The part a thing grows from and is anchored by. Also the base form a family of words descends from.
Old Norse rot (a root), related to Old English wyrt, from a Proto-Germanic root; the underground anchor of a plant and the ancestor-word a family of words grows from are the same metaphor, applied twice.
The part a thing grows from and is anchored by — used here for what a word descends from, and for what the Grammar itself calls its own most basic atoms.