Lichens are composed of fungi (Kingdom Fungi) and a photosynthesising partner, usually algae. The fungi are considered ‘dominant’ or ‘stable’ in this relationship and so lichens are named according to the fungus. A simple, but helpful model is to visualize the fungus as a ‘home’ wrapped around the photosynthesising partner or photobiont. The photobiont can also be a photosynthesising bacterium belonging to a group called cyanobacteria.
There is a great diversity of lichens and subsequently a great diversity of arrangements of lichen anatomy. An ‘idealized’ anatomy is described here, particularly for the leaf-like or foliose lichens.
The upper surface is a layer of tightly bound fungal threads or hyphae; it is referred to as the upper cortex. Below that is another layer of fungal hyphae, but in this situation they have a loose arrangement with plenty of air spaces between them; in some of the air spaces are found the algal cells or cyanobacteria. The layer is termed the medulla. Below the medulla is the lower cortex; again this is composed of tightly woven hyphae. Small root-like structures called rhizines project down into the substrate from the lower cortex. Rhizines do not specifically absorb water from the substrate; they function more to attach the lichen to the substrate and increase humidity around the lower surface.
The upper cortex is exposed to the air and light; photosynthesis occurs in the medulla; the lower cortex is attached to the substrate – rocks or soil or trees. This description is of the main body or thallus of a lichen.