Editing Lichenology (section)

===20th century===

In 1909 the Russian lichenologist [[Konstantin Mereschkowski]] presented a research paper "The Theory of two Plasms as the basis of [[Symbiogenesis]], A new study on the Origin of Organisms", which aims to explain a new theory of Symbiogenesis by lichens and other organisms as evidenced by his earlier work "Nature and Origin of [[Chromatophores]] in the Plant Kingdom". These new ideas can be studied today under the title of the Theory of [[Endosymbiosis]].<ref>{{cite journal | last1 = Cavalier-Smith | first1 = T | year = 2003 | title = Microbial Muddles | doi = 10.1641/0006-3568(2003)053[1008:mm]2.0.co;2 | journal = BioScience | volume = 53 | issue = 10 | pages = 1008 | doi-access = free }}</ref>

Despite the above studies the dual nature of lichens remained no more than a theory until in 1939 the Swiss researcher Eugen A Thomas<ref>[[Species:Eugen A. Thomas]]</ref> was able to reproduce in the laboratory the phenotype of the lichen ''[[Cladonia pyxidata]]''<ref>{{cite web | url=http://www.voyageurcountry.com/htmls/floweringplants/plants/pixiecups.html | title=Pixie Cups (Cladonia pyxidata)}}</ref> by combining its two identified components.

During the 20th century botany and mycology were still attempting to solve the two main problems surrounding lichens. On the one hand the definition of lichens and the relationship between the two symbionts and the taxonomic position of these organisms within the plant and fungal kingdoms. There appeared numerous renowned researchers within the field of lichenology such as [[Henry Nicollon des Abbayes]], [[William Alfred Weber]], [[Antonina Georgievna Borissova]], [[Irwin M. Brodo]], and [[George Albert Llano]].

Lichenology has found applications beyond [[biology]] itself in the field of [[geology]] in a technique known as [[lichenometry]] where the age of an exposed surface can be found by studying the age of lichens growing on them. Age dating in this way can be absolute or relative because the growth of these organisms can be arrested under various conditions. The technique provides an average age of the older individual lichens providing a minimum age of the medium being studied.<ref>{{cite book |title=Procesos biofísicos actuales en medios fríos: estudios recientes |language=es |trans-title=Current biophysical processes in cold environments: recent studies |editor=Antonio Gómez Ortiz |date=1998 |publisher=Edicions Universitat Barcelona |url=https://books.google.com/books?id=r_LQ70GnOnoC&q=lichenometry&pg=RA1-PA327|isbn=9788447519231 }} Miguel Mateo Garcia, The Growth Curve of Rhizocarpon geographicum in the Valley of Madriu Andorra, Biophysical Processes in Cold Media {{ISBN|84-475-1923-6}}</ref> Lichenometry relies upon the fact that the maximum diameter of the largest thallus of an [[epilithic]] lichen growing on a substrate is directly proportional to the time from first exposure of the area to the environment as seen in studies by [[Roland Beschel]]<ref>{{cite web|url=http://www.geomorphology.org.uk/assets/publications/subsections/pdfs/OnsitePublicationSubsection/91/4.2.7_lichenometry.pdf |title=Archived copy |access-date=2014-01-21 |url-status=dead |archive-url=https://web.archive.org/web/20140203072554/http://www.geomorphology.org.uk/assets/publications/subsections/pdfs/OnsitePublicationSubsection/91/4.2.7_lichenometry.pdf |archive-date=2014-02-03 }}</ref> in 1950 and is especially useful in areas exposed for less than 1000 years. Growth is greatest in the first 20 to 100 years with 15–50&nbsp;mm growth per year and less in the following years with average growth of 2–4&nbsp;mm per year.<ref>{{cite book |url=https://books.google.com/books?id=xASdKCoT6McC&q=lichenometry&pg=PA562 |last=Schaetzl Randall |first=J |author2=Sharon Anderson |year=2005 |title=Soils Genesis & Geomorphology |publisher=Cambridge University Press |page=562 |isbn=978-0521812016}}</ref>

The difficulty of giving a definition applicable to every known lichen has been debated since lichenologists first recognised the dual nature of lichens. In 1982 the [[International Association for Lichenology]] convened a meeting to adopt a single definition of lichen drawing on the proposals of a committee. The chairman of this committee was the renowned researcher [[Vernon Ahmadjian]]. The definition finally adopted was that lichen could be considered as the association between a fungus and a photosynthetic symbiont resulting in a thallus of specific structure.<ref name="Hawksworth 1989">David L. Hawksworth (1989) "Interactions Fungus and Alga in Lichen Symbiosis liquenoides" ''Annals of the Botanical Garden of Madrid'' (46).</ref>

Such a simple [[A priori and a posteriori|a priori]] definition soon brought criticism from various lichenologists and there soon emerged reviews and suggestions for amendments. For example, [[David L. Hawksworth]] considered the definition imperfect because it is impossible to determine which one thallus is of a specific structure since thalli changed depending upon the substrate and conditions in which they developed. This researcher represents one of the main trends among lichenologists who consider it impossible to give a single definition to lichens since they are a unique type of organism.<ref name="Hawksworth 1989"/>

Today studies in lichenology are not restricted to the description and taxonomy of lichens but have application in various scientific fields. Especially important are studies on [[environmental quality]] that are made through the interaction of lichens with their environment. Lichen is extremely sensitive to various air pollutants, especially to [[sulphur dioxide]], which causes [[acid rain]] and prevents water absorption. [[File:Merezhkovsky K S.jpg|thumb|K S Merezhkovsky Russian Lichenologist (1855–1921)]]