outer covering of tree trunk

例句：

1. 1.The outer covering of the bark is made up of cells that protect the tree from damage.
   树皮的外层由细胞组成，保护树木免受损害。
2. 2. The outer covering of the bark can be used to make paper or other materials.
   树皮的外层可以用来制作纸张或其他材料。
3. 3. The outer covering of the bark provides insulation and protection for the tree.
   树皮的外层为树木提供了隔热和保护。