You may have heard about lichens but not many have a good idea on what lichens are in reality. But this fact isn’t very surprising because lichens have been a riddle even for scientists for a long time and continue to remain so. Only in 1867 the dual nature of lichens was discovered by a Swiss botanist Simon Schwendener and it was a really unexpected disclosure in biology. The scientific society has been debating on the nature of lichens until today. There is no exact definition for the term “lichen” approved worldwide. However, according to the most accepted definition, lichens are composite organisms consisting of a fungus (the mycobiont) and a photosynthetic partner (the photobiont) growing together in a symbiotic relationship.The fungus builds the main part of the body of the lichen and encapsulates a population of algae or photosynthetic cyanobacteria. The fungus feeds off the sugars produced by its symbiotic partner. An interesting fact is that if isolated from the lichen, the algae and cyanobacteria continue to grow as independent organisms while the fungal partner at times, cannot. The body of the lichen is called a thallus. It is a complete organism despite being composed of the interaction of different organisms. Each lichen species (though there are a few exceptions) has a specific species of algae which is a very important fact for the taxonomic classification of lichens. Ascomycetes (phylum Ascomycota) are the major fungi found in lichens. The second common type of fungi are Basidiomycetes (phylum Basidiomycota) and a species called Geosiphon pyriforme, representative of the Phycomycota phylum.The fungi possess hyphae which look like long straight or branched filaments. The mycobionts of lichens also produce modified hyphae because of their functional peculiarities. Among the modifications are adipose cells, motor hyphae and plectenchyma. These fungi also possess other types of hyphae which take part in the absorption processes. For example – haustoria, impresoria, appresoria etc.The algae found in lichens are more diverse – Cyanobacteria, green algae, yellow-green algae and even brown algae. The most common genus is the green alga Trebouxia. There are also some interesting algae which possess other algae at the same time. Depending on the way of grouping of algae in the lichen, the thallus is either called heteromer or homomer. In the case the algae are grouped in one layer we call the thallus heteromer and in the case the algae are evenly distributed in the lichen we call the tallus homomer. In terms of evolution the homomer type is more primitive.Lichens are attached to the substrate in different ways. The primitive species are attached with the help of hyphae from the middle part of the thallus. More developed species have proper developed organs of attachment.Lichens are morphologically very different from fungi. Different species have various shapes, sizes, colours, structures and so on. There are red, yellow, green, black, brown and other lichens. Sizes also vary from one millimeter to two meters. In general there are four main morphological types of lichens – crustose lichens, squamulose lichens, foliose lichens and fruticose lichens. But each of these types divide into more subcategories.Lichens have three types of reproduction – sexual, asexual and vegetative reproduction.During the sexual reproductive process, spores are produced xospores are produced during the asexual reproductive process. In both cases only the mycobiont takes part in spore production. During the vegetative reproduction the lichen is able to restore from a very small fragment or specific structures.It’s estimated there are about 20.000 – 30.000 species on the Earth. Lichens don’t make a taxonomic group and its systematics is based on the identification of fungi . If the fungus is an Ascomycete, then the lichen also belongs to the phylum of Ascomycetes and so on.Lichens are commonly found on soil surfaces, on branches and trunks of trees, on rocks and stones. At times these substrates are fully covered by lichens. Lichens are found everywhere. Lichens adapted to live even in extreme conditions – in the coldest parts of Antarctica, in the driest deserts, in the wettest rainforests and on top of the highest mountains.