The cells that are on each side of a stomata and open and close it like a mouth are called guard cells meristem cells parenchyma cells collenchyma cells

The cells that are on each side of the stomatal pore and open and close it like a mouth is called guard cells. Guard cells are found almost all over the body part of the plant. The opening and closing of the stomata acts as a mediator by changes in turgor pressure of the guard cells. Turgor pressure pushes and sometimes out of the cell wall of the plant to allow ions of solution to come in and out. The stomatal pore of the guard cell becomes large when water is present around it and it is small when water is not present. 

It's cell theory. easy name i would think.