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Any of about 18,000 thallophytic organisms composed of an algae and a fungus growing in a symbiotic relationship on a solid substrate, such as a rock. Lichen have been used since ancient times as a source of food, medicine and dyes. Lichens grow relatively slowly (about 0.1 - 10 mm per year). Algae perform phtosynthesis producing carbohydrates and vitamins while the fungi absorb water vapour and provide a protective environment for the delicate algae. Their composite body is called a thallus and is classified as:

- squamulose: small, flat scales that do not adhere tightly to substrate
- crustose: flat, firmly attached to substrate
- foliose: leaf-like lobes, attached in the center to substrate by clusters of hyphae (rhizones); may reach several feet in diameter
- fructicose: plant-like growth attached at one point or cluseter called a holdfast


  • G.Caneva, M.P.Nugari, O.Salvadori, Biology in the Conservation of Works of Art, ICCROM, Rome, 1991

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