Atlas of plant and animal histology

Stomata are present in the aerial parts of the plants. They regulate gas and water exchange with the environment. Stomata are composed of guard cells and a substomatal chamber, located beneath the guard cells. Stomata are scattered among epidermal cells, mostly at the same level as epidermal cells (image A), though they can 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 associated cells called subsidiary or companion cells. Guard and subsidiary cells differ from epidermal cells by their morphology and size, along with the uneven thickening of their cell walls. Guard cells of an individual stoma may be more or less separated, allowing an opening between them, known as the pore or ostiole (Figure 1), which communicates the external environment with the substomatal chamber and then with the internal tissues of the plant. Guard cells, subsidiary cells, and the substomatal chamber form the stomatal complex.

The number and distribution of stomata in the leaves depend on the species and external factors influencing the leaf. Stomata are typically found in the lower surface of the leaf (abaxial or shaded surface). However, in aquatic plants with leaves floating on the water, the stomata are found in the upper surface (adaxial, exposed to the sunlight), while totally submerged leaves may lack stomata. The number of stomata is also variable. For instance, xerophyte plants, adapted to dry environments, show many stomata for an intense exchange of CO₂ and oxygen during the very short time of less-dry periods of the day. The disposition of stomata in the epidermis is apparently random in dicot plants, whereas in monocot plants they are organized in the long rows parallel to the veins, which extend from the petiole to the leaf tip.

More images