Lichens Rosemary Etheridge Lichens Are Small and Insignificant and Hard to Identify So They Are Often Ignored but They Can Be Beautiful
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Lichens Rosemary Etheridge Lichens are small and insignificant and hard to identify so they are often ignored but they can be beautiful. They are organisms that grow on surfaces such as rocks, bark and soil. Churchyards are particularly good places to look, especially limestone tombstones. Lichens on a tombstone What are lichens? A lichen is a small ecosystem in its own right comprising 2 or sometimes 3 organisms, a green alga or cyanobacterium and a fungus, living together in harmony in one body as an example of symbiosis. The fungus protects the alga or bacterium from drying out and from intense sunlight while the green alga or cyanobacterium makes food by photosynthesis. The bacteria also absorb atmospheric nitrogen and turn it into nitrates. It could also be argued that it is parasitism by the fungus rather than symbiosis, since the fungus gets most of the advantage from the relationship as the alga can live alone and as part of a lichen it gives up 80% of its food to the fungus. Lichens can occur in any habitat, from polar regions and mountain tops through temperate regions, deserts and the tropics. They can cope with extreme temperatures, drought, ultraviolet light and ionising radiation. However, air pollution kills lichens because they absorb water and any dissolved contents over the whole of their surface. This means that lichens can be used for bio-monitoring, one example being the complete disappearance of Usnea from the industrial areas of the north, Midlands and London and the South-east but with the improvements in air quality it is making a comeback. Usnea The replacement of the leucistic variants of the peppered moth by melanistic variants during the industrial revolution was a reflection initially of the disappearance of the light-coloured lichen from trees and as it has reappeared, so the leucistic variants have also returned. Uses of lichens Lichens are mainly carbohydrates with 1-5% protein and no fat and are eaten by some animals, including slugs and insects as well as reindeer in the Arctic tundra (reindeer moss). They are also eaten by some humans as emergency food, staple food or a delicacy and the manna from heaven may have been lichens. Iwatake (Umbilicaria esculent) is a traditional Korean and Japanese delicacy: it is quite expensive and is collected off the sides of cliffs. Lichen dyes have been known since ancient times. This was an important cottage industry in the Highlands and islands of Sotland, where Harris Tweed was traditionally dyed with lichen dyes. The lichens were scraped off rocks with spoons, hoops or sea shells. Litmus to distinguish acids and alkalis was derived from a lichen. Essential oils extracted from some lichens have a musk smell and were used in perfumes and cosmetics and about 50% of lichen species produce substances with antibacterial properties and have been used in folk medicine, in antibiotic creams, cough mixtures and pastilles. Features of lichens Lichen growth forms are of 4 types: Crustose (flat) lichens with an upper cortex overlying the algal layer and medulla; Foliose (leafy) lichens with an upper cortex, algal layer, medulla and a lower cortex, being attached to the substrate by many short rootlets (rhizinae); Fruticose (bushy) lichens with a central core surrounded by medulla the algal layer and outer cortex; and Squamulose (scaly ) lichens with small scales. There are a number of different means of reproduction. Spores are produced by fruiting bodies (apothecia) and contain fungal material only. Fungi are of 2 types, spore droppers and spore shooters and most of those in lichens are spore shooters. The spores float around until they land on a suitable substrate. After germination they may acquire free-living algae or take algae from existing lichens. Apothecia may be leconorine (“jam tarts”) or lecideine (“fruit gums”). “Jam tarts” and “Fruit gums” There is also vegetative reproduction with soredia containing algal cells wrapped in fungal filaments and isidia being peg-like outgrowths from the cortex containing both types of cell. Soredia and Isidia Lichens vary in their colours and look greener when wet because the outer cortex is opaque when dry and the inner algal layer is only visible when the cortex is wet. This also means that photosynthesis only occurs when the lichen is wet so lichens in desert regions actively photosynthesise only when wetted by early morning dew, low cloud or the infrequent rain. Some examples of lichens Lepraria incana is a leprose (scurfy) lichen which is pollution-resistant. It forms an amorphous crustose layer, particularly on tree trunks and is one of the commonest lichens in eastern Britain. Lepraria incana and Lecanora muralis The chewing gum lichen (Lecanora muralis) is also pollution-resistant. It is very common on man- made substrata such as paths and pavements, where it is often mistaken for discarded chewing gum. It is crustose but made up of tiny lobes and has “jam tart” fruiting bodies. The orange lichen (Xanthoria parietina) is one of the most resistant foliose species to sulphur dioxide and nitrogen dioxide air pollution. The lichens with whiskers (Physcia tenella and Physcia adscenens) have whiskers which are visible only with a hand lens. They grow on twigs, trees and street signs etc. Both Xanthoria and Physcia like nutrient-enriched substrates and often grow together. Xanthoria and Physcia growing on the top of a post box The Parmelia family are foliose lichens which grow in vast patches. There are a lot of different lichens in this group, which were formerly all classed as Parmelia but the genus is now split into lots of separate genuses. Examples include the hammered shield lichen (Parmelia sulcata), Flavoparmelia coperata, Parmotrema perlatun and Punctelia subrudecta. Parmelia sulcata and Evernia prunastri Oak moss (Evernia prunasti) is a foliose lichen which was used for perfume-making. It is common on Stanmore Common. Fruticose (bushy) lichens are usually intolerant of air pollution. The only two species that grow in the Harrow area are Ramalina farinacea and Ramalina fastigiata. Ramalina farinacea and R. fastigiata The Cladonias include a number of species that are not easy to distinguish. Cladonia coniocraea is the most common and tends to grow on old rotting stumps. There are quite a lot in Pear Wood. Cladonia cup lichens have fruiting bodies around the rim of the cups. Cladonia coniocraea and C. chlorophaea Conclusion Lichens are very difficult to identify. It cannot usually be done just by looking at them. There are chemical tests and then microscopic examination is needed. A useful book is Frank S. Dobson’s Lichens: an Illustrated Guide to the British and Irish Species. There are a number of Field Studies Centre guides on lichens and there is also the old standby of The Observer’s book of lichens. The speaker and Jane Skelly will be leading the Society’s lichen walk at the Bentley Old Vicarage nature reserve and All Saints churchyard on Saturday 2 April. .