19th century

In this scientific panorama appears the Swedish botanist Erik Acharius, a disciple of Linnaeus, who is today considered the father of lichenology by initiating the taxonomy of lichens with his pioneering studies of the flora of Sweden, such as in Lichenographiae Suecicae Prodromus of 1798 or in Synopsis Methodica Lichenum, sistens omnes hujus ordinis naturalis of 1814.[8] These studies and classifications are the cornerstone of subsequent research. In these years of restructuring of the new discipline, various works of notable scientific importance appear, such as Lichenographia Europaea Reformata published in 1831 by Elias Fries or Enumeratio Critica Lichenum Europaeorum published in 1850 by Schaerer in Germany.[9]​.

But these works suffer from superficiality as they lack physiological studies and are mere lists of new species because at this time there is no doubt about the vegetal nature of these organisms and therefore it is not considered necessary to carry out research with them that would be easier to do with higher plants.[10] A few years must pass until, from the middle of the century, various European researchers begin physiological and biochemical studies that allow progress in the study of these complex organisms; Thus Itzigsohn or Bayrhoffer") in Germany, Edmond Tulasne and Jean Camille Montagne") in France, Buhse in Russia, Leighton") in England or Edward Tuckerman in the United States began to publish various works of great scientific importance.

As works on the subject are published, numerous unknowns surrounding lichens are being resolved. The sexual reproduction of these organisms was resolved by the French botanist Tulasne in an article published in 1852 in Annales des Sciences Naturelles entitled Mémorie pour serve à l'Histoire Organographique et Physiologique des Lichens where the fungal reproductive organs or apothecia in lichens were identified.[11]​[12]​.

These new discoveries were increasingly contradictory for scientists, for example when the apothecium, typical of fungi and absent in any other known photosynthetic organism, was identified in lichens as the only reproductive structure. These contradictory characters were accentuated with the discovery of the alga within the lichen structure thanks to advances in microscopy. At first this presence of algae in the thallus was not interpreted as part of a symbiosis but as mere contamination of the samples having been taken in humid places, but as the analyzes multiply it becomes clear that these algae are more than mere artifacts.

Anton de Bary, a German mycologist specializing in phytopathology, was the first to propose in 1865 the possibility that lichens are nothing more than the result of the parasitism of various Ascomycetes fungi on algae of the Nostoc type and other similar ones. Successive studies such as those carried out in 1867 by Famintzin") and Baranetzky") demonstrated the non-dependence of the algar component, that is, its ability to live independently of the lichen thallus.[13]​

It was in 1869 when Simon Schwendener determined that all lichens were the result of fungal attack on algal cells when he observed that all the algae present in these organisms existed freely in nature. This researcher was the first to formulate the dual nature of lichens as a result of the capture of the algae by the fungus.[14] We only had to wait a few years for Jean-Baptiste Édouard Bornet to conclude in 1873 that the relationship between fungus and algae was purely symbiotic after observing a large number of different species of lichens. He also established that this symbiosis was not purely specific since the same species of alga was capable of associating with several different species of fungus to form lichens of different phenotypes. In this way, the nature of lichens was clarified, which over time will be clarified again.

The 20th Century

In 1909, the Russian lichenologist Konstantin Mereschkowski presented a research work entitled The Theory of Two Plasms as the Basis of Symbiogenesis, a New Study or the Origins of Organisms, which sought to explain a new theory of Symbiogenesis through lichens and extensible to other organisms as can be seen from his previous work Nature and origin of chromatophores in the plant kingdom; These novel ideas can be studied today converted into the theory of endosymbiosis.[15]​.

Despite the above, the dual nature of lichens did not go beyond being a theory until in 1939 the Swiss researcher Eugen A. Thomas") was able to reproduce in the laboratory the lichen phenotype of Cladonia pyxidata after the artificial union of its two identified components.[16]​.

During the century, botany and mycology continue trying to solve two of the main problems that exist around lichens, on the one hand the definition of lichen itself and the relationship between the two symbionts and on the other hand the taxonomic position of these organisms within the plant and fungal kingdoms. Numerous renowned researchers appear in the field of lichenology such as Henry Nicollon des Abbayes, William Alfred Weber, Antonina Georgievna Borissova, Irwin Murray Brodo or George Albert Llano.

