In plants, a stoma is a tiny pore in the surface of a leaf that is used for gas exchange. Most leaves are covered in these tiny pores, which allow the plants to take in carbon dioxide for use in photosynthesis and expel their waste oxygen.
The term “stoma” comes from the Greek word for “mouth.” Groups of more than one stoma are often called “stomata” rather than “stomas,” because “stomata” is the Greek way of pluralizing “stoma.”
Stomata are crucial for a plant’s life functions because they allow carbon-containing carbon dioxide gas to enter the plant’s tissues. These gas molecules are actually the source of the carbon atoms used by plants to create sugars, proteins, and other essential materials for life.
Without carbon dioxide reaching their tissues, plants could not create the organic molecules they need to survive and grow. Among land plants, only liverworts lack stomata.
Just like a mouth, the opening and closing of a stoma is controlled by a surrounding pair of lip-like structure called “guard cells.” These cells can grow larger or smaller, opening or closing the stoma as is required by the plant.
Scientists are still not sure what exactly triggers these guard cells to open or close the stomata, but it is thought that they might respond to external stimuli such as light and humidity.
Plants can have different strategies for opening and closing their stomata, depending on the demands of their environments.
For example, one challenge faced by plants is the need to balance carbon dioxide intake with water loss. When stomata open to let gas in, water can also evaporate out. For desert plants, that can be a very big problem.
Most plants open their stomata during the day, so that they can maximize carbon dioxide intake during the prime hours to harvest energy from sunlight.
But CAM plants, which evolved in desert environments, open their stomata at night when it’s cooler instead. That way they lose less water than they would if they opened their stomata under the scorching desert sun.
They then store CO2 in the form of a carbon-containing acid within their cells, so that they can use it for photosynthesis when the sun rises.
In medicine, the term “stoma” can also refer to an artificial opening to an organ, such as those created by doctors for colostomies and urostomies.
The primary purpose of stoma is to allow carbon dioxide gas to enter the air spaces in a plant’s tissues. Once inside these air spaces, the CO2 can be used by the plant’s photosynthetic tissues as sources of carbon to build sugars, amino acids, and more!
The very first photosynthetic organisms had to make all of their biological materials from scratch. They learned to do this by stringing carbon molecules from CO2 gas into complex organic molecules. These early photosynthesizers would then release waste oxygen gas, made from the oxygen atoms they didn’t need.
A molecule of glucose, for example, can be made using the equation:
6CO2 + 6H2O + SUNLIGHT → C6H12O6 + 6O2
Sugar molecules like glucose, in turn, can be used to make the backbone for amino acids, nucleotides, and lipids as well as for long-term energy storage.
Early photosynthesizers had a single cell, or just a few cells. But as land plants became more complex, it became harder for their many cells and thick tissues to get all the carbon dioxide they needed. Stomata evolved to ensure that enough CO2 could penetrate plant leaves and other tissues to ensure efficient photosynthesis.
As you can see from the equation above, plants require both carbon dioxide and water molecules to perform photosynthesis. This can be tricky for desert plants, which have the dual challenges of scarce water and high evaporation.
In most deserts, daytime temperatures can be very hot indeed. The air also tends to be very dry. That means that plants with open stomata can lose large amounts of water through them – something which plants in these almost rain-free environments cannot afford.
But at night, the lack of water vapor means that the heat of the day quickly escapes, and temperatures fall quite rapidly. Deserts at night can get downright cold – which means much, much less evaporation than during the daytime.
That’s why some desert plants have evolved something called Crassulacean Acid Metabolism, or CAM. CAM plants open their stomata at night, when the air is cold and evaporation rates are much lower. During the night, they take carbon dioxide into their cells and convert it into a carbon-containing acid.
When the sun rises, the stomata close – but the carbon stored in the plant cells’ vacuoles can be used to create glucose!
Types of Stoma
Biologists have used several classification systems to describe different types of stomata. These systems classify stomata based on their placement on plant leaves; based on the structure of surrounding subsidiary cells; or based on the developmental course of the guard cells and subsidiary cells.
Apple or Mulberry (Hypostomatic) Type
On many fruit trees such as apples, mulberries, peaches, and walnuts, stomata are found only on the lower surfaces of leaves. This may reduce water loss by taking in carbon dioxide through the cooler, shaded surfaces of the leaves.
The term “hypostomatic” comes from the term “hypo” for “under” or “below” and the term “stoma.”
On potatoes as well as beans, cabbage, and related plants, stoma are found primarily on the lower surfaces of leaves, but are also found in smaller numbers on the top sides of leaves.
Oat (Amphistomatic) Type
In oats and other grasses, stomata are equally distributed across all faces of the leaf, since all sides of grass blades get roughly equal exposure to the air.
The term “amphistomatic” comes from the Greek “amphi” for “both sides” and the term “stoma.”
Water Lily (Epistomatic) Type
Water lilies and other aquatic plants have stoma found on the upper surfaces of leaves – the surfaces that are typically above water.
The term “epistomatic” comes from the Greek “epi” for “over” or “on top of,” and the term “stoma.”
Potamogeton (Astomatic) Type
Potamogeton and submerged aquatic plants lack stomata entirely, or have vestigial stomata that do not function.
The term “astomatic” comes from adding the Greek word “a” for “without” to the word “stoma.”
Actinocytic stomata are surrounded by at least four cells, which form a circle with the stoma and its guard cells at the center. These types of stoma can resemble a daisy, with a single row of petals radiating out from a stoma at the center.
Anisocytic or Cruciferous
This type of stoma is surrounded by three cells of unequal size. These cells can resemble roses, with cells like petals of increasing size spiraling out from a central stoma.
Anomocytic or Ranunculaceous
Anomocytic cells are surrounded by a small number of subsidiary cells, which are identical to the cells of the surrounding epidermis.
These stoma are surrounded by at least four cells, arranged in a ring around the stoma. These subsidiary cells may resemble carnations, with multiple layers of cells ringing the central stoma like petals.
Diacytic or Caryophyllaceous
This type of cell is bordered by just two surrounding cells – one on each side of the stoma. The surrounding subsidiary cells are at right angles to the guard cells, meaning that they intersect in the middle of guard cells rather than at their edges. These arrangements can resemble brick walls, with stoma placed between subsidiary “brick” cells.
These types of stoma may be quite close together indeed, with the same subsidiary cell possibly bordering different stomata on different sides!
Graminaceous stomata have dumbell-shaped guard cells, which are sandwiched between subsidiary cells that run parallel to them.
Like diacytic cells, paracytic cells are surrounded by just two guard cells – one on each side. But in paracytic cells, the gaps between subsidiary cells align with the gaps between guard cells, instead of being at right angles to them.
Based on Plant Development
In mesogynous stomata, the guard cells and their surrounding subsidiary cells develop from the same mother cell.
In this type of stomata, guard cells are formed from one mother cell, while subsidiary cells form from different mother cells.
In mesoperigynous stomata, guard cells develop from one mother cell, while subsidiary cells develop both from that same mother cell and from neighboring cells.
As the name suggests, this type combines attributes of both mesogynous and perigynous stomata.