silk cotton vs plant fiber

silk cotton and plant fiber both are nouns.

Word	Noun	Adjective	Verb	Adverb
silk cotton	Yes	No	No	No
plant fiber	Yes	No	No	No

As nouns, plant fiber is a hypernym of silk cotton; that is, plant fiber is a word with a broader meaning than silk cotton:

silk cotton: a plant fiber from the kapok tree; used for stuffing and insulation
plant fiber: fiber derived from plants

Other hypernyms of silk cotton include plant fibre.

silk cotton (noun)	plant fiber (noun)
a plant fiber from the kapok tree; used for stuffing and insulation	fiber derived from plants