Related Clues:

• Forest growth
• Oakmoss, e.g.
• It may grow on rocks
• Rock growth
• Tree growth
• Tree-trunk cover, perhaps
• Symbiotic plant
• Symbiotic growth
• Fungus
• Fungus + alga
• It grows on rocks
• Fungus-alga union
• Reindeer moss, e.g.
• Tiny plants found on rocks, tree-trunks etc
• Crusty, tufty growth on trees and stones