剑桥词典on a leaf , one of a pair of cells positioned around each stoma (= a small hole in the surface ) that open it to release gases or close it to prevent water from being lost 植物的保卫细胞（保卫细胞组成环状，构成了叶子的气孔，能控制气流的进出，阻止水分流失。） 返回 guard cell