When encountering the various types, shapes, textures of Lichen one would come to a conclusion that attempting to gain a layman knowledge of Lichen would be a daunting task. Well, good news is, there are 6 basic categories of Lichen, depending on who you ask. I list four that are commonly found in the wild, One type not listed is the "gelatinous" type, which for me can be confused with fungal growth in general.
Tree Lichen about 3 inches long growing on a tree branch during the winter time
What is Lichen? Lichen is basically an organism that is formed by filaments of it's fungal partner, normally consisting of various shades of greens, grey, and in some instances reds. On the outer surface you see mostly the fungal mass, which can not by itself process light to generate its nourishment. Thus the underpinning of Lichen is the alga which provides food to the fungal matter. Together the fungal and alga depend on each organism in order to survive. In short the fungal protects the alga with a protective shell that absorbs ultraviolet light, allowing this fascinating partnership to survive in unlikely places.
Grey or Green - depending on the availability of moisture the fungal cortex will become transparent as moisture is about and what you see underneath is mostly the green alga doings it business, and thus appear green.
Lichens are a complex life form that is a symbiotic partnership of two separate organisms, a fungus and an alga.
One odd fact I learned about Lichen is some types will only flourish in the most pristine of air quality - which make it excellent indicator of the overall quality of the environment. Lichen also can be asexual e.g., a small bits floats off and begins a new Lichen or through a microscopic process called soredia. So, how long do Lichen live? The jury is still out on that - but we do know Lichen can live for many many years. What normally impacts the growth of lichen like so many other things in nature is human activity. It has been reported some growths of Lichen are still going after a 100 years.