Answer:

lichen

Answer	Crossword Clue
LICHEN	Fungus-alga symbiosis
LICHEN	Oakmoss, e.g.
LICHEN	Growth on a rock

LICHEN	Rock growth
LICHEN	Tree growth
LICHEN	Fungus-alga union
LICHEN	Woodsy growth
LICHEN	Symbiotic growth
LICHEN	Alga-fungus combination
LICHEN	Alga-fungus symbiosis
LICHEN	Beard moss, for one

LICHEN	Growth on trees and rocks
LICHEN	Rock fungus
LICHEN	Rock plant
LICHEN	Tree-trunk cover, perhaps
LICHEN	Growth on rocks
LICHEN	Fungus + alga
LICHEN	Growth on rock walls
LICHEN	Forest growth

LICHEN	Source of litmus
LICHEN	Plant life of Antarctica.
LICHEN	Thallophyte.
LICHEN	Iceland moss.
LICHEN	Moss.
LICHEN	Rock plant.
LICHEN	Reindeer moss.
LICHEN	Rock garden plant.

LICHEN	Rock greenery.
LICHEN	Mosslike plant.
LICHEN	Rock cruster
LICHEN	Fungus-algae alliance
LICHEN	It may grow on rocks
LICHEN	Mossy growth
LICHEN	Iceland moss
LICHEN	Growth on a tree

LICHEN	What a rolling stone is unlikely to gather