Lichens are a type of symbiotic organism made up of a plantlike partner and a fungus. There are three major types of lichen — crustose, foliose, and fructicose — each of which has its own shape, structure, and environmental preferences. Intermediate types include leprose and squamulose lichen, among others. These organisms may also be grouped by the kind of environment in which they prefer to grow.
Every individual lichen is composed of a mycobiont, or fungus, combined with a photobiont or phycobiont in the form of green algae or cyanobacteria. The algae or bacteria photosynthesize, providing nutrients for the fungus, and giving the lichen its characteristic greenish or blueish color. Both parts of the lichen get water and minerals from dust and rain, but some also obtain nutrients from their substrate via the fungal partner.
Not all types of lichen look the same. Crustose are flat and unlobed, with a close attachment to their substrate, and can be difficult to remove from the rock or tree on which they grow. Foliose lichen are leafier-looking, as their name implies, and are made up of two thin sheets of fungus with algae in the middle. They grow in round lobe formations and are easier to pull off of their substrate, since they attach only by small rootlets. Fructicose, or shrubby lichen, have small round branches made of fungus with algae inside, and an unusual pattern of vertical growth that may look beard-like or resemble a small bush.
Other types of lichen include leprose lichen, which form powdery, largely unstructured masses without a smooth surface. Placodioid lichens are lobed or unattached at the edges, and closely attached at the center, making them an intermediate form between crustose and foliose lichens. Another intermediate form, squamulose lichen, has many tiny lobes. Dimorphic lichens have characteristics of both squamulose and fruticose lichens, with small lobes that carry tiny stems or branches.
Environmental grouping divides lichens into seven major categories. Several types of lichen grow on plants, and are called epiphytic. This group includes the coricolous lichens, which prefer to grow on tree trunks, as well as the ramicolous lichens, which inhabit twigs. The musicocolous lichens grow on live moss, and the foliicolous lichens prefer evergreen leaves. Both of those types are ephiphytic, but the legnicolous, saxicolous and terricolous lichens, which inhabit wood, stones, and soil respectively, are not epiphytes.