"belonging to, pertaining to, or affecting roots; characterized by the presence of radicles," by 1815, from radicle or else from Modern Latin radicula, diminutive of Latin radix "root" (from PIE root *wrād- "branch, root") + -ar.
1670s, in botany, "rootlet, part of the embryo of a plant which develops into the primary root," from Latin radicula, diminutive of radix "root" (from PIE root *wrād- "branch, root"). Anatomical sense of "branch of a nerve, vein, etc. resembling a root" is by 1830.
It is the hypothetical source of/evidence for its existence is provided by: Greek rhiza, Lesbian brisda "root," Greek hradamnos "branch;" Latin radix) "root, radish;" Gothic waurts, Old English wyrt; Welsh gwraidd, Old Irish fren "root."
word-formation element meaning "pertaining to, of the nature of," from Latin -arem, -aris "of the kind of, belonging to," a secondary form (by dissimilation) of -alis, used after syllables with an -l- (such as insularis for *insulalis, stellaris for *stellalis).
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Harper, D. (n.d.). Etymology of radicular. Online Etymology Dictionary. Retrieved $(datetime), from https://www.etymonline.com/word/radicular