In plants, the epidermis (thin surface layer of tissue) is not continuous. It may be interrupted by minute pores called stomata. A stoma is formed by a pore which remains surrounded by two cells called guard cells, which by their changes in shape bring about closing and opening of the stoma. The pore of the stoma is called stomatal aperture through which gaseous exchange takes place.
In monocotyledons (grasses), these guard cells are dumb-bell shaped and the pores are slit-like. Sometimes, the guard cells are semi-lunar crescent-shaped cells and are attached to each other by the curved ends of their concave sides leaving a slit-like opening. In angiosperm plants, these two guard cells are kidney-shaped, and they remain attached to each other by their two ends of the concave sides, forming a central pore.
The walls of the guard cells facing the pore side are usually thicker than the other sides and the thinner parts are more extensible. The guard cells are usually covered by cuticle which generally continues on that wall which faces the stomatal pore and which also reaches the cells abutting on the sub stomatal chamber (just beneath each stoma, a prominent cavity or intercellular space is present that is known as the sub stomatal chamber).
Thickness of Cell Wall
Thickness of cell walls of guard cells and their size varies in different types. In grasses, the guard cells having narrow middle regions with thickened outer and inner walls; the guard cells are associated with subsidiary cells, one on each long side. In the Amaryllis type, found in both dicotyledons and monocotyledons, the cell walls of guard cells around each opening are thickened. In the Mnium type, found in bryophytes, the ventral wall of guard cells is thinner than the dorsal wall.
These cells contain prominent nucleus, dense cytoplasm, numerous mitochondria, elements of endoplasmic reticulum, Golgi bodies, vacuoles of various sizes and choloroplastids. These chloroplastids differ from the chloroplastids of the mesophyll cells. However, they perform photosynthesis normally. The guard cells of some species may lack photosynthetic pigments.
The epidermal cells, which are located next to the guard cells, are called subsidiary cells. These cells can be differentiated from other cells by their size and arrangement. In other words, the epidermal cells adjoining the guard cells often differ morphologically from the rest of the typical epidermal cells.