Answer:
Deep, deep, do they go. Spreading out as they go. Never needing any air. They are sometimes as fine as hair
What has to be broken before you can use it?
You have it even if there is none.
These are the bumpy parts of your digits
It goes up and down the stairs without moving.
Though not a plant, has leaves. Though not a beast, has spine. Though many wouldn't need this thing, It's more valuable than wine.