Organisms belonging to the lower plants consisting of a photosynthetic alga or cyanobacterium (blue-green alga) in symbiotic association with a fungus, making crust-like (crustose), leaf-like (foliose) or shrubby (fruticose) forms that are commonly blue-green or orange.

This definition is abridged from A – Z of tree terms: A companion to British arboriculture.
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