What are Stomata? Functions, Structure, and Types

Understanding what are stomata will help improve our knowledge of how plants exchange gases necessary for photosynthesis and how they transpirate. This means the definition, structure, types, and functions of stomata will be discussed in this article.

Table of Contents

Stomata definition in biology

Stomate, plural stomata, or stomas, are any of the microscopic openings or pores in the epidermis of leaves and young stems. These stomata are more abundant on the underside of leaves and they allow gases to be exchanged between the outside air and the leaf’s branched system of interconnecting air canals.

What is a stoma?

A stoma which is a singular form of stomata is a plant cell structure in the epidermis of tree leaves and needles that help plants exchange carbon dioxide and water with the atmosphere.

Stomata structure

This stoma diagram help in the understanding of what are stomata.
A stoma diagram of a leaf.


The pore is the primary opening through which all gaseous exchanges, vapor exchanges, and atmospheric absorption occur. All of the functions of the stomata would be rendered useless if the pores were not present because it is the main part that aids in stomatal function in plants.

Guard cells

These are the most crucial components of a Stoma. These cells are kidney-shaped and have a thick inner cell wall.

The entire functioning of the stomata is dependent on its capability in opening and closing, Hence the importance or the presence of guard cells in the structure of the stomata because guard cells are the only ones capable of inflating to open the stomata and deflating to close the stomata. The ability of the guard cells to open and close stomata is critical for plant maintenance.

The Guard cells’ outer cell walls are thin making them easily expand and as they do so, they absorb water by pulling the inner walls along with them, causing the stomatal pore to open. 
Guard cells expand when turgid and contract when flaccid. Hence, a  healthy rate of transpiration, gaseous exchanges, and a healthier process of plant growth would be achieved with proper guard cells.

Subsidiary cells

These are found all across the plant, but especially near the stomata. These cells are soft and allow the guard cells to expand thereby opening the pore. They essentially serve as a workspace for the stoma and without them, the pores will not be able to work efficiently for an extended period of time.

Epidermal cells

These are plant cells that provide physical and mechanical support to the plant. They are irregular in shape and form the plant’s outermost layer. These cells serve as the foundation of a plant and because they are stiff, they allow the stomata pores to close again.

Types of stomata

  1. By placement of the stomata
  2. Based on stomata structure
  3. By plant development

The different types of stomata in plants are classified based on their location on plant leaves, the structure of the surrounding subsidiary cells, or the developmental path of the guard cells and subsidiary cells.

Placement of the stomata

  • Apple or mulberry type (hypostomatic)
  • Oat type (Amphistomatic)
  • Potato type
  • Water lily type (Epistomatic)
  • Potamogeton type (Astomatic)

Hypostomatic type

The term “hypostomatic” is derived from the words “hypo” for “under” or “below” and “stoma.” These types of stomata are only found on the lower surfaces of leaves on many fruit trees, including apples, mulberries, peaches, and walnuts. The placement of these stomata helps to reduce water loss by absorbing carbon dioxide through the leaves’ cooler, shaded surfaces.

Amphistomatic type

The term “amphistomatic” is derived from the Greek words “Amphi” for “both sides” and “stoma.” In this type of placement, stomata are evenly distributed across all faces of the leaf as is found in oats and other grasses, as all sides of grass blades receive roughly equal exposure to air.

Potato type

In this placement, stomata are found primarily on the lower surfaces of leaves on potatoes, as well as beans, cabbage, and related plants, but they are also found in smaller numbers on the top sides of leaves.

Epistomatic type

The term “epistomatic” is derived from the Greek words “epi” for “over” or “on top of” and “stoma.” This means that the placement of the stomata is on top of the leaves. A good example of plants that possess such stomata is water lilies and other aquatic plants that have stomata on the upper surfaces of their leaves, which are usually above water.

Asomatic type

The term “astomatic” is derived from the Greek word “a” for “without” added to the word “stoma.” Examples of plants with this type of placement are Potamogeton and submerged aquatic plants that are either devoid of stomata or have vestigial stomata that do not function.

Based on structure of the stomata

  1. Actinocytic
  2. Anisocytic or cruciferous
  3. Anomocytic or ranunculaceous
  4. Cyclocytic
  5. Diacytic or caryophyllaceous
  6. Graminaceous
  7. Parcytic


At least four cells surround actinocytic stomata, forming a circle with the stoma and its guard cells in the center. These stomas can resemble daisies, with a single row of petals radiating out from a central stoma.

