Outer wall of guard cells is:

Explanation

Guard cells are pairs of specialized epidermal cells that work to control excessive water loss, closing on hot, dry, or windy days and opening when conditions are more favorable for gas exchange. The inner wall of a guard cell is thicker than the outer wall. When the guard cell is filled with water and it becomes turgid, the outer wall balloons outward, drawing the inner wall with it and causing the stomata to enlarge. The outer wall of guard cells is thin and elastic.