Stomata are tiny, specialized structures found in the outer layer of a plant leaf. These small openings regulate the exchange of gases necessary for life, specifically carbon dioxide intake and the release of oxygen and water vapor. Viewing them under a microscope transforms the flat surface of a leaf into a landscape dotted with these repeating structures. Understanding the visual components of these pores provides insight into how plants manage their internal environment.
Basic Anatomy and Location
The fundamental structure of a stoma centers around a pair of specialized cells, known as guard cells, which surround a central opening called the stomatal pore. Under a microscope, these guard cells present a distinct, curved appearance. They are readily distinguishable from the surrounding pavement-like epidermal cells, which are typically irregular in shape.
Unlike other epidermal cells, guard cells contain chloroplasts, giving them a slightly different coloration. The stomatal complex is most frequently observed on the lower surface of leaves, a placement that helps minimize direct exposure to sunlight and reduce excessive water loss. In some plants, however, these structures are distributed across both the upper and lower leaf surfaces.
Diversity in Stomatal Structure
The shape of the guard cells is the primary distinguishing feature across plant species. In many flowering plants, particularly broad-leafed dicots, the guard cells have a kidney-shaped or bean-shaped morphology.
A different look is observed in monocots, especially members of the grass family, where the guard cells are elongated with bulbous ends, giving them a characteristic dumbbell shape. The pore between these cells often appears as a long, narrow slit rather than the oval opening seen in kidney-shaped stomata. Subsidiary cells, specialized epidermal cells that flank the guard cells, contribute to the visual complexity by varying in number and arrangement. These flanking cells can be indistinguishable from other epidermal cells or be highly prominent, lying parallel to the guard cells, as is commonly seen in grasses.
How Appearance Changes During Function
The appearance of the stomatal complex is dynamic, changing based on the plant’s internal water status. This transformation is controlled by changes in turgor pressure, the internal water pressure pushing against the guard cell walls. When water moves into the guard cells, increasing their turgor, the stomata adopt their “open” look.
During the open state, the guard cells swell and bow outward, pulling apart the inner walls and creating a wide, visible pore for gas exchange. The unique shape and thickened inner walls of the guard cells facilitate this curving action. Conversely, when the guard cells lose water, they become flaccid, the turgor pressure drops, and the cells straighten or collapse against each other. This effectively seals the pore shut and represents the plant’s effort to balance carbon dioxide uptake with water conservation.