rhizoid vs fibril

rhizoid and fibril both are nouns.

Word	Noun	Adjective	Verb	Adverb
rhizoid	Yes	No	No	No
fibril	Yes	No	No	No

As nouns, fibril is a hypernym of rhizoid; that is, fibril is a word with a broader meaning than rhizoid:

rhizoid: any of various slender filaments that function as roots in mosses and ferns and fungi etc
fibril: a very slender natural or synthetic fiber

Other hypernyms of rhizoid include filament, strand.

rhizoid (noun)	fibril (noun)
any of various slender filaments that function as roots in mosses and ferns and fungi etc	a very slender natural or synthetic fiber