Atlas of plant and animal histology

Stomata are present in the aerial parts of the plants. They regulate gas and water exchange with the atmosphere. Stomata are composed of guard cells and a substomatal chamber, located under the guard cells. Stomata are scattered among epidermal cells, mostly at the same level as epidermal cells (image A), but they may also protrude (image B) or be somewhat sunken (image C). In the latter case, they can even be found in invaginations of the epidermis called stomatal crypts. Commonly, the guard cells are surrounded by a variable number of attached cells called subsidiary or companion cells. Guard and subsidiary cells are differentiated from epidermal cells by their morphology and size, as well as by the uneven thickening of their cell walls. Guard cells of the same stoma may be more or less separated, allowing an opening between them, known as pore or ostiole, which communicates the external environment with the substomatal chamber, and then with the internal tissues of the plant. Guard cells, subsidiary cells and substomatic chamber form the stomatal complex.