A filamentous lichen is a lichen that has a growth form like a mass of thin, stringy, non-branching hairs or filaments. Filaments of the photosynthetic partner (Trentepohlia ) or trichome-forming (cyanobacteria) are surrounded by sheaths of the fungal filaments. So unlike in most other lichen growth forms, the filaments of fungus do not determine the shape. Filamentous lichens may look like matted black hair.
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Filamentous lichen Wikipedia(Text) CC BY-SA