Plants produce their food through photosynthesis, a chemical reaction involving carbon dioxide, water, light and the green pigment called chlorophyll. During photosynthesis, the stoma opens in the presence of light, allowing carbon dioxide to enter the leaf tissue, where it is transformed into sugars that will feed the plant's cells. Oxygen is a by-product of photosynthesis, and it exits the leaves through the stoma. The opening of the stoma also causes the loss of water vapor through a phenomenon called transpiration, which is important to regulate the temperature of the plant.
Like most living creatures, plants need oxygen to survive. Although plants take carbon dioxide and give out oxygen during photosynthesis, the opposite occurs during respiration. Respiration is not a light or chlorophyll dependant process like photosynthesis; therefore it can happen during the night. Low levels of oxygen in the plant tissue help to trigger the opening of the stoma in the dark.
The size and shape of stoma varies according to plant species and environmental conditions, such as temperature, quantity of light and humidity. Most stoma are elongated, while the surrounding guard cells look like dumbbells in grasses and like kidney beans in shrubs and trees. The quantity of stoma per squared millimeter also depends on species, light and environmental conditions, ranging from 100 to 1,000. Grasses generally have lower stoma density in comparison to trees.
The stoma is also a gateway to many plant pathogens, which enter the plant tissues during photosynthesis and respiration. However, most plants can detect the presence of pathogens and close the stoma. Some pathogen bacteria are able to force the opening of the stoma by releasing coronatine and other chemicals. Sulfur dioxide, oxides of nitrogen, carbon monoxide and other air pollutants can enter the leaf through the stoma. These gases can cause reduction in growth rates in some plants.