and algae, each lichen species tends to
be a specific union of one kind of each.
(However, "triads," composed of one
kind of algae and two kinds of fungi,
have been discovered.)
Known as a symbiotic "mutualism,"
lichen are considered to have a gener-
ally equitable relationship. The fungi,
with its reinforced cell walls, multicel-
lular tissues, and resistance to desicca-
tion, can withstand some dry or harsh
conditions, and so provide an insulated
home to the more sensitive photobi-
ont. Inside this shelter, the algae enjoys
higher and more regulated moisture
levels than outside, a temperature
buffer, and screening from potentially
damaging UV rays.
For its part, the photobiont whips up
breakfast, lunch, and dinner out of
literal thin air. A photobiont algae has
membranes that are more perme-
able than its free-living (non-symbi-
otic) cousins, and its sugars flow into
the tissues of the surrounding fungi.
Many of the cyanobacteria are also
capable of nitrogen fixation and pro-
vide this essential nutrient for their
fungal hosts.
The body of a lichen is called a thallus,
and these are found in several distinct
forms. Crustose lichen, as suggested,
form a thin "crusty" layer on rocks
and other substrates. Foliose lichen
have thicker, wavier thalli that are
"leafier" than the crustose. Fruticose
lichen are miniature thickets of hol-
low, branching fungal tubes that can
grow upright or be "pendant"—i.e.
hang down from trees.
Given the tough conditions they find
themselves in and their limited photo-
synthesis production capacity in rela-
tion to body size, lichen grow very
slowly. The annual radial growth of
crustose and foliose species is some-
where in the neighborhood of 0.5-5
mm; growth in the fruticose species
averages 1-2 cm per year. The typical
lichen body is something of an algae
sandwich; two slices of thick, dense,
protective fungal cortex, top and bot-
tom, with a nice spread of green algal
butter, and a spongy layer of fungal
31
Lichens collage from Perlmutter and Rivas Plata, NC State presentation
