Final answer:
Both bark and waxy covering help reduce water loss in plants.
Step-by-step explanation:
The word that correctly fills in the blank is water. Both bark and waxy covering help reduce water loss in plants. Bark serves as a protective layer, preventing water from evaporating and protecting the plant from physical damage. Waxy covering, also known as the cuticle, prevents water loss through evaporation and acts as a barrier to pathogens.