Information about lichens


1. What are lichens?

Lichens are in fact a symbiotic lifeform between a fungus and a alga. This symbiosis is so intens that seemingly a new lifeform generates: the lichen.
For a long time lichens even were concidered to be a seperate taxonomic group. Until in the 18th century lichens were even concidered to be mosses.
A difference between mosses and lichens is that lichens are able to make dishlike fruitbodies, just like fungi: apothecia (7)
A symbiosis is a lifeform where all the partners have profit of each other. Often the partners get so dependant on each other, that they are not able to live separately any more. In the case of the lichens, the fungus delivers water and minerals to the alga, while the alga takes care of the fotosynthesis ans supplies suchars to the fungus. The fungus sucks these products out of the cells of the algae by means of haustoria (suction-pipes) which penetrates the algal cells. Due to this special liveform, lichens are able to survive extreem conditions: they live on rocks, in freezing conditions, very dry regions...




2. Structure of lichens.

Although a lichen looks like a 'normal' plant, the organism has a thallus.
Fungi consist of widely dillatated filaments (hyfens). In the lichen these hyfens surround the algae. In most lichenspecies these algae are situated in a layer: the algaelayer (2). This layer is identifiable with a good magnifier. The axtreem abiotic actors force lichens to form a protective layer: the cortex (1) . This layer is formed by the fungus. Here by short, thick hyfens cling togerther.
On the bottomside to there is a protective layer: the pith (3). Mostly the pith consist of a loosely weaved hyfennetwork. Under the pith you'll find the undercortex (4) which has the same structure as the uppercortex.
On the bottom you'll often find 'rootlike' structures: the rhizines (5). Generally these also are bundels of hyfens.

This layered structure makes sure that lichens have a long lifetime. Even after years of severe dryness, some species are able to 'revive' by water absorption. Under normal circumstances, a bushlike lichen can live for more then 10 years, crusty lichens even can get older then 100 years...
That same layered structure obstructs the change of gasses, necessary for the fotosynthesis. Therefore with a lot of species the cortex is often interupted so respirationpores arise: cyphella en pseudocyphella (6)).
An important problem is the procreation of the lichens. The fruitboddies that arise with a lot of species, are formed by the fungus. When they are dishlike, they are called apothecia (7). This production of fungusspores, however, is not an optimal way of reproduction. The spores do have to find the right algae to start a new lichen-symbiosis.
A joint dispersion of the two symbionts is more significant. The easiest way is the accidential dispersing of small pieces of the lichen. When dry, the lichen is very fragile so small fragments of the lichen can break of and get spread by the wind.

A lot of species don't wait on this accidental dispersion: they constrict small pieces of the thallus. Hereby outgrows (composed of fungi and algae) can arise: isidia (8) .


Even more favourable are structures where the algae are surrounded by hyfens to form germing pellets or soredia (10) . After dispersion these soredia can grow fast and form a new thallus.
Herewith the cortex tears up so the algal layer can come out: soralen (9)arise.



3. Making microslides.

1. leave a small piece of a leaflike lichen to dry;
2. drop een thin layer of paraffine (e.g. melting candle) on a piece of balsawood;
3. as soon as the paraffine starts to coagulate (it forms a film on top of the layer), you bring the fragment of the lichen on the paraffine;
4. instantly cover up the lichen with more fluid paraffine;
5. let coagulate completely (use frigerator);
6. chop the paraffine and the embedded lichen with a razor-blade in very, very thin slices; use your forefinger to guide the razor-blade
7. bring the cuttings in a petridish with a solvent (aceton, toluol,...) and leave it there till all the paraffine has dissolved;
8. carry the thallusfragments with a brush in an other dish with diluted eosinesolution and leave them there for about 5 minutes;
9. rinse the coloured slices in water and bring them in a diluted solution of astrablue (ca 3 minutes);
10. rinse again in water;
11. bring in ethanol (eth.20%, eth.40%, eth.80%, tolueen, and xyleen) each time for 5 minutes;
12. embed in a Euparal.
The algae are red, while the hyfens are blue.

4. Importance of lichens.

Lichens are very good bio-indicators to survey the air pollution. The different species are specificaly sensitive for certain concentrations of sulfurdioxid, that the appearance of some species indicates this concentration.

Altough lichens are very good usable as air pollution detector, they have less economical value.
The most important is Reindeermoss (Cladonia rangiferina). This lichen is not only food for the Reindeer, but in Lapland it's also used as cattle-fodder.
In former days lichens played an important role in the fabrication of pīgments (litmus for instance); Nowadays its only used for the dying of wool of very good quality.
Also as a medicine lichens use to have more importance. Lungmoss (Lobaria pulmonaria) was used against lungdiseases while Leatherrmoss (Peltigera sp.) was a cure against rabies. Icelandic moss (Cetraria islandica) is still processed in coughing-mixtures. Species like Evernia prunastri and Pseudevernia furfuracea are used in parfums while Umbilicaria esculenta is on the banquet-menu in Japan.


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