Choose the statement not true for stomatal apparatus

Usually a stomatal apparatus has a pore or stoma, surrounded by two guard cells which are kidney shaped in dicots and dumbbell shaped in grasses like maize. Guard cells are usually surrounded by special epidermal cells called subsidiary cells. In some cases, the subsidiary cells are absent. Movement of stomata is due to variation in turgor pressure. The outer walls of guard cells are thin and inner walls are thick.