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Lichens are symbiotic organisms made up by the association of microscopic green algae or cyanobacteria and filamentous fungi. Lichens take the external shape of the fungal partner and hence are named based on the fungus. The lichen fungus is typically a member of the Ascomycota—rarely a member of the Basidiomycota. Some lichen taxonomists place lichens in their own division, the Mycophycophyta, but this practice ignores the fact that the components belong in separate lineages.

Lichens live on various surfaces: soil, trees, rocks, and walls. They are often the first to settle in places lacking soil, constituting the sole vegetation in some extreme environments such as found at high mountain elevations and at high latitudes. Some survive in the tough conditions of deserts, and others on frozen soil of the arctic regions. Some lichens have the aspect of leaves ( foliose lichens); others cover the substratum like a crust ( crustose lichens); others adopt shrubby forms ( fruticose lichens); and there are gelatinous lichens (see lichen forms below).

When seen under magnification, a section through a typical foliose lichen thallus reveals four layers of interlaced filaments (fungus). The upper layer is formed by densely agglutinated fungal hyphae building a protective outer layer. It is called the cortex. Beneath the upper cortex is an algal layer composed of algal cells embedded in rather densely interwoven fungal hyphae. The algal cells contain chlorophyll, permitting the plant to live in a purely mineral environment. Mostly the fungus protects the alga against drying out. Beneath the algal layer, is a layer of loosely interwoven fungal hyphae without algal cells. This layer is called medulla. Beneath the medulla, the bottom surface resembles the upper surface and is called the lower cortex , consisting of densely packed fungal hyphae. The lower cortex often bears structures, such as rhizinae or a tomentum , serving to attach the thallus to the substratum on which it grows. Soredia (sing. soredium ), which contain algal cells as well fungal filaments, come loose from the lichen and serve as a means of dispersal by vegetative reproduction.

Lichens are the only food available for many animals living in arctic regions, such as reindeer. Although lichens can grow in harsh environments in nature, many lichens are sensistive to certain man-made pollutants. Hence, they have some potential as pollution indicator organisms. When growing on rocks, lichens, by their acids, further the process of weathering, by which rocks are slowly turned into soil.

Classification

Lichens are informally classified by growth form into:

• crustose (paint-like, flat), e.g., Caloplaca flavescens
• filamentose (hair-like), e.g., Ephebe lanata
• foliose (leafy), e.g., Hypogymnia physodes
• fruticose (branched), e.g., Cladina evensii, C. subtenuis, and Usnea australis
• leprose (powdery), e.g., Lepraria incana
• squamulose (consisting of small scale-like structures, lacking a lower cortex), e.g., Normandina pulchella