What protects the branch and trunk of a tree from mechanical injury and desiccation?

The bark serves as the protective outer layer of a tree's trunk and branches, playing a critical role in safeguarding these parts from mechanical injury and desiccation. This layer consists of several components, including the outer bark, which is typically rough and can help shield against physical damage, pests, and diseases. Additionally, the bark acts as a barrier to moisture loss, helping to prevent desiccation in the inner layers of the tree.

While wood provides structural support and defense against some types of harm, it is not directly involved in protecting the tree from environmental stressors. Leaves, on the other hand, primarily serve the purpose of photosynthesis and gas exchange rather than protection. The phloem is responsible for transporting nutrients and sugars produced by the leaves throughout the tree, but it does not serve as a protective barrier. Thus, the bark is distinctly suited for its protective role, making it the correct answer.