Anisocytic or cruciferous

This stoma is surrounded by three cells of varying sizes. These cells can look like rose petals, with cells of varying sizes spiraling out from a central stoma.

Anomocyctic or ranunculaceous

Anomocytic cells are enclosed by a limited number of subsidiary cells that are identical to the epidermis cells.


In the cyclocytic structure, at least four cells surround the stoma, forming a ring around the stoma. These subsidiary cells may resemble carnations, with multiple layers of cells resembling petals ringing the central stoma.

Diacytic or caryophyllaceous

This type of cell is surrounded by only two other cells, one on each side of the stoma. The surrounding subsidiary cells are perpendicular to the guard cells, which means they intersect in the middle of the guard cells rather than at their edges. These configurations can resemble brick walls, with the stoma sandwiched between subsidiary “brick” cells.

These stomata can be quite close together, with the same subsidiary cell potentially bordering different stomata on opposite sides.


Guard cells in graminaceous stomata are dumbell-shaped and are sandwiched between subsidiary cells that run parallel to them.


Paracytic cells, like diacytic cells, are encircled by only two guard cells, one on each side. The gaps between subsidiary cells in paracytic cells, on the other hand, align with the gaps between guard cells rather than being at right angles to them.

Plant development

  1. Mesogynous type
  2. Perigynous type
  3. Mesoperigynous type

Mesogynous type

The guard cells and their surrounding subsidiary cells develop from the same mother cell in mesogynous stomata.

Perigynous type

Guard cells form from a single mother cell, while subsidiary cells form from multiple mother cells in this type of stomata.

Mesoperigynous type

This type, as the name implies, combines characteristics of both mesogynous and perigynous stomata. Guard cells develop from a single mother cell in mesoperigynous stomata, while subsidiary cells develop from both that same mother cell and neighboring cells.

Functions of stomata

Stomata are responsible for allowing carbon dioxide gas to enter the air spaces in the tissues of a plant. Once inside these air spaces, CO2 can be used as a carbon source by the plant’s photosynthetic tissues to build sugars, amino acids, and more.

Therefore it can be said that the function in the stomata photosynthesis or photosynthesis stomata relationship is the stomata opens to take in carbon dioxide that is necessary for photosynthesis.

Stomata are also used to release oxygen and water vapor into the environment in other to maintain the balance of water in the plant.

Exchange of Gases in Stomata

Stomata which is the plural of stoma are openings in the surface of plants that are mostly found on the leaves but can also be found on stems and other organs. They are pores surrounded by guard cells, which are specialized parenchymatic cells.

Stomata come in different shapes and can change to adapt to different environmental parameters, ensuring optimal photosynthesis criteria are met.

Green plants require a source of carbon dioxide and a way to expel oxygen in order to continue photosynthesis. Also, these green plants cells require oxygen and a way to dispose of carbon dioxide in order to continue cellular respiration (just as animal cells do).

Thus, the gaseous exchange for which they are responsible facilitates photosynthesis by allowing in the necessary CO2.

Carbon dioxide is used as a fuel to power photosynthesis, which produces oxygen as a byproduct and is then released into the atmosphere. So, how do stomata help with photosynthesis?

The main function of stomata in photosynthesis is by contributing significantly to transpiration (which is the intake of water into the plant, its distribution within it, and its final release to the atmosphere from the areal parts). Transpiration through stomata increases the plant’s water potential, which favors passive water absorption in the roots, which is then conveyed all across the plant by the Xylem.

Plants require 6 molecules of water and 6 molecules of CO2 to produce sugar and oxygen through photosynthesis. As previously stated, stomata play an important role in the entry of water and CO2 into the plant, thereby facilitating photosynthesis.

Importance of stomata in plants

All of the biological materials used by the first photosynthetic organisms had to be created from scratch. These biological organisms discovered how to do so by stringing carbon molecules from CO2 gas into complex organic molecules. These early photosynthesizers would then emit waste oxygen gas, which was created from the oxygen atoms they didn’t require. Plants require both carbon dioxide and water molecules to perform photosynthesis but this can be difficult for desert plants, which face both scarcity of water and high evaporation.

