The fungus and algae of lichen are associated together in a spongy thallus which can range in diameter from less than 1 mm to more than 2 m. The thallus consists of 3 or 4 layers of cells or hyphae. The upper layer, called the upper cortex, contains gelatinous substances which perform protective functions. Below the upper cortex is the algal layer, in which algal cells are scattered among strands of hyphae. Next comes the medulla, which consists of loosely packed hyphae and within which a number of substances produced by the lichen are stored. The fourth layer, called the lower cortex, may not always be present. It resembles the upper cortex but is thinner and is often covered in anchoring strands of hyphae called rhizines.
Lichens have been categorized into three major groups depending on their type of growth. Crustose lichens are attached or embedded on the substrate and form coloured, crusty patches. Foliose lichens have leaf-like thalli whose edges are crinkly or divided into lobes. Fruticose lichens may resemble tiny upright shrubs or may hang down in festoons from tree branches.