These are a special kidney shape which opens and closes the stomata in the leaf.
When the plant has lots of water the guard cells fill with it and go plump and turgid. This makes the stomata open so gases can be exchanged for photosynthesis.
When the plant is short of water the guard cells lose water and become flaccid, making the stomata close. This helps stop too much water vapor escaping.
Thin outer walls and thickened inner walls make the opening and closing of the stomata work.
They are also sensitive to light and close at night to save water without losing out on photosynthesis.
Guard cells are adapted to their function of allowing gas exchange and controlling water loss within the leaf.
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