Some plants add stiffening material to some of their cell walls. A "secondary" cell wall is an additional layer of cellulose which increases wall rigidity. More layers may be added containing lignin in xylem cell walls, or containing suberin in cork cell walls. These compounds are rigid and waterproof. They make the secondary wall stiff. Both wood and bark cells of trees have secondary walls. Other parts of plants such as the leaf stalk may get reinforcement to resist the strain of physical forces.