lichen
noun/ˈlaɪkən/, /ˈlɪtʃən/
/ˈlaɪkən/, /ˈlɪtʃən/
[uncountable, countable]- a very small grey or yellow plant-like organism (= living thing) that spreads over the surface of rocks, walls and trees and does not have any flowers compare mossWord Originearly 17th cent.: via Latin from Greek leikhēn.
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lichen