What forms the stomatal apparatus?

Hint: Stomatal apparatus is a structure found in the epidermis of leaves. The epidermis is the outermost covering of the whole plant body and it consists of epidermal cells, stomata, and epidermal appendages- trichomes and hairs. Epidermis is covered by a thick cuticle layer, except in roots to prevent excessive loss of water

Complete answer:
Stomata is a structure present in the epidermis of leaves that regulates the process of transpiration and gaseous exchange.
The three basic parts of the stomata form the stomatal Apparatus.
They are the stomatal pore, guard cells, and surrounding subsidiary cells.
Stomatal pore- This is a pore present in stomata for uptake of carbon dioxide for the process of photosynthesis to make glucose and to remove water through them. They also release oxygen through them during photosynthesis.
Guard cells- Each stoma is composed of two bean shaped guard cells that enclose the stomatal pore. In grasses, the shape of the guard cell is dumb-bell. The outer wall of guard cells is thin and the inner wall is highly thick. The guard cells regulate the opening and closing of stomata. Microfibril present in guard cells are radially oriented which helps in stomata opening.
Subsidiary cells- Sometimes a few epidermal cells in the proximity of guard cells become specialized in their shape and size and are known as subsidiary cells.

Note:
Dicot and monocot leaves have different anatomy.
In Dicot leaf, the abaxial surface (lower surface) generally has more stomata than the adaxial surface (upper surface). Sometimes adaxial surfaces don’t have any stomata while in monocot leaves stomata is present on both adaxial and abaxial sides.