The question of whether a stoma is the same as a stomata arises frequently due to their similar appearance and sound. This linguistic confusion touches on a fundamental aspect of plant biology, concerning the structures that allow plants to interact with the atmosphere. This article will define the distinct meanings of “stoma” and “stomata” and explore the anatomy and function of these microscopic structures.
The Difference Between Stoma and Stomata
The distinction between the two terms is immediate and definitive, resting on the difference between singular and plural forms. A stoma (pronounced STOH-muh) is the singular form, referring to one individual opening or pore found on the surface of a plant’s leaves or stems. This pore is where the exchange of gases and water vapor occurs. In contrast, stomata (pronounced STOH-muh-tuh) is the plural form, describing the multiple pores collectively across the plant’s surface.
The difference is purely grammatical, similar to the relationship between bacterium and bacteria. Understanding this rule clarifies the functional units responsible for plant respiration and hydration.
The Anatomy of the Stomatal Complex
Every stoma is an integral component of a larger, specialized structure called the stomatal complex. The core of this complex consists of the pore itself, which is surrounded by two specialized cells known as guard cells. These guard cells are distinct from the surrounding epidermal cells because they contain chloroplasts and are capable of photosynthesis.
The paired guard cells have a unique, curved shape, often described as kidney-shaped. Their cell walls are unevenly thickened, which is important for their function. In some plants, the stomatal complex also includes accessory cells called subsidiary cells, which border the guard cells. These subsidiary cells provide mechanical support or act as a reservoir for water and ions, aiding the guard cells in their movements.
How Stomata Facilitate Plant Life
The primary role of the stomata is to manage the plant’s gas exchange, which is a constant balancing act between two necessities. The first function is to allow carbon dioxide (\(\text{CO}_2\)) from the atmosphere to enter the leaf for photosynthesis, while simultaneously releasing the oxygen (\(\text{O}_2\)) produced as a byproduct. This exchange is only possible when the pores are open.
The second, and often conflicting, function is the regulation of water loss through a process called transpiration. When the stomata are open to take in \(\text{CO}_2\), water vapor inevitably escapes from the moist interior of the leaf. This water loss creates a suction force that helps pull water and nutrients up from the roots through the xylem.
The opening and closing of the stoma are precisely controlled by changes in the turgor pressure within the guard cells.
Stomatal Opening
When the plant needs to open the pore, it actively transports solutes, such as potassium ions (\(\text{K}^+\)), into the guard cells. This influx of solutes causes the water potential inside the guard cells to decrease, which causes water to move into the cells via osmosis. As the guard cells swell and become turgid, their unique wall structure causes them to bend outward, widening the stomatal pore.
Stomatal Closing
Conversely, when the plant needs to conserve water, such as during drought stress or high temperatures, signals like the plant hormone abscisic acid trigger the transport of ions out of the guard cells. The subsequent loss of water causes the guard cells to become flaccid, which collapses the pore and effectively seals the leaf surface to minimize dehydration. The plant must continuously weigh the need for \(\text{CO}_2\) for energy production against the risk of excessive water loss.