The minute pores present in the epidermis of leaves are called stomata. A stoma is composed of two guard cells; which are bean-shaped. The guard cells are dumbbell-shaped in grasses. The outer wall of guard cells is thin and the inner walls are highly thickened. Chloroplast is present in the guard cells. The guard cells regulate the opening and closing of stomata. A few specialized epidermal cells may be present near the guard cells. These specialized cells are called subsidiary cells. The stomatal aperture, guard cells and subsidiary cells together make the stomatal apparatus. Transpiration and exchange of gases are regulated by stomata.
