The beginnings Lichens as a group have received less attention in classical treatises on botany than other groups although the relationship between humans and some species has been documented from early times. Several species have appeared in the works of
Dioscorides,
Pliny the Elder and
Theophrastus although the studies are not very deep. During the first centuries of the modern age they were usually put forward as examples of
spontaneous generation and their reproductive mechanisms were totally ignored. For centuries naturalists had included lichens in diverse groups until in the early 18th century a French researcher
Joseph Pitton de Tournefort in his
Institutiones Rei Herbariae grouped them into their own genus. He adopted the Latin term lichen, which had already been used by Pliny who had imported it from Theophrastus but up until then this term had not been widely employed. The original meaning of the Greek word λειχήν (leichen) was moss that in its turn derives from the Greek verb λείχω (liekho) to suck because of the great ability of these organisms to absorb water. In its original use the term signified
mosses,
liverworts as well as
lichens. Some forty years later
Dillenius in his
Historia Muscorum made the first division of the group created by Tournefort separating the sub-families
Usnea, Coralloides and
Lichens in response to the morphological characteristics of the lichen
thallus. After the revolution in
taxonomy brought in by
Linnaeus and his new system of classification lichens are retained in the
Plant Kingdom forming a single group
Lichen with eight divisions within the group according to the morphology of the
thallus. The
taxonomy of lichens was first intensively investigated by the
Swedish botanist
Erik Acharius (1757–1819), who is therefore sometimes named the "father of lichenology". Acharius was a student of
Carl Linnaeus. Some of his more important works on the subject, which marked the beginning of lichenology as a discipline, are: •
Lichenographiae Suecia prodromus (1798) •
Methodus lichenum (1803) •
Lichenographia universalis (1810) •
Synopsis methodica lichenum (1814) (
Evernia prunastri) Later lichenologists include the American scientists
Vernon Ahmadjian and
Edward Tuckerman and the
Russian
evolutionary biologist Konstantin Merezhkovsky, as well as amateurs such as
Louisa Collings. Over the years research shed new light into the nature of these organisms still classified as plants. A controversial issue surrounding lichens since the early 19th century is their reproduction. In these years a group of researchers faithful to the tenets of Linnaeus considered that lichens reproduced sexually and had sexual reproductive organs, as in other plants, independent of whether asexual reproduction also occurred. Other researchers only considered asexual reproduction by means of
Propagules.
19th century Against this background appeared the Swedish botanist
Erik Acharius disciple of Linnaeus, who is today considered the father of lichenology, starting the taxonomy of lichens with his pioneering study of Swedish lichens in Lichenographiae Suecicae Prodromus of 1798 or in his Synopsis Methodica Lichenum, Sistens omnes hujus Ordinis Naturalis of 1814. These studies and classifications are the cornerstone of subsequent investigations. In these early years of structuring the new discipline various works of outstanding scientific importance appeared such as Lichenographia Europaea Reformata published in 1831 by
Elias Fries or Enumeratio Critico Lichenum Europaeorum 1850 by
Ludwig Schaerer in Germany. But these works suffer from being superficial and mere lists of species without further physiological studies. It took until the middle of the 19th century for research to catch up using biochemical and physiological methods. In Germany and
Johann Bayrhoffer, in France
Edmond Tulasne and
Camille Montagne, in Russia
Fedor Buhse, in England
William Allport Leighton and in the United States
Edward Tuckerman began to publish works of great scientific importance. Scientific publications settled many unknown facts about lichens. In the French publication
Annales des Sciences Naturelles in an article of 1852 "Memorie pour servir a l'Histoire des Lichens Organographique et Physiologique" by
Edmond Tulasne, the reproductive organs or apothecia of lichens was identified. These new discoveries were becoming increasingly contradictory for scientists. The
apothecium reproductive organ being unique to
fungi but absent in other
photosynthetic organisms. With improvements in
microscopy,
algae were identified in the lichen structure, which heightened the contradictions. At first the presence of algae was taken as being due to contamination due to collection of samples in damp conditions and they were not considered as being in a
symbiotic relation with the fungal part of the thallus. That the algae continued to multiply showed that they were not mere contaminants. It was
Anton de Bary a German
mycologist who specialised in
phytopathology who first suggested in 1865 that lichens were merely the result of parasitism of various fungi of the
ascomycetes group by
nostoc type algae and others. Successive studies such as those carried out by
Andrei Famintsyn and
Baranetzky in 1867 showed no dependence of the algal component upon the lichen thallus and that the algal component could live independently of the thallus. It was in 1869 that
Simon Schwendener demonstrated that all lichens were the result of fungal attack on the cells of algal cells and that all these algae also exist free in nature. This researcher was the first to recognise the dual nature of lichens as a result of the capture of the algal component by the fungal component. In 1873
Jean-Baptiste Edouard Bornet concluded form studying many different lichen species that the relationship between fungi and algae was purely
symbiotic. It was also established that algae could associate with many different fungi to form different lichen
phenotypes.
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. Despite the above studies the dual nature of lichens remained no more than a theory until in 1939 the Swiss researcher Eugen A Thomas was able to reproduce in the laboratory the phenotype of the lichen
Cladonia pyxidata 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. 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 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 mm growth per year and less in the following years with average growth of 2–4 mm per year. 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. Such a simple
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. 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.
21st century ==Lichens in pharmacology==