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16. What causes the opening and closing of guard cells of stomata during transpiration?

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Stomata are the tiny pores present on the surfaces of leaves that help in the exchange of gases. A stoma consists of bean-shaped (in dicot) or dumbbell-shaped (in monocots) guard cells that are surrounded by the modified epidermal cells called subsidiary cells. The opening and closing of the stomata are mainly due to change in the turgidity of guard cells. The inner walls of the guard cells are thick and elastic, while the outer walls are thin. Numerous microfibrils are present in the cell walls of guard cells to facilitate the opening and closing of the stomata.

Stomatal opening- At the time of the opening of the stomata, the turgidity of the guard cells increases. As a result, the outer walls bulge and the inner walls become crescent-shaped. The stomatal opening is facilitated by the radial arrangement of the microfibrils. 

Stomatal closing- At the time of the closing of the stomata, the guard cells lose their turgidity, the outer and inner walls retain their original shapes, and the microfibrils get arranged longitudinally.

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