Lichens are not plants. They are fungi that live in symbiosis with photosynthetic algae. The fungal part of the lichen collects water and nutrients, whereas the algal part of the lichen uses these materials to produce energy in photosynthesis.

Lichens have no roots, stems or leaves.

Lichens usually thrive in dry and sunny growing locations. They can often be seen growing on the trunks of park trees. However, lichens are very sensitive to air pollution. This is why they are not as common in cities as they are in the wild. 

Different lichen species can be distinguished by looking at their color and structure. This is why they are usually quite easily recognizable. 

Trees are home to various lichen species. The most common lichen species living on the trunks of park trees are the lichen of the genus Parmelia, such as the one pictured on the right.