Tough skinned
The visible dead layer that covers the tree is bark, of course – exterior bark, to be precise. Botanists consider bark to have an inner part, as well, composed of phloem. What's that? It's vital conducting tissue that carries the sugars produced by photosynthesis to nourish all parts of the tree, from its highest branches to its deepest roots.
The outer bark, the periderm, is an impermeable physical barrier a bit like an animal's hide. It protects the tree from parasites, wounds, the noonday sun and bitter cold. In some trees, it is even thick enough to withstand forest fires.
