General Introduction to Lichens
Lichens are the organism found in the wood,Stone or the wet walls which are said to be the symbiotically associated orgsnisms between Algae and the fungi.Depending upon the nature and physiology they are may be of three type as crustose, foliose and fruticose.They are the indicator of air pollutant being most sensitive towards sulphurdixide(SO2) and pioneer in vegitation.
The morphology, physiology and biochemistry of lichens are very different from those of the isolated fungus and alga in culture. Lichens occur in some of the most extreme environments on Earth—arctic tundra, hot deserts, rocky coasts, and toxic slag heaps. However, they are also abundant as epiphytes on leaves and branches in rain forests and temperate woodland, on bare rock, including walls and gravestones, and on exposed soil surfaces (e.g., Collema) in otherwise mesic habitats. The roofs of many buildings have lichens growing on them. Lichens are widespread and may be long-lived;however, many are also vulnerable to environmental disturbance, and may be useful toscientist
in assessing the effects of air pollution, ozone depletion, and metal contamination. Lichens have also been used in making dyes and perfumes, as well as in traditional medicines.

| lichen growing on large rock |
The body (thallus) of most lichens is different from those of either the fungus or alga growing separately. The fungus surrounds the algal cells, often enclosing them within complex fungal tissues unique to lichen associations. In many species the fungus penetrates the algal cell wall, forming penetration pegs or haustoria similar to those produced by pathogenic fungi. Lichens are poikilohydric, capable of surviving extremely low levels of water content. However, the re-configuration of membranes following a period of dehydration requires several minutes at least.
Symbionts:Living as a symbiont in a lichen appears to be a very successful way for a fungus to derive essential nutrients, as about 20% of all fungal species have acquired this mode of life. The largest number of lichenized fungi occur in the Ascomycota, with about 40% of species forming such an association.[16] Some of these lichenized fungi occur in orders with nonlichenized fungi that live as saprotrophs or plant parasites (for example, the Leotiales, Dothideales, and Pezizales). Other lichen fungi occur in only five orders in which all members are engaged in this habit (Orders Graphidales, Gyalectales, Peltigerales, Pertusariales, and Teloschistales). Lichenized and nonlichenized fungi can even be found in the same genus or species. Overall, about 98% of lichens have an ascomycetous mycobiont. Next to the Ascomycota, the largest number of lichenized fungi occur in the unassigned fungi imperfecti. Comparatively few Basidiomycetes are lichenized, but these include agarics, such as species of Lichenomphalia, clavarioid fungi, such as species of Multiclavula, and corticioid fungi, such as species of Dictyonema.
Types And Groeth form:
Lichens are informally classified by growth form into:
- crustose (paint-like, flat), e.g., Caloplaca flavescens
- filamentous (hair-like), e.g., Ephebe lanata
- foliose (leafy), e.g., Hypogymnia physodes
- fruticose (branched), e.g., Cladonia evansii, C. subtenuis, and Usnea australis
- leprose (powdery), e.g., Lepraria incana
- squamulose (consisting of small scale-like structures, lacking a lower cortex), e.g., Normandina pulchella
- gelatinous lichens, in which the cyanobacteria produce a polysaccharide that absorbs and retains water.
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