The outside covering of a tree trunk is called bark.
moss
Bark
bark
the stem is only on flowers and isn't covered in bark while the trunk is on a tree and is covered in bark
Bark covers a tree trunk.
Tree trunk is a compound word
the outer part of a tree trunk
The trunk of a tree is called a 'bole'.
the outer part of a tree trunk
The Root.(this answer is incorrect) The tree trunk (also called the bore) is the main wooden axis that supports the branches and is directly attached to the roots which in turn support the tree.
Cellulose makes the trunk of a tree sturdy.