The thin, outer layer of a tree is called the bark. The bark serves multiple purposes, including protecting the tree from pests, diseases, and physical damage. It acts as a barrier to prevent water loss and insulates the tree against temperature extremes. Bark consists of several layers, and its outermost layer is made up of dead cells that provide the protective function. The inner bark, or phloem, is responsible for transporting nutrients produced in the leaves down to the roots. The appearance and texture of bark can vary significantly between different tree species.