Guard cells are well adapted to perform their function which is regulating the transpiration from the cell.
Guard cells are bean shaped in dicots and dumb bell shaped in monocots.
They are a pair of cells that regulates the opening and closing of the stomata by blocking or allowing water to pass through.The most important adaptation of a guard cell is the thicker inner cell wall compared to outer cell wall.
In day, the chloroplasts perform photosynthesis at a high rate and thus make the protoplasm hypertonic. This causes endosmosis of water from the surrounding specialized epithelial tissue in its surroundings called subsidiary cells thus making the guard cells turgid.As the inner cell wall is thicker, the cell cannot expand uniformly lengthwise so instead expands forming a banana shape. At night, they become flaccid and close the stomata.