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Plant of the Week
Foliose Lichens
Latin: Parmelia sp.
In the midst of a record cold winter it might be
encouraging to examine an organism that actually flourishes during this type
of weather. Lichens love cold weather. In fact, they only grow in the winter
when temperatures are below 50 degrees. They go dormant during the summer
heat.
Lichens are everywhere in our environment, but few of us know, or perhaps
care, much about them. But like all of Mother Nature’s bounty, they have a
fascinating story to tell.
What makes lichens unique is that they are a symbionts – sort of the Borgs
of the plant kingdom. The fungal part of the association primarily falls to
one of the species of ascomycetes, or cup fungi. The algal part of the
symbiosis relies on just two species that are found in 70 percent of all
lichens.
The algae function in photosynthesis and sometimes nitrogen fixation, while
the fungus provides structure and support for the algae. These species have
evolved to the point that they cannot live without their partner. Being
somewhat prudish, they don’t have sex but reproduce vegetatively.
More than 13,500 kinds of lichens have been identified. These are broken
down into three major categories. The foliose (leaf-like) lichens are the
most common types that grow on the trunks of trees or on rocks in the shady
woods. They’re usually gray-green and form more or less circular colonies.
The glamorous lichens are the fruticose (shrub-like) lichens that grow more
or less like real plants. Of this group, the two most prominent in Arkansas
are reindeer moss (Cladonia sp.), which forms brittle gray buns 6-8
inches across in sunny spots in our mountainous counties, and
old-man’s-beard (Usnea longissima), which looks like Spanish moss
and grows in isolated locations where the moisture and temperature
conditions are perfect.
The Rodney Dangerfield’s of the lichen clan are the crustose (crustose)
lichens that grow closely adhered to or inside rocks. These are often
colorful shades of yellow, green, orange or black. They are hard to study.
Arkansas probably has about 300 kinds of lichens counting the foliose and
fruticose types, but no one knows for sure how many crustose types there
are.
Lichens are edible – well sort of. In the tundra region of the far north,
the principle winter browse of reindeers is reindeer moss, the same species
that occurs here. European explorers of the arctic regions learned that
foliose lichens growing on rocks could be eaten if all other food sources
were depleted. No doubt it tasted better if they called it by its French
name, tripe de roche, which translates roughly as "guts of the rock."
The foliose lichens found on rocks are very slow growing. They usually grow
less than a millimeter a year. By comparing the size of a colony with the
known growth rate of a given species, fairly accurate dating is possible. In
fact, a special branch of archeology uses the growth rate of lichens as a
means of dating structures.
Lichen encrusted stone is more valuable to the stone pickers of Arkansas
than field stone without a comparable patina. But unfortunately,
transplanted lichens seldom survive being moved from a cool shaded forest to
the side of your house. Of course, dead lichens look almost like living
lichens, so hardly anyone notices.
Eventually, a new species of lichen will colonize your native stone facade,
so just have patience. Of course, at a millimeter a year, it will be some
time before you have much to brag about.
By: Gerald Klingaman, retired
Extension Horticulturist
- Ornamentals
Extension News
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