Probably many may have heard about lichens but only few of them 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 even 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. So there is no single and exact definition for the term “lichen” approved of 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. Though different opinions and hypotheses dominated previously. 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 independant organisms while the fungal partner is able to grow separately just a little time.
The body of the lichen is usually 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. And that’s 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 there is also one species called Geosiphon pyriforme, whose fungal partner is represented by the fungi of Phycomycota phylum.
The fungi possess hyphae which look like long straight or branched filaments. But mycobionts also produce some modified hyphae because of their functional peculiarities. Among the modifications are the adipose cells, motor hyphae and plectenchyma. These fungi also possess some other types of hyphae which take part in absorption processes. For example – haustoria, impresoria, appresoria etc.
The algae found in lichens are more diverse – anobacteria, green algae, yellow-green algae and even brown algae. The most common species is the green alga Trebouxia. There are also some interesting algae which possess several 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 of the middle part of the thallus. More developed species have proper organs of attachment.
Lichens are morphologically very different. Different species have various shapes, sizes, colours, structures and so on. There are red, yellow, green, black, brown and other lichens. The size is also very different and varies 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 divides into more subcategories.
Lichens have three types of reproduction – sexual, asexual and vegetative reproduction.
In the result of sexual process spores are produced during the sexual reproduction. Exospores are produced during the asexual one. In both cases only the microbiont takes part in spore producing. During the vegetative reproduction the lichen is able to restore even from a very small fragment or special 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 fungi identification. If the fungus is an ascomycete correspondingly the lichen also belongs to the phylum of ascomycetes and so on.
Lichens are found on soil surfaces, on branches and trunks of trees, on rocks and stones. At times these substrates are fully covered by lichnes. Lichens are found everywhere. Lichens adapted to live even in extremal conditions – in the coldest parts of Antarctica, in the driest deserts, in the wettest rainforests and in the highest mountains.