No, it is a mulberry "crown" not a root. Mulberry crowns are the knobs of scar tissue formed on the ends of major limbs when the trees are pollarded (all branches cut off) every year.
Another hint: make the lower barrel longer than the upper barrel. How much longer? Start with 1/4 inch and adjust to your liking. The longer lower barrel with Sharon's hint of a larger upper barrel makes for a really nice looking pen.
Do a good turn daily!
Don