What are the specialised structures of plants which control the opening and closing of stomatal pores ?

The correct answer is Guard cells. Stomata are microscopic pores found on the surface of leaves and stems that facilitate gas exchange (CO2 and O2) and transpiration. Each stomatal pore is surrounded by two specialized kidney-shaped (in dicots) or dumb-bell shaped (in monocots) cells called guard cells.

• Function: The opening and closing of the stomatal pore is regulated by changes in the turgor pressure of the guard cells. When water flows into these cells, they swell and become turgid, causing the pore to open. When they lose water, they become flaccid, and the pore closes.
• Other Options: Chloroplasts are organelles for photosynthesis; vacuoles store fluids and help maintain turgor; and cell walls provide structural support. However, the guard cells are the specific structures that act as the gatekeepers for the stomatal aperture.