Lichenology has also turned out to have applications beyond biology itself; in geology, the technique called lichenometry is used, which allows establishing the age of different substrates based on the age of the lichens that develop on them. The dating of this age can be absolute or relative since the growth of these organisms can be arrested in various conditions. The technique establishes an average of the ages of the oldest individuals to give a minimum age to the medium analyzed.[17] The technique is based on the fact that the maximum diameter of the largest thallus of an epilithic lichen developed on a substrate is proportional to the time elapsed since exposure to the medium of that surface according to studies developed by Roland Beschel") in 1950 and is especially useful on surfaces less than 1000 years old. Growth is greatest in the first 20 to 100 years. years, with 15 to 50 millimeters/year and lower in the following years with averages of 2 to 4 millimeters/year.[18]​.

The difficulty of giving a definition applicable to each and every one of the known lichens is a topic that lichenologists have been debating since the symbiotic nature of these organisms became evident. In 1982 the International Association of Lichenology held a convention with the aim of approving a single definition of lichen based on proposals made by a committee. The president of that committee was the renowned researcher Vernon Ahmadjian. The result, finally approved, was that lichens could be considered the association between a fungus and a photosynthetic symbiont that gives rise to a thallus of specific structure.[19]​.

Such a a priori simple definition did not leave any researcher on the subject indifferent and criticism and proposals for amendment soon arose. David L. Hawksworth, for example, considered the definition imperfect since it was impossible to determine what a thallus of specific structure was when observing that various lichens produced changing phenotypes depending on the substrate and the conditions in which they developed. This researcher represents one of the majority trends within lichenologists by considering that it is impossible to give a single definition because lichens are not a single type of organisms.[19]​.

Lichenological studies today are not only used for the description and taxonomic classification of lichens but have multiple applications in various scientific fields. Studies on environmental quality carried out through the growth and development of lichens in ecosystems are especially important. These studies are based on the extreme sensitivity of these organisms to various air pollutants, mainly SO2, which causes the so-called acid rain and prevents the absorption of water.

19th century

In this scientific panorama appears the Swedish botanist Erik Acharius, a disciple of Linnaeus, who is today considered the father of lichenology by initiating the taxonomy of lichens with his pioneering studies of the flora of Sweden, such as in Lichenographiae Suecicae Prodromus of 1798 or in Synopsis Methodica Lichenum, sistens omnes hujus ordinis naturalis of 1814.[8] These studies and classifications are the cornerstone of subsequent research. In these years of restructuring of the new discipline, various works of notable scientific importance appear, such as Lichenographia Europaea Reformata published in 1831 by Elias Fries or Enumeratio Critica Lichenum Europaeorum published in 1850 by Schaerer in Germany.[9]​.

But these works suffer from superficiality as they lack physiological studies and are mere lists of new species because at this time there is no doubt about the vegetal nature of these organisms and therefore it is not considered necessary to carry out research with them that would be easier to do with higher plants.[10] A few years must pass until, from the middle of the century, various European researchers begin physiological and biochemical studies that allow progress in the study of these complex organisms; Thus Itzigsohn or Bayrhoffer") in Germany, Edmond Tulasne and Jean Camille Montagne") in France, Buhse in Russia, Leighton") in England or Edward Tuckerman in the United States began to publish various works of great scientific importance.

As works on the subject are published, numerous unknowns surrounding lichens are being resolved. The sexual reproduction of these organisms was resolved by the French botanist Tulasne in an article published in 1852 in Annales des Sciences Naturelles entitled Mémorie pour serve à l'Histoire Organographique et Physiologique des Lichens where the fungal reproductive organs or apothecia in lichens were identified.[11]​[12]​.

These new discoveries were increasingly contradictory for scientists, for example when the apothecium, typical of fungi and absent in any other known photosynthetic organism, was identified in lichens as the only reproductive structure. These contradictory characters were accentuated with the discovery of the alga within the lichen structure thanks to advances in microscopy. At first this presence of algae in the thallus was not interpreted as part of a symbiosis but as mere contamination of the samples having been taken in humid places, but as the analyzes multiply it becomes clear that these algae are more than mere artifacts.

