Lichens
are 'dual organisms.' Every lichen is a partnership between
members of two different kingdoms which live together in
a special, mutually beneficial relationship (a symbiosis).
Each lichen is made up of a fungus (usually an ascomycete)
and an alga (green or blue-green).
There
are almost 20,000 lichens, each involving a different fungus,
but the same algal partner can be found in many different
lichens, so many fewer algae are involved.
The
body of the lichen is built up by tough fungal hyphae, and
the algae live inside that framework.
The
fungus protects the alga from the harsh world outside, and
provides it with water and mineral nutrients. The alga makes
its own food by photosynthesis, and leaks some of this food,
which is then absorbed by the fungus (which cannot make
its own food).
This
partnership is so tough and self-reliant that lichens can
grow in places like bare rock in deserts, where nothing
else can survive. When it is too dry, too hot, or too cold,
lichens go into a state of suspended animation until conditions
improve. Since the algae make up only about 5% of each lichen,
and are out of action for much of the time, you can imagine
that lichens grow very slowly -- only a few millimetres
per year. They make up for this by living for centuries,
or in a few cases, millennia.
Lichens
have only one serious weakness -- they must absorb their
mineral nutrients from the rain. So if the air is polluted
with sulphur dioxide, this dissolves in the rain and is
absorbed by the lichens which often die as a result.
See
pictures of many lichens at Chapter
7 of The Fifth Kingdom.
A much
more detailed account can be found in the book and CD-ROM.
