The exchange of carbon dioxide and oxygen is known as respiration and the loss of water to the environment is known as transpiration. Neither of these processes will be discussed in this paper. The guard cells are the main structure of the stomatal apparatus and are next in line for discussion. Dicotyledons have kidney-shaped structures and like all other cells in the epidermis are covered by a layer of wax known as the cuticle. This cuticle is used to prevent diffusion of water through the entire epidermal layers and may be located over the stomata in order to help prevent excessive water loss while the stomata is open.
Monocotyledons are diacytic and the stomatal complex contains two subsidiary cells.
The stomata of gymnosperms are different from angiosperms because they are commonly sunken. The guard cells usually appear suspended from the subsidiary cells. In addition, the guard cells contain lignin, which is a compound often thought to aid in support. Subsidiary Cells Subsidiary cells are cells that are located centrifugal to the guard cells. They can be classified into three different origins.