Lichen is a simple slow growing plant.
Characteristically, it forms a low crust-like, leaf-like,
branching growth typically on rocks, walls and trees
. They are composite plants comprising a
fungus that has photosynthetic algal cells. Lichens get
their supply of water and nutrients from the atmosphere
and they may be sensitive indicators of atmospheric
pollution.
Lichen is an ecologically obligate, stable mutualism
between an exhabitant fungal partner and an inhabitant
population of unicellular or filamentous algal or
cyanobacterial cells. Lichen plays an important role in
soil formation. They impart their grey, yellow or green
color to the surface they cover.
The fungal partner of lichens is usually an ascomycete
(sometimes a basidiomycete), the algal partner being a
green (chlorophyta) or a blue-green (cyanophyta) alga.
About 19% of all fungi and 42% of all ascomycota are
lichenized. The number of photosynthetic partners
involved in lichen formation is relatively small; out of 40
genera involved, 25 are algae and 15 are cyanobacteria.