Daytime temperatures in most deserts can be extremely high and the air is also extremely dry. This means that plants with open stomata can lose lots of water through them. The lack of water vapor in the desert environment also means that the heat of the day quickly escapes at night, and temperatures drop dramatically. Deserts can become dangerously cold at night, resulting in far less evaporation than during the day.

As a result of this drop in temperature at night, some desert plants have developed a process known as Crassulacean Acid Metabolism or CAM . The stomata of CAM plants open at night, when the air is cold and evaporation rates are much lower, and they take carbon dioxide into their cells at night and convert it into a carbon-containing acid. When the sun rises, the stomata close, but the carbon stored in the vacuoles of plant cells can be used to produce glucose.

Mechanism of opening and closing of stomata

Stomata open and close depending on the demands of their environments, plants can employ a variety of strategies for opening and closing their stomata. Plants, for example, face the challenge of balancing carbon dioxide intake with water loss because water can evaporate when stomata open to allow gas in.

Two guard cells surround the stomata, which control their opening and closing. During the day, stomata open to allow gas exchange and to release water vapor through transpiration. The change in turgor pressure of the guard cell causes the stomata to open and close.

Water is absorbed by the roots during the day due to increased transpiration pull, and it is transferred to various regions of the plant via the xylem. The guard cell swells and becomes turgid after receiving this water and as a result, the stomatal pore becomes open. Likewise, the guard cell becomes floppy and shrinks as the roots absorb less water at night, and the stomatal pores close as a result of this.

FAQ on stomata

How do stomata open and close?

The stomata are made up of two guard cells. The walls of these cells are thicker on the inside than on the outside. Because of the unequal thickening of the paired guard cells, the stomata open when water is taken up and close when water is lost. When a plant opens and closes its stomata, it is maintaining a balance between gas exchange and water loss. Also, the closing of a plant’s stomata will prevent water from escaping through open pores.

How do guard cells open and close stomata?

Each stoma opening is surrounded by a pair of guard cells and one of many possible environmental or chemical signals causes the cells to open. For instance, strong sunlight or higher-than-average levels of carbon dioxide inside the cell are examples of the previously mentioned environmental or chemical changes.

Guard cells take in sugars, potassium, and chloride ions (i.e., solutes) through their membranes in response to these signals and a rise in solute concentration causes an influx of water across the guard cell membrane. The guard cells “inflate” into two kidney-bean-like shapes as their volume increases and as they expand, the stoma opening in the center of the two guard cells is revealed (similar to a hole in the center of a doughnut). Once fully expanded, the stoma is open, allowing gases to flow between the cell and the surrounding environment.

The pore of the stoma closes in the opposite direction when there is excessive water loss through the stoma, such as during a drought, which causes chemical reactions that signal water and ions to exit the guard cells. As solutes pass through the guard cells, the pair “deflates,” closing the stoma like two flat balloons.

On a whole, osmotic pressure is used by guard cells to open and close stomata, allowing plants to control the amount of water and solutes within them.

What is most likely to happen if you add drops of saltwater on top of stomata?

NaCl (saltwater) inhibits plant growth and reduces crop yield by reducing soil water potential and limiting water uptake. But salinity stress, on the other hand, as a result of the application of saltwater raises ABA (Abscisic acid) levels in the shoot, which promotes stomatal closure, which reduces water loss. So in summary, the most likely result of what happens when saltwater drops on top of stomata is the closing or closure of the stomata to reduce water loss.

What is the function of stomata in plants?

A plant may open and close its stomata to prevent excess water loss and maintain the exchange of gases that goes in and out of the plant through the stomata.

What two gases move in and out of the leaf stomata?

Carbon dioxide (CO2) and oxygen (O2) are the 2 gases that move in and out of the leaf stomata. This is because Carbon dioxide and oxygen cannot pass through the leaf cuticle in large amounts that are necessary for optimum photosynthesis.

Do hornworts have stomata?

Yes, hornworts do have stomata but these tiny microscopic openings While stomata are common in hornworts and mosses, they are not ubiquitous.

What are the plants that keep their stomata open only at night?

CAM plants which are usually xerophytes (plants that grow in the desert region) keep stomata closed in the daytime and open it only at night. This is because of the hot dry environment during the day that will cause water to leave the plant through transpiration.
On a whole, CAM plants have adapted to very dry climates by opening their stomata only during the night.

