Branch and twig samples showing lichen (pronounced "liken") growth often arrive at the Plant and Insect Diagnostic Clinic to be diagnosed for disease. These interesting organisms, however, do not cause disease problems. They live and gather sunlight on twigs or branches but do not infect the tree. Many lichens grow rapidly when exposed to full sunlight, which explains their common occurrence on dead or dying trees. In addition to growing on tree parts, lichens can be found on dead wood, rocks, soil, tombstones, or other sunny places.
A lichen is an unusual organism because it consists of two unrelated organisms, an alga and a fungus. These two components exist together and behave as a single organism. When two organisms live together in this way, each providing some benefit to the other, they are known as symbionts. The alga, because it is a green plant, can photosynthesize and provide energy for the lichen. The fungus contributes to the relationship by obtaining water and minerals and by protecting the algal cells from desiccation. Together the fungus and the alga make up what is known as the lichen thallus.
The color and growth form of the thallus is used to group and classify the lichens. The most common species of lichens on trees tend to be a gray-green color, but other species may be orange, yellow, slate blue, or black. There are three major growth forms of lichens: foliose, fructicose, and crustose. Foliose lichens have leaf-like lobes. These are the gray-green structures that can often be seen growing on tree trunks or branches. They are slightly raised and can grow and coalesce with other lichen thalli, covering several inches or more of bark. If moistened, they become somewhat rubbery and can be removed. Fructicose lichens have hair- like or stringy thalli and are less common. Finally, as the name implies, crustose lichens have crust-like thalli. Cruostose lichens can often be found tightly embedded on rocks or lower tree trunks. (Rocks with lichens are used to add interest to rock gardens.)
Besides being fascinating in their own right, lichens have medicinal uses, some food value, use as oils in perfumes, and are indicators of air pollution. Because lichens are extremely sensitive to air pollution, their absence can be used as a measure of how much an area is polluted.