A lichen is a community between an alga and a fungus. So a lichen is not a plant. Such a community is also called a symbiosis. It comes from a Greek and means “living together”. The algae provide the fungus with nutrients that it cannot produce itself. The fungus gives the alga support and supplies it with water because it has no roots. In this way, they both help each other.
Lichens come in a wide variety of colors. Some are white, others are yellow, orange, deep red, pink, teal, gray, or even black. That depends on which fungus lives with which algae. There are around 25,000 lichen species worldwide, of which around 2,000 are found in Europe. They grow very slowly and can get very old. Some species even live for several hundred years.
Lichens have three different growth forms: Crustacean lichens grow tightly together with the substrate. Leaf or deciduous lichens grow flat and loose on the ground. Shrub lichens have branches.
Lichens are just about everywhere. They can be found in the forest on the trees, on garden fences, on stones, walls, and even on glass or tin. They endure a lot of heat and cold. They feel most comfortable when it is a bit cool for us humans. So lichens are not demanding in terms of habitat or temperature, but they respond poorly to polluted air.
Lichens absorb dirt from the air but cannot release it again. Therefore, where the air is bad, there are no lichens. If the air is a little less polluted, only crustacean lichens grow. But if it has crust lichen and leaf lichen, the air is less bad. The air is best where lichens grow, and the other lichens like it there too. Scientists take advantage of this and use the lichen to identify the level of air pollution.