38 Nebraskaland • April 2020
that retains moisture, while some species are restricted
to iron or other metal-rich rocks. Interestingly, the latter
species can often tolerate metals, such as copper, that kill
other organisms.
Lichens, which have no mechanism to prevent desiccation,
survive on exposed rocks and other extreme habitats by
drying out and not photosynthesizing during periods of
extreme heat, cold or drought. In arid climates, such as
deserts, they often subsist only on dew, capturing what little
moisture there is each night and photosynthesizing until
mid-morning when the lichen dries out. Because conditions
for optimal photosynthesis may not last long each day, most
lichens grow slowly, often only a few hundredths of an inch
per year. On the fl ipside, they have long life spans. A map
lichen growing in the Arctic is estimated to be 8,600 years
old, making it one of the Earth's oldest living organisms.
Reproduction
Lichens reproduce sexually and asexually (vegetatively).
As for the latter, dry lichens can simply break into fragments
that are wind dispersed and, if they land on a suitable
substrate, resume growth when moisture returns. Many
lichen species also have specialized outgrowths containing
both alga and fungus cells designed to break off and form
new lichens.
In lichens undergoing sexual reproduction, spores are
usually produced in open disc-shaped or closed dome-shaped
structures called ascomata. During sexual reproduction, only
the fungus is reproduced, and the young fungus must fi nd
and capture the appropriate alga for a new lichen to form.
This is a chancy event, and may explain why lichens put
more stock in the less risky, asexual reproduction.
A pixie cup lichen, a fruticose species, grows on a downed
log in a spring-branch canyon on the central Niobrara River
Valley in Brown County. The trumpet-like formations are
stalks topped with disc-shaped ascomata, reproductive
structures.
