This way for clean air

Lichens can't cope with air pollution, largely because they get most of their nutrients from the air; particulates clog up their pores and they grow too slowly to work around this, unlike regular plants.

Not that they're plants - they are a symbiotic pairing of fungus and algae.  Some of them contain an interesting hallucinogen called usneic acid, and in Scandinavia, some people like to smoke them.  But there's not much to do over there...