Stomata: Definition, Types, Functions & Mechanism

Stomata: Do you know what role nostrils play in our body? They help us in the process of breathing. Have you ever wondered do plants breathe or not? Yes, tiny pore or stomata present on the surface of leaves commence the process of breathing in plants. A stoma is a small hole in the surface of a leaf that is utilised for gas exchange in plants. The majority of the leaves have these small holes, which allow plants to take in carbon dioxide for photosynthesis and discharge waste oxygen.

There are thousands of stomata present on the surface of leaves. Most of them are found on the lower side of the leaves. We now know that plants breathe, so numerous questions, such as what stomata look like? Can plant close these pores etc., do cross our mind. In this article, we’ve provided in-detail information on stomata; their structure, types, diagram, functions, mechanism, etc. Scroll down to read more.

What are Stomata?

Stomata are the tiny pores present on the epidermis of leaves. The pores of stomata facilitate gaseous exchange and are mostly present under the leaf’s surface. 

Each stoma is surrounded by two kidney or bean-shaped cells called guard cells. We can see the stomata under the light microscope. The stomata may occur on any part of the plant except the roots.

Stomata Diagram

Below we have provided the well-labelled diagram of stomata for your reference:

Stomata Structure

Let’s now discuss the structure of stomata now:

Epidermal Cell

It is the outermost layer of a plant made up of specialised cells originating from the dermal tissues. Epidermal cells tend to be irregular in shape, and their function is to provide mechanical support to the plant.

Guard Cells

These cells resemble the shape of a kidney or dumbbell-shaped that consists of the chloroplast. They contain chlorophyll and capture light energy. The primary function of guard cells is to properly carry out the opening and closing mechanism of the stoma.

1. Stomatal Pore refers to the minute ports as the openings found under the leaf’s structure. These pores play a significant role in exchanging gases like Oxygen and Carbon dioxide.
2. The inner wall of these guard cells is thicker than the outer. 

Subsidiary Cells: It is located adjacent to guard cells in the stroma of a leaf, subsidiary cells provide support, which helps in the moment of guard cells. Subsidiary cells are generally formed in proximity to mother cells. However, they can be seen growing independently too.

Types of Stomata

There are different types of stomata depending upon the type of the guard cells and the arrangement of subsidiary cells. Evolutionarily, stomata are divided into four types:

1. Moss-type stomata are found in the capsules of certain mosses, like Physcomitrium patens.

2. Gymnospermous type stomata are found in naked seeded plants. They have sunken stomata to reduce transpirational water loss.

3. Coniferous type stomata are sunken. In their median parts, the guard cells appear to be elliptical in section and have narrow lumina.

4. Gramineous type stomata are found in the grass family. The stomata have two guard cells surrounded by two lens-shaped subsidiary cells.

5. Dicotyledonous type: Out of these, the Dicotyledonous type of stomata is of diagnostic significance

Name	Description	Example
Paracytic or Rubiaceous or Parallel-celled stomata	Two subsidiary cells are parallel to the longitudinal axis of pore and guard cells.	Senna and Coca.
Diacytic or Caryophyllaceous or Cross-celled Stomata	The Pores of the stomata remain surrounded by a pair of subsidiary cells whose common wall is at a right angle to the guard cells.	Peppermint, Spearmint, Vasaka.
Anisocytic or Cruciferous or Unequalcelled Stomata	The stomata remain surrounded by three subsidiary cells, of which one is distinctly smaller than the other two.	Belladonna,Datura,Stramonium,Hyoscyamus. Vinca.
Anomocytic or Ranunculaceous or Irregular-celled Stomata	The stomata remain surrounded by a limited number of subsidiary cells like the remaining epidermal cells.	Buchu, Clove, Digitalis, Lobelia, Phytolacca americana.
Actinocytic or Radiatedcelled Stomata	These stomata are surrounded by four or more subsidiary cells, elongated radially to the stomata.	Members of Ebenaceae.

Functions

Below we have provided the significant functions of stomata:

1. Stomata allow the exchange of gases \(CO_{2\;}\; and\;O_2\) with the atmosphere.
2. Evaporation of water from the leaf surface occurs through the stomata. Thus, the stomata help in the process of transpiration.
3. It closes or opens its pores to maintain the moisture balance based on climatic conditions.

Mechanism

1. Light is the main source for the stomatal opening. 
2. The opening and closing of stomata depend upon the turgid or flaccid state of guard cells.
3. The stomatal aperture closes when the guard cells are in a flaccid state.
4. During the daytime, due to photosynthesis (guard cells have chloroplast), the concentration of carbohydrates rises, leading to osmotic uptake of water by the guard cells. Thus, the guard cells swell.
5. The walls of guard cells of the stomatal pore are thicker outside, and the walls present inside are thinner, guard cells bulge due to the inflow of water, thus widening the stomatal opening.
6. When the osmotic pressure of the guard cells becomes lower (during the night), the water leaves these cells due to exosmosis and moves to the neighbouring epidermal cells having cell sap of higher concentration.
7. The guard cells become flaccid and shrink, and the stomatal aperture closes.

Stomatal Transpiration

Transpiration is the water loss from the aerial portions of the plants, mainly leaves, in the form of vapour. This takes place from the lower surface of a dicot leaf because there are more stomata present as compared to the number of stomata on the upper surface.

Summary

Stomata are minute pores on the epidermal layer of leaves. These are the main sites for gaseous exchange and transpiration. The pore is surrounded by guard cells that can close and open the pore. Some plants may have well-developed stomata, while others may not have stomata.

Environmental and internal factors control the opening and closing of these. So, to sum up, stomata play an important role in plant physiology that helps the plants survive by eliminating the excess water from their body through transpiration.

FAQs

Q.1. When does the opening and closing of stomata take place?
Ans: Stomata close or open their pores to maintain the moisture balance based on the climatic conditions.

Q.2. How do the opening and closing of stomata take place?
Ans: The opening and closing of stomata depend on the turgor pressure caused by the osmotic flow of water into the guard cells. When the guard cells are turgid, they expand, resulting in stomata opening. When the guard cells lose water, they become flaccid leading to stomatal closure. Stomata normally open when the light strikes the leaf and close at night. 

Q.3. What is the function of stomata?
Ans: 1. Stomata allow the exchange of gases (CO2CO2 and O2O2) occurs through the stomata.
2. Thus, the stomata help in the process of transpiration.
3. It closes or opens its pores to maintain the moisture balance based on climatic conditions.

Q.4. How do plants respire at night when stomata are closed?
Ans: Plants keep their stomata closed at night to prevent or control excess water loss from their pores. Plants release carbon dioxide, take in oxygen, and oxidise stored food by absorbing oxygen. 

Q.5. What is the role of stomata?
Ans: Stomata are the specialised pores or openings present in the epidermis of plant cells, which play a crucial role in gaseous exchange during photosynthesis and respiration. They also help in transpiration.

Q.6. Draw a neat structure of Stomata
Ans:

Practice CBSE Class 9 Questions

Now that you are provided with all the necessary information on Stomata, we hope this article is helpful to you. If you have any queries on Stomata or its functions, ping us through the comment box below, and we will get back to you as soon as possible.