The structure of lichen thallus is divided into two groups based on its internal structure viz. 1. Homoiomerous and 2. Heteromerous.
1. Homoiomerous thallus (Internal Structure):
In fruticose lichen, the thallus exhibits a simple structure with little differentiation. It consists of loosely arranged fungal hyphae in which algal cells are equally distributed throughout.
Example: Leptogium, Collema
2. Heteromerous thallus (Internal Structure):
The thallus belonging to this category shows considerable differentiation. The algal component is confined to a specific region. The thallus can be differentiated into four zones if the vertical section (V.S.) is observed. They are
Upper cortex: It forms the upper surface of the thallus. It is thick and protective in nature and consists of fungal hyphae. The fungal hyphae are compactly interwoven without any intercellular spaces to produce tissue-like layer called plectenchyma.
Algal zone: It is a zone below the upper cortex and is formed of algal cells belonging to Chlorophyceae or Cyanophyceae intermingled with fungal hyphae. The algal zone is the photosynthetic region of the lichen.
Medulla: It is the central core of the thallus consisting of loosely arranged fungal hyphae with intercellular spaces. The hyphae run parallel to the long axis. The walls of the hyphae are strong and thick.
Lower cortex: It is formed of densely grouped hyphae which run perpendicular to the surface. The bundle of hyphae (rhizinae) arise from the lower surface and penetrate the sub-stratum functioning as anchoraging and absorbing organs. In some lichens, the lower cortex is absent and is replaced with a sheet of hyphae forming hypothallus.