In this article we will discuss about the thallus structure of lichens with the help of diagrams.
In a lichen thallus, the phyco-symbiont and the myco-symbiont are intimately associated, but their identity is maintained. On the basis of gross morphology, lichens are distinguished into three types — crustose, foliose and fruticose. The crustose lichens grow as a crust adhering closely to a substratum which may be a rock, bark of trees, mortars of walls, roof-tops or even soil.
In these lichens, the thallus is a flat, dorsiventral structure, closely adpressed to the substratum, e.g. Parmelia acetabulum. The foliose lichens have leaf-like appendages resembling the liverworts. They remain attached to the substratum with a relatively smaller area compared to the crustose types, e.g. Cetraria islandica.
The fruticose lichens have a bushy, more or less upright or pendulous habit, e.g. Cladonia rangiferina. The basidiolichens have a quite different type of thallus. They resemble the basidiocarps of thin bracket- fungi projecting from the branch of trees which serves as the substratum, e.g. Cora pavonica.
The different forms of lichen thalli are shown in Fig. 5.27:
The internal structure of most lichen thalli shows differentiation into several layers of fungal tissues with interlocked algal or cyanobacterial cells or filaments. In the foliose thalli, these layers are more or less well-defined and arranged dorsiventrally. In crustose forms, the layers are not so well demarcated. In fruticose lichens, the dorsiventrality is absent.
The tissue layers of lichen thalli, in general, are the upper cortex consisting of a more or less compact hyphal layer, an algal or cyanobacterial layer below the upper cortex, a medulla composed of very loosely interwoven hyphae and a lower cortex consisting of a compact hyphal layer.
The algal layer contains the algal cells or filaments interwoven with fungal hyphae. A generalized internal organization of these four layers is diagrammatically represented in Fig. 5.28. In addition to the above tissue-layers, lichen thalli have rhizoid-like structures, called rhizines which attach the thallus to the substratum and help in absorption of water and mineral salts. The algal zone often exhibits presence of characteristic haustoria produced by the fungal hyphae which are sent into the algal cells, or in some cases, each algal cell is surrounded by hyphae (Fig. 5.29A and B).