树皮

例句：

1. 1. The tree's bark is rough and tough.
   这棵树的树皮粗糙而坚韧。
2. 2. The sapwood of the tree is soft and easily damaged.
   这棵树的内层软且容易受损。
3. 3. Heartwood is harder and stronger than the sapwood.
   心材比韧皮部更硬更强。