They have three surrounding subsidiary cells, one of which is smaller than the other two.
Example: Commonly seen in Solanaceae (potato, tomato).
Paracytic Stomata
Two subsidiary cells lie parallel to the guard cells.
Example: Often observed in plants like members of the Rubiaceae family.
Diacritic Stomata
Two subsidiary cells are arranged at right angles to the guard cells.
Example: Typical in plants of the Caryophyllaceae family.
Gramineous (Dumbbell-Shaped) Stomata
Common in grasses and cereals (family Poaceae). Their guard cells look like dumbbells.
Function of Stomata
The function of stomata is critical for plant survival. They:
Regulate Gas Exchange: They allow CO₂ entry for photosynthesis and release O₂ into the atmosphere.
Control Water Loss: By opening and closing, they manage the amount of water vapour that escapes, helping plants maintain proper water balance.
Support Cooling: Transpiration via open stomata cools the plant surface, especially in warm conditions.
Stoma vs Stomata
Stoma: Refers to a single pore guarded by two guard cells.
Stomata: Plural form, referring to all such pores on a leaf.
This distinction often helps students differentiate between one opening (stoma) and the collective openings (stomata).
Stomata in Photosynthesis
Stomata in photosynthesis serve as the gateway for carbon dioxide to enter the leaf. Once inside, CO₂ is used by the chloroplasts in the mesophyll cells to produce glucose and oxygen. In turn, oxygen exits the leaf through these pores.
Stomata in Plants: Key Points
Present in the epidermis (mainly underside of leaves in most plants).
May also be found on stems and other aerial parts, though less commonly.
The opening and closing mechanism is largely driven by water movement in and out of the guard cells.
Practise Quiz on Stomata
Try these multiple-choice questions. Answers are given below.
1. Which cells directly control the opening and closing of the stomatal pore?
A. Epidermal cells
B. Guard cells
C. Subsidiary cells
D. Mesophyll cells
2. What is the main function of stomata in plants?
A. Absorption of nutrients
B. Defence against pathogens
C. Gaseous exchange and transpiration
D. Support for stem growth
3. Which type of stomata has two subsidiary cells lying parallel to the guard cells?
A. Anisocytic stomata
B. Anomocytic stomata
C. Paracytic stomata
D. Diacytic stomata
Quiz Answers
B. Guard cells
C. Gaseous exchange and transpiration
C. Paracytic stomata
Related Topics
FAQs on Stomata Diagram, Structure, and Functions
1. Why are stomata mostly found on the underside of leaves?
In most plants, the underside of the leaf receives less direct sunlight, reducing water loss through evaporation. Hence, stomata on the lower surface help control transpiration effectively.
2. Can stomata be seen with the naked eye?
No, stomata are too small to be observed without a microscope or a strong hand lens. Under a microscope, each stoma appears as a tiny opening flanked by guard cells.
3. Does the number of stomata differ between plant species?
Yes, different species have varying numbers of stomata. Environmental conditions, plant types, and adaptations also influence stomatal density.
4. How do stomata help in transpiration?
When stomata open, water vapour escapes from the leaf’s internal tissues to the atmosphere. This process is called transpiration, which helps cool the plant and drives water and nutrient transport from the roots.
5. Do all plants have the same type of stomata?
No, different plants may exhibit different types of stomata, such as anomocytic, anisocytic, paracytic stomata, diacytic, or gramineous. The arrangement of subsidiary cells around the guard cells determines the type.
