fern ally vs whisk fern

fern ally and whisk fern both are nouns.

Word	Noun	Adjective	Verb	Adverb
fern ally	Yes	No	No	No
whisk fern	Yes	No	No	No

As nouns, whisk fern is a hyponym of fern ally; that is, whisk fern is a word with a more specific, narrower meaning than fern ally:

fern ally: pteridophytes of other classes than Filicopsida
whisk fern: chiefly tropical clump-forming plants of skeletal appearance resembling whisk brooms; lacking roots

Other hyponyms of fern ally include horsetail, club-moss, club moss, lycopod, quillwort.

fern ally (noun)	whisk fern (noun)
pteridophytes of other classes than Filicopsida	chiefly tropical clump-forming plants of skeletal appearance resembling whisk brooms; lacking roots