flax vs plant fibre

flax and plant fibre both are nouns.

Word	Noun	Adjective	Verb	Adverb
flax	Yes	No	No	No
plant fibre	Yes	No	No	No

As nouns, plant fibre is a hypernym of flax; that is, plant fibre is a word with a broader meaning than flax:

flax: fiber of the flax plant that is made into thread and woven into linen fabric
plant fibre: fiber derived from plants

Other hypernyms of flax include plant fiber.

flax (noun)	plant fibre (noun)
plant of the genus Linum that is cultivated for its seeds and for the fibers of its stem	fiber derived from plants
fiber of the flax plant that is made into thread and woven into linen fabric