What is Root?

Legal Definition
In vascular plants, the root is the organ of a plant that typically lies below the surface of the soil. Roots can also be aerial or aerating, that is growing up above the ground or especially above water. Furthermore, a stem normally occurring below ground is not exceptional either (see rhizome). Therefore, the root is best defined as the non-leaf, non-nodes bearing parts of the plant's body. However, important internal structural differences between stems and roots exist.
-- Wikipedia
Legal Definition
That part of a tree or plant under ground from which it draws most of its nourishment from the earth.

2. When the roots of a tree planted in one man's land extend into that of another, this circumstance does not give the latter any right to the tree, though such is the doctrine of the civil law; Dig. 41, 1, 7, 13; but such person has a right to cut off the roots up to his line. Rolle's R. 394, vide Tree.

3. In a figurative sense, the term root is used to signify the person from whom one or more others are descended. Vide Descent; Per stirpes.
-- Bouviers Law Dictionary
Legal Definition
-- Ballentine's Law Dictionary