When they grow on plants, they do not live as parasites, but instead use the plants as a substrate.
Lichens occur from sea level to high alpine elevations, in many environmental conditions, and can grow on almost any surface.
The thallus form is very different from any form where the fungus or alga are growing separately.
Colonies of lichens may be spectacular in appearance, dominating much of the surface of the visual landscape in forests and natural places, such as the vertical "paint" covering the vast rock faces of Yosemite National Park.
The underside of the leaf-like lobes of foliose lichens is a different color from the top side (dorsiventral), often brown or black, sometimes white.
They are among the first living things to grow on fresh rock exposed after an event such as a landslide.
The long life-span and slow and regular growth rate of some lichens can be used to date events (lichenometry).
Squamulose lichens may appear where the edges lift up. The thallus is not always the part of the lichen that is most visually noticeable.
Some lichens can grow inside solid rock between the grains (endolithic lichens), with only the sexual fruiting part visible growing outside the rock.Generally, the fungal mesh surrounds the algal or cyanobacterial cells, often enclosing them within complex fungal tissues that are unique to lichen associations.The thallus may or may not have a protective "skin" of densely packed fungal filaments, often containing a second fungal species, which is called a cortex.A fruticose lichen may have flattened "branches", appearing similar to a foiliose lichen, but the underside of a leaf-like structure on a fruticose lichen is the same color as the top side.The leaf-like lobes of a foliose lichen may branch, giving the appearance of a fruticose lichen, but the underside will be a different color from the top side.The combined lichen has properties different from those of its component organisms. The properties are sometimes plant-like, but lichens are not plants.