Stomata

There are certain parts in all green plants which are essential and play a critical role in different life processes. Stomata is one of the essential parts that is used for gas exchange. It functions as the mouth of a plant and is also called a stoma, or stomas.

In this article let us explore more about the stomata, its types, structure, and functions along with the opening and closing of stomata

Definition of Stomata

When a leaf is examined under the microscope, we can observe many tiny pores, which are collectively called stomata.

Stomata are the minute openings, generally found in the epidermis of leaves. They are typically found in plant leaves and can also be found in stems and other parts of plants.  Stomata play an important role by permitting the movement of gases such as oxygen, carbon dioxide, and water vapour to diffuse between the interior and outer surface of the plant tissues.

Explore more Morphology of Leaves

Types of Stomata

There are different types of stomata and are mainly classified based on their number and characteristics of the surrounding subsidiary cells. Listed below are the different types of stomata.

Anomocytic Stomata: They are surrounded by the epidermal cells, which has fixed shape and size.  The stoma appears to be embedded in epidermal cells. There are no definite number and arrangement of cells surrounding the stoma.

Anisocytic Stomata: Each stoma is surrounded by the unequally sized three subsidiary cells. Among which,  one is distinctly smaller in size than the other two.

Diacytic Stomata: The stomata are surrounded by a pair of subsidiary cells that are perpendicular to the guard cells

Paracytic Stomata: The stomata are continuously surrounded by two subsidiaries, which are arranged parallel to the stomatal pore and the guard cells.

Gramineous Stomata: Each stoma possesses two guard cells, which are shaped like dumb-bells. The subsidiary cells are parallel to the guard cells. The guard cells are found narrow in the middle and wider at the ends.

Structure of Stomata

The stomata consist of minute pores called stoma surrounded by a pair of guard cells. This stoma functions as a turgor, an operated valve which functions by opening and closing according to the turgidity of guard cells. The cell wall surrounding the stoma is tough, flexible and thinner. The shape of guard cells usually differs in both monocots and dicots though the mechanism continues to be the same.

The Guard cells are specialized, bean-shaped cells, which are found surrounding to the stoma and are connected at both ends. These cells enlarge and contract to open and close the stomata pores. Guard cells also contain chloroplasts, the light-capturing organelles in plants.

The subsidiary cells also called accessory cells. They are the accessory cells to guard cells and are found in the epidermis of plants. These cells are surrounded and supported by the guard cells and act as a buffer between guard cells and epidermal cells, which functions by protecting epidermal cells against the expansions of the guard cells.

The average number of stomata is about 300 per square mm of the leaf surface.

Also, refer to Distribution of Stomata in the Lower and Upper Surfaces of the Leaves

The table given below explains the total number of stomata present on the upper and lower surfaces of leaves of different plants.

Total Number of Stomata / mm2
Upper Surface Lower surface
Monocotyledon
Wheat 50 40
Barley 70 85
Onion 175 175
Dicotyledon
Sunflower 120 175
Alfalfa 169 188
Geranium 29 179

Functions of  Stomata

The main functions of stomata are:

  1. Helps in the exchange of gases by opening and closing the pores in the leaves.
  2. It helps to expel the excess water out from the leaves in the form of water vapour.
  3. Based on the weather conditions, it closes or opens its pores to maintain the moisture balance.
  4. Allow the uptake of carbon dioxide and to give out oxygen during the process of photosynthesis.
  5. Stomata remain open during the day and closed at night. This closure prevents water from escaping through open pores.

Mechanism of Stomata Action

The mechanism of stomata – the opening and closing of stomata depends against the turgor pressure, caused by the osmotic flow of water in the guard cells. When the guard cells are expanded, the stoma opens and closes, when the guard cells lose water.

Stomata normally open when the light strikes the leaf and close during the night.

Also read-Transpiration

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