"root-like, resembling a root," 1858, from Greek rhiza "root," literal and figurative (see rhizo-) + -oid. As a noun in botany, "organ resembling a root on the underside of some plants," from 1875. Related: Rhizoidal; rhizoideous.
word-forming element meaning "like, like that of, thing like a ______," from Latinized form of Greek -oeidēs (three syllables), from eidos "form," related to idein "to see," eidenai "to know;" literally "to see" (from PIE *weid-es-, from root *weid- "to see"). The -o- is connective or a stem vowel from the previous element. Often implying an incomplete or imperfect resemblance to the thing indicated.
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."
<a href="https://www.etymonline.com/word/rhizoid">Etymology of rhizoid by etymonline</a>
Harper, D. (n.d.). Etymology of rhizoid. Online Etymology Dictionary. Retrieved $(datetime), from https://www.etymonline.com/word/rhizoid