The tissue which forms a covering layer over leaves, the softer portions of stems and roots, etc., is called epidermal tissue. Its thickness is usually one cell. On the stems and leaves it often has a waxy substance, cutin, on its outside walls. The function of this epidermis is largely for protection to prevent excessive evaporation of water and aid the root in its absorption of substances from the soil. On leaves and some stems, this epidermal tissue contains openings or pores called stomata, where gases are exchanged.