pillwort vs water fern

pillwort and water fern both are nouns.

Word	Noun	Adjective	Verb	Adverb
pillwort	Yes	No	No	No
water fern	Yes	No	No	No

As nouns, water fern is a hypernym of pillwort; that is, water fern is a word with a broader meaning than pillwort:

pillwort: European water fern found around margins of bodies of water or in wet acid soil having small globose sporocarps
water fern: ferns that grow in water

Other hypernyms of pillwort include aquatic fern.

pillwort (noun)	water fern (noun)
European water fern found around margins of bodies of water or in wet acid soil having small globose sporocarps	ferns that grow in water