What cell controls the opening and closing of stomata?

A stoma is a minute pore on the epidermis of aerial parts through which exchange of gases and transpiration take place. Each stoma is surrounded by a pair of kidney-shaped guard cells (in dicot) and a dumbbell-shaped guard cell (in monocot). Each guard cell is a modified epidermal cell showing a prominent nuclease, cytoplasm, and plastids, the wall of the guard cell is differentially thickened. The inner wall of each cell facing the stoma is concave and thick and rigid. The outer wall is convex and is thin and elastic. The guard cells are surrounded by a variable number of epidermal cells called subsidiary cells or accessory cells. The average length of stomata 20 to 28µm and breadth 5µm.

Mechanism of Opening and closing stomata: Opening and closing of stomata occur due to turgor changes in guard cells. Due to endosmosis, an increase in the turgor of the guard cells takes place which finally results in stretching and bulging out of outer thin walls. This results in the pulling apart of the opposed inner thicker walls creating an opening or pore in guard cells of stomata. When the turgor pressure of guard cells decreases inner walls sag, leading to the closure of space between them. This is due to the loss of water from guard cells, resulting in thicker walls to move closer and finally shut the opening.