- Afungus,usuallyoftheclassAscomycetes,thatgrowssymbioticallywithalgae,resultinginacompositeorganismthatcharacteristicallyformsacrustlikeorbranchinggrowthonrocksortreetrunks."Lichensmayformathin,crustlike,tightlyBoundcoveringovertheirsubstrate(e.g.,cr...
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