Stomata: The Leaf's Breath
The Anatomy of a Stoma
Each stoma (the singular of stomata) is a brilliantly simple yet highly efficient structure. It is not just a hole in the leaf; it is a complex pore surrounded by two specialized kidney-shaped or bean-shaped cells called guard cells.
The primary role of guard cells is to control the size of the stomatal pore. When they are full of water, or turgid, they swell and bend, pulling the pore open. When they lose water and become flaccid, they relax and collapse together, closing the pore. This mechanism is like inflating and deflating two long balloons side-by-side; when inflated, they curve away from each other, creating an opening.
In many plants, the stomata are also surrounded by subsidiary cells that help the guard cells function more effectively. The entire structure is typically found on the underside of leaves, which helps reduce water loss by being sheltered from direct sunlight and wind.
The Vital Functions: Gas Exchange and Transpiration
Stomata perform two critical and interconnected jobs for the plant.
1. Gas Exchange for Photosynthesis and Respiration: Plants need carbon dioxide ($CO_2$) from the air to perform photosynthesis. The chemical equation for photosynthesis is:
Carbon dioxide enters the leaf through the open stomata. At the same time, the oxygen ($O_2$) produced as a waste product of photosynthesis exits through these same pores. At night, when photosynthesis stops, plants still respire (like animals do), taking in oxygen and releasing carbon dioxide through the stomata.
2. Transpiration: This is the process of water movement through a plant and its evaporation from the leaves, primarily through the stomata. This creates a “pull” that helps draw water and essential nutrients up from the roots through the stem and to the leaves. This flow is called the transpirational pull. While crucial for nutrient transport and cooling the plant, transpiration also means the plant can lose a tremendous amount of water, creating a constant dilemma.
How Guard Cells Control the Pore
The opening and closing of stomata is a spectacular example of a plant responding to its environment. Guard cells are sensitive to various signals:
- Light: Stomata typically open at dawn. Blue light receptors in the guard cells trigger the activation of pumps that bring in potassium ions ($K^+$).
- Carbon Dioxide: Low levels of $CO_2$ inside the leaf, which occur during active photosynthesis, stimulate stomata to open.
- Water Availability: When a plant is dehydrated, a hormone called abscisic acid (ABA)1 is produced. This hormone signals the guard cells to close the stomata, conserving water.
The process of opening is driven by osmosis. When potassium ions ($K^+$) are pumped into the guard cells, the concentration of solutes inside increases. This causes water to follow by osmosis, flowing into the guard cells from the surrounding cells. The influx of water increases turgor pressure, swelling the cells and opening the stoma. The reverse process causes closing.
Stomata Across the Plant Kingdom
Not all plants have the same number, type, or distribution of stomata. These differences are adaptations to their specific environments.
Plants in hot, dry climates (like cacti) face a greater risk of water loss. They have evolved strategies to minimize this loss:
- Fewer stomata.
- Stomata located in sunken pits on the leaf surface to trap water vapor.
- Stomata that open at night (CAM plants2) when it is cooler and more humid. They take in $CO_2$ at night and store it for use in photosynthesis during the day while their stomata are closed.
Conversely, plants in cool, moist, and shady environments often have more stomata to maximize gas exchange. The number and density of stomata can even be used by scientists to study historical climate conditions, as plants that grew in high-$CO_2$ atmospheres tend to have fewer stomata.
| Plant Type | Environment | Stomatal Adaptation |
|---|---|---|
| Cactus | Hot, Dry Desert | Very few stomata; often open only at night (CAM) |
| Water Lily | Aquatic | Stomata only on the top surface of the leaf (the part exposed to air) |
| Pine Tree | Temperate, Seasonal | Stomata sunken below the surface of the needle to reduce wind and water loss |
| Fern | Cool, Moist, Shady | High density of stomata to maximize gas exchange in low-light conditions |
Observing Stomata in the Real World
You can see stomata for yourself with a simple experiment. Take a leaf from a plant like a spider plant or a begonia. Paint a thin layer of clear nail polish on the underside of the leaf. Let it dry completely. Then, carefully place a piece of clear tape over the dried polish, press down, and peel the tape off. The polish will come with it, creating a perfect impression of the leaf’s surface. Place the tape on a microscope slide and observe it under a microscope. You will see the tiny, mouth-like stomata and the guard cells surrounding them! This activity shows how scientists make impressions to study plant anatomy without damaging the specimen.
Common Mistakes and Important Questions
In a way, yes, but it’s more accurate to say they exchange gases. Animals breathe, meaning they inhale and exhale air through a single system primarily for respiration. Plants use stomata for both taking in $CO_2$ for photosynthesis and for the gas exchange required for respiration (taking in $O_2$, releasing $CO_2$). It’s a two-way street for different processes.
It’s a classic trade-off. A plant needs $CO_2$ to make food, but opening its stomata to get it means losing precious water. When water is scarce (a hot, dry afternoon), the risk of dying from dehydration is greater than the benefit of making a little more food. So, the plant closes its stomata to survive, temporarily halting photosynthesis. It’s a clever survival strategy.
Primarily, yes, but they can also be found on green stems. For example, the green stems of a cactus perform photosynthesis and have stomata. They are almost always absent on roots, which are designed for water and nutrient absorption, not gas exchange with the air.
Footnote
1 Abscisic acid (ABA): A plant hormone that functions in many developmental processes, including bud dormancy, and is a major player in initiating stomatal closure in response to drought stress.
2 CAM plants (Crassulacean Acid Metabolism plants): Plants like cacti and pineapples that have adapted to arid conditions by opening their stomata at night to take in carbon dioxide and closing them during the day to reduce water loss.