The structure of a foliose lichen is described as-
1. Upper cortex: The uppermost layer of lichen is called upper cortex. It is formed by fungal hyphae. The hyphae are densely packed so there are no empty spaces. But if there is empty space it is filled by mucilage. This region is covered with cuticle. It is thick and acts like epidermis.
2. Algal layer: The next layer above the lichen cortex is called algal layer. Green or blue-green algae are abundant in these areas. Green algae include Chlorella, Pleurococus and blue green algae include Gleocapsa, Rivularia, Nostoc etc. Being green, this area produces food through the process of photosynthesis. The fungus absorbs nutrients from this region with the help of haustoria.
3. Medulla: The hollow area formed by fungal hyphae below the algal zone is called medulla. Hyphae divide into branches and spread in different directions. But the hyphae of the thallus are thin in the peripheral region and densely arranged in the central region. The hyphae in this region are called calaroplektenchyma.
4. Lower cortex: The lowest layer of lichen is called lower cortex. This area is densely populated. There are no blank spaces. The cells in this region are composed of pseudoparenchyma.
5. Rhizain: Multicellular basal rhizin is formed from the lower cortex region. Rhizine binds the thallus to a substrate and absorbs water and mineral salts.