Since fungi are non-hierarchical organisms (right?), what happens if you take a garden variety mushroom and cut it in half without letting either half leave the earth (I’ll leave the exact logistics as an exercise for the reader)? Do the halves grow together? Do they die? Does each half become a separate organism?
The little thing that pokes up out of the ground is a fruit body of the fungus, which grows in a mat of fibers below ground. Any small part of the mat of fibers can become another mat of fibers, and eventually produce fruit bodies of its own.
Tris