Stomate (stoma)

A stomate (stoma) is a tiny opening that allows for the exchange of gas between the interior of a plant and the atmosphere. Found most abundantly on the surface of leaves, these mouth-like pores are opened and closed by special cells known as guard cells that expand and contract in response to a variety of different factors, including carbon dioxide levels, the availability of sunlight, and changing environmental conditions. When open, the stomata enable plants to take in carbon dioxide for use in photosynthesis while also allowing for the intake of oxygen and release of carbon dioxide during respiration. When closed, the stomates allow plants to prevent excess water loss and retain moisture as necessary. Under normal circumstances, the stomates typically stay open while photosynthesis is taking place during the day and close only when sunlight ceases to be available at night. In any event, stomates play a key role in plants’ ability to adapt to changing conditions and ultimately survive.

Brief History

Research suggests that stomates first came into existence more than four hundred million years ago. In fact, it was likely the emergence of stomates that initially enabled aquatic plants to overcome the inhospitable conditions of the terrestrial environment and begin colonizing Earth’s great landmasses. The subsequent spread of plant life across the planet effectively laid the groundwork for the evolution of the many complex ecosystems that exist around the world. As a result, the emergence of stomates was vital not only to the development of plants but also to that of all forms of terrestrial life.

The historical study of stomates is thought to date back as far as the seventeenth century. The first known investigation of stomates was undertaken by French physicist Edme Mariotte in 1660. English scientist Robert Hooke is credited with the formal discovery of stomates after he observed the structures with the aid of a simple microscope around 1665. The earliest published description of stomates appeared in Italian biologist Marcello Malpighi‘s Anatome Plantarum in 1675. Although Malpighi offered valuable insight into the physical appearance of stomates, he was unable to draw any firm conclusions about their function. Among the first scientists to tackle the question of what purpose the stomates served was English plant anatomist Nehemiah Grew. A contemporary of Malpighi’s, Grew studied stomates and published his findings in 1682. He suggested that the stomates were simply openings through which plants took in air, water, and nutrients. As a result, stomates were generally thought of as “breathing holes” until around the mid-nineteenth century. A more accurate description of the function of stomates was first put forth in a landmark paper written by German botanist Hugo von Mohl in 1850. In his work, von Mohl accurately concluded that the opening and closing of stomates was the result of changes in guard cells. Later in the century, scientists like N.J.C. Muller, Simon Schwendener, and Gottlieb Haberlandt built on von Mohl’s work and further explained the mechanics of stomates’ movements. From there, the modern scientific understanding of stomates and their functions gradually evolved as study and observation continued through the twentieth and into the twenty-first centuries.

Overview

Stomates primarily serve to regulate transpiration and the exchange of gases, in plants. They accomplish this by allowing for the intake of carbon dioxide and the emission of water vapor and oxygen as required by varying conditions and circumstances. Much of the function of stomates is tied to photosynthesis, which is the process through which plants use sunlight to create food from carbon dioxide and water. During photosynthesis, plants convert carbon dioxide, water, and sunlight into glucose, water vapor, and oxygen. While plants use the glucose as a source of energy, the oxygen and water vapor are excreted through the stomates. Thus, stomates are critical to photosynthesis at both ends of the process.

Although stomates can be found on many different plant surfaces, most are located on the surface of the leaves. Any given leaf can have thousands of stomates. On most plants that live on land, the stomates are mainly found on the underside of each leaf. This placement helps to protect the stomates from overexposure to heat and air currents. On aquatic plants, the stomates are found on the upper surface of leaves.

The action of stomates is controlled by large, crescent-shaped cells called guard cells. A pair of guard cells connected to each other at both ends surrounds each stomate and opens or closes it by expanding or contracting. Unlike other cells present in the structure of plant leaves, guard cells also contain chloroplasts, which are the special organelles found in certain plant cells that capture light for use in photosynthesis. The expansion and contraction of guard cells—and therefore the opening and closing of stomates—is a reaction to various stimuli, including the presence of light, the level of carbon dioxide within the plant, and certain environmental conditions, such as humidity and excessive heat. When humidity levels rise, for example, the excess water causes the guard cells to expand and open the stomates to allow water vapor to be released. When conditions are drier, however, lack of water will cause the guard cells to shrink and close the stomates to prevent too much moisture from escaping.

Research has shown that stomata play an important role in plant responses to global environmental change. Rising atmospheric carbon dioxide concentrations, increasing temperatures, and changing patterns of drought can alter stomatal behavior and influence the movement of carbon dioxide and water between plants and the atmosphere. Scientists continue to study these responses because they affect plant growth, ecosystem function, and the global water cycle.

Structurally, the guard cells are surrounded and supported by subsidiary cells that separate the guard cells from the epidermal cells that make up the rest of the leaf’s surface. The subsidiary cells protect the epidermal cells from the expansion of guard cells. In different plants, the subsidiary cells exist in different sizes and shapes. Based on the varying characteristics of these cells, there are several different types of stomates. Anomocytic stomates have irregularly shaped cells between the guard cells and epidermal cells. Anisocytic stomates have two large subsidiary cells and a small third one surrounding the guard cells. Diacytic stomates are surrounded by a pair of subsidiary cells that run perpendicular to the stomate itself. Paracytic stomates have a pair of subsidiary cells that run parallel to the stomate. Gramineous stomates have unique guard cells that are much narrower in the middle than at the ends. They also have subsidiary cells that run parallel to the stomate.

Bibliography

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