Same word as radices:

basis, bedrock, beginnings, bottom, cause, center, crux, derivation, essence, essentiality, footing, foundation, fountain, fountainhead, fundamental, germ, ground, groundwork, heart, inception, infrastructure, mainspring, marrow, motive, nub, nucleus, occasion, origin, pith, provenance, provenience, quick, quintessence, radicle, radix, reason, rhizome, rock bottom, seat, seed, soul, source, starting point, stem, stuff, substance, substratum, tuber, underpinning, well