Lichens on a tree. Two types of lichen growing on a tree trunk. Lichens are mutualistic (beneficial) associations between a fungus and an alga, the alga occupying a single layer of cells within the body of the fungus. They are found growing on the surfaces of trees and rocks. On trees they are epiphytes: they live on, but do not harm, their host. Seen here is Parmelia sulcata (upper frame, blueish-green), common throughout the northern temperate zone. Also pictured is Rinodina roboris (centre, blue).