As of 1970, plant stomata have been divided into eight types: actinocytic, anisocytic, anomocytic, cyclocytic, diacytic, hexocytic, paracytic and tetracytic. A typical stomata formation found in the dicotyledons, or flowering plants, is the anisocytic type that is comprised of a stoma surrounded by three cells of different sizes. The stomata are the microscopic pores on the epidermal layers of land plants that enable them to exchange oxygen, a by-product of photosynthesis, for carbon dioxide.
The presence of stomata on the external portions of land plants is a critical factor in their survival. The stomatal pores protect a plant by opening and closing in response to changes in environmental conditions. In times of drought, for example, the stomatal pores will close to keep water inside the plant and to prevent wilting and dehydration. At the same time, the stomata regulate the carbon dioxide and oxygen gas exchange portion of the respiratory process of land plants that plays a vital role in the global environment.
The development of stomata on plants is considered one of the primary evolutionary advances in the plant world. An event occurring about 400 million years ago, the appearance of stomata on land plants enabled them to survive and adapt to the terrestrial environment.
Studies of the fossil remains of plants has led to the general acceptance that the density and number of stomata appearing on the external portions of plants decreases in response to an increase in the concentration of carbon dioxide in the atmosphere.