Anton de Bary, a German mycologist specializing in phytopathology, was the first to propose in 1865 the possibility that lichens are nothing more than the result of the parasitism of various Ascomycetes fungi on algae of the Nostoc type and other similar ones. Successive studies such as those carried out in 1867 by Famintzin") and Baranetzky") demonstrated the non-dependence of the algar component, that is, its ability to live independently of the lichen thallus.[13]​

It was in 1869 when Simon Schwendener determined that all lichens were the result of fungal attack on algal cells when he observed that all the algae present in these organisms existed freely in nature. This researcher was the first to formulate the dual nature of lichens as a result of the capture of the algae by the fungus.[14] We only had to wait a few years for Jean-Baptiste Édouard Bornet to conclude in 1873 that the relationship between fungus and algae was purely symbiotic after observing a large number of different species of lichens. He also established that this symbiosis was not purely specific since the same species of alga was capable of associating with several different species of fungus to form lichens of different phenotypes. In this way, the nature of lichens was clarified, which over time will be clarified again.

The 20th Century

In 1909, the Russian lichenologist Konstantin Mereschkowski presented a research work entitled The Theory of Two Plasms as the Basis of Symbiogenesis, a New Study or the Origins of Organisms, which sought to explain a new theory of Symbiogenesis through lichens and extensible to other organisms as can be seen from his previous work Nature and origin of chromatophores in the plant kingdom; These novel ideas can be studied today converted into the theory of endosymbiosis.[15]​.

Despite the above, the dual nature of lichens did not go beyond being a theory until in 1939 the Swiss researcher Eugen A. Thomas") was able to reproduce in the laboratory the lichen phenotype of Cladonia pyxidata after the artificial union of its two identified components.[16]​.

During the century, botany and mycology continue trying to solve two of the main problems that exist around lichens, on the one hand the definition of lichen itself and the relationship between the two symbionts and on the other hand the taxonomic position of these organisms within the plant and fungal kingdoms. Numerous renowned researchers appear in the field of lichenology such as Henry Nicollon des Abbayes, William Alfred Weber, Antonina Georgievna Borissova, Irwin Murray Brodo or George Albert Llano.

Lichenology has also turned out to have applications beyond biology itself; in geology, the technique called lichenometry is used, which allows establishing the age of different substrates based on the age of the lichens that develop on them. The dating of this age can be absolute or relative since the growth of these organisms can be arrested in various conditions. The technique establishes an average of the ages of the oldest individuals to give a minimum age to the medium analyzed.[17] The technique is based on the fact that the maximum diameter of the largest thallus of an epilithic lichen developed on a substrate is proportional to the time elapsed since exposure to the medium of that surface according to studies developed by Roland Beschel") in 1950 and is especially useful on surfaces less than 1000 years old. Growth is greatest in the first 20 to 100 years. years, with 15 to 50 millimeters/year and lower in the following years with averages of 2 to 4 millimeters/year.[18]​.

The difficulty of giving a definition applicable to each and every one of the known lichens is a topic that lichenologists have been debating since the symbiotic nature of these organisms became evident. In 1982 the International Association of Lichenology held a convention with the aim of approving a single definition of lichen based on proposals made by a committee. The president of that committee was the renowned researcher Vernon Ahmadjian. The result, finally approved, was that lichens could be considered the association between a fungus and a photosynthetic symbiont that gives rise to a thallus of specific structure.[19]​.

Such a a priori simple definition did not leave any researcher on the subject indifferent and criticism and proposals for amendment soon arose. David L. Hawksworth, for example, considered the definition imperfect since it was impossible to determine what a thallus of specific structure was when observing that various lichens produced changing phenotypes depending on the substrate and the conditions in which they developed. This researcher represents one of the majority trends within lichenologists by considering that it is impossible to give a single definition because lichens are not a single type of organisms.[19]​.

Lichenological studies today are not only used for the description and taxonomic classification of lichens but have multiple applications in various scientific fields. Studies on environmental quality carried out through the growth and development of lichens in ecosystems are especially important. These studies are based on the extreme sensitivity of these organisms to various air pollutants, mainly SO2, which causes the so-called acid rain and prevents the absorption of water.