This picture shows lichen on a tree, seen through a magnifying glass, at the Rennes 1 university in Rennes, western France, on March 26, 2021. - Lichens can be discreet, insignificant, almost invisible, but constitute an unknown reservoir of molecules with medical potential. Fragile or highly resistant to the most hostile environments, they are sentinels of air quality and indicators of climate change. (Photo by Damien MEYER / AFP)