Growing on your stone wall, your roofing shingles, on bare rocks and most commonly on tree trunks or limbs are unique organisms called lichens. Lichens are rather unusual because they are actually two organisms living together, fungi and some type of algae.

Lichens survive because they have a symbiotic relationship. Symbiosis in simpler terms means: “living together.” There are quite a few examples of symbiosis. One like parasitism is harmful to the host while the parasite benefits.

The symbiosis that is occurring in lichens is referred to as mutualism. Mutualism is when both organisms in the relationship benefit. Another simple example of mutualism would be the relationship between a tick bird and a rhinoceros. Where the tick bird benefits by eating pests from the hide of the animal and obviously the rhino benefits by having their parasites removed.

Lichens can survive in some harsh conditions because the fungi “part” of the lichen is good at retaining water. The algae, which is usually only a few cell layers thick beneath the surface of the lichen contains chlorophyll, captures the sun’s energy and makes food. The food is made for the algae and the fungus part of the plant. Lichens do not have vascular tissue like celery, shrubs or trees to carry food and water and thus remain rather small plants. They don’t have roots either.

However, a lichen has rhyzines, tiny rootlike structures that are able to grab onto the rocks or walls where they grow. These rhyzines over a very long time can actually help to break down a rock into tiny soil-producing particles.

I wanted to avoid the specifics and biology of lichens so this column didn’t end up being too technical. However, know that there are three types of lichens: crustose, foliose and fruticose.

Crustose lichen species usually grow flat on tree trunks or rocks and are a often little more than a few sheets of paper thick. Foliose lichens also grow on trunks and rocks but are attached only in spots by their rhyzines and will appear leafy above the rock face. The fruticose lichen is like a miniature shrub, a half inch, one inch, maybe a few inches in height. They can grow upward or hang. The Arctic Tundra supports “reindeer moss” which is actually a lichen that caribou and musk oxen feed upon.

Lichens are generally beneficial. One aspect of that is they can grow where little else can and slowly begin the process of soil production. They are also great indicators of our atmospheric health. Lichens do poorly or even die when the air contains too much sulfur dioxide. Sulfur dioxide combines with moisture to produce acid rain.

Our efforts to control smokestack emissions has resulted, at least in the Times News area, in cleaner air. Although I didn’t conduct a scientific study, it appears that in my lifetime, lichen growth on trees has definitely increased. On your nature walks take note of the variety of lichen species that can be found almost everywhere. Get out there and enjoy.

Test Your Outdoor Knowledge: Root beer, an old time favorite soda, was once made from the roots of a ____ tree. A. yellow birch, B. witch hazel, C. hickory, D. sassafras.

Last Week’s Trivia Answer: The firefly is Pennsylvania’s state insect.

Contact Barry Reed at breed71@gmail.com.