A layer of dead cells on the outer surface of a tree or woody plant. Bark can be paper thin, like Birch trees, or two feet thick, like a mature Redwood tree.
The exterior layer of bark that is most familiar to us is a layer of mostly dead cells, called rhytidome. It helps support the erect, massive shape of trees.
Bark also provides a layer of protection for the inner workings of the plant. It can keep out insects, fungus, and bacteria that would otherwise attack the fragile living cells in the trunk, branches, or stems. When an insect or a foreign object manage to pierce through the bark, the trees will cordon off the wound. They send more bark tissue to surround the wound where it threatens the plants’ inner layers.
In the slice of wood pictured here you can see several holes surrounded by the darker tissue similar to that of the outer bark. When these wounds happened, the tree compartmentalized the damage and continued to grow around it. Tree wounds don’t get better, as they say in the business: trees don’t heal, they seal.
That’s why sometimes it appears that the tree has swallowed ropes, chains, or even an entire bicycle. It simply continues to grow around any damage.