This particular classification system is often referred to as the Linnean classification , though the use of some of the elements predates Linnaeus. I have emphasized that there is some notion of similarity behind any classification scheme but I have not yet said how similarity is defined. That is a critical point which will be the subject of the next section.
For the moment I will assume that there are definitions of similarity for lichens and plants and I will finish this section with some examples of classifications and a few more definitions. For the purpose of illustration here are classifications of the lichen Pannaria sphinctrina , Acacia pycnantha Australia's national flower, the Golden Wattle and the grass Themeda triandra Kangaroo Grass. Moreover, it is a taxon at species rank.
Genus, family, order, class, phylum and kingdom are also examples of taxonomic ranks. Lecanorales, Physciaceae, Fabales, Poaceae, Acacia and Themeda triandra are more examples of taxa, each being a taxon at some rank.
In this hierarchical ordering of living organisms, the total number of species is greater than the number of genera, which in turn is greater than the number of families and so on. There are rules governing the creation of taxonomic names and one is that when a biologist proposes the creation of a new taxon at some taxonomic rank, he or she must ensure that there is at least one new taxon at each lower taxonomic rank. For example, a botanist who wishes to define a new order must ensure that within the new order there is at least one family, which in turn must contain at least one genus, which in turn must contain at least one species.
Warning : If you're looking for the full classification hierarchy for every lichen, this page is not for you. Nor will this page help you identify lichen specimens. Rather, the intent here is to give you some examples of the features or tools used in lichen identification and some general observations about lichen classification.
Lichens are often referred to as lichenized fungi and it is the fungal partner on which the classification of a lichen is based. Therefore lichen classification is part of the subject of fungal classification. On that page click on current outline for the latest classification scheme, down to generic level. At any given time, anyone attempting to classify lichens can use only those features that the available technology reveals.
Several hundred years ago all that was available was the naked eye so features such as growth form, colour and substrate were used to differentiate lichens and by the concepts of genus and species were in use. Since the s the development of tools or techniques such as optical microscopes, chemical testing, growth studies, electron microscopes and DNA analysis have revealed details about aspects such as lichen structure, physiology, metabolic products, ecology and genetics.
Each new tool or method has provided information previously unobtainable. Sometimes this is simply an improvement in seeing something previously known. Early optical microscopes allowed the detection of spores, but with no fine detail of spore appearance, whereas today's optical microscopes reveal finer spore detail.
Spores vary in shape, colour, size and septation depending on species. In other cases the new information is astounding and can generate considerable controversy. Until well into the s lichens had been thought of as single organisms. With the help of the optical microscope Simon Schwendener was able to reveal the dual nature of lichens in a paper published in , yet this idea was vehemently attacked by some influential lichenologists for some decades.
A multiseptate spore. Non-septate spores. Whenever new information has become available it has inevitably supported some existing ideas of lichen classification but contradicted others. As long as new information is capable of being found lichen classification is based on unavoidably incomplete information and so classification schemes are 'works in progress'.
A newly found contradiction simply means that some earlier concepts of taxon demarcation need to be re-examined since all the available evidence should be assessed. I have said that early classification was based on naked eye features. In the 19th century microscopic examination showed that some seemingly quite different species were very similar microscopically. Studies in the field showed there to be good evidence that at times the environment could influence thallus morphology, whereas microscopic features remained constant.
This meant that heavy reliance on macroscopic morphology as the basis for a classification scheme was untenable since no scientific classification should be based on features that were not fixed, but could vary depending on where a thallus grew or which photobiont occurred in the thallus. By the end of the 19th century increased weight was given to microscopic features in the definition of taxa, though macroscopic morphology was not ignored. One result of this work was that some species, once thought to be distinct, were shown to be identical.
Such groups of species were lumped into one. Conversely, there were also cases where what had been thought to be one species, based on naked eye features, could be divided into two or perhaps more based on the evidence of the optical microscope. Thus, what had been one species was split into two or more. The advent of electron microscopes in the 20th century allowed the study of very fine morphological detail and cellular structure. In particular the structure of apothecia, perithecia and asci became important in classification of the ascomycetes in general.
