Tree Bark
Tree bark is composed of waterproof cells which protect the interior from temperature variations, diseases, animals and insects. This outside layer is more or less thick depending on the age and species of the tree. The cork oak, for example, has a thick bark that protects it from fire.
As it ages, the bark thickens and cracks, causing crevices of varying sizes as on the pine tree. Or it can renew itself gradually, leaving the old bark to fall in thin layers continually, like on the silver birch.
Other barks like that of the sand tree have prickles as a defence.
