Lichens

I learned recently that lichens are a symbiotic relationship formed between algae and fungus. This means that the many varieties that exist of lichen are actually each distinct collaborations between these two organisms. Isn't that amazing? 

Last weekend I was on a walk through the Homewood cemetery in Pittsburgh and I noticed some lichens growing on tombstones and on trees. What struck me was that the lichens seemed to grow selectively on one side of the stone only, which made me wonder if their ecosystems are so sensitive that even the tiniest changes in the light conditions might lead to hospitable conditions for them, or not. On trees, they grew in a gradient where they were plentiful on one side of the tree and increasingly less plentiful along the curve into the sunlight.

I also wonder if we see lichens growing on trees and stones because of the nature of these materials as water-retaining. If you touch a stone, especially here in Pittsburgh, sometimes it feels wet. The same is true for bark. I don't know about fungus, but I wonder if that's what the algae part of the collaboration is able to grow far from the water. I generally think of the ocean when I think of algae, don't you?