Habit and Habitat 4. Associated Members 5. Nature of Association 6. Classification 7. Structure of Thallus 8. Reproduction 9. Generally the fungal partner occupies the major portion of the thallus and produces its own reproductive structures.
The algal partner manufactures the food through photosynthesis which probably diffuses out and is absorbed by the fungal partner. Lichens are a group of plants of composite thalloid nature, formed by the association of algae and fungi. Based on the morphological structure of thalli, they are of three types crustose, foliose and fruticose.
Only fungal component is involved in sexual reproduction. Basidiospores are produced in Basidiolichen, generally look like bracket fungi and basidiospores are produced towards the lower side of the fruit body. The growth of lichen is very slow, they can survive in adverse conditions with high temperature and dry condition.
There is about genera and 15, species of lichens, widely found in different regions of the world. The plant body is thalloid; generally grows on bark of trees, leaves, dead logs, bare rocks etc. They grow luxuriantly in the forest areas with free or less pollution and with abundant moisture. In India, they grow abundantly in Eastern Himalayan regions. They do not grow in the highly polluted regions like Industrial areas.
The growth of lichen is very slow. The algal members belong to Chlorophyceae Trebouxia, Trentepohlia, Coccomyxa etc. The fungal members mainly belong to Ascomycotina and a few to Basidiomycotina. There are three views regarding the nature of association of algal and fungal partners in lichen:. According to some workers, the fungus lives parasitically, either partially or wholly, with the algal components.
According to others, they live symbiotically, where both the partners are equally benefitted. The algal member synthesises organic food sufficient for both of them. It is like a master and slave relationship, termed helotism. They are classified on the nature and kinds of fruit bodies of the fungal partner. Based on the structure of fruit bodies of fungal partners, Zahlbruckner classified lichens into two main groups:. It is also known as Discolichen e. The fruit body is flask-shaped i.
It is also known as Pyrenolichen e. The plant body of lichen is thalloid with different shapes. They are usually grey or greyish green in colour, but some are red, yellow, orange or brown in colour.
Later, based on detailed structures,. This is the simplest type, where the fungal mycelium envelops either single or small cluster of algal cells.
The algal cell does not envelop all over by fungal hyphae. The lichen appears as powdery mass on the substratum, called leprose Fig. They are either wholly or partially embedded in the substratum, e. These are leaf-like lichens, where thallus is flat, horizontally spreading and with lobes.
Some parts of the thallus are attached with the substratum by means of hyphal outgrowth, the rhizines, developed from the lower surface Fig. These are shrubby lichens, where thalli are well developed, cylindrical branched, shrub-like Fig.
They are attached to the substratum by a basal disc e. In this type, algal members are filamentous and well-developed. The algal filaments remain ensheathed or covered by only a few fungal hyphae. Figure 1. This cross-section of a lichen thallus shows its various components.
The upper cortex of fungal hyphae provides protection. Photosynthesis occurs in the algal zone. The medulla consists of fungal hyphae. The lower cortex also provides protection. The rhizines anchor the thallus to the substrate. In some ways, the symbiotic relationship between lichens and algae seems like a mutualism a relationship in which both organisms benefit. The fungus can obtain photosynthates from the algae or cyanobacterium and the algae or cyanobacterium can grow in a drier environment than it could otherwise tolerate.
However, most scientists consider this symbiotic relationship to be a controlled parasitism a relationship in which one organism benefits and the other is harmed because the photosynthetic organism grows less well than it would without the fungus.
It is important to note that such symbiotic interactions fall along a continuum between conflict and cooperation. Lichens are slow growing and can live for centuries. They have been used in foods and to extract chemicals as dyes or antimicrobial substances. Some are very sensitive to pollution and have been used as environmental indicators. Annotated Bibliography Alexopoulos, Constantine J.
Mims, M. A well-paced book describing the fascinating world of lichens and lower plants. The author discusses their structure, life, history, and uses in a very accessible manner. He even includes a basic identification guide at the end.
A fairly technical field guide describing around of California's 1, known lichens. The keys are supplemented with numerous black and white photos as well as color plates.
The scientific names mentioned above came from this book. Page 16 of 38 We've changed our address to www. Please update your bookmarks! We've changed our address to www.
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