Question

Why are stomata and guard cells called turgor operated valves?

Answer 1

because their opening and closing is determined by their turgidity.The stomata  acts as a turgor operated valve which closes and opens according to the turgidity of guard cells. The guard cells are the only epidermal cells that contain chloroplasts

Answer 2

Each stomata consists of a minute pore called stoma surrounded by two guard cells. The stomata acts as a turgor operated valve .it closes and opens according to the turgidity of guard cells.