In many woody plants, the outer layer of bark is made up of dead tissue, so it does not grow. Underneath the outer bark is a layer of living tissue, which does grow. As the tree grows, its trunk gets thicker, pushing outwards on the dead layer of bark and breaking it. For some trees, such as Sycamores, the bark peels off in layers, exfoliating the old, dead tissue. For others, it cracks and continues to build up, as in this photo.