Stomates

AbstractStomates or stomata (singular: stomate or stoma) are structures on the surfaces of plant leaves that allow gas exchange (transpiration, i.e., loss of water vapor, uptake of CO2, and emission or uptake of O2) between the interior of a leaf and the atmosphere.Plant leaves are covered by a waxy layer, the cuticle, that prevents water loss from the cells on the leaf surface, or epidermis. The cuticle does not completely cover the leaves, however; it is interrupted by microscopic pores, surrounded by pairs of specialized guard cells. A stoma is a unit composed of a pore and its guard cells. Guard cells, unlike regular epidermal cells, usually contain chloroplasts (photosynthesizing organelles containing chlorophyll). In some plants, there are also additional, specialized subsidiary cells that differ in shape from regular epidermal cells and participate in the osmotic changes that drive guard cell movement.Stomata can be found in all aboveground parts of plants but are more frequent in leaves. They usually appear on one (the lower) side of leaves but may appear on both. When stomata occur on the lower (abaxial) leaf surface, the plant is said to be hypostomatous; hyperstomatous plants have stomata only on the upper (adaxial) leaf surface, and plants with stomata on both surfaces are called amphistomatous. In this latter case, abaxial stomata are generally more numerous. The number of stomata per unit area is called the stomatal density. This figure may vary when plants are grown under different environmental conditions such as atmospheric CO2concentration.