Guard cells line the openings of stoma and other organs in plants, opening and closing to moderate the process of respiration. The chief role of guard cells is to prevent an excess loss of water through respiration, allowing the plant to trade oxygen and carbon dioxide without becoming dehydrated.
Guard cells are part of the system that maintains drought resistance in plants. When the plant is filled with moisture, the guard cells become filled with fluid, causing the stoma to open. When the plant loses enough moisture, the stomata wither and draw closed automatically.
In addition, drought-resistant plants often have thick stems in order to store as much moisture as possible and deep root systems to draw moisture from far below the ground.