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countable and uncountable, plural lichens or lichen
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. quotations examples
The Beaches of Lukannon–the winter wheat so tall, / The dripping, crinkled lichens, and the sea-fog drenching all!
1894 May, Rudyard Kipling, “Lukannon”, in The Jungle Book, London, New York, N.Y.: Macmillan and Co., published June 1894, page 122
It was the same rich green that one sees on forest moss or on the lichen in caves: plants which like these grow in a perpetual twilight.
1895 May 7, H[erbert] G[eorge] Wells, chapter XI, in The Time Machine: An Invention, New York, N.Y.: Henry Holt and Company
The nibble marks of the stone adze were still visible, though crusted over with scale lichens in most places.
1915, John Muir, chapter V, in Travels in Alaska
(figurative) Something which gradually spreads across something else, causing damage. quotations examples
Meanwhile, abiding a day of judgment, she fought ceaselessly to deny the bitter drops in her cup, to tear back the slow, the intangibly slow growth of a hot, corrosive lichen eating into her heart.
1912 January, Zane Grey, “Shadows on the Sage-slope”, in Riders of the Purple Sage […], New York, N.Y., London: Harper & Brothers Publishers, page 202