Guard Cells

Guard cells are cells that control the opening and closing of stomata. When filled with water and swollen, the guard cells contract and open the stomata, and when dry and shrunk, the guard cells relax, closing the stomata. This regulates gas exchange, allowing water to leave the plant when the plant is watered, and reducing water loss when the plant is dry. Guard cells also respond to environmental conditions, closing when the air is dry, and opening when the air is humid.