A lichen is created by the union of a fungus and an alga. As in
the case of a successful marriage, these partners live together
in harmony. Although some say that the fungus is parasitic
on the alga, thereby distorting the alga, such an association
is sufficiently successful to enable a single lichen to survive for several
Lichens have scientific names, as do their components; thus, the fungus
has a name, the alga has a name, and the association has a name
Lichens grow in harsh environments: on rocks and the trunks of trees: in
salt spray, the desert, the arctic, and tropical forests. Yet lichens are sensitive,
and their presence or absence can be used as an indicator of pollution. For
example, lichens often do not grow well on the trunks of trees in the city.
Lichens may be crustose, foliose, or gelatinous. Some crustose lichens
seem to be painted on the substrate. Verrucaria maura
is a lichen that forms
a distinct black band just above high tide along the New England coast.