Past participle form; having been joined or united together.
From coadunate + -ed (past participle marker). Functions as an adjective describing something that has undergone the process of coadunation.
This passive form appears in technical botanical descriptions where you need to emphasize that structures 'have been' fused together, versus describing their permanent state with the simple form.
Complete word intelligence in one call. Free tier — 50 lookups/day.