Basic Morphology

Internal structure of a crustose lichen.

While their gross appearance differs, the inner structures of all three lichen groups are similar. Each lichen is composed of a number of layers. The outer layer (farthest from the substrate) is known as the cortex and is composed of fungal cells. The next layer – the algal layer – is dominated by algal cells in a matrix of fungal threads that hold them in place, close to the surface. The third layer is the medulla, which is largely made up of fungal cells. In crustose lichens, this layer is directly connected to the substrate, but the other two groups differ: foliose lichens have another layer, the lower cortex, and are attached to the substrate by means of rhizinae, hair-like strands; in fruticose lichens, the cortex and algal layer are repeated below the medulla. The entire structure of any lichen is known as a lichen body, or thallus.

Internal structure of a fruticose lichen.
Internal structure of a foliose lichen.