1. A fungus, usually of the class Ascomycetes, that grows symbiotically with algae, resulting in a composite organism that characteristically forms a crustlike or branching growth on rocks or tree trunks.
地衣一种真菌类植物，通常属于子囊菌纲，与藻共生形成一种复合有机体，生长于岩石或树干上，多为硬壳状或枝叉状