Stomata are microscopic pores on the surfaces of plant leaves that allow gases to move in and out. They serve as the plant’s breathing system: carbon dioxide enters through these tiny openings to fuel photosynthesis, while oxygen and water vapor exit. A single leaf can have anywhere from 5 to 1,000 stomata per square millimeter, depending on the species and growing conditions, and plants lose roughly 97% of the water they absorb through these pores under normal conditions.
How Stomata Are Built
Each stoma (the singular form) is formed by a pair of specialized cells called guard cells. These guard cells are kidney-shaped and sit on either side of the pore, controlling its size like two curved hands that can press together or pull apart. The walls of guard cells are not uniform in thickness. The inner walls facing the pore are thick and rigid, while the outer walls are thinner and more flexible. This uneven construction is what allows the pore to open and close as the guard cells change shape.
Surrounding the guard cells, many plants have additional helper cells called subsidiary cells, which assist with the mechanical movement of the pore and help supply water and ions. Inside the guard cells themselves are small structures that store starch, along with a large central water-filled compartment called a vacuole. The entire unit, guard cells plus subsidiary cells, is called the stomatal complex.
Where Stomata Appear on Leaves
Not all leaves carry stomata in the same places. Most broad-leaved plants are “hypostomatous,” meaning their stomata are concentrated on the underside of the leaf. This placement reduces direct sun exposure on the pores, which helps limit water loss. A smaller number of species have stomata only on the upper surface, but the most common arrangement overall is “amphistomatous,” with stomata on both sides.
Grasses, including wheat, tend to have roughly equal numbers on both surfaces, and the stomata on the top and bottom can actually respond independently to their local environment. One side of the leaf might partially close its pores while the other side stays open, depending on differences in light or CO2 concentration hitting each surface. Plants like common beans follow a more typical pattern, with most stomata on the lower surface.
How Gas Exchange Works
The core job of stomata is moving CO2 from the air into the leaf’s interior, where photosynthesis happens. The path is surprisingly complex. CO2 first crosses a thin layer of still air hugging the leaf surface, then passes through the stomatal pore into an air-filled cavity just below. From there, it diffuses through the spongy network of air spaces inside the leaf until it reaches the walls of photosynthetic cells. At that point, CO2 dissolves into water in the cell wall, crosses the cell membrane, and finally reaches the enzyme that captures carbon for sugar production.
Oxygen and water vapor travel the same route but in reverse. Oxygen produced by photosynthesis diffuses out of the cells, through the internal air spaces, and exits through the stomatal pore. Water vapor follows the same outward path, and this constant stream of evaporating water, called transpiration, also helps pull water up from the roots like a straw. The trade-off is steep: for every molecule of CO2 a plant captures, it can lose hundreds of water molecules through the same opening.
How Stomata Open and Close
Stomatal movement comes down to water pressure inside the guard cells, known as turgor pressure. When guard cells absorb water, they swell and bow outward, pulling the pore open. When they lose water, they go limp and the pore closes. The plant controls this by pumping charged particles, especially potassium ions, in and out of the guard cells.
To open stomata, the plant activates channels in the guard cell membrane that pull potassium ions inward. As potassium accumulates inside the cell, water follows by osmosis, inflating the guard cells and widening the pore. To close stomata, a different set of channels activates, pushing potassium and other ions back out. Water follows, the guard cells deflate, and the pore shuts. This system responds within minutes to changing conditions.
The Role of Stress Hormones
When a plant is running low on water, its roots and leaves ramp up production of a stress hormone called abscisic acid (ABA). ABA triggers a rapid signaling chain inside the guard cells that activates the ion channels responsible for pushing potassium out. The whole process, from hormone signal to pore closure, can begin within about one minute. This fast response is critical during drought, allowing the plant to slam its pores shut before too much water escapes.
What Makes Stomata Open or Close
Light is the strongest trigger for stomatal opening. Blue light in particular drives guard cells to absorb potassium and swell. Under bright conditions, stomata open wide to maximize CO2 intake for photosynthesis. Conductance (a measure of how freely gases pass through stomata) typically saturates at about half of full sunlight intensity, meaning stomata don’t keep opening wider beyond that point.
Humidity also plays a major role. When the air around a leaf is dry, the difference in water vapor between the inside of the leaf and the outside atmosphere increases. Plants respond to this drying gradient by partially closing their stomata to conserve water. Higher temperatures can amplify this effect, making stomata more sensitive to CO2 levels and humidity changes simultaneously. The interplay between these factors means that stomatal behavior on a hot, dry afternoon looks very different from a cool, humid morning, even on the same leaf.
How Rising CO2 Is Changing Stomata
Before the Industrial Revolution, atmospheric CO2 held steady at around 280 parts per million for thousands of years. Today it exceeds 420 ppm, a level not seen in at least 420,000 years and possibly not in the past 20 million years. Plants are responding. Studies of herbarium specimens collected over the past 200 years show that at least eight tree species have measurably reduced the number of stomata on their leaves since the Industrial Revolution.
The logic is straightforward: when CO2 is more abundant in the air, each individual stoma can capture the same amount of carbon with a smaller opening or fewer pores. Plants are taking advantage of this by producing fewer, smaller stomata, which cuts water loss through transpiration while still meeting their photosynthetic needs. Lab experiments growing plants at pre-industrial CO2 levels confirm the pattern. Plants given less CO2 develop more stomata, while those given elevated CO2 develop fewer. Under the most pessimistic climate projections, atmospheric CO2 could reach 970 ppm by 2100, which would likely push stomatal density even lower across many species.
Stomatal Adaptations in Extreme Environments
Desert plants, known as xerophytes, have evolved some of the most dramatic stomatal modifications. Many develop sunken stomata, recessed into pits below the leaf surface where a pocket of humid air reduces evaporation. Others grow dense hairs (trichomes) around their stomata that trap moisture and slow airflow across the pore. Some desert species maintain unusually small guard cells, which physically limits how wide the pore can open and puts a ceiling on water loss.
Certain desert plants go further by flipping the normal schedule entirely. Instead of opening stomata during the day when photosynthesis is most active, they open them at night when temperatures drop and humidity rises, storing CO2 chemically for use during daylight hours. This strategy, used by cacti and succulents among others, can cut water loss dramatically. Some sodium-adapted desert species, like the Central Asian shrub Zygophyllum, even reduce their stomatal density under drought stress, producing fewer pores per leaf to minimize transpiration at the cost of slower growth.