What happens when the stomata on a plant’s leaves are open?

When stomata on a plant’s leaves are open, water vapor and other gases, such as oxygen, are released into the atmosphere through them. Simultaneously, carbon dioxide is taken into the plants to be used and stored for photosynthesis.

What are the cells surrounding the stomata called?

The cells that surround the stomata are called guard cells and these cells are the ones that are responsible for the opening and closing of the stomata in a leaf of a plant. these guard cells are the cells that are on each side of the stomata and open and close it like a mouth.

What are stomata?

Stomate, plural stomata, or stomas, are any of the microscopic openings or pores in the epidermis of leaves and young stems. Stomata are more abundant on the underside of leaves and they allow gases to exchange between the outside air and the leaf’s branched system of interconnecting air canals.

Do bryophytes have stomata?

In answering this question of whether bryophytes have stomata one has to note that not all members of the bryophytes have stomata. But those that have stomata are hornworts and mosses and the stomata present are not much like the ones found on higher plants.

The stomata in bryophytes are located on sporangia and are restricted in their occurrence across phylogeny. The only member of the bryophytes that do not have stomata are the liverworts and they are the only extant land plants that lack stomata entirely.

Why does the lower epidermis have more stomata than the upper epidermis of a leaf?

Terrestrial plants have stomata on the surface of their leaves and the reason why most of the stomata are on the bottom side of the leaf is that that side is the cooler part of both sides of the leaves. The bottom side of a leaf is less susceptible or less prone to water loss through direct contact from the sun on the leaves like the upper side does.

Where are the stomata located?

Stomata are found on the lower side of the leaf in most plants, but trees such as pine have stomata all-around their needles. However, if the leaf is floating (as in a water lily), the stomata will be on top.

Completely submerged leaves may not have any stomata at all (because of them being useless). The location of the stomata aid the air spaces in the spongy layer of a leaf connect with air through stomata in the epidermis.

How does stomata density vary with CO2 concentration?

The greater the number of stomata per unit area (stomata density), the more CO2 can be taken up and the greater the amount of water that can be released. As a result, increased stomata density can greatly increase the potential for behavioral control over water loss rate and CO2 uptake.

What cells form stomata?

The epidermal cells of a leaf that contains chloroplast which is called guard cells are the cells that form the stomata.

If a plant’s stomata were always closed, how would this affect the plant’s ability to make glucose?

Plants with closed stomata will have less glucose produced but it can also lead to the plants producing enough glucose that is needed for their function. For instance, plants like liverworts with no stomatal opening still produce glucose. on a whole, closed stomata will not greatly affect the plant’s ability to make glucose.

What is the relationship between stomata and photosynthesis?

The stomata photosynthesis or the photosynthesis stomata relationship is based on the opening of the stomata to take in CO2 into the plant. This carbon dioxide is a primary ingredient for plants in the manufacturing of their food in the form of starch. It is then fair to say that the relationship between the stomata and photosynthesis is that the more the stomata open in the day for normal plants the more they take in CO2 for photosynthesis. While CAM plants are based on the opening of the stomata in the nighttime to take in carbon dioxide for photosynthesis during the day when the stomata are usually closed.

What do stomata look like under the microscope?

Stomata under a microscope look like a tiny doughnut hole in the shape of an “O” when they are open. These same stomata look like the shape of the letter “I” when they are closed. This microscopic description can be used to answer the question of what stomata look like.

What is the role of stomata in photosynthesis?

Stomata regulate the flow of gases into and out of the leaves. During the day, when air temperatures rise and CO2 levels are normal or higher than normal, the stomata open, allowing carbon dioxide to enter and photosynthesis to occur.

What is the function of stomata in transpiration?

The function of stomata in transpiration is that they help in the intake and release of water in the plant. Especially using the opening and closing mechanism.

What is the function of stomata in water gain and loss?

One of the main functions of stomata in water gain and loss is their ability to detect changes within the plant when it comes to releasing hormones such as the ABA hormone that triggers water gain and reduces water loss when the stomata close. Another scenario is when stomata detect environmental changes like humidity which will either trigger water gain or loss.

What is the function of stomata in roots?

The root stomata function to facilitate increased gaseous exchange during respiration and/or increased transfer of some nutrients and water in the rapidly growing primary root.

A video showing stomata structure and how they work.