basidium vs promycelium

basidium and promycelium both are nouns.

Word	Noun	Adjective	Verb	Adverb
basidium	Yes	No	No	No
promycelium	Yes	No	No	No

As nouns, promycelium is a hyponym of basidium; that is, promycelium is a word with a more specific, narrower meaning than basidium:

basidium: a small club-shaped structure typically bearing four basidiospores at the ends of minute projections; unique to basidiomycetes
promycelium: the basidium of various fungi

basidium (noun)	promycelium (noun)
a small club-shaped structure typically bearing four basidiospores at the ends of minute projections; unique to basidiomycetes	the basidium of various fungi