As plants made their historic transition from aquatic to terrestrial biomes, they developed several adaptations that facilitated life processes. One of their initial challenges was the loss of water to air. As a result, many plants developed a waxy cuticle that covered their aerial parts. Coverage for stems and leaves, the cuticle contains tiny pores called stomata that enable gas exchange, since this process cannot occur directly through the cuticle layer. Surrounding cells regulate the opening and closure of these pathways, in response to outside aerial conditions.