What structure do plants have that prevents water loss?
To prevent water loss from free surface of leaves epidermal cells are provided with thick cuticle, radially elongated epidermal cells and presence of waxy deposition. Further water loss is prevented by sunken stomata and epidermal motor cells etc.
What functions as a protective covering that resists harmful substances and organisms and prevents extreme water loss?
not necessarily . It depends upon the type of the plant,its environment, availability of water etc . certain plants (especially desert plants) have a waxy coating on their leaves to prevent water loss. Even though they have larger leaves their waxy coating prevents the loss of water than the plants having smaller leaves in the tropics.