Lichen
/lˈaɪkən/
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Definition
(noun) Any of many symbiotic organisms, being associations of algae and fungi, often found as white or yellow-to-blue–green patches on rocks, old walls, etc.
Sentence Examples
Caribou eat almost only lichen during the winter.
CEFR Practice Quiz
We saw some colorful ____ growing on the rocks while we were hiking in the mountains on Saturday morning.